Gate 4 Group 15 2011

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In this section of the project we will address any problems we have had throughout the semester in the corrective action section. We will go over the entire reassembly the product, and suggest multiple design revisions for our product. This will be the final analysis of our project and we will make detailed conclusions about the product.
Figure 4.1: Product before reassembly


Cause for Corrective Action

Unresolved Problems

The most pressing unresolved issue in the completion of our project is the limited amount of time left before the end of the semester. As the semester comes to an end we are all encountering extreme time pressure; the work keeps piling up. This will make it increasingly challenging to meet the deadlines for the 4th and 5th Gates. With the previous gates we had the luxury of being able to dedicate entire weekends to finalize our submissions, but now everyone in our group has to prepare for final exams and projects for other courses, and this will obviously cut back on the time we will be able to devote to this course. To combat this we will be extremely proactive as a group and get all our work done far in advance. This will mean working over the Thanksgiving break and meeting on weekends to make sure that we are all on the same page. This might seem overzealous from some perspectives, but in the long run it will most definitely pay off.

In looking to forward to Gate 5 there are issues with our previous gates that we must address. In our future meetings we will have to budget time so that we can discuss the necessary revisions needed on our previous gates.

Addressed Problems

For Gates 1 and 2 our group was able to get our work done well before it was due, ensuring that if we ran into any unforeseen issues we would have time to address them. However, in the submission for Gate 3 we did not have the same luxury as in the earlier gates. For that gate our group set Saturday afternoon as the deadline for every member of the group to send their section of the project to the editor so it could be read over as a single document. When the due date was pushed back to Friday the 18th we extend the deadline by one day to give everyone a little more time to work on their respective parts. However, select members of the group did not send their work to the editor until Tuesday night which delayed completion of the entire gate. Since the project was sent to the editor late both he and our wiki expert had to work until the last minute to get the project done. This is something we are committed to avoiding for the completion of Gate 4. As a group we decided that the two group members who could meet on Thursday to reassemble our weed wacker should do so, so we would be able to make additional progress during the Thanksgiving break. This let us set a due date more than a week prior to when everyone would have to send their part to the editor, which should be more than enough time to get all our work done. This also allows our group some leeway; if one (or more) members sends his part to the editor late the editor will still have time to read over it and make the necessary corrections.

Product Reassembly


Table 4.1: Difficulty Scale

Rating Description Example
1 Very simple task that can be completed by an individual with little to no familiarity with tools or machines Screwing a lid on a fuel tank
2 Simple task that may require minimal familiarity with tools or machines Screwing an easily accessible screw with a driver
3 Task that may be difficult for uninitiated individuals and/or requires the use of multiple tools, but not excessive force Putting a bolt and nut in place using two wrenches
4 Mildly difficult task that may require the input of significant force or a high degree of precision Inserting a press fit bearing with a mallet
5 Difficult task that requires a significant amount of expertise or force input Putting a c-clamp inside a small cylinder

Factory Assembly

There are many different ways a weed wacker could have been assembled by the manufacturer. Basing our conclusions on what our product looks like, the prevailing technology at the time of manufacture (1991), and the relative complexity of the pieces, we believe the column on the far right of the table below is how our weed wacker was originally put together. When it says “Manual” that means a worker assembled the respective components by hand (or with the use of basic tools), and for any step that says “Machine” we believe the procedure was done utilizing a machine or automated process.

