Group 10 - Line Trimmer

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Contents

Overview

This Wiki page serves to present all deliverables in a concise and organized setting for MAE 277 Group 10 of the Fall 2009 semester. Group 10's product is the Line Trimmer. All sections may be accessed using the Table of Contents or by standard scrolling. This page will be updated at the conclusion of each Gate.

Request for Proposal

Delivery Date: 10/09/2009

Work Proposal

Introduction

We plan to completely disassemble the line trimmer as far as possible with the tools we have readily available. From there we will select some of the major components that make the line trimmer run and function the way that it does. We will make solid models of these components on a computer, perform an engineering analysis of them and try to develop possible revisions that could be made to make these components even better. Lastly, we will reassemble the line trimmer into the same condition we started with.

Required Tools for Dissection

Socket Wrench Set - Used to remove several nuts that are on the product

  • Bottom of the trimmer
    • There is a nut holding in place the plastic disk which holds the trimming pieces
    • There is a nut between the plastic disk and the metal shaft
  • Spark plug
    • We will need a wrench to remove the spark plug

Flat head screwdrivers - These will be used to remove flat headed screws and pry open certain areas of the line trimmer

  • Bottom of the trimmer
    • There are two screws holding the plastic foot guard on
  • Top of the trimmer
    • External coverings that would be best removed by prying with a flat headed screwdriver
    • There are some visible flat head screws that are used for making tuning adjustments to the engine which will need to be removed

Phillips head screwdrivers - These will be used to remove Phillips head screws

  • Top of the trimmer
    • Four Phillips head screws keeping the trigger hand grip on
    • Two Phillips head screws holding the cover of the choke box on
    • Under the external coverings we expect to find additional Phillips head screws holding down more coverings and pieces to the main frame

Allen Wrench Set – These will be used to remove screws with an Allen head on them

  • Top of the trimmer
    • There are six Allen wrench screws on the bottom of the engine and tank cover
    • There are four Allen wrench screws on the top of the engine and gas tank cover

Small Hammer - It will be used for loosing parts that need to be pried off
Needle-nose pliers - These will be used for removing springs and the throttle cable

Potential Challenges and Outcomes

Refer to Gantt Chart and Timeline in the Management Proposal

Durability of Parts - If we break a piece during our disassembly or reassembly, it will take time to replace the piece properly. This will also present a challenge as it will be necessary to find an almost exact replica of the part to ensure proper reassembly.


Scheduling Conflicts Among Group Members - It is possible that tests, homework, sports and various other activities could interfere in the ability of the group to coordinate meeting times. This could slow down any phase of the project because full group cooperation is what is needed to create and provide a complete and well coordinated analysis.


Possible Emergency - Emergencies happen and the ability of the group to maintain on the predicted timeline may be a challenge as major emergencies are not accounted for in the timeline.

Ability to Assemble/Reassemble Product - Based on experience in the group, the amount of time to assemble the trimmer will vary. While minor variations are accounted for in the timeline, a major problem in our ability to disassemble/reassemble will cause our group to fall behind schedule.

Member’s Capabilities and Shortcomings

Andrew Engelbach-Schafer

  • Capabilities
    • Some Autodesk Inventor experience
    • Some experience in disassembling products
    • Group project experience from PLTW courses in High School and Architecture internship
  • Shortcomings
    • No experience with a Wiki
    • Never disassembled a small engine

Ryan Adams

  • Capabilities
    • Experience with group projects
  • Shortcomings
    • No experience with solid modeling

Damitha Abeykoon

  • Capabilities
    • Experience with automotive engines
  • Shortcomings
    • Not familiar with small engines
    • No experience with engineering analysis

Patrick Smith

  • Capabilities
    • Wiki administration experience
    • Autodesk Inventor experience
    • Exposure to reverse engineering techniques from PLTW courses
    • Experienced in group settings
  • Shortcomings
    • No experience in disassembly of gas engines
    • Poor coordination when dealing with small components

Justin Storms

  • Capabilities
    • Experience with group projects through clubs, engineering internship and previous courses
    • Knowledge of tools from previous jobs and projects
    • Quick learner to Wiki software
  • Shortcomings
    • Little experience with small gas engines
    • No experience with 3D modeling

Management Proposal

Group Meetings

ADD FORMATTING

The group plans to manage the work through a sustained effort during the semester. The roles listed below assign responsibilities to each member and hold them accountable for the completion of certain tasks. The tasks that need to be done are listed in the timeline. The timeline and Gantt chart will serve as a guide for the group to gauge and evaluate the overall progress towards the final completion of the project throughout the semester. The group will have regular meetings each Thursday at 3:30pm in 101 Bell to discuss the previous week’s work and the next week’s work. Each member has a copy each others’ schedule and contact information so it will be easy to have many informal meetings and communications for work done in smaller groups. The point of contact or communication liaison for the group will be the Project Manager Justin Storms.

Group Roles

Project Manager - Justin Storms

The Project Manager of the group will oversee that the project is moving along at the scheduled pace. He keeps up communication and organization of the group in order to keep everyone on the same page. Communications and questions with the course instructor and teaching assistants can be handled through the project manager.

Lead Wiki Developer - Patrick Smith

The Lead Wiki Developer of the group will assure that the Wiki site is being maintained and properly formatted. He will assist other group members in learning how to post to the Wiki correctly in order to make optimal use of it.

Dissection/Assembly Leaders - Andrew Engelbach-Schafer and Ryan Adams

The Dissection/Assembly Leads of the group will work together to manage the disassembly and recording of each step. They will also oversee the reassembly of the product and are primarily responsible for the successful reassembly of the product back to working order. The leaders will verify that documentation is done for both the dissection and reassembly processes. Any major obstacles encountered in the dissection or assembly of the product will be directed to the Dissection/Assembly Leaders to solve.

CAD Model Leader - Damitha Abeykoon

The CAD Model Lead will do a large portion of the three dimensional solid models of the components of the dissected product. He will teach and show other group members working on the modeling what must be done. The CAD Model Lead monitors the progress of the models and assures that they will be accurate and complete by the deadline.

Along with the above mentioned roles, each member of the group will take some part in each of the major areas of disassembly, assembly, component and engineering analysis, creating design revisions, CAD modeling, documenting the steps, and posting to the Wiki. The Dissection/Assembly Leads will keep each other up to date and work together to ensure that what is taken apart can be put back together the proper way. Each member is responsible for reading and reviewing written parts that will be handed in and posted to the Wiki. One of the main management goals of the group is for each member to take on a larger responsibility in one area while still receiving help and helping others successfully fulfill their roles.

Timeline

1. Request for Proposal - Due 10/9

1.1 Work Proposal (9/27-10/7)
The work proposal was discussed during a group meeting. Andrew took on the responsibility of expanding on what we discussed and typing up a formal proposal to be submitted. It will be read and reviewed by the rest of the group before submission.
1.2 Management Proposal (9/27-10/7)
The management proposal was discussed during a group meeting. Justin took on the responsibility of going into detail and giving formal descriptions of the things discussed. He will send the report around for review by the rest of the group before submission.
1.3 Initial Product Assessment (9/27-10/7)
The initial product assessment was handled by Damitha and Ryan. They put the responses into a technical writing and will send around the final report for review by the other group members.
1.4 Wiki Posting (10/7-10/9)
Patrick will post the entirety of the request for proposal in proper format on the Wiki site as the Lead Wiki Developer.

2. Preliminary Product Review - Due 10/30

2.1 Dissection Process (10/10-10/23)
The dissection process will be managed by the Dissection/Assembly Leads. They will work to solve many of the problems the group members encounter with taking apart each part of the line trimmer. This will include finding out what the proper tools are for each part in the dissection process.
2.2 Write Dissection Report (10/17-10/28)
The group will develop a scale for ease and complexity. It will then explain and discuss each step of the disassembly and relate it to the established scale. Pictures and videos will be included on the Wiki site.
2.3 Causes for Corrective Action (10/24-10/28)
By the last week of the preliminary product review the group will meet and assess its standing compared to the previously drafted timeline. Decisions will be made on whether to adjust or change responsibilities, roles and work. A formal write-up will be done if there are major changes by the Project Manager.
2.4 Wiki Posting (10/26-10/30)
The Wiki posting of the dissection report will be posted in the last week of the preliminary product review. All of the group members will try to help Lead Wiki Developer to get everything posted on the Wiki in presentable format including text, pictures and videos.

3. Coordination Review - Due 11/23

3.1 Component Summary (10/31-11/20)
All of the group members will work on the components summary to answer the questions listed on the coordination review assignment sheet. The Project Manager will type up these results into a presentable manner for the Wiki.
3.2 Design Revisions (10/31-11/20)
All of the group members will discuss ideas for possible design revisions. After deciding on three design changes, one of the Dissection/Assembly Leads will write a technical report explaining the design changes. This will be then posted to the Wiki.
3.3 Solid Model Assembly (10/31-11/20)
The CAD Model Lead will create solid models of four components that he and the group choose from the line trimmer. Other group members will try and assist in drawing some of the components as much as they can. The final assembly of the components in sequence will be passed on to the Lead Wiki Developer to post to the Wiki.
3.4 Engineering Analysis (10/31-11/20)
All of the group members will discuss and give engineering analysis on the line trimmer. One of the Dissection/Assembly Leads will write a technical report explaining the engineering analysis that was done on the product. This will be then posted to the Wiki.
3.5 Causes for Corrective Action (11/18-11/22)
By the last week of the coordination review the group will meet and assess its standing compared to the previously drafted timeline. Decisions will be made on whether to adjust or change responsibilities, roles and work. A formal write-up will be done if there are major changes by the Project Manager.
3.6 Wiki Posting (11/5-11/23)
Many of the group members should be posting to the Wiki as they finish up their parts of the report. The Lead Wiki Developer will make sure that everything is posted by the due date in a neat and professional looking manner.

