Group 11 - Weed Eater Featherlite 20C - Gate 3

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Group 11 - Weed Eater Featherlite 20CGroup 11 - Weed Eater Featherlite 20C - Gate 3
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Figure 1: Fully assembled view of the product. }}
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Name of Artifact Featherlite 20C string trimmer
Manufacturer Weed Eater
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 [[[|v]]  [[|d]]] 

Group 11 - Weed Eater Featherlite 20C String Trimmer

Contents

Introduction

After completely dissecting the Weed Eater Featherlite 20C and documenting the process, the group performed a thorough analysis of the individual parts of the product and how they work together to form the various subsystems. This allows each member of the group to further understand the engineering decisions that went into creating the product and the different concerns that were taken into consideration. Each component has certain attributes that contribute to the overall function of its subsystem and the product as a whole. The function that each subsystem performs has an impact the material it is made of and how each part fits together. The group will examine how the global, economic, societal, and economic factors have impacted the design. This analysis will allow the group to create design revisions to improve the product.

Coordination Review

Cause for Corrective Action

As the project has progressed, the members of the group have continued to work well together to complete each task. Unlike Gate 2, this gate requires less hands-on work and more analysis of the individual components and subsystems of the product. Meeting at 86 Winspear each week has proved to be extremely successful because this was also the location where the actual dissection took place making the parts readily available.

Although there has not been conflicts between the different members, we have faced a large conflict with time management. This gate in particular has been difficult because performing a complete analysis requires a great deal of time making it difficult to balance the work of this project with the workload from other classes. Since the members of the group take the same classes, they each have similar workloads. Not having a long period of time to complete the gate, there was also a large amount of tests scheduled during the week leading up to its due date. We were still able to complete the assigned tasks, but in a different manner. In order to prevent the members from getting overwhelmed, the group broke up the tasks into smaller portions. Having these smaller tasks assigned more often has made it easier to follow the schedule previously laid out in Gate 1 in the Gantt Chart. We has also changed some of the meetings to Capen Library during the day when the members have a break in between classes. This has been beneficial because it opens up the night for work in other classes and allows the more frequent tasks and deadlines to more easily be assigned

In Gate 2, the group members completed their assigned tasks and sent them to Erika Salem to be edited and uploaded onto the wiki - the group soon realized how time consuming it is to upload and format this work. To make sure that one person was not doing more work than the other members, the group divided up the task of editing the compilation of work. This is beneficial because it allows each member to improve their technical writing skills, which was a weakness of many. We have also decided to schedule additional time editing and uploading the information making tasks due earlier than in previous gates. This earlier deadline made completing the overall gate easier and less stressful.

Although the group has only faced problems involving time management and heavy workloads, addressing these issues and making certain adjustments has made completing the gate less strenuous. For completing Gate 4, meeting at a group will prove to be an issue because of Thanksgiving break. The members of the group will be returning home for five days, which will prevent the group from being able to have the usual weekly meetings. This can be resolved either by assigning a greater amount of work over the break to be completed or by having a group video chat to discuss the progress that has been made thus creating a meeting as similar as possible to the regular meetings.

Product Evaluation

Component Summary

The Weed Eater Featherlite 20C can be broken down into three sections by which to dissect the product: the engine, the driveshaft, and the head. The driveshaft and head are composed of few parts and required a fraction of the time to dissect compared to the engine. The engine is made up of numerous screws and small parts that cannot be seen without removing the outer casing resulting in a lengthy dissection time.


Partslistwhole.jpg
Fig. 3-1: This is an exploded view of the parts of the string trimmer excluding the engine. The number of each part corresponds to the parts listed in Table 3-1.


Table 3-1: Component Summary For Figure 3-1
Component 1: Throttle Cable Assembly Component 6: Drive Shaft Housing Component 11: Line Limiter Component 18: Hub
Component 2: Trigger Component 7: Drive Shaft Component 12: Screw - Line Limiter Component 19: Compression Spring
Component 3: Throttle Housing (Right) Component 8: Throttle Housing (Left) Component 13: Dust Cap Component 20: Clip/Retainer
Component 4: Screw - Throttle Housing Component 9: Vertical Grip Component 14: Bolt Component 21: Screw - Clip/Retainer
Component 5: Bolt - Vertical Grip Component 10: Wingnut - Vertical Grip Component 16: Wingnut - Shield Component 22: Spool With Line


Manualengine.jpg
Fig. 3-22: This is an exploded view of the engine of the string trimmer. The number of each part corresponds to the parts listed in Table 2-2.