Table 4.2: Step by step reassembly

Step Description Rating Tool(s) Used in Our Reassembly Original Factory Assembly Picture
1 Compress the piston ring using your hands and insert the piston into the cylinder. 4 Hands Machine
Ring is located on the side of the piston
2 Insert the crankshaft into the engine housing. Then use a mallet to insert press fit bearings into the proper position. 4 Mallet Machine
Here, the crankshaft has been inserted through the bottom of the housing.
3 Insert c-clamps to prevent excess and unwanted movement of the bearings. 5 Needle Nosed Pliers Machine
The clamps are located on the crankshaft in the center of the photograph.
4 Align the crankshaft housing and the engine block, ensuring the large gasket is between the parts. Do not attach the two parts together in this step. 1 Hands Machine
Be sure to center the gasket.
5 Insert the offset pin of the crankshaft into the ring on the piston arm. 2 Hands Manual
The pin connects to the right of the center of the crankshaft
6 Rotate the crankshaft until the piston moves into the correct position. This should allow the engine block and housing to fit together properly. 2 Hands Manual
Rotate the crankshaft by hand
7 Insert two machine screws through the engine block fins and into crankshaft casing and then tighten with the Allen Wrench. 2 Allen Wrench Machine
Here, the screws are attached through the bottom of the product, as oriented in the photo.
8 Attach the magneto to the engine block with two machine screws. Also attach magneto wire to the correct spot on the engine casing. (The clip of our wire broke so we were unable to complete this step in our actual reassembly) 2 Allen Wrench Machine
The magneto is the irregularly shaped device to the left of the crankshaft
9 Align and attach the flywheel to the crankshaft. Rotate the shaft so the Woodruff key is aligned so that it will fit into the slot in the flywheel. 2 Hands Manual
Watch closely to align the Woodruff key correctly
11 Reassemble the muffler by placing the cylinder in between the two pieces of protective casing. 2 Hands Manual
Be sure to align the cylinder correctly, otherwise the part will not fit together
12 Attach the muffler to the cylinder using the two springs. The use of a long screwdriver may be needed to create the leverage needed to pull the springs into the appropriate holes. 4 Hands, Long Screwdriver Manual
Exercize caution when attaching the springs
13 Insert the partially assembled engine into the protective plastic engine casing. 1 Hands Manual
Rotate engine until it fits correctly
14 Reinsert the kill switch lever into the hole in engine housing and align it with the appropriate slots in the housing and the casing. 2 Hands Manual
The lever is on the lower right of the product in the photo.
15 Attach the front of the engine casing with the six corresponding screws. 2 Allen Wrench Manual
Manipulate plastic casing until it fits correctly
16 Place the pull start mechanism around the crankshaft. 1 Hands Manual
Manipulate part until it sits level.
17 Insert the tube spacer over the crankshaft. Then place clutch on crankshaft 1 Hands Manual
This part is located under the nut at the end of the crankshaft in this photo.
18 Attach the washer and the nut to the end of crankshaft. 2 Crescent Wrench Manual
Be careful not to cross thread the nut
19 Attach the clutch shroud with four machine screws. 2 Allen Wrench Manual
The screw holes are located at the four corners of the part
20 Feed the throttle cable through the hole in engine and casing. Attach it to the designated hole on the carburetor. 3 Hands Manual
Manipulate the cable until it slides through.
21 Align the carburetor with the engine block opening at the back of the engine casing with the small shroud in between the parts. 2 Hands Manual
Be sure to center the gasket.
22 Align the choke plate and the carburetor. Insert one machine screw into the side without dimples. Place the choke arm over the other hole and insert a second machine screw. 3 Hands, Allen Wrench Manual
Be sure to avoid over tightening, as it will render the choke inoperable
23 Attach the air filter on top of the choke plate with two machine screws. 2 Allen Wrench Manual
Be sure to place the foam inside of the filter
24 Feed the throttle cable through the trigger housing and attach it to the trigger. 2 Hands Manual
Manipulate the knob at the end of the cable to fit correctly
25 Attach the trigger mechanism to the main shaft. 2 Allen Wrench Manual
Be sure to leave enough room for the user\'s hand
26 Attach the main shaft to the engine by inserting it into the clutch shroud. Be sure to align the square drive shaft into the square hole in the shroud. Secure with two screws. 3 Hands, Allen Wrench Manual
Manipulate the shaft until it settles
27 Attach the forward handle with a bolt and nut. 3 Two Crescent Wrenches Manual
This can be placed wherever it is most comfortable for the user
28 Attach the sling attachment ring to main shaft with a bolt and nut. 3 Two Crescent Wrenches Not Factory
This can also be placed wherever it is most comfortable
29 Attach the protective shield to the main shaft with a bolt and nut. 3 Two Crescent Wrenches Manual
Be sure to place the shield between the head and the user
30 Place the large head piece on the end of drive shaft. 1 Hand Manual
Rotate until the part fits over the hex nut
31 Place the cylinder with the flange on end of drive shaft, make sure that the flange end is towards engine. 1 Hand Manual
Do not let the spring fall out
32 Place the spool on end of shaft, around the flanged cylinder. 1 Hand Manual
Place the ribbed side toward the ground
33 Secure the head pieces to the main shaft with a Phillips screw. 2 Phillips Screwdriver Manual
Do not strip this screw, as it will make the machine difficult to maintain
34 Attach the head cap to main head piece. Apply pressure toward the main piece and rotate clockwise. 4 Hands Manual
A lot of pressure may have to be applied.