4. Critical Project Review - Due 12/7

4.1 Reassembly Process (11/24-12/2)
The reassembly process will be managed by the Dissection/Assembly Leads. They will work to solve many of the problems the group members encounter with putting each part of the line trimmer back together in the proper way. This will include finding out what the proper tools are for assembling each part and making sure the product works.
4.2 Write Reassembly Report (11/28-12/5)
The group will explain and discuss each step of the assembly with specific detail on unique challenges. Pictures and videos will be included on the Wiki site of the assembly process.
4.3 Wiki Posting (12/2-12/7)
Many of the group members should be posting to the Wiki as they finish up their parts of the report. The Lead Wiki Developer will make sure that everything is posted by the due date in a neat and professional looking manner.

5. Delivery - Due 12/11

5.1 Compliance Matrix (12/6-12/10)
The group will meet and go over the compliance matrix. The Project Manager will type any notes that need to be made with respect to the compliance matrix.
5.2 Oral Presentation Preparation (12/4-12/9)
All of the group members will meet to devise the points that need to be covered in the oral presentation. Each member will receive a topic to discuss and present.
5.3 Finalize Wiki (12/3-12/11)
This includes the addition of an executive summary, introduction and list of references. Everything else in the Wiki must be in good order and format and ready for final submission.

Gantt Chart

Figure 1 - Gantt chart of projected reverse engineering project execution

Group 10 Gantt Chart

Initial Product Assessment

FIXES: Wall of text, clear definition of component complexity #4 Internal components #5 Smells more and creates more noise than an Electric line trimmer #7

  1. The line trimmer can be used for finer purposes such as cutting, trimming, and edging grass. The user can also cut through thicker brush in smaller areas that lawn mowers cannot reach or fit.
    • This particular line trimmer appears to be mainly for home use based on the size of it. The line trimmer seems to be medium size compared to other ones. Larger and heavier duty line trimmers are normally used by professionals that do landscaping.
    • The line trimmer can be used for landscaping around the yard. It works great for tidying up the edges the lawn mower cannot get close to and the areas that are too small for it to fit at all. The thin plastic line that normally comes with the product can be changed with other attachments to give it other uses. For example, the line might be replaced with a plastic blade for heavy work like cutting thick brush.
  2. A gas line trimmer uses a small two stroke internal combustion engine. This engine is fueled by an unleaded gasoline and two stroke oil mixture. The fuel is mixed with air in the carburetor and flows into the combustion chamber where the spark plug ignites it. This drives the piston which turns a shaft that translates this energy down the shaft of the line trimmer to spin the cutting device on the bottom1. The user can control the speed of the device using the throttle trigger located on the handle below the engine.
    • There are many different types of energy used in a line trimmer. These include some of the major forms of energy such as chemical energy, internal energy, kinetic energy, electrical energy, thermal energy and mechanical energy.
    • These energies are all used and transformed to carry out the function of the line trimmer. There is chemical and internal energy contained in the two-stroke gasoline and oil mixture. This fuel containing the chemical energy is fed into the carburetor where it mixes with outside air. This mixture then moves into the internal combustion engine. A spark plug supplies a spark of electrical energy which ignites the fuel and air mixture. This converts the chemical, internal and electrical energy into kinetic energy and work through combustion. The energy of combustion process drives the piston down which rotates a shaft. The process also involves kinetic and mechanical energy. This energy is then translated down the shaft of the line trimmer to the bottom cutting piece which rotates and does shaft work, which is a form of energy. During these conversions some energy is lost to frictional forces. Another byproduct of the combustion process is thermal energy, which is dissipated into the surroundings in the form of heat. Essentially all of the controls on the line trimmer are mechanical devices. The trigger on the line trimmer uses a thin steel cable to attach to the throttle which is help back by springs. By pulling the trigger for the throttle this uses mechanical energy. When pull starting the device there is mechanical energy being used to get the engine going.
  3. The line trimmer looks as though it is in good shape, however it does not run.
    • After fueling up the line trimmer with the appropriate fuel and trying to start it for 20 minutes there was no success. The group tried adjusting the choke to different positions, but did not see any hope.
    • The pull start string on the device does not retract all the way back into the product. This seems like it could be part of the problem. The line trimmer does not sound like it is igniting inside. The spark plug seems to be good on the device and fuel was being pumped into the combustion chamber. The group’s best guess is that the problem lies somewhere in the pull start, but we will not know until we disassemble the device.
  4. In terms of landscaping tools, the most complex thing would be a large tractor and this would rate as a 10 on a scale from 1 to 10. The simplest type of landscaping tool would be something like a pair of gardening shears, which would rate at a 1. The line trimmer falls at around a 3 on this complexity scale. It is not a purely human power device and it has a small gasoline internal combustion engine.
    • There are probably around 15 to 20 major components on the line trimmer. These include the gas tank, the carburetor, the combustion chamber, the choke, clutch guards, drive cables, throttle cables, throttle trigger, gear boxes, spark plug, pull start, handles, foot guard, cutting disk and covers to name many of them. If we consider screws and other small components like nuts and washers there are probably around a hundred things.
    • Most of the parts on the line trimmer are not very complex at all. They have been around for many, many years. Perhaps the most complex components are the parts of the engine if the user is not familiar with small gasoline engines. However, to an individual with mechanical knowledge this would be rather simple and basic.
  5. The materials used in the product are plastic, steel, iron, copper, rubber, and nylon.
    • The visible materials are the plastic, metals, rubber and nylon. The plastic makes up the casing around the engine, the handle, the gas tank, and the guard around the trimming part. Steel cable is used in the throttle and drive cables. The shaft and chassis of the line trimmer are made up steel most likely. Copper is used for the wires that connect the spark plug and rubber insulates those wires. Nylon is what the string for the pull start is made of.
    • Some of the internal materials that are assumed are cast iron for the engine and combustion chamber. The oil and gasoline mixture that is inside serves as both a fuel and lubrication for the moving parts.
  6. This is a product that most users would be pleased with if we could get it to run. In its current state the user would probably be disappointed because it will not start up. It seems like it is a rather minor problem that could be fixed.
    • This line trimmer would be comfortable for use on a small lawn. The device is heavier than many of the other comparable and newer models, so it would not be ideal for trimmer a large lawn for a long period of time because it would get heavy. The device seems to be on the medium to small side, so for taller users it might not be as comfortable. For individuals less than six feet tall it seems like it would be the perfect size. The addition of a sling to help support the weight of the engine on the user’s shoulder might be a good addition.
    • The product was not able to start, so in this regard it is not easy to use. However, similar models are relatively easy to use. The user simply needs to adjust the choke to the proper position, pull in the throttle trigger and pull the starter. These engines typically take a few pulls before they begin running. Once they are running though and heat up for a few second the user simply needs to adjust the choke and then it can be used to start trimming. The line trimmer extends at a good angle to allow for easy cutting. To turn off the device there is a stop button located right by the pull start that the user simply needs to switch to shut it off.
    • The line trimmer does require some maintenance. The trimming tools attached on the bottom that do the cutting need to be replaced every so often. It needs to be refueled with a gasoline and oil mixture after every couple hours of use. After a few years, the spark plug may need to be changed depending on how heavily the device is used. After a decade or so, the line trimmer might need an engine tune up or need to be cleaned up inside to keep it running smoothly. This type of service would be more difficult and something that most users would not be able to do on their own. However, most of the maintenance such as refueling and replacing the trimming attachments can easily be done by the user.
  7. There are other brands and models of line trimmers that exist that can do the same task. Some are better, while some are not quite as good. There are more expensive gasoline line trimmers that are light weight, heavy duty and more comfortable to use. There are also electric line trimmers that are smaller, cheaper and quieter.
    • Our particular line trimmer probably costs around $80. Comparable models can be bought for the same price by the same company and other companies. Some of the more expensive line trimmers that run up to $200 offer features such as being more light weight or comfortable to hold. The electric line trimmers are cheaper than our gas line trimmer; however, the user must drag an electrical cord around them or use a battery that can drain quickly. The gasoline line trimmer offers more freedom and extended use.
    • The gasoline line trimmer compared to electric ones gives the user more freedom as mentioned before. The user is not attached or restricted to a certain area of use by an extension cord. The device does not have to recharge, it can just be refueled instantly when it runs low. The device is powerful and can cut through thick brush with the correct attachments.
    • Some disadvantages of the gasoline line trimmer are the size and weight of it. Carrying the device around can get tiring for the user. The product is a little noisy when it runs and it vibrates a lot. Electric line trimmers quieter and often times lighter. The engine can get hot and leak fuel or oil on the user.

