Group 10 - Line Trimmer/Preliminary Design Review

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Contents

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 - Dissection Procedure

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.

References

[1] McMaster Carr

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


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