Gate 2 Group 15 2011

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Contents

Introduction

In Gate Two, we began the physical dissection of our 1991 Craftsman weed wacker. For the cause for corrective actions portion of the gate we assessed the plan we constructed in Gate One. We discussed what went according to plan and what things we did differently and why. We also talked about the various conflicts the groups face and how we resolved them. In the physical dissection of the product we completely disassembled the weed wacker to see how all of the various subsystems were connected to each other.


WholeChebang.JPG

Picture 2.1 - Disassembled Product.

Reflection and Corrective Action

Following Management Plan from Gate 1:

Following the initial management plan set forth in Gate 1, our group had a great deal of success in the dissection of our weed wacker, and ultimately with the finalization of Gate 2. Our plan was to set strict due dates and meet as a group often (at least once a week) to discuss progress and determine what to do in order to accomplish our goals. This course of action was chosen so that we would not have to cram the work into a single weekend. Instead we would simply have to add finishing touches to our final report because the majority of the work would be already completed. Our management plan also set up a way for us to quickly resolve any conflicts that arose while working on the project. If two group members strongly disagreed, and the conflict could not be directly resolved there would be a group vote in which all members of the group had equal weight. If one of the involved individuals was still dissatisfied with what was going on that individual was encouraged to consult either a TA or one of the instructors (no conflict has required these actions). A list of conflicts we encountered and how they were resolved is provided below.


Table 2.1 -Conflicts that our group had experienced and how they were resolved.

Conflict Resolution
Disassembly of the spring shaft. A group discussion that lead to the group moving forward on the dissection without taking the spring shaft out of its casing. (After consulting Instructor Cormier we returned to the lab and removed the spring shaft).
Time conflicts regarding meetings. There have been multiple conflicts between group members’ schedules.

Members that are available meet and the missing group member(s) catch up at a later date. In the case of more important meetings (such as lab work) we move the time of the meeting so that everyone can attend.

Incompleteness of portion of Gate 1. When the project manager saw that a portion of Gate 1 was incomplete he informed the group member responsible for that section of the gate and the issue was promptly corrected.
No group member was familiar with wiki design. Brent Haseley took it upon himself to figure out how to format the group’s wiki page, and before he started on the page for Gate 2 he met with an instructor during office hours to figure out how to better present the group’s information.


Distribution of Work

One of the biggest problems most groups are faced with is the distribution of work. It is a very difficult task to decide what constitutes a fair amount of work is for each person in the group. We also found it challenging to ensure that nobody feels as though they are responsible for completing the entire project on their own. Our management profile states that the project manager has the responsibility to delegate tasks from the gates and to set the deadlines for the group. However, if anyone feels as though they have been assigned too much work they are encouraged to speak up and propose an idea on how to improve the delegation of the gate. Thankfully there have been no issues with the distribution of work. This was due in large part to Charles Kalbfell, who as project manager made sure all group members stayed on task and completed their respective parts in a timely manner.


Team Dynamic

Our group has developed great chemistry and camaraderie, even in the face of the inevitable minor setbacks and mistakes. We are now comfortable enough with each other to offer constructive criticism, which has let group members improve in areas where they were relatively weak. One clear example of the group benefitting from constructive criticism was during the completion of Gate 1. One group member had not sufficiently answered all of the questions pertaining to his section and other group members informed him of this inadequacy. He then promptly remedied his mistake, and in doing so he drastically improved his part by more fully developing his ideas and adding more description and figures. Due to respect for one another and constant communication our group has developed a tremendous working dynamic that has enabled us to collaborate on this project with very few conflicts. We all are willing to listen to each other’s opinions and we are able to resolve problems before they escalate. A problem faced by many groups involves the workload from other classes, but this has not had too much impact on our group or our deadlines because we have stayed on track and been very proactive.


Group Member Roles

Everyone completed their tasks and volunteered their strengths during the dissection phase of our project. Frances Kalbfell did a great job on Gate 1 editing everyone’s parts and providing feedback. Charles Kalbfell provided the leadership backbone of the group by making sure everyone completed their tasks in a timely manner. Brent Haseley was very proficient in learning how to use the wiki page and putting the project up, as well as playing a major role in the product dissection. James Ziccarelli and James Quirk were vital in the physical dissection of the weed wacker. James Ziccarelli proved to be a technical expert while conducting the majority of the engine disassembly, and James Quirk documented everything that was done in the lab.