Preliminary Design Review

Delivery Date: 10/30/2009

Causes of Corrective Action

Overall, the progress of the reverse engineering of the line trimmer has gone smoothly. The dissection and dissection instructions were completed in the time frame established in the work and management proposal. Problems have arisen with meeting times and work distribution, but are being addressed as follows:

  • Meetings
    • Issue - As a group we have had a hard time coordinating a set meeting time in which every group member has been able to show up.
      • On Tuesday October 20, 2009 the group performed the dissection. Some members were not able to make this meeting due to illness and exams.
      • The weekly scheduled meeting of Thursdays at 3:30 PM does not seem to work well with all of the group members due to a recitation conflict.
    • Resolution - We are going to meet and find a new scheduled weekly meeting time.
      • We will meet after MAE 277 on Monday November 2, 2009 to form a schedule that coordinates around all group members.
      • This new schedule will go into much more detail as to specific times as to when we will be able to meet, rather than the general meetings originally established.
  • Work distribution
    • Issue - Some of the group members were unable to assist in dissection.
      • Due to reasons mentioned above a couple group members were unable to attend dissection.
    • Resolution - Some adjustments will be made to the management proposal to adjust the work done by each group member.
      • Patrick will assist Damitha in the solid modeling because he has a lot of experience with Autodesk Inventor.
      • Ryan will work on the design revisions while giving Justin assistance with the component summary for coordination review.
  • No other setbacks
    • Aside from the meeting coordination our group has molded well and we are getting all our deliverables done on time or early.
    • The chemistry within the group is good and all members are contributing a fair amount and doing their assigned jobs.

Product Dissection Plan

Introduction

The dissection of a product can assist in developing a better understanding of how something functions. In order to gain a better understanding of how the Weed Eater LT7000 works it will be completely dissected. The dissection process will by described step-by-step so that it could be repeated by another individual. Assessment of Group 10’s overall experience, including challenges and problems, will be given. Lastly, some minor analysis of the fasteners chosen in the product will also be included.

Difficulty Scale

In order to properly describe the dissection, a scale of difficulty will be established and used throughout the dissection outline. The scale will range from 1 to 5 with 1 being the most simple a step could be and 5 being the most difficult. A difficulty rating of 1 would be something as simple as loosening and removing a screw that is easy to reach. A rating of 5 would involve removing something that is very hard to reach, needs a special tool and needs special treatment such as being heated up. The time taken to perform a step has a relationship to its rating of difficulty. Something that can be in a minute would have a difficulty rating of 1 or 2 in most cases. Steps that are more time consuming, say 5 to 10 minutes, would have a difficulty rating closer to 4 or 5. Steps requiring a special tool might rate slightly higher on the difficulty scale, but at the same time a step that still only takes a minute with a special tool would not have a rating of 5, it would most likely be a 2.

Tools Required

  • The following tools are required for this dissection:
    • 1/4” flathead screwdriver
    • #2 Phillips head screwdriver
    • 5/32” hex key wrench
    • 3/16” hex key wrench
    • 9/16” hex head socket
    • Pliers
    • Needle nose pliers
    • Small hammer
    • Penetrating oil
  • The following safety equipment should be used for this dissection:
    • Safety glasses
    • Protective gloves

Dissection Steps

Skip Dissection Steps and continue to Post-Dissection Topics


Table 1 - Step by step process of the dissection of the line trimmer

Step Description Tools Required Time Required Difficulty Picture
1 Foot Guard Removal

Remove the two flat head screws which clamp the foot guard onto the metal shaft at the bottom of the line trimmer.

1/4" flathead screwdriver 1 minute 1
Click to View
2 Handle Removal

Unscrew the wing-nut on the line trimmer handle midway up the shaft. The handle with then be able to slide off the shaft.

None 1 minute 1
Click to View
3 Trigger Handle Removal 1

Unscrew four #2 Phillips head screws from the one side of the plastic handle with the throttle trigger.

#2 Phillips screwdriver 1 minute 1
Click to View
4 Trigger Handle Removal 2

Lift half of the plastic handle off, exposing the throttle cable and trigger connection inside.

None 1 minute 1
Click to View
5 Trigger Removal

Slide the plastic trigger off of its pivot point inside the handle. Remove the throttle cable hammer-head from the plastic trigger.

None 1 minute 1
Click to View
6 Drive Shaft Removal 1

Unscrew the two socket head cap screws with a hex drive recessed into the green casing half an inch above the trigger handle. Watch for the nuts attached to the other side of the screws that will fall once the screws are loose.

5/32" hex key wrench 1 minute 1
Click to View
7 Drive Shaft Removal 2

The metal drive shaft housing may be pulled out of the engine case. This exposes the drive shaft. Wear protective gloves when pulling the drive shaft out of the drive coupling because there may be small metal shards on it.

Protective gloves 1 minute 2
Click to View
8 Spark Plug Removal

Pull the rubber cap off of the spark plug. Loosen and unscrew the spark plug with a wrench and pull it out of the combustion cylinder.

3/4" wrench 1 minute 2
Click to View
9 Outer Case Removal

Remove the socket head cap screw with a hex drive next to the spark plug.

5/32" hex key wrench 1 minute 1
Click to View
10 Air Filter Case Removal 1

Remove the two Phillips head screws holding down the air filter cover.

#2 Phillips screwdriver 1 minute 1
Click to View
11 Air Filter Removal

Pull out the foam air filter and the metal filter plate inside the air filter box.

None 1 minute 1
Click to View
12 Air Filter Case Removal 2

Unscrew the two socket head cap screws with a hex drive inside the air filter box and lift up it up. Be aware that this also leaves the carburetor below it free to move.

5/32" hex key wrench 1 minute 2
Click to View
13 Throttle Cable Removal

Slide the end of the throttle cable out of the carburetor. Use a pair of needle nose pliers to then pinch the plastic cover of the throttle cable that squeezes through the lower engine cover.

Needle nose pliers 2 minutes 2
Click to View
14 Top Outer Case Removal 1

Remove the four socket head cap screws with a hex drive on the top of the engine cover.

5/32" hex key wrench 2 minutes 1
Click to View
15 Top Outer Case Removal 2

Carefully lift off the top engine cover and attached gas tank. Be sure to hold the carburetor that is attached by the fuel line.

None 1 minute 2
Click to View
16 Top Gasket Removal

Remove the gasket that is positioned in between the top cover and the open window exposing the connecting rod assembly.

None < 1 minute 1
Click to View
17 Bottom Outer Case Removal 1

Remove the four socket head cap screws with a hex drive on the bottom of the engine cover. Then the cover will be able to separate from the engine.

5/32" hex key wrench 2 minutes 1
Click to View
18 Bottom Outer Case Removal 2

Disconnect the ignition module wires from the inside of the power switch The spades may be difficult to pull off the terminals so it is easier to use needle nose pliers.

Needle nose pliers 2 minutes 2
Click to View
19 Pull Start Removal 1

Remove the two socket head cap screws with a hex drive around the perimeter of the pull-cord disk. Lift the plastic pull-cord disk out of the bottom engine cover and set it off to the side with a little slack in the cord.

5/32" hex key wrench 1 minute 1
Click to View
20 Pull Start Removal 2

Remove the single Phillips head screw on the outside of the starter spring disk.

#2 Phillips screwdriver 1 minute 1
Click to View
21 Pull Start Removal 3

Carefully pull out the starter spring disk. CAUTION: If the starter spring disk is dropped, it will unravel and need to be rewound.

#2 Phillips screwdriver 1 minute 2
Click to View
22 Ignition Module Removal 1

Unscrew the two socket head cap screws with a hex drive on the ignition module.

5/32" hex key wrench 1 minute 1
Click to View
23 Ignition Module Removal 2

Pull the ignition module off the crankcase assembly along with the spacer below it. NOTE: The ignition module may seem stuck if the magnets on the ignition module and flywheel are lined up and attracting each other. Just simply give the drive coupling a quarter turn to make this easier.

None 1 minute 2
Click to View
24 Carburetor Adaptor Removal

Unscrew the two socket head cap screws with a hex drive on the carburetor adaptor located on the side of the combustion cylinder opposite the muffler.

5/32" hex key wrench < 1 minute 1
Click to View
25 Cylinder Gasket Removal

Remove the carburetor adaptor which was unfastened in the previous step. Be sure to also remove the cylinder gasket which lies between the cylinder and the carburetor adapter.

None < 1 minute 1
Click to View
26 Cylinder-Muffler Removal 1

Remove the two socket head cap screws with a hex drive which fasten the combustion cylinder to the crankcase assembly. These must be accessed through the two holes in opposite corners of the combustion cylinder on the end where the spark plug enters.

3/16" hex key wrench 2 minutes 1
Click to View
27 Cylinder-Muffler Removal 2

Looking down at the end of the combustion cylinder with the spark plug opening, twist the combustion cylinder and muffler piece clockwise a quarter turn. Gentle wiggle and lift the twisted unit up from the crankcase assembly pulling the piston out of the combustion cylinder.