Table 3-2: Component Summary For Figure 3-22
Component 1: Muffler Kit Component 18: Air Filter Cover Component 35: Kill Switch Component 53: Flywheel Assembly
Component 2: Muffler Spring Component 19: Screw Component 36: Starter Handle Component 54: Washer
Component 3: Spark Arrestor Kit Component 20: Spacer - Choke Component 37: Rope Kit Component 55: Screw - Retainer
Component 4: Screw - Cylinder Component 21: Washer Component 38: Screw - Pull String Component 56: Retainer
Component 5: Spark Plug Component 22: Piston Kit Component 39: Drive Coupling Component 57: Starter Pulley Kit
Component 6: Cylinder Component 23: Piston Ring Component 40: Starter Dog Spring Component 58: Starter Spring
Component 7: Cylinder Gasket Component 24: Piston Ring Retainer Component 41: Retainer Ring - Crankshaft Component 59: Fan Housing
Component 8: Carburetor Adaptor Component 25: Connecting Rod Assembly Component 42: Outer Bearing Component 62: Screw - Fuel Tank
Component 9: Screw Component 26: Gasket-Cylinder Kit Component 43: Crankcase Seal Component 63: Fuel Cap
Component 10: Carburetor Gasket Component 27: Ignition Module Component 44: Inner Bearing Component 64: Fuel Pickup
Component 11: Carburetor Kit Component 28: Screw - Ignition Module Component 45: Shroud Component 65: Fuel Line (Small)
Component 12: Choke Plate Mounting Component 29: Ground Wire Assembly Component 46: Screw - Shroud Component 66: Fuel Line (Large)
Component 13: Choke Shutter Component 30: Lead Wire Assembly Component 47: Screw - Outer Casing Component 67: Fuel Tank Assembly
Component 14: Air Box Component 31: Nose Cone Clamp Assembly Component 48: Muffler Guard
Component 15: Screw - Choke Plate Mounting Component 32: Nose Cone Screw Component 49: Crankcase O-Ring
Component 16: Plate-Filter Support Component 33: Nut - Nose Cone Component 50: Crankshaft Assembly
Component 17: Air Box Foam Component 34: Screw - Front Case Component 51: Crankcase Assembly

Complexity Scale

In order for the complexity of the components to hold any meaning, a scale had to be created to compare the components to each other. To determine the overall complexity of each component of the Weed Eater Featherlite 20C, the group utilized a complexity scale based off of three characteristics - geometry complexity, manufacturing processes complexity, and function complexity - which are the three main topics outlined in Table 1 in the Gate 3 handout. Since the parts do not greatly vary in these three categories, the complexity scale used only extends from 1-3.

Geometry Complexity:

  • 1. The part is made of one or two basic shapes (circle, square, etc.)
  • 2. The part is made of numerous simple shapes put together.
  • 3. The part is made of complex shapes

Manufacturing Processes Complexity:

  • 1. The part is manufactured using only one process.
  • 2. The part is manufactured using two different processes.
  • 3. The part is manufactured using three or more processes.

Function Complexity:

  • 1. The part performs one function that does not involve movement during use.
  • 2. The part performs one function that receives at least one flow (mass, signal, or energy) during use.
  • 3. The part performs two or more functions.

The complexity rating for each component can be found in the component summary located in Table 3-1 and Table 3-2. The two tables are a record of every part of the string trimmer each having a description that includes the complexity rating based on the scale outlined. Since the components work together and contribute to the overall function of the product, a broad scale is not needed. Components that received a low rating in geometry complexity include piston ring and the gaskets, which also received a low rating in manufacturing processes complexity and function complexity. However, certain components such as the crankcase received a low manufacturing processes rating while receiving high geometry complexity. The complexity of the geometry of the crankcase is because of the function is performs and the amount of parts that attach to it. The complexity ratings in the various categories greatly depend on the location of the component as well as its function.

Product Analysis

Aside from the complexities of the individual components stated above and described in this section, there is also different levels of complexity for the physical interaction between components. The levels range from a very simple connection that requires no screws or fasteners - parts that simply slide on to one another - to more complex connections that involve numerous screws.

Drive Shaft & Housing

Component Function:
The function of the drive shaft is to transfer the energy produced by the engine to the cutting head to allow it to rotate the cutting line successfully performing the overall task of the string trimmer. As the engine is running, the movement of the piston causes the crankshaft and the flywheel to begin to spin creating rotational energy. As this happens, the energy is transferred from the the engine assembly to the drive shaft. The shaft is not composed of solid metal; rather it is a thickly braided steel cable. This allows for the preservation of kinetic energy thus giving the string trimmer a higher efficiency, while still allowing for the curved shaft desired in the design. Once the flywheel, and thus the shaft, begins to rotate, the rotational kinetic energy is then transferred to the cutting head. The housing of the drive shaft possesses a very different function. The drive shaft connects to the engine, but is not exposed to much heat. Since this component is always moving during use, material was chosen that would remain durable and have a long lifespan. The housing is an aluminum tube that completely encircles the drive shaft. It protects the drive shaft from debris and contaminant keeping it in better condition for a longer period of time thus having a longer lifetime. Even more importantly, the housing protects the operator from the fast-spinning drive shaft. Without the presence of the drive shaft housing, the string trimmer would be inoperable making it a vital component of the product.

Component Form:

Figure 3-88: How drawing a wire works

The drive shaft is not made of basic shaped materials as one might think. This being said the drive shaft is composed of numerous drawn wires that are braided together, making it three-dimensional, in order to make the drive shaft very strong and durable. This is important because the drive shaft is constantly in motion during use. \ Most of the pieces of the drive shaft are formed using forging with additional drilling done in order to adds the slots that allow for the other components to connect to it. These pieces also need to be very long and skinny in order to fit into the driveshaft casing that travels down to the head. The way that the driveshaft is formed is extremely related to the purpose that it serves.