Group Meetings

Our success in meeting our deadlines was due in large part to our willingness to meet as a group. As stated previously, we originally planned to meet as a group at least once a week. It turned out that we consistently meet three or four times a week to make sure everything gets done on time. At the end of each lecture we meet for a minimum of 15 minutes so that everyone gets caught up on the progress of the other members and to discuss what still has to be completed. Outside of class we have relied heavily on e-mail to coordinate and extend group meetings. So far our group has spent three hours (over two days) in the lab completing the dissection of the product. While we were in the lab we decided we would rather go for one longer lab session than two short ones, which meant we did not meet our original goal of going to the lab every week. However, since one lab session represented double the time it equaled about the same amount of actual lab time we originally planned on. In Gate 1 we originally showed that dissection of the product would only take twenty minutes, but we had more difficulty in taking off the casing screws because of unanticipated rust and corrosion. The process of documenting every step of the dissection also slowed down the group more than we initially thought. The knowledge and experience we received from this part of the project will help us greatly in the reconstruction phase; when we reassemble the product we will know exactly where everything goes.


Table 2.2 - Group meeting dates and completed tasks.

Date of Meeting Accomplishments
10/13/12 We removed all the external casing and fasteners on the weed whacker.

We took apart every component of the engine.

10/20/12 We removed the drive shaft from its housing in the curved shaft and removed the bearings around the magneto using a hammer and other hand tools.


Potential Setbacks

By being proactive we were able to avoid problems arising from group members not cooperating. We were also extremely fortunate in that we didn’t have any unresolved conflicts in completing Gate 2. We went through the project smoothly because we invested many hours in preparing ourselves for our tasks, and because we were all willing to work hard and help one another. Whenever an individual encountered a difficult time with their part of the project they only needed to e-mail the rest of the group, and someone would respond by helping as much as they could.

Going forward we foresee very few problems, however they may be rather difficult to overcome. The most worrisome of these is that only one of our members has proficiency in AutoCad, but this potential speed bump can be easily avoided by being proactive and going to office hours if we ever have any issues. Another potential problem we may have going forward is that there may be difficulty in reassembling the weed wacker. For example, some parts of the engine (such as the piston rings) are pressure fit and require the use of specialized tools to install correctly.

Product Disassembly

Step Difficulty Rating

When assembling or disassembling a product it is helpful to know how difficult a given task will be. This warns the user if assistance will be required or if special equipment is going to be necessary. A simple number scale with no examples is not optimal – without examples there is no frame of reference for how difficult a "5" is or how simple a "1" is. We chose a scale that will rate the relative difficulty of disassembling our 1991 Craftsman weed wacker. We chose our upper and lower bounds based on what we thought the easiest and hardest procedures would be in the dissection of our product. (However, in our disassembly we never actually reached level 5.)

Table 2.3 - Difficulty scale and an example constitutes each rating.

Rating Description Example
1 Very simple task that can be completed by an individual with little to no familiarity with tools or machines Unscrewing a lid on a fuel tank
2 Simple task that may require minimal familiarity with tools or machines Removing 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 Removing a bolt and nut using two wrenches
4 Mildly difficult task that may require the input of significant force or a high degree of precision Removing a press fit bearing with a mallet
5 Difficult task that requires a significant amount of expertise or force input Removing a c-clamp inside a small cylinder


Table 2.4 - A step by step description of disassembly with difficulty ratings.