Derivations From Disassembly

For the most part the reassembly of the product mirrored the disassembly from Gate 2. The only changes from the disassembly can be found in steps 3 and 8 of the reassembly. In step 3 we used a pair of needle-nosed pliers to position the c-clamps, whereas in the disassembly we used a screwdriver to pry the clamp free. In step 8 we were unable to reattach the ground wire because the attachment point of the ground wire broke during disassembly. We did not expect there to be many differences from our reassembly and disassembly because most of the parts were taken apart in a specific order and this was the best way to reassemble the product. We were also very mindful of how old and brittle the weed wacker was when taking it apart; this enabled us to avoid damage that would force us to devise alternative ways to reassemble it components.

Challenges During Assembly

The product was reassembled exactly as it had been originally assembled, with the exception of the magneto ground wire, and fuel line. The attachment point of the magneto ground broke during disassembly, so the wire has been placed in the casing so that it grounds out to the main engine housing. This should not affect the function of the engine. It should also be noted that the primer and fuel cable running from the fuel tank to the carburetor were not present at the beginning of the project, so they have not been included in the dissection or reassemble of our product.

Design Revisions

Backpack Weed Wacker

Weed wackers are traditionally designed with a very narrow focus; they affix all sub-assemblies to a long shaft that also transmits power from a 2-cycle motor to the cutting head, and everything is geared to efficiently cut unwanted vegetation. However, recently there have been some changes in the look and design of weed wackers. Some companies (Honda being the most notable example [1]) are exploring fundamental changes in weed wackers, including wheeled models you push along the ground - similar to a conventional lawn mower. This shows that some manufacturers are willing to seek out novel solutions to gain a competitive advantage. While we can appreciate these efforts, we feel there are better ways to modify the design of a weed wacker to benefit a traditional homeowner.

A primary reason weed wackers might need modification is that they can be very difficult for a casual user to operate. Even though it is not a particularly heavy tool a weed wacker can be cumbersome and unwieldy for certain demographic subsets (including the elderly, people with disabilities, women, young people, and even smaller adult males). These market niches explain why companies are exploring ways to produce machines that do not have to be physically carried by the operator. Unfortunately, even the most advanced example currently available is not a viable alternative because in practice there is a tremendous loss of maneuverability compared to a hand-held unit. As a way to keep the versatility of a hand-held unit while making a more manageable tool for all potential users, a worthwhile design change to consider would be creating a backpack weed wacker.

A backpack weed wacker would look similar to a leaf blower in that the engine would be mounted on a cradle the operator carries on their back (see Figure 4.2).

Figure 4.2:The inspiration behind a backpack weed wacker.


Although this is a leaf blower it is similar to the design we envision for our backpack weed wacker. The main difference being that instead of a hollow tub coming from the engine we would have a shaft with a spring shaft inside of it [1].

Then the power would be transmitted to the cutting head by a bent shaft (similar in appearance to the way the nozzle/controls come out of a current leaf blower although with a solid bent shaft and not a hose). This would mean the user would still have carry the weight of the machine (a clear disadvantage compared to the push weed wacker), but since it is carried by your torso it would be much more manageable than a traditional hand-held weed wacker. Carrying the weight on your back and legs rather than with your arms would make it much easier for weaker users to handle such a tool. This design modification would not add insurmountable costs to the common weed wacker because there are not that many alterations that would have to be made – and much of those accommodations would only entail minor modifications to off-the shelf blower designs. In essence, the only additional components that would need to be added to create a backpack weed wacker would be a set of straps and a back plate, and these are inexpensive when compared to the overall cost of a weed wacker.However, it is possible that we would need to slightly alter the motor assembly of our product, and we would certainly need to test the new products safety.

