Group 10 - Line Trimmer

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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


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

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.


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

Group 10 Gantt Chart

Initial Product Assessment

  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


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 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


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

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

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)




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

Metal Stamping

Die casting and machining

Foot Guard
2 Trimming Head 1 Trimming Head (1)

Hex Nut (1)



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

Die casting and machining

Trimming Head
3 Lower Handle 1 Handle (1)

Shape 1 wing nut (1)

Clamp (1)




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

Lower Handle
4 Throttle Handle 2 Throttle housing (2)

Throttle lever (1)

Throttle cable (1)

  1. 2 Phillips tapping screws (4)


Woven 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

Throttle Handle
5 Drive Shaft 1 Drive shaft (1)

Drive shaft housing (1)



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

Drive Shaft
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)







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

Engine Housing / Fuel Tank
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





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

Pull Start
8 Crankcase Assembly 4 Crankshaft (1)

Drive coupling (1)

Flywheel (1)

Ball bearing (1)

Washer (1)

Gasket (2)



Aluminum alloy

Aluminum alloy



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

Crankcase Assembly
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


Piston Assembly
10 Combustion Cylinder 4 Combustion cylinder (1)

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

Aluminum alloy


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

Combustion Cylinder
11 Air Filter 2 Air filter (1)

Air filter plate (1)

Air filter housing (1))




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

Air Filter
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







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

13 Muffler 3 Muffler cover (2)

Muffler baffle (2)

Tension springs (2)




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

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)



Rubber, copper




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

Ignition Module

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.

Many of the components visible on the exterior of the line trimmer are not subject to any major forces aside from the vibration from the engine. The components subjected to larger forces include the inner workings of the internal combustion engine and the drive shaft. The combustion cylinder is subjected to higher pressures created by the expansion of air during the combustion of the air and fuel mixture. The piston is also subjected to this high pressure as it is forced down. The connecting rod has compression forces and torques acting on it as it drives up and down with the piston as well as around the crankshaft. The driveshaft experiences torque which is transmitted to the trimming attachment. Lastly, the trimming attachment experiences reaction forces from all of the objects it hits and chops while spinning.

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.

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.

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.

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.

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.

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 engine block. 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 there lawn and use a line trimmer shirtless and this strap could cause irritation to the skin. To overcome this the strap would be removable.

Engine Block: We are going to change the material the engine block is made out of from iron to aluminum. This will reduce weight and require less work for the user to operate. The length of time an operator could use this product could be drastically increased by doing this because the bulk of the weight of this product is in the engine block.

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.


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


[1] McMaster Carr

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