Manufacturing Methods:
The drive shaft is composed of multiple layers of steel wire. Each individual wire is drawn thin through a die. Once multiple wires are drawn, the group is then spun tightly together, forming one thin cable. Two cables, each being sixty inches in length, are wound together creating one thick, tightly-wound steel braided cable that is both rigid, yet compliant. The drive shaft is manufactured by cold-drawing in particular because it increases the already strong material making it extremely durable. This manufacturing process is possible for this application because of the high ductility, yet rigidity of steel. Numerous individual steel wires that are strung together allow for the flexibility required in the curved shaft design, yet have enough combined strength to transfer the high amount of kinetic energy from the engine to the cutting head for operation. The drive shaft housing, on the other hand, is composed of lightweight aluminum what is manufacturing my forging. Forging creates the overall shape of the housing and drilling creates the additional holes that the housing possesses.

Material:

Figure 3-89: The bend on the shaft contributes to ease of use
The materials involved in the drive shaft assembly are steel and aluminum, which are both strong, cheap, and readily available in most areas of the world. The drive shaft is composed entirely out of drawn steel, braided into one single, thick cable. This design was chosen because it allowed for flexibility of a curved drive shaft while still being able to handle the strain placed on it by the engine. The drive shaft housing is composed of aluminum, rolled into a hollow tube to separate the shaft and the operator. Aluminum was chosen because it is incredibly lightweight compared to steel. This greatly contributes to the low weight of the string trimmer and thus its ease of use. Although aluminum is not as strong as steel, the housing is not subjected to the same stresses as the driveshaft, and thus rigidity was not as pivotal of an issue in design consideration. The manufacturing processes used to create these components were chosen because they are quick and cheap, while still remaining accurate and keeping the material strong (or strengthening it in the case of drawing).

Aesthetics:
These components were not produced to be aesthetically pleasing. One reason for this is because of the fact that the entire drive shaft is completely enclosed by the drive shaft housing and is not designed to ever be disassembled by the operator and thus has no aesthetic importance. If one were to remove it from the housing, it would be a dark grey colored covered entirely with the lubricant that is used to keep it rotating smoothly. The design and "look" of the drive shaft housing is directed more toward making the string trimmer easy to use rather serving an aesthetic function. The aesthetic characteristic of the housing is in its smooth finish that keeps it looking have a smooth and new. Although the aesthetic characteristics of the components were not really taken into account, it does not hinder the product by deterring the market that they are selling to.

Component Complexity:
Although the drive shaft assembly is not very complex, it is more complex than one might think based on seeing the drive shaft housing. This is because the drive shaft is composed of a series of steel wires braided and wounded together. The drive shaft housing, however, is essentially just a long tube of metal forms an angle in order to make using the product easier. The physical connections made by the drive shaft and housing are not complex. The drive shaft connects to the drive coupling on the flywheel on the top side and the cutting head on the bottom side. This physical connection allows for the rotational energy to be transferred from the engine to the cutting wheel.

Engine Assembly

Component Function:

Figure 3-90: The engine on the side of the flywheel

The engine is the major assembly of components that the entire string trimmer depends on in order to function. The engine imports a combination of a fuel/oil mixture and pressurized air that undergoes combustion resulting in translational kinetic energy that gets transferred to rotational kinetic energy. The engine is composed of various parts including the cylinder, piston, piston connecting rod, crankshaft, flywheel and the crankcase. Aside from the engine being made up by these parts, it also connects to numerous other components such as the carburetor, fuel tank, exhaust, drive shaft, and air box. The fuel stored in the fuel tank is transported to the carburetor via the small fuel line and ignites in the cylinder. This ignited fuel creates pressure which in turn pushes the piston down turning the flywheel. This acts in a circular motion which pushes the piston back up the cylinder in order to expel the hot exhaust gases that the combusted fuel produced as well as accept more of the gasoline/oil fuel. The circular energy created by the piston is contained by the cylinder in order to direct the energy in a fluent motion. This rotational energy created by the fly wheel is then transferred to the driveshaft. Without the engine there would be no production of energy and thus no supply of energy to the cutting head to rotate. The engine creates all of the energy and motion for the string trimmer and transfers it to the other components to be utilized for complete the function of the product.

Component Form:
The engine is not made up of simple shapes but is instead it is composed of a very intricate arrangement of symmetrical and three-dimensional parts. There are various different shapes and screw holes within the engine block that act to hold the assembly of components together so they can act as one component. There are also many screw holes on the exterior of the block that allow other components of the machine to attach and bond to it. The engine also has many slots cut into it in order to allow for heat to escape in a more efficient way and to let the rotational energy be as efficient as possible. The different shapes and geometries of the components were created largely as a result of their function as well as so they can fit together in a small area to keep the engine relatively small and easier to maneuver. The engine assembly operates in a hot environment due to the combustion of the fuel, which impacted the decision to construct the engine out of steel and aluminum that can withstand the heat without changing its original properties.