Step Description Rating Pictures
1 Start by removing the rotating head cover by hand. There are two plastic pieces, one on each side, which just needed to be pushed in, and then pulled down for the cover to be removed. 1
1 - Group 15 2011.JPG
2 Remove the string casing and spring using a Phillips head screw driver in the center hole of the remaining part of the head. 2
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3 Remove protective shield over the head by unscrewing two (2) flat head screws. 2
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4 Now remove the actual head from the shaft by turning it by hand. The head and string casing will come apart. 1
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5 Now move up the shaft to the handle. It has a knob that has to be loosened by hand all the way. Now the handle can be removed. 1
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6 Remove the harness attachment loop. Use channel lock pliers or a wrench to hold the nut on the back while using a flat head screw driver to remove the screw. 3
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7 Remove pipe clamp using a flathead screwdriver. Then take off the grip. 2
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8 Remove the trigger casing screw from the shaft using a 5/32 Allen key. 3
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9 Remove the plastic casing protecting the motor. Use a 5/32 allen key to remove the thirteen (13) allen screws. 4
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10 After the bottom of the casing is unscrewed and removed the shaft will fall out of place and it can be separated from the engine. Be careful to slide the weed wacker trigger down the shaft while removing the shaft in order not to damage the cable. 1
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11 With the shaft off the inner drive shaft located inside the aluminum shaft can be lifted out by hand. 1
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12 After all casing screws have been removed with the 5/32 allen key lift and take off the cone shaped protective casing from the engine. 3
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13 Remove the clutch assembly from the engine. Place vice grips around the semicircular piece of the transmission and hold it firmly to prevent the transmission from turning. Then using a 9/16 wrench remove the bolt holding the transmission in place. 5
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14 Once the bolt has been removed from the shaft lift up and carefully remove the transmission and the washer. 1
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15 Lift up and remove the rest of the casing containing the rip cord starter. The rip cord wind up spool will then separate from the casing. 1
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16 Unplug the cord connecting the spark plug and magneto. Also unplug the ground wire coming off the magneto. This will allow you to remove the magneto from the engine since the screws that held it in place came out when you unscrewed the casing. 1
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17 Remove the spark plug from the engine using a 13/16 socket attached to a ratchet. 3
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18 Lift and remove the key shaped piece located on the engine shaft that goes through the flywheel. 1
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19 Use a 5/32 Allen key to remove the two (2) allen screws holding the air filter in place and then remove the plastic guard and the filter. 2
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20 The choke handle located on the back of the engine is held in place by an allen screw. Use a 4mm allen key to remove the screw and then the choke handle and choke plate can be taken off. 3
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21 The carburetor is positioned right under the choke plate. It is now free and will be able to be lifted off for removal. 1
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22 Remove the muffler guard on the back of the casing that is held in place by two (2) Phillips screws. 2
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23 The rest of the casing screws are now going to be removed. Under where the carburetor was located are four (4) deep set allen screws. The long end of a 4mm allen key may be necessary to reach the screw. It may be hard to turn the keys to initially break the connection, but a small pipe can be placed over the end of the allen key to gain leverage and provide more torque. 4
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24 The throttle cable can now be removed by sliding it through the hole that it goes through on the engine casing. 1
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25 On top of the engine next to the flywheel is a small diameter rod that’s approximately four (4) inches long. This is the engine kill lever and it can be lifted up and removed. 1
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26 Now you are able to take the rest of the assembled engine out of the casing. It is easily lifted off of the engine when held with the engine facing down. 2
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27 The muffler is now ready for removal. It is held on by two (2) tension springs that need to be removed. With a firm grip holding the engine in place a pair of pliers can be used to get a grip on the end of the spring located nearest the engine and then the springs can be removed. (Use caution in this step to make sure the spring does not snap back and strike the hand holding the engine in place.) 4
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28 The engine block in which the piston moves in contains two (2) allen screws which can now be removed using a 3/16 allen key. After these are removed the casing lifts off the piston. 3
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29 The piston can now be removed by sliding the arm that enters the bottom of the piston out of the inside of the engine. No removal of screws or C-rings is necessary for this. 1
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30 The flywheel can be removed if it is struck with considerable force. The flywheel extends out of the engine housing which can be hit with a hammer. Then it can be slid up the crank shaft for removal. 5
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31 Where the crank shaft enters the bearings there is a c-clamp that needs to be removed. A screwdriver can be used to pry out one side of the clamp and then slid around the rest of the c-clamp for removal. 5
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32 Place the remaining assembled engine on a sturdy surface with the crank shaft pointing upwards. With a metal hammer strike the top of the crank shaft firmly to push the crankshaft through the bearings and out of the remaining engine casing. 5
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33 There is now only one bearing left in the engine casing (the other is attached to the drive shaft). Inside the casing where the bearing was sitting is a bearing buffer that was between the two bearings to protect them. Needle nose pliers can be used to grab hold of the rubber protector and pull the bearing protector out. Be careful not to pull quickly or the plastic might rip. 1
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34 Inside the hole that used to house the bearing protector is a c-clamp that now needs removal in order for the other bearing to be released. Needle nose pliers are needed again to grip the end of the c-clamp and pull it out of place. 5
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35 There is one more bearing ready for removal. A final c-clamp is located just above the bearing but too far down to be removed. A small sized deep wall socket can be placed through the inside of the engine casing and over the bearing. The socket needs to be narrow enough not to be touching the c-clamp; otherwise the bearing will not come out. Now with the socket in place strike the back of it with hammer and the bearing will fall out. 5
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Is that part of the product intended to be disassembled?