None 2 minutes 2
Click to View
28 Connecting Rod Removal

Slide the connecting rod off the pin located inside the opening of the crankcase assembly.

None 1 minute 1
Click to View
29 Muffler Removal

The muffler is attached to the combustion cylinder by two small high-tension springs. To release one of the springs get a solid grip of the spring with a pair of needle nose pliers at the end where it goes in the muffler. Pull the spring out with the pliers while pushing the muffler and cylinder away until the one end of the spring is free from the it’s hole. CAUTION: The spring may fly out if it slips out of the needle nose pliers.

Needle nose pliers 3 minutes 3
Click to View
30 Muffler Disassembly

The muffler assembly is not held together by any other means than the tension springs. A tap from a small hammer will loosen up the muffler body, cover and baffles, so that they can be pulled apart by hand.

Small hammer 1 minute 2
Click to View


31 Drive Coupling Removal

The drive coupling needs to be loosened up with penetrating oil to help break it free. After the penetrating oil is applied around the coupling, a 9/16" extended ratcheting socket can be placed over it. A pair of pliers needs to grip the pin located inside the opening of the crankcase assembly in order to keep the flywheel and crankshaft from spinning with the drive coupling. A fairly large torque needs to be applied to break the drive coupling loose (around 80 lb*ft).

9/16" extended hex head socket and pliers 5 minutes 4
Click to View
32 Fly Wheel Removal

Once the drive coupling is removed, tap the fly wheel lightly around the edges to break it loose. It will then slide off of the crankshaft.

Small hammer 2 minutes 2
Click to View
33 Starter Dog Removal

The two starter dogs and springs can be removed by unscrewing the Phillips head screw for each one on the underside of the flywheel.

#2 Phillips screwdriver 1 minute 1
Click to View

Post-Dissection Topics

Challenges

The major challenges with the line trimmer presented themselves once the dissection process got to the major parts of the small gasoline engine. One challenge was figuring out how to remove the muffler that was held to the combustion cylinder by two small highly tensioned springs. The group was unable to disconnect the muffler on the first day. With a little help from one of the group member’s father and a good pair of needle nose pliers we were able to successfully release the spring and disconnect the muffler. The better needle nose pliers allowed us to grip the spring without slipping off so we could apply a sufficient force to pull and release it. The overall problem solving time for this step took around 20 minutes.
A second major challenge in the dissection process was getting the drive coupling off of the crank shaft. The group had no success trying to break the drive coupling loose on dissection day one. On the second dissection day, one of the group members used penetrating oil to loosen up the coupling. With the correct tools of a 9/16” socket, a pair of pliers and sufficient force the drive coupling was able to be broken loose. To break the coupling loose a force was applied almost equal to one of the group member’s body weight of 160 lb. The force was applied to a wrench approximately half a foot away from the center of the drive coupling, so this translates to approximately a 80 lb*ft torque. The problem solving and removal for this step took around 25 minutes.

Dissection Evaluation

The most common fasteners used in the line trimmer were Phillips #2 and socket head cap screws with 5/32” hex drives. The Phillips #2 screws were used for fasteners that were shorter in length and not subjected to much force. It is not necessary to use a fastener that is really large if a smaller one can adequately withstand the forces applied to it. Choosing the appropriate size screws and not over doing things saves money for the consumer and manufacturer. One example of where this screw was used is on the trigger handle to hold the two halves of the handle together. The handle experiences some vibration but other than that it is not subjected to many other forces.
The socket head cap screws[1] with 5/32” hex drives were longer in length compared to the Phillips screws. They were used on parts that were subjected to greater forces or vibrations like the engine case or the combustion cylinder. The choice of a hex drive on these could have been done to allow those screws to be tightened down to greater torques. Although the line trimmer was probably assembled by a machine, when maintenance is performed on it a person takes it apart. It is easier to control the torque placed on a screw with a hex key by using one of a different length to get different leverages. A hex key also provides a better fit than a Phillips head screw because there is less tendency for the drive tool to want to slip out of the fastener. Phillips head screws were originally designed for the automobile industry so that machines would slip of out the slot before over tightening the fasteners[2].
Thicker flat head screws were used to clamp the foot guard onto the drive cable shaft. These were probably chosen because that area is subjected to a lot of vibration and bumping. A thicker screw can withstand a higher shear force that might be generated if the foot guard is hit against a building. Tension springs were most likely used to fasten the muffler to the cylinder because they clip well around the fins of the cylinder and a screw would be difficult to use. A 9/16” hex drive coupling because it is relatively easy to find an extended socket that will fit over. Most socket sets contain extended sockets that will reach over a shaft like the drive coupling a few inches. These are usually included to give the user greater reach. It is also easier to apply a greater torque to this type of fastener often times because you can get extra leverage from a longer ratchet handle. The drive coupling needs to be very secure on the crankshaft so it does not spin off.
Overall the dissection process was moderately difficult. The gasoline powered line trimmer seems as though it is intended to be taken apart easily. There were not any hard-to-find tools needed. The manufacturer did not use rivets or weld many parts together, which would have been a more permanent assembly and difficult to take apart. The only components that seemed as though they were not intended to be taken apart were the carburetor and the gasoline tank. The carburetor had several visible gaskets and membranes attached with adhesive and many small screws. The gasoline tank is not meant to come apart because it needs to hold the fuel without leaking.
The group was able to get the line trimmer 90% disassembled on day one in around 90 minutes. This time included the actual dissection, pictures taken and notes for each step. The major problems left after day one were to figure out how to remove the drive coupling and how to detach the muffler from the combustion cylinder. With the photographs and instructions provided it would be reasonable for an average person, without experience with small gas engines, to dissect the line trimmer down as far as step 28.
The second dissection day was more difficult because it involved more problem solving. Around an hour was spent figuring out how to remove the drive coupling and muffler, as well as cleaning things up. Adequate records and documentation was taken at each step allowing the group to create an effective plan for the dissection of the line trimmer. With some handy skills I would expect most individuals to be able to complete steps 29 through 33 to entirely disassemble the line trimmer.

Coordination Review

Delivery Date: 11/30/2009

Causes for Corrective Action

Overall, the progress of the reverse engineering of the line trimmer has gone smoothly. The component summary, design revisions and solid modeling were all roughly completed in the desired time frame mentioned in the work proposal. The engineering analysis was be to be adapted from question three in assignment four. The issue of meeting time was resolved from the previous gate. The group decided not to meet collectively as often, but rather to contact each other via text message to meet and exchange information individually. The project manager, Justin Storms, still monitors the overall progress of the project and establishes a common meet time if necessary.


Family Emergency

  • There was one obstacle that was encountered during the Coordination Review. One group member had to travel home due to a family emergency for a couple weeks. He was still able to communicate with the rest of the group through email and completed his portion of the design revisions.


Solid Modeling

  • One group member who assigned to be the leader of the solid modeling was struggling with figuring out how to make the components, so another group member took over the responsibility of creating the solid models for the line trimmer.


Presentation

  • A meeting to discuss and create a presentation will be organized by the project manager. The slide show will be made during this meeting and the speaking roles will be divided up. The meeting will take place on Tuesday, December 1, 2009 at 4:15pm in 101 Bell.


Component Summary

Below is a summary of the components and the parts that make up those components in a line trimmer. The function of each component is described briefly. Information is given about the material these parts are primarily made of and the manufacturing process involved in making them. Each component is rated on a scale of complexity described below. Further discussion of the components is included after Table 2.

In order to compare the components, a scale of complexity will be established. The scale will range from 1 to 5, with 1 being the least complex component of the line trimmer and 5 being the most complex component of the line trimmer. A complexity rating of 1 corresponds with a simple component like a foot guard that can be made quickly and easily in manufacturing. It is a component that is easy to understand how it functions. A rating of 5 corresponds to a component like the carburetor on the line trimmer. This component is made up of many small parts. It is not easy for the average user to understand how it works just from looking at it. It has several gaskets and small screws.

Note - Table 2 is best viewed in 1920 x 1200 resolution. Lower resolutions may distort table formatting.