Manufacturing Methods:
The engine consists of the crankcase, piston, piston connecting rod, flywheel, cylinder head, crankshaft, as well as numerous screws used to hold everything together. The crankcase is manufactured using die casting. This is evident by some tapering as well as part lines that show where the molds connected and where the tool removed the part from the mold. The cylinder head is also made utilizing the die cast method given that it also has a part line that divides the cylinder in half showing where the molds joined together. Both of these parts are also subject to drilling to create the amount of holes needed for screws. Since screws are used to connect the components, the drilled holes also had to get turned on the lathe in order to create interior threading. The flywheel, on the other hand, does not have the identifying characteristics of something made using die casting because it does not have any part lines and is produced much like a turbine blade. As a result, the flywheel was manufactured using investment casting. This is also because it is very difficult to create the arced teeth the flywheel possesses with any other manufacturing process. Different from all of the previously mentioned components, the piston was not manufactured using any sort of mold. Instead, it was produced by turning on a lathe because of its axial symmetry. The piston also has two holes that go through it, which are produced by drilling because it is a very fast yet still moderately accurate process. The crankshaft is manufactured by forging, which strengthens the material by compressing it. This process is used because it is very dimensionally accurate. It is then turned in order to create the threads that are on the end that connects to the flywheel. The connecting rod is also made by forging coupled with drilling because of the holes it possesses that allow it to connect to the piston on one side and to the crankshaft on the other side. These different processes can be performed because of the material of the parts. Die casting requires a metal of high fluidity, which is achieved by the aluminum and steel within the engine. Other major factors that played a role in deciding which process to use was that these processes are able to produce a high volume of parts in a relatively cheap and efficient manner.

Material:

Figure 3-91: The cylinder that connects to many of the other parts of the engine

The two materials that make up the engine are aluminum and steel. These particular materials were chosen because they are relatively cheap to work with, easy to obtain, and can also withstand the pressure produced as a result of combustion. These hard metals also act as a shock resistor so if the weed whacker is dropped there will not be detrimental damage to the machine. These parts are also strong enough to put up with the amount of heat they are subject to as well as being easily replaceable to the average person because of the relatively low cost of production. These materials are also readily available in most countries so they can be reproduced almost anywhere in the world and if not they can be shipped. The crankcase, flywheel, and connecting rod are made of aluminum. The flywheel and crankcase, which are the largest parts in the engine are made of aluminum because it is a lot lighter than steel. In this case, societal factors played a large role in the decision of what material to use because the use of aluminum keeps the weight of the string trimmer relatively low making the trimmer less strenuous to use. The piston and crank are made out of steel, which is stronger and heavier and thus can withstand more extreme conditions. Both materials are relatively available in most countries that would use this product contributing to the final decision of material.

Aesthetics:
The engine block, like the drive shaft, was not designed with aesthetics in mind because except for certain circumstances the engine is not visible to the operator. As a result, the steel and aluminum are dull grey in color. These blocks can be painted, but because of the heat they are subject to the paint would probably peel off so this process is not done. The engine is contained by the outer casing preventing the operators from being able to see it. Instead of making the engine aesthetically pleasing, the outer casing that houses it is aesthetically pleasing. In essence, the aesthetics of the engine assembly are not really relevant to how the product looks or functions.

Component Complexity:
On the outside the engine just seems like a simple block that many people would be able to take apart. When one begins to take it apart they will realize that it is a lot more complex than they originally thought it would be. Every part that is taken out needs to be re-lubricated before it is put back into its position in the engine block or else it will create way too much running friction further decreasing the overall efficiency of the string trimmer. It is also easy to break parts as you take the product apart because some of the pieces can be very small and fragile such as the piston ring retainer. Also various processes need to be used in order to form all the parts that the engine consists of. The parts needed to be made relatively cheap as well as sturdy, which is why aluminum and steel were used to make them and turning was used to smooth out most of the parts. The engine is very complex in that it deals with numerous interactions - it is composed of numerous parts that transform the fuel and air originally input into rotational energy that the string trimmer can use to perform its job.

Plastic Casings

Component Function:
The main purpose of the plastic casings, namely the fan housing, shroud, and cutting shield, is to protect the operator. The cutting shield extends out from the end of the drive shaft housing to keep the cut grass from flying upward back at the user. The end of the shield also bends downward protecting the legs of the user from the cut grass and weeds getting pushed radially outward. This downward bend serves an addition purpose in that it guards the operator from the line, which is constantly rotating on the spool. As a result of the design of the shield, it is located between the cutting head and the operator. The fan housing and shroud also serves multiple purposes. Their main function is to protect the user not only from the heat produced by the engine, but also from its moving parts such as the flywheel, which is constantly rotating during use. The shroud is located on the side of the user and is manufactured with horizontal spaces with no material as shown in Figure 3-67. This allows for the warm/hot air to be released without harming the user - this helps to keep the engine from getting excessively hot. The plastic casings have little to do with the flow of material and energy. The cutting head receives the rotational energy from the drive shaft and transfers it to the cutting line that allows the string trimmer to successfully complete its job.

Component Form:

Figure 3-92: Part lines on the yellow air box

These components are not made up of basic shapes. They are three dimensional, but rarely axially symmetric. Rather they are composed of various holes and details that allow them to fit together with the various parts they are connected to. For example, the fan housing and shroud have large empty cavities of particular dimensions that allow the engine to fit within that space. The component form also contributes to the function each part and the tasks it performs. This is very evident with the shield - it extends out a long distance and bends downward at the end in order to fully protect the operator during use. If the part simply extended outward from the drive shaft housing it would not fully protect the user because with the angle at which the string is held, the cut grass could easily still come into contact with the lower legs of the user. The form of the components is greatly attributed to the purpose it serves.