After disassembly of the weed wacker we determined that only a few pieces are intended for consumer removal. First and most importantly is the head of the weed wacker. This part has clearly been designed to be removed because in order to use the machine for an extended amount of time you need to replenish the string, and this calls for removal of the head. The ability to remove this part by hand greatly simplifies routine maintenance - once this piece is removed the line on the spindle can be replaced as needed. The handle on the shaft was also obviously designed so that a user could easily remove it. The screw holding it in place was designed with a knob big enough so that it could be adjusted by hand. Once the screw is loosened the handle can be moved up and down the shaft to the desired length. The harness attachment loop was also designed so that each user could set it at whatever height he/she desired. The only other pieces designed to be taken off (or adjusted) regularly by the owner are the air filter and sparkplug. The only thing holding the casing of the air filter to the engine is two allen screws. Once these screws are removed the air filter can simply be pulled out and either replaced or cleaned depending on the wishes of the user. The owner’s manual of our specific weed wacker calls for the air filter to be cleaned out once a month to achieve maximum performance. The sparkplug requires a ratchet or wrench, but given the stresses it has to endure there is no viable alternative. The manual suggests replacing the plug annually [1].

No other parts of the weed wacker have been designed for casual removal. The inner workings of a weed wacker consist of many parts, most of which are very small and would be easy to lose track of. The average consumer does not have the knowledge to take apart the sub-assemblies and perform routine maintenance. These components were designed so that the product does not fail internally. Many of the pieces found inside of the casing could not break without disastrous repercussions, meaning that if they did break the machine is almost certainly useless from that point on (there is very little that can be done for the weed wacker if the piston breaks). If any of the components should fail the consumer is expected to either take the weed wacker to a small engine repair shop or replace the unit.

Connection of Subsystems

In order for a weed wacker to turn gasoline into useful rotational kinetic energy it needs a series of subsystems. These subsystems can be connected in a variety of ways, but for every subsystem there is a clear, well thought out solution that the designing engineers feel is the optimal solution. These solutions arise from balancing global, societal, economic and environmental concerns.

Fuel tank to carburetor

Picture 2.2 - Fuel tank to carburetor connection
    • Physical –A rubber fuel line attaches the fuel tank to the primer bulb (missing from our product, presumably removed by the previous owner), and another fuel line attaches this to the carburetor.
    • Mass – A mixture of gasoline and oil are poured into the tank as fuel. This is transferred through the fuel lines to the primer, through the carburetor into the cylinder, and eventually out through the exhaust system to the atmosphere. This process is initiated by squeezing the primer bulb and is sustained by the vacuum created by the combustion process.
    • Energy – The gasoline starts as a liquid with chemical potential energy. Therefore, the energy is transported to the carburetor via the mass transfer of the fuel.
    • Signals – There are two main signals between the fuel tank and the carburetor during normal operation. The first comes from activating the choke, which alters the gasoline to air ratio to facilitate starting the engine. More commonly, the operator controls the throttle, which moves a plate in the carburetor that allows more air and fuel into the combustion chamber, which directly controls the power output of the engine.
    • Purpose for Connection – There must be a fuel reservoir to enable multiple engine cycles. The fuel tank serves this purpose, but introduces the problem of transporting fuel from the location where it is stored the tank) to the location where it is used (the combustion chamber). (The solution is not as simple as it first appears. A tube sized for the amount of fuel a 26cc engine requires is also thin enough for the surface tension of the gasoline to hinder gravity induced flow. To combat this the primer is added between the tank and the carburetor to pump the fuel before vacuum is created.)
    • Influence of Global, Societal, Economic, and Environmental Concerns – Gasoline and motor oil are expensive, combustible, harmful to the environment, and have an unpleasant aroma. For all of these reasons, it is necessary to isolate the fuel from the user, the environment, and the rest of the systems. This is achieved by engineering the tank, lines, primer, and carburetor to be as leak-proof as possible, and also by ensuring all connections between these parts are secure and minimally prone to breakage.
    • Performance Influence- The use of plastic for the fuel line provides for sufficient flexibility that it can be woven through and around the other parts of the weed wacker. This also makes it so that any torqueing of the engine will not break the fuel line - it will just temporarily bend or stretch the tubing.