Table 2- List and overview of components and parts contained inside the line trimmer

Item # Component Complexity Parts Included (Quantity) Materials Function Manufacturing Process Picture
1 Foot Guard 1 Foot guard (1)

Foot guard clamp (1)


1/4" Slotted tapping screws with a hex washer head (2)

Plastic

Steel


Steel

The foot guard serves to protect the user from the spinning trimming attachment. Injection molding

Metal Stamping


Die casting and machining

Click to View
2 Trimming Head 1 Trimming Head (1)

Hex Nut (1)

Plastic

Steel

The trimming head spins to cut weeds and brush. Injection molding

Die casting and machining

Click to View
3 Lower Handle 1 Handle (1)

Shape 1 wing nut (1)


Clamp (1)

Plastic

Steel


Steel

The lower handle serves for a place to hold and support the line trimmer to allow for better control. Injection molding

Die casting and machining


Metal stamping

Click to View
4 Throttle Handle 2 Throttle housing (2)

Throttle lever (1)


Throttle cable (1)


  1. 2 Phillips tapping screws (4)
Plastic

Plastic


Woven Steel


Steel

The throttle handle serves as a grip for the user to support the device and control the speed of the spinning trimming attachment. Injection molding

Injection molding


Cable weaving


Die casting and machining

Click to View
5 Drive Shaft 1 Drive shaft (1)

Drive shaft housing (1)

Steel

Steel

The drive shaft transfers the power from the crankshaft spun by the motor down to the trimmer attachment. The housing protects the user from the fast spinning drive shaft. Die casting and machining

Die casting and machining

Click to View
6 Engine Housing / Fuel Tank 2 Upper engine housing (1)

Fuel tank (1)


Fuel line (1)


Lower engine housing (1)


Button head 5/32" hex socket cap screws (10)


Hex nuts (2)

Plastic

Plastic


Plastic


Plastic


Steel


Steel

The engine housing encloses most of the components of the line trimmer. It protects the user from the moving parts inside. The fuel tank and line hold and deliver the fuel to the carburetor. Injection molding

Injection molding


Injection molding


Injection molding


Die casting and machining


Die casting and machining

Click to View
7 Pull Start 2 Pull cord (1)

Pull cord disk (1)


Starter spring (1)


#2 Phillips button head machine screw (2)


Button head 5/32" hex socket cap screw (2)

Nylon string

Plastic


Steel


Steel


Steel

The pull start is used to start up the line trimmer by spinning the flywheel. The cord retracts back into the engine housing due to tension in the starter spring. Weaving

Injection molding


Metal stamping


Die casting and machining


Die casting and machining

Click to View
8 Crankcase Assembly 4 Crankshaft (1)

Drive coupling (1)


Flywheel (1)


Ball bearing (1)


Washer (1)


Gasket (2)

Steel

Steel


Aluminum alloy


Aluminum alloy


Steel


Rubber

The crankcase assembly transfers the power from the piston moving up and down to the drive shaft. The ball bearing allows this to spin very fast and smooth without generating too much heat. Die casting and machining

Die casting and machining


Die casting and machining


Die casting and machining


Die casting and machining


Injection molding

Click to View
9 Piston Assembly 3 Piston (1)

Piston ring (1)


Piston pin (1)


Piston pin retainer (1)


Connecting rod (1)

Aluminum alloy

Aluminum alloy


Aluminum alloy


Aluminum alloy


Aluminum alloy

The piston assembly is the device that moves up and down in the combustion cylinder as explosions occur. The connecting rod transfers this power to the crankshaft. Die casting and machining

Die casting and machining


Die casting and machining


Die casting and machining


Forging

Click to View
10 Combustion Cylinder 4 Combustion cylinder (1)

Standard head 3/16" hex socket cap screw (2)

Aluminum alloy

Steel

The combustion cylinder is where the fuel is injected and ignited by the spark plug. The combustion process that occurs here is the source of power. Sand casting and machining

Die casting and machining

Click to View
11 Air Filter 2 Air filter (1)

Air filter plate (1)


Air filter housing (1))

Foam

Steel


Plastic

The air filter is responsible for trapping dust that may enter with the air coming into the carburetor. Baked chemical mixture

Metal stamping


Injection molding

Click to View
12 Carburetor 2 Carburetor (1)

Choke shutter (1)


Carburetor spacer (1)


Gaskets (2)


Standard head 5/32" hex socket cap screw (2)


Button head 5/32" hex socket cap screw (2)


Phillips #2 button head machine screws (2)

Steel / Plastic

Steel


Plastic


Rubber


Steel


Steel


Steel

The carburetor mixes air with fuel for the combustion process. Air comes in through the air filter and fuel from the fuel, then depending on how much the throttle is engaged this air and fuel mixture is drawn into the combustion cylinder. The gaskets help to give a tight seal with the carburetor to the different parts so now fuel is lost. The choke shutter controls the amount of air in the fuel mixture. Metal casting, machining, injection molding

Metal stamping


Injection molding


Die casting and machining


Die casting and machining


Die casting and machining

Click to View
13 Muffler 3 Muffler cover (2)

Muffler baffle (2)


Tension springs (2)

Steel

Steel


Steel

The muffler system functions to quiet the combustion process for the user as well as slow down the combusted gas exiting. Metal stamping

Metal stamping


Wire drawing and winding

Click to View
14 Ignition Module 5 Ignition module (1)

Ignition module spacer (1)


Wire leads (3)


Spark plug (1)


On/off switch (1)


Standard head 5/32" hex socket cap screw (2)

Plastic

Steel


Rubber, copper


Plastic


Steel


Steel

The ignition module works to produce the spark that ignites the fuel mixture. There is a magnet on the flywheel and when this passes by the ignition module it induces a current with is fed to the spark plug inside the combustion cylinder. This occurs once every revolution when the piston reaches the top of the combustion chamber. Injection molding

Die casting


Wire drawing


Die casting and machining


Die casting


Die casting

Click to View


Due to the great number of components in the product, only ten will be chosen to be looked at more closely. Components that are thought to be particularly interesting or used a lot in the line trimmer will be focused on. Many of the insights for these components may also be true for the components not focused on.


Component- Foot guard

Part number- 530-069299
Choice of material- Plastic was chosen because it is relatively light weight compared to steel. High density plastics can offer relatively good strength to absorb impact from rocks kicked up by the trimming attachment. Aluminum could have offered the same light weight advantages and strength at plastic but would have been more costly.
Forces applied- around 20-30 lb from rocks kicked up
Shape- The foot guard has a triangular shape with a lip on it to provide a barrier between the user and the trimming attachment. The shape was chosen to minimize size and weight while still protecting the user from the spinning trimming attachment.
Choice of manufacturing process- Injection molding was chosen because it very effective for plastics. These guards are probably produced on a large scale for many other line trimmers, so this was a cost effective method.
Functional/cosmetic- The part is functional because it serves as a safety device to protect the user from the spinning trimming attachment and debris that is kicked up.


Component- Trimming head

Part number- Unknown (aftermarket)
Choice of material- A high density plastic was chosen for this part because it is relatively strong for its purpose of trimming small brush and grass. A metal trimming attachment would be more dangerous to the user considering the speed that it is spinning at. A metal trimming attachment would be for professional use, where as the plastic trimming material is more for home use. Most individual do not need to trim extremely thick brush.
Forces applied- around 30 lb
Shape- The trimming head is circular with little plastic strings sticking out. This is because circular objects usually spin best due to their inertia. The extended strings allow for a greater for a greater tip velocity because they have a farther distance to travel at a given angular velocity.
Choice of manufacturing process- Injection molding was chosen because it very effective for plastics. This trimming attachment is probably produced on a large scale for many other line trimmers, so this was a cost effective method.
Functional/cosmetic- The trimming head is functional because it is the actual device that cuts the grass and brush. The choice of colors of the plastic is cosmetic because it is not as essential to the function.


Component- Shape 1 wing nut

Part number- 530-091373
Choice of material- Steel was chosen to make the wing nut. This is because steel is a very strong, rigid material and will not deform very easily after being spun off and on a screw. The wing nut is so small that the decrease in weight by choosing aluminum would not be cost effective due to the greater expense.
Forces applied- around 12 lb
Shape- A wing nut was chosen because it is easy to loosen by hand. This is advantageous for the lower handle because it allows the user to easily adjust the position of the handle to the most comfortable spot.
Choice of manufacturing process- Die casting and machining were chosen. These are common processes for most fasteners. The shape of the wing nut was most likely achieved purely through die casting. Some machining was probably necessary to get the threads done.
Functional/cosmetic- The wing nut is functional because it holds the lower handle in place and allows for the user to easily adjust the position of it.


Component- Button head 5/32” hex socket cap screw

Part number- 530-015934
Choice of material- Steel was chosen to make all of the fasteners on this particular line trimmer. This is probably because steel is relatively inexpensive compared to aluminum and does not add a lot of weight considering fasteners are relatively small compared to the line trimmer. Plastic fasteners would break more easily and deform, where as steel fasteners are more durable and will last longer if they are removed for maintenance procedures.
Forces applied- varies- most are not subjected to more than a couple lb
Shape- The choice of a hex socket is probably because they are less prone to stripping than a Phillips or slot head screw. At the same time, most people who have somewhat of a toolbox will have a set of hex keys. A button head was most likely chosen because it allows for a lower profile considering most of these fasteners are on the case of the line trimmer.
Choice of manufacturing process- Die casting was chosen because these are produced on a very large scale. This fastener is used in many other things besides this particular line trimmer. Machining was probably made minimal for the threads to cut down costs and time of production.
Functional/cosmetic- These fasteners are functional because they serve to hold together many of the parts and components of the line trimmer. The choice of making them black is cosmetic rather than having them be grayish like steel.