Manufacturing Methods:
The manufacturing method used to make the plastic casings including the air box, handles, and the different components of the cutting head was injection molding. The identifying characteristics possessed by the various parts include part lines and riser marks located on the underside of most of the parts as well as some parts left with flash. The use of plastic greatly influenced which manufacturing process would be used because plastic has high fluidity allowing the liquid plastic to easily conform to the mold and harden into that shape and design. All of these components are composed of a variety of shapes and numerous holes, which can easily be achieved through injection molding. Global factors did not play a role in the decision to choose injection molding for the plastic components; however, societal and economic factors played a vital role. Safety is always the main concern with any product. Injection molding creates a durable product that is reliable and will last for a long time making it appealing to consumers. Economic factors had the greatest impact on the decision to manufacture the products using injection molding because this process is good for creating a high volume of small/medium parts with dimensional consistency. Although the initial cost of the machine is expensive,the molds used are reusable making the overall cost of the process relatively cheap. This also ensures that the parts are dimensionally consistent from string trimmer to string trimmer, which makes the consumer confident that the particular
string trimmer he/she is purchasing is of high quality. Environmental factors play a small role because the excess plastic from each part can be recycled. Other than the electricity used to power
the manufacturing plant and the equipment there are no other negative effects of this process.

Material:

Figure 3-93: The flash left on the fan housing

Plastic was chosen as the material to manufacture these parts. This was done for numerous reasons: plastic is a lighter material keeping the string trimmer from becoming too heavy and strenuous for the operator to use, plastic is a lot cheaper than most other materials including metals and is readily available in most countries, and the material properties of plastic allow it to be manufacturing easily and cheaply. The use of metal in this situation is not necessary because these parts are not being exposed to high temperatures that would cause the plastic to melt and deform. Plastic allows for cheap manufacturing, which leads to the price of the product being lower and more affordable for consumers.

Aesthetics:
Plastic is available in a wide range of colors allowing it have both a functional and aesthetic purpose. All of the plastic components in the string trimmer are one of three colors: black, green, or yellow. This is because these are the colors of the company and its logo. The vast majority of plastic components on Weed Eater products are either black or green and the logo is written in yellow. This is no different on this Weed Eater Featherlite 20C. Of the visible plastic components, the shield and parts of the cutting head as well as the shroud are black while the fan housing is green. The yellow components of the product are trigger and the air box, which is connected to a black plastic component outside of the engine casing. Since the color of the plastic has no impact on its functionality, the choice of color is strictly for aesthetic reasons - they represent the company.

Component Complexity:
These plastic components range in geometric complexity. On the outside of each component the geometry seems very simple, but various intricate shapes are formed on the inside of these parts depending on their function. Figure 3-92 shows various shapes molded on the inside of it that allow it to hold the air filter foam and connect to the air filter cover (component 18). Although some parts contain intricate shapes, injection molding is capable of manufacturing such shapes. This further explains why injection molding was chosen as the manufacturing process to
produce these parts. The interactions made are simple in that they are all connected by screws and fasteners and the majority of the parts are stationary throughout use.

Fasteners

Component Function:
The Weed Eater Featherlite 20C is composed of a total of twenty-nine fasteners only two of which are bolts. The bolts are used to connect the cutting shield and the vertical grip to the drive shaft housing. Bolts are used as opposed to screws because they fully extend through the two components that are being connected allowing them to be tightened by hand with a wingnut. The function of screws and bolts is to connect two or more parts together and to secure certain parts in specific positions and do not play a part in the transfer of material and energy. This can be seen in the exploded view of the parts in both Figure 3-1 and Figure 3-22. The product uses different types of screws, Phillips-Head, hexagonal, and Torx, to connect different components because the screws are designed for different uses. The Torx screws are used for parts within the engine that are not designed to be removed because Torx screws are designed with the star-shaped head that resists cam-out better than other screws. Cam-out is the process by which a screwdriver slips out of its position during tightening. Torx screws limit this allowing the screws to get tightened to a great extent. Phillips Head and hex screws, however, are used to fasten the components that may need to be removed at some point. The shape of Phillips Head screws prevent them from being tightened too tight while the shape of the hexagonal screws results in a greater surface are allowing for it to be tightened effectively and easily by an Allen wrench. These different screws are utilized to connect the majority of the product components together because they are both strong and durable. This wide variety of components that screws are used for shows that they can function in different environments and under a wide variety of conditions and that heat does not really have an effect on its reliability and function.