Carburetor to cylinder and piston

Picture 2.3 - Carburetor to cylinder connection
    • Physical – The carburetor attaches directly to the cylinder, with a gasket between the two steel parts.
    • Mass - The fuel changes phases inside the carburetor. After flowing through a small orifice it goes from a liquid to a vapor, and then it flows directly into the cylinder. At this time air is also introduced in to the system; the ratio of air to fuel is controlled via adjustment screws on the carburetor.
    • Energy – The fuel has not materially changed from its original state yet, so it still holds potential chemical energy. Therefore, energy flows with the moving fuel mass.
    • Signals – The carburetor determines the correct ratio of fuel to air, but this is within the carburetor and so it is not transferred over system boundaries. While not a traditional control signal, there is a pseudo-signal sent from the carburetor to the cylinder; the carburetor can send increased volumes of combustion materials into the cylinder, which directly results in increased engine power when the operator uses the throttle trigger.
    • Purpose for Connection – In order for the fuel to burn, it must be mixed with the correct compliment of air. For this reason, fuel cannot be simply dumped from the reservoir tank into the cylinder. The main function of the carburetor is to regulate the flow of fuel and air into the cylinder to give the most efficient combustion.
    • Influence of Global, Societal, Economic, and Environmental Concerns – The carburetor allows for the ratio of fuel to air to be set to provide the most efficient combustion. This minimizes fuel use, which is beneficial because gasoline is an expensive, finite resource that is harmful for the environment, not to mention that it is politically controversial (both domestically and in an international context).
    • Performance Influence- The carburetor is held onto the engine by two long metal screws. There is a gasket between the carburetor and the cylinder that keeps extra air from getting into the cylinder. There are no extreme loads or high stresses on this connection, but it does experience some thermal shock because of its proximity to the combustion chamber.

Magneto to Spark Plug

Picture 2.4 - Magneto to spark plug connection
    • Physical- The magneto is located on the edge of the crankcase, and there are magnets on the edge of the flywheel. The magneto is connected to the spark plug by an insulated wire, and the spark plug is grounded to the cylinder by direst metal-to-metal contact.
    • Mass – The only mass flow in this system is that of the electrons but compared to the mass of the system this is negligible.
    • Energy - When the rip-cord is pulled, the flywheel turns, and the magnets passing over a coil creates an electric current, which creates a large electric potential. When this charge is released it flows along the wire to the spark plug, where the charge ultimately jumps the gap, creating a spark that initiates combustion.
    • Signals – There is no true signal sent between the magneto and the spark plug however the current sent from the magneto to the spark plug is what allows the spark plug to fire.
    • Purpose for Connection – Gasoline will not ignite unless it has been heated beyond its flash point. The simplest method of accomplishing this is to build up a large potential difference and allow it to arc across a small gap in a gasoline rich environment. This small spark is hot enough to ignite local gasoline vapor molecules, which set off a nearly instant chain reaction, resulting in combustion.
    • Influence of Global, Societal, Economic, and Environmental Concerns – There are no obvious influences in this subsystem. It can be hypothesized that this method is used to combust the gasoline because it is cheap compared to other possible methods, but there is little supporting evidence because no comparable product uses a different method of starting the combustion process. (Diesel engines use a different paradigm, but they are too heavy and expensive to be a viable alternative for a weed wacker.)
    • Performance Influence- High voltage runs through the wire that physically connects the magneto to the sparkplug. It is very well insulated to make sure that no electricity is lost, and so that the operator is not shocked. The wire is fairly flexible so that it can bend. It also has a boot on the end of it so that it can hook directly to the spark plug. This boot keeps the spark contained because it is very well insulated and it is designed to stop short circuits. It also has a metal clamp so that there is a very secure metal to metal connection between the spark plug and the wire.