Component- Drive shaft cable

Part number- 530-094671
Choice of material- Steel was chosen for the drive shaft cable because it was the most practical option. A plastic drive shaft would wear down very fast and could deform due to heat from friction. An aluminum drive shaft would be more costly and unless a good alloy was used, the aluminum drive shaft could deform. Steel is able to hold its form very well, but at the same time be flexible as it was designed to be for this part.
Forces applied- moments of around 70 lb*ft
Shape- The drive shaft cable was made to be flexible so that it could follow a variety of shapes from the engine to the trimming attachment. This allows for a variety of designs of the line trimmer that will cater to the comfort of the user. The cable is also square at the ends and circular in the middle section. The square ends allow for a good connection between it and the drive coupling and trimming attachment. This will prevent slippage of the drive cable in each of these pieces. The cable is circular in the middle section because this will allow it to spin more smoothly inside the drive shaft.
Choice of manufacturing process- The cable was most likely die cast and machined. The small notches along the entire cable appear to be too sharp to be done in the casting process. Some sort of machine most likely cut in all of the small grooves along the cable.
Functional/cosmetic- The drive shaft cable is purely functional. The user cannot see the drive shaft cable while the line trimmer is in operation because it is inside its housing.


Component- Starter spring

Part number- 530-029395
Choice of material- Steel was chosen for the starter spring. Steel is chosen for nearly all springs. Steel is resistant to deforming from its current shape, so it tends to coil back to its shape when it is pulled on (if it is deformed a lot or bent it will not return to its shape).
Forces applied- around 10 lb
Shape- The spring was chosen to be wound in a spiral so that when the pull start was pulled it compressed the spring. The spring then pushed back on the pull cord disk which spun it in a direction so that it retracted the pull string. This shape is a relatively compact way to retract the pull cord.
Choice of manufacturing process- The spring was most likely stamped out of a large thin sheet of steel. The strip of thin steel that was stamped out was then wound into a spiral and the coil was placed into a disk.
Functional/cosmetic- The starter spring is functional. It serves to retract the pull cord after the line trimmer has been started (or attempted to be started).


Component- Flywheel

Part number- 530-039149
Choice of material- Aluminum alloy was chosen to make the flywheel. This is most likely because the flywheel is a more crucial part of the device. A lighter weight flywheel is easier to spin and power. A steel flywheel would be heavier. Steel could also cause a problem with the ignition module. There is a magnet on the flywheel which triggers the spark plug on each revolution. A steel flywheel could become magnetized and induce an electric field on the ignition module at the wrong time and cause combustion at the wrong moment.
Forces applied- moments of around 70 lb*ft
Shape- The flywheel has fins around the edge rather than being a solid disk to save weight and material. There is a notch in the center of the flywheel so that it only fits onto the crankshaft one way. This is to ensure that the magnet on the flywheel will induce a magnetic field on the ignition module only at the desired moment when the piston is near the top of the combustion chamber.
Choice of manufacturing process- The flywheel was die cast to get the rough shape of it. Most of the surfaces are not finished, so the only machining was probably done to make sure that it fit well around the bearings and crankshaft.
Functional/cosmetic- The flywheel is purely functional. It is not seen by the user during operation. It serves to regulate the timing of the combustion process.


Component- Combustion cylinder

Part number- 530-012309
Choice of material- The combustion cylinder is made of aluminum alloy. Aluminum alloy offers a good strength to weight ratio. A steel combustion cylinder would have added a considerable amount of weight to the line trimmer. Aluminum is also better at dissipating excess heat from the combustion process to the surroundings.
Forces applied- around 80 lb
Shape- The combustion cylinder was designed with fins all around it. This was done intentionally to give added strength by having a thicker outer support as well as provide a larger surface area to dissipate excess heat from the combustion process. It can be see that the fins taper as the go out from the combustion cylinder. This is a sign that the cylinder was most likely die casted.
Choice of manufacturing process- The cylinder was die cast because it was most likely produced on a larger scale for other small gasoline engines. It is evident from the mold lines that can be seen up the side of the cylinder that this was a permanent mold cast. Machining was required to achieve a very smooth finish inside the combustion cylinder so that the piston could slide smoothly up and down in the cylinder while reducing losses to friction.
Functional/cosmetic- The combustion cylinder is function. It serves to house the combustion process. It is not painted to designed to be aesthetically pleasing because it is hardly visible to the user while the line trimmer is assembled.


Component- Muffler

Part number- 530-069389
Choice of material- The muffler is made from a relatively thin sheet of steel. The muffler does not need to be very strong because there are not many forces of great magnitude applied to it. It does need to be able to handle warm gas coming from the combustion cylinder, which makes steel a more favorable choice than plastic. Aluminum would have been rather expensive and unnecessary because the weight saved is not significant.
Forces applied- around 15 lb (tension springs that hold it to the cylinder)
Shape- The muffler has a box shape with rounded corners. Inside there are baffles with staggered holes. The purpose of this shape is to slow down the exhaust coming out, giving it some extra time to cool down as well as decrease the amount of noise that is produced by the combustion inside the cylinder. This shape makes a less direct route for the exhaust gas to exit.
Choice of manufacturing process- The muffler was most likely shaped by metal stamping. Sheet steel is relatively cheap and easy to stamp out compared to casting. The muffler is not an extremely sensitive part, nor does it need to be exceptionally strong in a line trimmer, making stamping sufficient.
Functional/cosmetic- The muffler is a functional component. It serves to quiet the noise produced by combustions, as well as slow down and cool exhaust gases.

Component- Carburetor

Part number- 530-035308
Choice of material- The carburetor contains plastic, rubber and steel. Steel was chosen as the main frame of the carburetor due to cheaper price. Rubber was chosen to make gaskets and other connections because of its ability to make good seals. Good seals are necessary because the carburetor mixes fuel with air. Plastic was used to cover some of the internal workings of the carburetor. Perhaps plastic was chosen to make one less set of parts that could rust together.
Forces applied- none
Shape- The carburetor has both a fuel and air inlet and a single outlet for the mixture of these two. It is relatively small because the line trimmer does not require a lot of fuel since it is a small engine with only one cylinder.
Choice of manufacturing process- Injection molding was chosen for the plastic and rubber parts because it is very common for these materials. It is cost effective on the larger scale that these would be produced at. Die casting and machining were chosen for the steel frame of the carburetor. Machining was necessary to get a smooth finish on the inlets and exits of the fuel and air, so these substances could flow smoothly through and be delivered to the engine.
Functional/cosmetic- The carburetor is purely functional. It serves to mix air with fuel before flowing into the cylinder. It has many small parts and seals, thus is not meant to be tampered with a lot. It is concealed by the air filter and engine housing.


Materials Chosen

The components of the line trimmer are made of common materials that are relatively inexpensive. One of the most common materials of the components was steel. Steel is relatively strong and is cheap in price. One of the sacrifices with steel is weight. Steel has a higher density than many other metals such as aluminum and magnesium alloys. Many of the steel components are relatively small, such as hardware, springs, and sheet metal, so a significant amount of weight would not be saved by making some of these components out of aluminum alloys, which are more expensive. The larger components like the combustion cylinder, piston, and flywheel are made of a lighter weight aluminum alloy. Choosing an aluminum alloy for these components most likely saved a couple pounds, which makes the line trimmer more appealing to the user justifying the extra cost. Plastic was used for the entire engine casing, trimming attachment and handles because it is relatively lightweight and strong. Plastic is cheap and can be molded into many shapes, colors and give a smooth finish. Rubber was used for many of the gaskets and lines because it can seal off connections well making them close to air tight. The foam air filter is a simple device than can keep large dust particles out of the internal workings of the engine and prevent it from getting even dirtier inside. The foam air filters were most likely cut out of a very large block of foam that was cooked.


Manufacturing Process Chosen

The choice of material has a significant effect on the manufacturing process. The components made of plastic needed to be injection molded. The components made of steel and aluminum alloys were mostly die casted. Die casting leaves a relatively smooth finish, but some surfaces that needed more precision were then machined. Machining can but costly for metal parts because it is time consuming, so the amount of it is usually minimized. Some components that were cast and have a rougher finish were most likely sand cast. Other steel components that are relatively thin were most likely stamped out. This is a relatively fast and inexpensive way to make parts that are made of sheet metal. Foam is a chemical mixture that has to be baked in the size of a large block. It can then be cut up into nearly any shape or size for use. Many of the wires and cables were drawn through a wire die where they were forced to a smaller diameter. For the electrical wires, they were then put inside rubber tubing.

Permanent mold casting was chosen for many of the metal components because it is one of the best options for components that are going to be made on a moderately large scale. Many of the components are common in small gas engines so they are probably used in engines other than that of a line trimmer. Machining was required for many of the screws and fittings between components because permanent casting alone is difficult to attain a high level of precision. It is more realistic to machine threads onto a screw that has been cast. Plastic components were chosen because they are light weight and plastic can be molded into nearly any shape. Injection molding is relatively cheap considering its flexibility.


Shape

Shape has a significant effect on the manufacturing process. Components that need square edges or intricate details can be difficult to manufacture in some processes. For plastics, these details can usually be achieved with injection molding. For metals, permanent castings typically cannot attain high levels of small detail and machining is necessary. Investment casting is capable of having higher levels of detail, but it is more expensive. The fluidity of metal is often a limiting factor when casting. On the combustion cylinder it is easy to see that the fins around the outside taper and the corners are round, which is typical of permanent mold casting. Components that can be made of thin sheet metal are most easily stamped out. This is a relatively fast and cheap process and is desirable when it is appropriate.