Component Form:

Figure 3-94: The process of forging a screw

Aside from the various head shapes, screws all contain threads that allow it to tighten and connect components together. These threads keep the fasteners from being axially symmetric because of their pitch, but it is the pitch is essential in creating friction between the threads that keep the screws tightened over time. The pitch among screws generally remains constant allowing different screws to be used interchangeably. However, screws vary greatly in length based on what they are specifically designed for. This is greatly evident with this product because contains very small screws such as the one utilized to secure the clip/retainer in the cutting head in place (component 21) as well as very long screws such as the two choke plate mounting screws (component 15) that is long because it connects two parts that are not directly connected to each other, which also explains why why the threads are only located at the bottom half of these screws. The different head shapes possessed by these screws have different qualities that connect to their usage. The Torx screws are designed to be tightened the maximum amount without causing any damage to the tool due to camming out; however, the Phillips Head screws are designed with the opposite idea in mind. The head shape of these screws prevents the screws from being tightened too much thus usually being used for components that will more likely be removed at some point. Hexagonal shaped screws head are designed to be easily tightened because of its high surface area. Furthermore, Allen wrenches are also designed in an L-shape allowing the screws to be tightened with greater forced because it allows the person to apply more force. Global factors do not effect the shape of these screws, but does impact the size based on English or metric units.

Manufacturing Methods:

Figure 3-95: Creating threads by rolling

The initial shape of screws is manufactured by a specific forming and shaping process known as forging. Forging is when the shape of the component is formed due to compressive forced applied through a die. After the bar stock is cut to the specified length, the shape of the head is added by forging. Forging can produce a variety of head shapes depending on the shape of the punch, which is what applies the force to the top of the screw as seen in Figure 3-94. The process of forging can be done on a high volume scale making this allowing it to be a cost effective and efficient process. After forging, the thread are created by a thread rolling. During this process, the screw is rolled over two dies while it is still hot enough to be shaped. Thread rolling was chosen for the screw because that is the most efficient way to produce a thread for screws in large volumes. For most screws, including the ones utilized by this product received a final coating to prevent corrosion. This is mainly done by hot-dip galvanizing, which is the process of coating the screw with steel with a thin zinc layer protecting the screw from rusting. This allows the screws to remain durable and have a long length of life.

Material:
All of the fasteners used in the Featherlite 20C are made out of steel. This material was chosen because of the relatively cheap cost as well as the conditions and use the screws and bolts have to endure. Steel does not wear under climate conditions such as the heat of the engine and is not effected by heavy loads. It also keeps the screws from rusting allowing them to last for a long time. A main issue with screws is their tendency to become stripped over time and use by being screwed and unscrewed. Utilizing steel provides long-lasting durability to avoid this from occurring.

Aesthetics:
The screws and bolts used by this product are either black or silver. The majority of the black screws are the ones that are used the fasten the outer parts such as the throttle housing (ergonomic grip) and the engine casing. These screws are aesthetically pleasing because they blend in with the product parts making them less noticeable and giving the product as a whole a more appealing look. The screws used to connect the various components of the engine, however, are silver because they are not seen during operation and are only seen if a repair needs to be made and therefore the aesthetics of this does not matter. The screws fit into premade depressions that further allow the screws to be barely visible. Although the fasteners are designed mainly for their function, they also contribute to some aspects of the aesthetics of the product.

Component Complexity:
At first glance fasteners seem like simple components; however, it requires three manufacturing processes to produce. Most screws are relatively similar in complexity, but vary only in the shape of their heads, which can increase or decrease its geometric complexity. However, these different head shapes do not have an impact on the manufacturing process because different punches are used to form different shapes. The concept of the screw is also very simple and thus is used by a large portion of products manufactured and used in everyday life. The interactions are not complex in that the screws and fasteners simply tighten two or more components together.

Pull Starter Assembly

Component Function:

Figure 3-96: How the starter pulley kit fits in the fan housing

The pull start assembly contains all the contents that are necessary to spin the magneto in order for engine to start. Contained in the pull start assembly is the synthetic fiber and handle that the operator manually pulls. This is connected to a pulley system containing a plastic cylinder that the rope wraps around as seen in . The plastic cylinder fits into the magneto which turns the engine over. The pull start assembly also contains the spring that recoils the synthetic fiber of the pull start, so the operator does not have to keep recoiling the rope to start it. The overall function of the pull start assembly is simple - it is to start the engine. However, since there are parts in between the pull start assembly and the engine, the pull start simply spins the magneto as a result of the pull force from the operator on the rope which translates to turning the engine over. The starter pulley kit is attached to the fan housing and the string is fed through the part to connect to the pull start handle. Between the starter pulley kit and the fan housing is the spring, which allows the string to return to its original position after being pulled by the operator to start the engine. Although most of the plastic components serve no purpose in the flow of material and energy, the starter pulley kit transfers human energy into kinetic energy, and the cutting head transfers the rotational energy from the drive shaft to the line that cuts the grass.

Component Form:
The main form of the pull start assembly is a plastic cylinder with a grove around the edge for the rope to sit in. The cylinder translates the pulling force from the operator that is starting the engine to the magneto has a hole in the middle so it can connect to the drive shaft. The hole is circular to fit around the housing for the engine to hold it into place and is extended out towards the magneto so it can fit around and spin it. Also on that cylinder there are notches so the steel coil called the spring can fit in so it can be attached to recoil the rope and handle back to the pull start assembly. There is also the handle on the pull start assembly which is not a basic shape, but a shape to fit your hand for maximum grip to pull.