By making our weed wacker a backpack unit we are also still able to go any place a standard weed wacker could go; we are not sacrificing mobility for convenience as we would with a push model. The only major disadvantage that could present itself would be reduced efficiency if ergonomics dictate we have to drastically change the degree of curvature. The resulting power loss could be mitigated by utilizing a differential/transmission unit, but that would add considerable cost. Our preliminary estimate is that a workable design would result in a power loss that would be unnoticeable to a casual user.

Factors Addressed

  • Societal – Making a backpack weed wacker would certainly make the product much easier to use for people of all ages.
  • Societal- You do not give up any of the versatility of a standard weed wacker.
  • Economic- Making the weed wacker a backpack weed wacker would slightly increase the price of the machine.

Four Cycle Engine

Nearly all weed wackers sold today utilize a traditional 2-cycle engine to produce power; however these machines are not as efficient or clean as other options. Many companies have started using electric motors in their smaller, household products. However, a 4-cycle engine would be a better power source than either of these two alternatives. This is the reason why have decided that a worthwhile design change would be to have our weed wacker utilize a 4-cycle rather than 2-cycle engine.

2-cycle engines are cheaper to produce, but we believe that over the life of the machine you would actually save money with a 4-cycle engine. For instance, a 4-cycle engine is more efficient at extracting power from fuel than a 2-cycle engine, meaning that a user would not have to fuel their machine as often if it had a 4-cycle motor. Additionally, 4-cycle engines tend to last substantially longer than their 2-cycle counterparts [2]; it is very likely the initial cost disadvantage would be more than offset by longer product life. You would also save a considerable amount of money because a 4-cycle engine uses much less oil than a 2-cycle engine. With a 2-cycle engine you need to add oil every time you fill up the gas tank, and this is a disadvantage on multiple fronts. Given the small volumes involved it is difficult to achieve the optimal gas/fuel ratio. Adding too little oil drastically reduces service life, and adding too much produces excess emissions with no accompanying benefit (which is part of the reason 2-cylce engines are viewed as dirty engines). However, with a 4-cycle engine there is a completely separate reservoir for the oil. While this oil needs to be periodically changed it is not consumed by the engine through the combustion process, which means less oil usage in the long run. While this results in a favorable cost benefit ratio, at least as important are the environmental considerations – many trade groups are warning that the familiar 2-cycle engines may soon be outlawed because of their exhaust emissions, 4-cycle engines may soon be mandatory. If this is so whichever company invests early on in 4-cycle research is poised to reap major benefits through licensing and consulting arrangements.

We are not claiming 4-cycle engines are superior in all respects to 2-cycle engines. It is undeniable that it costs more to manufacture a 4-cycle engine because they are more complex and mandate more sub-assemblies than 2-cycle engines (such as a pressurized lubrication system). They also require more routine maintenance (oil changes) though this is mitigated by how simple those changes are, by the elimination of multiple fuels (and the complete elimination of mixing gas and oil), and by the reduced fouling resulting from cleaner combustion. If our weed wacker were to make the move from a 2-cycle to a 4-cycle engine there would also be a substantial power loss, at least on a horsepower-per-unit-weight basis. Depending on how the relative states of tune a 2 cycle engine could easily produce twice the power of a similar displacement 4-cycle engine because it creates power on every revolution of the crankshaft whereas it take two revolutions of the crankshaft to create power in a 4 cycle engine. Another issue with 4-cycle engines is that they tend to be substantially heavier than 2-cycle engines. However, if we also adopt a backpack mount (the previous design revision) this is not really a major consideration because if the weight was on the users back they would not notice it as much. It is also pertinent to note that environmental concerns are limiting the amount of research and development being lavished on general use 2-cycle engines. Looking at motorcycles, snowmobiles, power generators, outboard motors, and many other consumer goods it seems 4-cycle engines are the way of the future.

In conclusion, our group feels the switch from a 2-cycle engine to a 4-cycle engine would be a beneficial design change because: they are more efficient overall, they are cheaper over the long run, and they produce fewer harmful emissions. We do not need to worry about whether or not a 4-cycle engines will hold up when placed on a weed wacker because many companies such as Honda and Toro already sell 4-cycle weed wackers[5]. We feel the increase in the initial cost of the machine will be made up over the life of the product, and that any power loss will not be a major concern for the average homeowner.