Many of the components have a particular shape. The components are shaped in a way that they can be assembled together in a way such that they do not interfere with each other’s function. The combustion cylinder has fins on around the cylinder that taper. This allows for better heat transfer from the combustion cylinder with the surrounding air so it does not overheat. The shape of the drive shaft housing is curved in a way that it makes the line trimmer more comfortable for the user to hold and trim with. The handles are designed in a way that the user can support the line trimmer easily.


Functional vs. Cosmetic

Nearly all of the components in the line trimmer are functional. One of the cosmetic components is the plastic engine housing. The line trimmer would run without the engine housing, but it would not look very user friendly because there would be lots of moving parts exposed. The engine casing is functional in a sense also that it protects the user from the moving parts of the small gas engine as well as some of the components that might be hot or dangerous for the user to touch.


Note- Part numbers are taken from the LT 7000 Part List, which can be found on the Weed Eater website.

Adobe Acrobat or Reader software is required to view PDF files, and can be downloaded from the Adobe Website.

Design Revisions

Our revisions to the line trimmer mainly focus on ease of use and transportability. The three components we chose to focus on were the starter, adding a sling, and the material of the drive shaft. The reason we chose to focus on these parts because we felt as users of this product at one time or another would be the best place to enhance the product to make it more marketable.

Electric Starter: This would replace the pull cord with an electric button of which would start the engine. This would greatly improve ease of use for the consumer because the pull start can be very difficult to use correctly and often malfunctions. The target group for this product is for families who often times have teenagers mow the lawn and a pull cord start can sometimes be challenging and harmful. To prevent shoulder injury and labor a electric start will ease the effort you have to put into the machine in order to use it. As a downside to this improvement, it would cause an increase in price. Also with the electronic components the machine is more susceptible to water damage and shocks. As with any additional components added to a machine it will also add maintenance.

Sling: This would add a sling just above the engine. This would increase the comfort level for the consumer because after extended use the line trimmer can be very uncomfortable. This sling will more evenly distribute weight across the body allowing you to use more major muscles to help support the bulky device. This will also reduce back injury as the stress on the back will be dramatically minimized. Often times an individual that has to support or carry a device of this nature will put the majority of the load on their back muscles and this is the most common source of back injury with such devices. The sling will prevent any hunching over and overall just increase comfort to the user. This improvement would only cause a slight increase in price. Also some people prefer to mow their lawn and use a line trimmer shirtless and this strap could cause irritation to the skin. To overcome this the strap would be removable.

Drive Shaft Housing: It would be beneficial to make the drive shaft housing out of an aluminum alloy rather than steel. Steel is heavier than an aluminum alloy would be, so this would assist in decreasing the overall weight of the line trimmer. This would be an advantage to the user because it would increase comfort during operation. A disadvantage to doing this is that aluminum alloys are more expensive than steel, so the cost of the line trimmer would most likely increase slightly. More extensive calculations would have to be made to see if the increased cost would outweigh the decrease in weight.

Conclusion: All of these revisions coupled together will increase ease of use and comfort to the customer while not significantly increasing the price. This will make the product much more marketable to a wider range of potential buyers.

Solid Model Assembly

Choice of Components:

  • Combustion Cylinder - The component is essential to the functionality of the Line Trimmer. There are several associated components which interact with the Combustion Cylinder, making it a logical choice. Combustion Cylinder Model
  • Muffler - While not being an essential component for basic functionality, the muffler represents Engineering problem solving. The muffler both makes the engine run quietly as well as slows down the combusted gas that exits the Combustion Cylinder. Muffler Model
  • Spark Plug - This is responsible for transferring the induced current to the Combustion Cylinder. It shows how the simple chemical process of combustion is initiated with real components. Spark Plug Model


Assembly

  • Muffler Attached - The Muffler connects by fitting inside an orifice on the Combustion Cylinder and is additionally held on by compression springs (not shown). Muffler Attached View
  • Spark Plug Attached - The Spark Plug connects by inserting the threaded end into a threaded orifice on the Combustion Cylinder. Spark Plug Attached View


Choice of CAD Package:

Autodesk Inventor 2010 was used for all solid modeling. Inventor is free to college students, thus making it useful for group collaboration. Computers in the Bell Computing Lab additionally have Inventor installed, providing a useful backup solution in case of individual computer failure. Inventor also features easy-to-use manipulation tools such as simple threading which was important for the chosen components.

Engineering Analysis

Due to formatting constraints in wiki code, this section is linked below as a PDF file. Adobe Acrobat or Reader software is required to view PDF files, and can be downloaded from the Adobe Website.

Engineering Analysis

Critical Project Review

Delivery Date: 10/09/2009

Product Reassembly Plan

Introduction:

The purpose of the critical project review is to provide detailed directions and outlines challenges that were encountered in the reassembly of the product that was reverse engineered. The line trimmer was able to be successfully reassembled with only a couple challenges and bumps during the process. Table 3 below outlines all of the steps taken, along with the tools required, time required, a difficulty rating and a picture.


Difficulty Scale:

In order to give an idea of how hard each step is, a scale of difficulty will be established and used throughout the reassembly outline. The scale will range from 1 to 5 with 1 being the most simple a step could be and 5 being the most difficult. A difficulty rating of 1 would be something as simple as refastening a screw that lines up easy and is easy to access. A rating of 5 would involve reassembling something that is very hard to do for most people. An example of this is the starter spring. Rewinding the starter spring takes more time and patience for someone who has not done it before. The spring needs to be gripped tightly while it is wound up and can result in hand cramps. The time taken to perform a step has a relationship to its rating of difficulty. Something that can be done in a minute would have a difficulty rating of 1 or 2 in most cases. Steps that are more time consuming, say 5 to 10 minutes, would have a difficulty rating closer to 4 or 5. Steps requiring a special tool might rate slightly higher on the difficulty scale, but at the same time a step that still only takes a minute with a special tool would not have a rating of 5, it would most likely be around a 2.


Tools Required:

  • Tools needed for reassembly include:
    • 1/4” flathead screwdriver
    • #2 Phillips head screwdriver
    • 5/32” hex key wrench
    • 3/16” hex key wrench
    • 9/16” hex head socket
    • Pliers
    • Needle nose pliers
  • Safety equipment that should be used includes:
    • Safety glasses
    • Protective gloves


Reassembly Steps

Skip Reassembly Steps and continue to Post-Reassembly Topics


Table 3 Reassembly procedure

Step Description Tools Required Time Required Difficulty Picture
1 Starter Dog

The two starter dogs must be reattached to the flywheel using a screwdriver. Be sure that small torsion spring is positioned properly around the small extrusions on the flywheel.

#2 Phillips screwdriver 1 minute 2
Click to View
2 Flywheel

The flywheel can be slid on over the crankshaft. Be sure to line up the notch on the crankshaft with the key in the flywheel hole. If not the flywheel will not slide down fully into position.

None 1 minute 2
Click to View
3 Drive Coupling Fastening

Place the washer around the crankshaft so that it rests on the flywheel. Begin twisting the drive coupling on by hand until it cannot go any tighter. Next place a 9/16” extended ratcheting socket over the drive coupling to tighten it and with a pair of pliers grip the pin located inside the opening of the crankcase assembly in order to keep the flywheel and crankshaft from spinning with the drive coupling.

9/16” extended hex head socket and pliers 2 minutes 3
Click to View
4 Muffler

The muffler assembly needs to be put back together with the baffles positioned correctly and not upside down. Refer to the picture to the right to verify the correct position.

None 1 minute 2
Click to View
5 Muffler Attachment

Next, the tension springs must reattach the muffler to the combustion cylinder. Attach one spring with the longer hook latching around the combustion cylinder fin and the shorter hook latching into the small muffler case hole. The second spring should then first be attached to the combustion cylinder fin at the longer latch end. Then using a pair of needle nose pliers, get a good grasp on the spring and apply a tension on it so that the smaller latch can be placed into the small hole on the muffler body. CAUTION: The spring may fly if it slips out of the needle nose pliers.

Needle nose pliers 2 minutes 4
Click to View


6 Piston and Connecting Rod

Slide the hole in the connecting rod of the piston assembly back over the pin located inside of the opening of the crankcase assembly.

None 1 minute 1
Click to View
7 Cylinder-Muffler

With the combustion cylinder oriented such that the muffler is farthest from the flywheel, carefully slide the piston into the combustion cylinder (Note: be sure the piston ring is properly lined up and pinched in slightly so it will fit inside the combustion cylinder). Once the combustion cylinder has been slid over the piston, turn the cylinder-muffler assembly a quarter-turn counterclockwise looking at the top of it.

None 2 minutes 2
Click to View


8 Cylinder-Muffler Fastening

Once the cylinder-muffler assembly is sitting on the crankcase assembly appropriately, refasten the two socket head cap screws with a hex drive. These can be tightened via the two holes in opposite corners of the combustion cylinder on the end where the spark plug enters.

3/16” hex key wrench 1 minute 1
Click to View
9 Cylinder Gasket

Replace the cylinder gasket that lies between the cylinder and the carburetor adapter. Be sure that the appropriate holes line up correctly.