Manufacturing Methods:

Figure 3-97: This shows the location of the starter spring that must remain secure to keep from unwinding

The pull start assembly is primarily made of plastic except for the synthetic fiber that attaches to the cylindrical pulley to the handle. Also the spring is made of thin steel that is coiled around the cylinder so it can recoil back to the pull start assembly. since the steel is required to be thin it was manufactured by rolling. This is also because this process is quick, cheap, and can create this component in high volume. The plastic cylinder and the handle of the pull start assembly are both manufactured through injection molding because polymer has a high fluidity allowing it to be fed into the mold. Injection molding was chosen because it can produce components in high volume with good dimensional consistency and is good for manufacturing smaller parts. The synthetic fiber is made from a forming and shaping method called drawing. The manufacturers draws the various strands of fiber into bundles, which are then braided to increase its strength.

Material:
The pull start assembly is made of a few different materials including plastic, steel, and synthetic fiber. The pulley assembly containing the cylinder is composed of plastic because it is cheap, can be produced in high volume, strong enough for the job it has to do, and is accessible all over the globe. Plastic was also chosen for these components because it is very lightweight keeping the overall weight of the product down making is easier to use by the operator.The coil spring is made of steel for strength, durability, and flexibility. Steel is used for the coil spring so it can recoil back to where it was and still be able to be stretched without breaking. Steel is able to straighten out as well as become coiled again until plastic. Synthetic fiber is chosen because of how flexible, accessible, cheap, and easy it is to produce. Synthetic fiber is the best material for what it does because of its durability to be pulled and stretched without breaking. Synthetic fiber is becoming increasingly popular for this use. If these parts break or wear down, the plastic and steel can both be recycled.

Aesthetics:
When the pull start was designed it aesthetics were taken into account for a few reasons. Since the pull start assembly is almost hidden from view, manufacturers chose black as the color of the handle to match the shroud and most other plastic components and to further represent the logo colors of the company. The pull start handle is located outside of the fan housing, which is green. This allows the handle to match while still being easily noticed by the operator. The handle is also made so your hand fits around it perfectly for maximum pulling comfort. The parts of the pull start assembly that are located within the outer casing are simply white because it will not be seen by the operator unless the string trimmer is taken apart

Component Complexity:
The individual components are simple in their geometry, manufacturing, and function, but when put together becomes very complex. This is because the pull start assembly has a very large function it has to do and without it, the engine would not start. The pull start translates your pulling force into rotational energy for the engine to turn over. It consists of a pulley system with a tightly wound flat steel starter spring (component 58) that recoils the string after the operator pulls it. Without this component, the string would have to be manually rewound each time the string needed to be pulled. If the spring is not placed in its designated position in the correct direction the the string trimmer will not be able to function properly. The pull start assembly is what actuates the entire function of the trimmer.

Solid Modeled Assembly

The Computer Aided Design program used to model the components of the piston assembly and connecting rod was Auto Desk Inventor 2010. Inventor was chosen over AutoCAD because a member of the group had extensive experience using this program since high school. This group of components was chosen because the piston is a relatively small component that is composed on numerous parts that serve and important function. The piston assembly and connecting rod are the what convert the energy produced by the engine from the fuel to kinetic rotational energy that gets transported through the drive shaft to the cutting head.

Individual Components

Table 3-3: Individual Solid Models of the Piston Assembly System
Part Name Solid Model of Part
Piston Bottom
Figure 3-98: Bottom part of the piston
Connecting Rod
Figure 3-99: Connecting Rod
Piston Ring Retainer
Figure 3-100: Piston Ring Retainer
Piston Ring
Figure 3-101: Piston Ring
Piston Inner Bar
Figure 3-102: Piston Inner Bar
Final Assembly Together
Figure 3-103: Final Assembly Together


Fig.3-104: Exploded View- This shows the assembly of the components in isometric view.
Fig.3-105: Exploded View- This shows the assembly of the components in front view.

Engineering Analysis

The most intricately engineered component of the machine is the motor assembly, primarily focused on the path of the fuel, from entrance via the carburetor until the conclusion of the path out of the exhaust assembly. The key problem with the combustion process of the engine is the fact that it is a small, two-cycle motor. The nature of a two-cycle motor tends to lead to an excess amount of unburnt fuel, and high levels of pollutants in the form of exhaust gasses. The other problem wi th the small, lightweight nature of the engine assembly is that there is little room for an adequate enough exhaust muffling system in order to keep noise levels down. Because of this, engineers had to utilize all of the space and weight constraints in the most efficient way possible.

With the weight of the machine being limited to such a small value, there is no room to assemble a catalytic converter, which would neutralize pollutants in the exhaust gasses. Because of this, engineers had to most effectively utilize the gasoline. This motor is of “light-duty,” meaning high-performance is not required in this assembly. Accordingly, with the demand for power reduced, so is the demand for fuel. As a result, engineers were able to run smaller jets in the carburetor, which are small holes that are responsible for allowing fuel to be drawn into the combustion chamber, allowing for better fuel efficiency. In order to provide adequate lubrication without pollutants, engineers decided upon a fuel-to-oil mixture of 40:1. This mixture has a higher percentage of oil than that recommended for high-efficiency two-cycle motors of 50:1, however is much leaner than the 32:1 ratio required in high-performance two-cycle motors.