Figure 4.3: Differences in complexity between 4 (left) and 2 (right) cycle engines


The figure above depicts a common 4 cycle engine (left) and a standard 2-cycle engine (left). The difference in complexity is the reason why it cost more to manufacture a 4 cycle engine. [3] [4]

Factors Addressed

  • Economic: 4 cycle engines are more expensive initially, but over the life of the machine have the ability to save the consumer money. They last much longer and are more efficient tools than their 2 cycle counterparts.
  • Environmental: 4 cycle engines produce less harmful emissions than 2 cycle engines.

Weed Killer Sprayer

The primary function of a weed wacker is to cut weeds around immoveable objects and the perimeter of a lawn. However, current weed wackers cannot do anything to prevent the growth of future weeds. That is why a good redesign consideration for a weed wacker would be the addition of a weed killer spray attachment. This would allow users to spray weed killer on areas at the same time they are cutting unwanted weeds. This would add an entire new function to our weed wacker; the ability to prevent future weed growth.

In order to accommodate the addition of such a feature a nozzle would be placed at the bottom of the shaft of the weed wacker. This nozzle would then be connected to a reservoir of weed killer (our design would closely mirror that of a Swiffer duster with the spray feature). However, whenever you are going to add an entirely new system to a product, especially a product that the user has to carry, you have to be wary of adding too much weight or bulkiness. However, we believe this could easily be avoided if a company were to add a sprayer to its product. A small tank of weed killer could be mounted right next to the gas tank – a location with sufficient heat shielding to ensure the engine will not produce enough heat to damage to the actual liquid. To control when the weed killer is sprayed, a button would be placed either on the handle that’s attached to the shaft or next to the trigger that controls fuel flow for the engine.

Adding a feature like a weed killer sprayer would undoubtedly add manufacturing and other costs to the weed wacker, but adding a secondary function to a weed wacker cannot come without a certain cost to both the user and manufacturer. There would be a number of components that would need to be put on our weed wacker for this addition to take place, such as a trigger mechanism, a weed killer reservoir, a spray nozzle, and tubing to connect the nozzle and the reservoir. Individually, none of the components would add substantial cost to the machine, but the combined costs of these things would result in a noticeable price difference in the weed wacker. One way to prevent these costs from reaching all consumers would be to make the weed control system an optional modular sub-assembly. By making this system an attachment we allow the user to kill all the weeds they want and then they can take the sprayer system off. This way users do not have to carry the sprayer around when they are not using it. Homeowners who would want to avail themselves of such versatility would gladly pay the incremental increase if it meant they could forego a stand-alone weed killer delivery system, but purchasers who did not want that particular feature would have the option to not pay for it until they encountered the need for it.

The ability for a weed wacker to be able to prevent weeds would be a huge marketing tool for the producer. By adding this capability to the weed wacker consumers would see major convenience benefits. Most importantly, it would save users a great deal of time because they would be able to trim their weeds and use preventive measures at the same time instead of having to do the two jobs separately. Also, adding this system would mean you would be able to trim much less often. This would cut down on the number of times that a user needs to trim their lawn per year, and this would increase the effective life of the machine, reducing long-term operating costs. Even though this redesign would cost both the producer and the consumer more money we feel this is a very worthwhile endeavor because of the time, energy, and money (in the long run) it would save for the user.

To make our weed killer sprayer work we would recommend that a small electric motor, most likely powered by batteries, spray the herbicide out of a nozzle behind the back side of the protective shield; making sure that the spray is not shot all over the trimming lines and end up killing grass that the user wants to keep. The system would operate the same way that a Swiffer WetJet works, and you would press a button to release the herbicide.

Factors Addressed

  • Social: Adding a weed killer spray to the weed wacker would save time for the user because it would allow them to get two jobs done at the same time.
  • Economic: The weed wacker with the weed killer sprayer would increase the price of the tool. However, not by as much as you would think because anyone who would want this would probably invest on a stand-alone weed killer spray. So it would take the cost from that and put add it onto the price of the weed wacker. The spray would also increase the life of the weed wacker meaning that you would save money because you would have to buy weed wackers less often.