None 1 minute 1
Click to View


10 Carburetor Adaptor

Place the carburetor adaptor over the gasket previously replaced making sure that it is oriented such that the screw holes line up. Next fasten the two socket head cap screws with a hex drive that go through the adapter and tighten into the crankcase.

5/32“ hex key wrench 1 minute 1
Click to View
11 Ignition Module

The ignition module attaches to the crankcase on the other side of the muffler-cylinder assembly. Place the ignition module spacer on the crankcase over the holes it lines up with. Next place the ignition module on top of the spacer with the two holes lined up. Feed the two socket head cap screws with a hex drive through the ignition module and spacer. It may be necessary to wiggle around these components to align the screws with the appropriate holes. Once they are lined up, tighten them down with a hex key.

5/32“ hex key wrench 2 minutes 2
Click to View
12 Top Gasket

Replace the gasket that is positioned in between the top cover and the crankcase window exposing the connecting rod of the piston.

None 1 minute 1
Click to View
13 Upper Engine Housing

Carefully place the engine assembled in the previous steps into the upper engine housing. Line up the crankcase window with the extrusion in the engine housing where the gasket was previously placed. Be sure not to damage the carburetor that is attached by the fuel line.

None 1 minute 1
Click to View
14 Upper Engine Housing Fastening

Refasten the four socket head cap screws with a hex drive on the top of the engine cover. These should line up well, if they do not, try starting them one at a time with only a couple turns.

5/32“ hex key wrench 1 minute 1
Click to View


15 Starter Spring Winding

If the starter spring unraveled during disassembly, it needs to be rewound, if not proceed to the next step. To rewind the start spring begin from the inside of the coil and begin wrapping it in as tight of a spiral as possible. Be sure not to let the spring slip while winding it or it will not fit back in the disk. It may be easier to have an additional pair of hands assist in holding the spring tightly while it is being wound. Once it is wound up, carefully place it into the start disk. Make sure only the hook at the end of the spring is hanging out of the disk.

2+ individuals 10 minutes 5
Click to View
16 Starter Spring

Replace the starter spring disk, with the spring coiled inside, into the bottom of the lower engine housing. Line up the hook on the end of the starter spring with the notch on the inside of the casing so that it can grab it when it is spun. Replace the small Phillips #2 head screw in the appropriate hole to hold down the spring.

#2 Phillips screwdriver 1 minute 2
Click to View
17 Pull Start

Replace the pull cord disk over the starter spring in the lower engine housing. Refasten the two socket head cap screws with a hex drive around the perimeter of the pull-cord disk.

5/32“ hex key wrench 1 minute 1
Click to View
18 Lower Engine Housing

Reconnect the ignition module wires to the power switch located on the inside of the cover. The spades may be difficult to slide over the terminals so it is easier to use needle nose pliers. (Note: it does not matter which spade is connected to which terminal.)

Needle nose pliers 2 minutes 2
Click to View
19 Lower Engine Housing Fastening

Place the lower engine housing over the bottom half of the engine such that it lines up with the upper engine housing. Refasten the four socket head cap screws with a hex drive on the bottom of the engine cover. These should line up well, if they do not try starting them with only a couple turns one at a time.

5/32“ hex key wrench 1 minute 1
Click to View
20 Throttle Cable

Slide the throttle cable back through the opening in the lower engine housing. The one end can then be slid back into the carburetor to control the fuel and air flow.

None 1 minute 2
Click to View


21 Air Filter Case

Place the air filter box over the carburetor. This might be a bit tricky because the screws need to go through the air filter box, the carburetor and then into the crankcase assembly. Start by getting one of the two socket head cap screws with a hex drive lined up properly and give it a few turns. Then move around the air filter box and carburetor slightly to line up the other hole. Finally, tighten these screws down.

5/32“ hex key wrench 2 minutes 2
Click to View
22 Air Filter

Replace the steel air filter plate and to the foam air filter in the air filter box.

None 1 minutes 1
Click to View
23 External Air Filter Case

Place the black external air filter case back over the air filter. This can be fastened down via the two Phillips head screws.

#2 Phillips screwdriver 1 minutes 1
Click to View
24 Lower/Upper Engine Housing Attachment

Fasten the socket head cap screw with a hex drive next to the spark plug.

5/32“ hex key wrench 1 minutes 1
Click to View


25 Spark Plug

Screw the spark plug back into the combustion cylinder using a wrench. Push the rubber cap back onto the spark plug.

3/4” wrench 1 minute 1
Click to View
26 Lower Handle

Slide the lower handle over the drive shaft housing and put the provided screw through the clamp and handle so that it extrudes from the handle. Screw the wing-nut on the extruded screw to tighten the handle down in the desired position.

None 2 minutes 1
Click to View
27 Drive Shaft

Place the metal drive shaft in the housing. Slide the drive shaft and housing into the bottom of the engine and engine housing. Be sure to twist and wiggle the drive shaft housing a little bit so that it slides in all the way and the notches line up. (Note: wear protective gloves when handling the drive shaft because there may be small metal shards on it.)

Protective gloves 2 minutes 2
Click to View


28 Drive Shaft Fastening

Fasten the two socket head cap screws with a hex drive recessed into the green casing half an inch above the trigger handle. Use a small screw Phillips screw driver to hold the nuts in place on the other side. Once they are tightened a little the nuts will be held in the notches designed for them in the case.

5/32“ hex key wrench 2 minutes 2
Click to View
29 Trigger Throttle

Slide the throttle cable hammer-head back into the plastic trigger. Then slide plastic trigger back onto its pivot point inside the handle.

None 1 minute 2
Click to View
30 Trigger Handle

Place half of the handle on the underside of the drive shaft housing. Place the other half of the handle on top of the drive shaft housing. They should line up easily and sit nicely around the housing. Pay attention to make sure that the throttle cable is able to exit the end of the handle.

None 1 minute 1
Click to View
31 Trigger Handle Fastening

Screw in the four #2 Phillips head screws on the appropriate side of the plastic handle with the holes.

#2 Phillips screwdriver 1 minute 1
Click to View
32 Foot Guard

Place the foot guard just above the trimming attachment such that it extends in towards the curve of the drive shaft housing. Place the clamp around the drive shaft housing and line up the holes in it with those in the foot guard. Screw the two flat head screws back in.

1/4” flathead screwdriver 2 minutes 2
Click to View

Post-Reassembly Topics

Challenges:

There were several unique challenges during the reassembly that differed from those during the dissection. One particular challenge was rewinding the starter spring back into its disk. The starter spring accidentally unraveled out of the starter spring disk during dissection of the product, therefore the group had to figure out how to wind a 20 foot piece of thin steel into a disk that was about four inches in diameter. It was difficult to hold the spring tightly together after a few windings had been done and it was strenuous on the hands. One of the problems was that the spring had to be wound so small that it was difficult to try and get two pairs of hands on it to help, so it had to be done by one person. Eventually, after a few attempts on several different occasions the group was able to get the starter spring back together. Another more mild challenge was reattaching the muffler to the combustion cylinder via the two tension springs. These springs were very stiff and did not have much space to grab onto. It was difficult to get a firm grasp of the spring with a pair of pliers so that a sufficient tension could be applied to it to place it back in the appropriate hole on the muffler body and combustion cylinder. After a couple attempts this obstacle was overcome without seeking any further assistance. One area that the group noted could also cause trouble was the reassembly of the muffler. The muffler could fit together in a variety of different ways, but they all are not correct. Thus it was a good thing that photographs were taken of the dissection to make sure that the reassembly of the muffler was done correctly. Not reassembling the muffler properly could cause the line trimmer to run louder or emit exhaust gas at a higher temperature posing a danger to the user.


Post Dissection:

The group was not able to get the line trimmer running before it was dissected. After dissection and reassembly, the line trimmer still does not run. This could be from a variety of problems. The group noted that the pull start mechanism seemed to have a problem in it because the cord would not retract after each pull. After dissection the group was not able to identify what exactly the problem was and upon reassembly it is still the same case. After attempting to start the device several times, the group did note that it smelled as though the fuel mixture was being pumped into the combustion cylinder. The group also tested the spark plug in a different line trimmer and it worked.

The dissection and reassembly processes were nearly mirror images of each other aside from the couple of unique challenges mentioned above. The dissection and reassembly differed in how the starter spring was dealt with and the muffler assembly. These differences are described above. The same set of tools was used for both processes with the exception of one or two items. Reassembly did not require the use of a small hammer and penetrating oil. The entire product was able to be successfully reassembled over the time span of less than an hour.

After completing the analysis of this product a couple recommendations can still be made. Improvements to the start up system would be beneficial because that is where the group thought that the problem lied in the line trimmer. The addition of a primer would be a helpful design revision. This would be a relatively cheap addition and would be able to get fuel into the combustion cylinder for the first pull. Adding an electric start up system could also assist in getting the device running. One way to alleviate some of the weight and cost added with an electric start system would be to use a detachable-rechargeable battery pack. By doing this the user could recharge the batteries inside his or her home between uses. This would eliminate the need for an alternator on the line trimmer.

Resources

This section contains supplemental material as well as linked content from the main body of the deliverable sections.

References

[1] McMaster Carr

[2] Different Types of Screws, Drives, and Heads