With the issue of fuel usage addressed, the issue of next utmost importance for operation is the volume of noise produced by the engine. Engineers had to adequately muffle the motor noise without using an excessive amount of space or weight. The motor, not being of a high-performance nature, does not require a tuned exhaust system, which reduces the amount of exhaust back pressure, to create extra power. The goals of the exhaust manifold in this assembly are to channel hot exhaust gasses away from the operator, and to muffle noise. With these functions in mind, engineers chose to go with a “suitcase” muffler design, with the entrance and exit relatively close to each other (from an outside glance). However, the path of the exhaust starts at the entrance on the broad side of the muffler while flanges work to channel the gasses from one part of the muffler to the other dissipating noise in the process. A layer of fiberglass “packing” on the inside of the muffler further silences noise by absorbing pressure from each engine explosion, and thus, the noise attributed.

Analysis Problem

Problem Statement:
The Weed Eater Featherlite 20C string trimmer has an engine efficiency of 18% as seen in the diagram below.[1] The power output of the engine is 1 horsepower and the string trimmer is used for 30 minutes. Determine the amount of energy input into the engine and the amount of energy lost due to heat. How many gallons of gas/oil mixture (40:1) is needed to run the engine for this period of time?


Diagram:
Diagramm.PNG
Assumptions:

  • 1 Horsepower = 746 J/s
  • Weed Eater is ran on full throttle for 30 minutes without letting off
  • Heat released by exhaust system
  • The energy of gasoline (Eg) is 132 MJ/L
  • 3.2 oz of motor is used per gallon of gasoline
  • The motor oil provides no energy to the system – it simply keeps the engine lubricated
  • “V” = the amount of fuel
  • The remaining energy not used by the head is lost as heat from friction


Governing Equations:

  • Efficency=output/input
  • E=P*t
  • Emech = Echem
  • P*t = E = Eg*V

Product Revisions

Figure 3-106: The arrows show the direction of the air

Design Revision 1

The object of this machine is to be a simple to use, lightweight string trimmer. The design has high functionality, yet is flawed in some components. Simple changes could completely alter the machine for the better. Due to the fact that this particular model is built to be used by an average consumer around a residential property, a key factor is safety of the operator. Many of the potentially dangerous parts are protected quite well, however an issue that is not completely resolved is the issue of excess heat emanating from the engine. The solution that is most viable for this problem is to implement intake and exhaust holes in the engine shroud. Intake holes would be placed on the underside of the engine on the front (closer to the drive shaft) while exhaust holes would be placed on the upper rear side. The heat generated by the engine operating would cause the hot air around it to exit the exhaust holes, pulling in cool air from the underside. When in operation, the operator holds the machine such that the engine assembly sits about twenty degrees above the horizontal. Because of this, The exhaust holes will be almost directly above the intake holes, allowing for smooth exhaustion of hot gasses in a direction that is away from the operator. This design revision allows the operator be remain safe from heat from the engine, while simultaneously allowing the engine to run cooler, and thus longer and more efficiently. The consumer cost will likely not increase at all because the molds, and manufacturing methods will change minimally. The driving factor of this design revision was societal.


Figure 3-107: Implementation of an electric starter

Design Revision 2

The Weed Eater is designed for the average homeowner, in the way that the unit is lightweight and simple to operate. The only issue with the current system as far as operating goes is the fact that ignition of the machine requires strenuous amounts of effort in order to pull the starter. The implementation of electric start would broaden the market of this machine by means of allowing virtually anybody to start and operate the Weed Eater. The usage of Lithium-Ion batteries and high efficiency electric motors would not compromise the lightweight nature of the machine. This type of technology is already implemented in such applications as motocross bikes, radio-controlled cars, where saving weight is a crucial concern. Thus implementing an electric starter to the string trimmer would not have a significant impact on the overall weight of the product.




Design Revision 3

Figure 3-108: The current size of the exhaust system that would have to be enlarged

With the nature of the machine being user-friendly, and available for everybody, a key issue is noise. This machine will be used in residential neighborhoods, where, for the most part, houses are all quite close together. Because of this, it is necessary to reduce engine noise as much as possible. This not only benefits not only the user, eliminating the need to wear ear protection during operation, but also benefits neighbors and wildlife. When the engine is operating at high RPMs, during normal operation, the noise is deafening at close proximity, but annoyingly audible for hundreds of yards surrounding. With a quieter exhaust assembly, neighbors will not only hear a difference for the better, but also, local wildlife might not be as fearful about treading or nesting near the property. The only design changes necessary to accomplish this would be the manufacturing of the newer, larger exhaust manifold. Consumer cost will likely not increase because although the molds will need to change for the exhaust parts and the shroud, the manufacturing processes and facilities do not need to change. The influences behind these revisions will be both environmental and societal.



References

[1] http://en.wikipedia.org/wiki/Internal_combustion_engine#Two-stroke

[2] http://www.ehow.com/how-does_5502005_piston-manufacturing-process.html

[3] http://www.madehow.com/Volume-1/Spark-Plug.html

[4] http://wiki.ece.cmu.edu/ddl/index.php/One-handed_weed_whacker

[5] http://www.itpower.co.uk/investire/pdfs/flywheelrep.pdf

[6] http://www.jp.com.au/Made.html