Piston to Crankshaft

Picture 2.5 - Piston to crankshaft connection
    • Physical - The piston is connected to the crankshaft with an offset arm, to allow translational motion that can be converted into rotational motion.
    • Mass – No mass is transferred.
    • Energy – The combustion in the cylinder converts the chemical energy stored in the gasoline into translational energy. This occurs when the rapidly expanding gasses push the cylinder. The cylinder has a pivoting arm attached to the top of it. The other end of this arm is attached to the crankshaft, but is offset slightly. This converts translational energy into rotational energy, which is the output of the crankshaft.
    • Signals- The translational motion of the piston causes the crankshaft to rotate.
    • Purpose for Connection- The combusting gasoline pushes the cylinder towards the open end of the system. This is linear motion and a type of mechanical energy, but the desired output is rotational motion. The crankshaft and offset arm enable this energy transformation.
    • Influence of Global, Societal, Economic, and Environmental Concerns – This type of connection is the most economical and efficient way to convert translational energy of the piston into rotational energy that can be utilized by the drive shaft. Since this is the most direct way to transform the energy it also allows the weed wacker to be lighter than if it was designed any other way meaning that a wider range of people in America and all over the world can use it.
    • Performance Influence- The connecting rod connects the piston to the crank. The end of the rod is a solid ring that goes around a knob on the crank. The piston moves the connecting rod up and down which in turn turns the crank (similar to the way the drive wheels on a train move).

Crankshaft to Driveshaft

    • Physical – The crankshaft is attached to a centrifugal clutch which eventually engages the driveshaft.
    • Mass – No mass is transferred between these components.
    • Energy – Rotational energy is transferred from the crankshaft to the drive shaft, but it is linear and not scaled.
    • Signals – The centrifuge clutch will only engage if it receives a signal from the throttle otherwise the engine will idle, but the clutch will not connect it to the drive shaft.
    • Purpose for Connection – There are situations where the user desires the machine to idle; where the engine runs, but the head does not rotate. This would be impossible to accomplish if the crankshaft was directly attached to the driveshaft because the engine would stall if the crankshaft was forced to stop rotating. A centrifugal clutch is introduced to remedy this problem. When the crankshaft is spun quickly enough, weighted half- disks are forced away from it by the centrifugal force. Once they have been forced far enough away from the shaft they come into contact with the inside of a drum connected to the driveshaft. This, in turn, causes the drive shaft to rotate.
    • Influence of Global, Societal, Economic, and Environmental Concerns – The incorporation of a centrifugal clutch is mainly for safety purposes. It is important to be able to disengage the driving force without shutting down the engine. By enabling the user to stop rotation by simply releasing the trigger, the head’s rotation can be stopped instinctively, rather than having to search for a kill-switch. The clutch also makes the engine more efficient and economical because it takes less gasoline to idle the engine than it does to re-start it (saving money).
    • Performance Influence- The engine drive shaft is connected to the clutch, and causes it to spin. When the clutch spins centrifugal force pushed the flats of the clutch outward and they grips the walls of the shroud which also spins. The clutch shroud is connected to the driveshaft, making the driveshaft spin.

Driveshaft to Head

    • Physical – The flexible driveshaft runs through the main shaft and is connected to the head with a hex nut.
    • Mass – No mass is transferred.
    • Energy – Rotational energy is transferred linearly, until it reaches the head. There the tips of the strings have a much larger radius than that of the driveshaft, yet they have the same angular velocity. Therefore, the linear velocity of the tips of the strings is much higher than that of the crankshaft, which results in a much higher kinetic energy (especially since velocity is squared in the kinetic energy equation; the new energy is exponentially larger than that of the shaft).
    • Signals – The rotation of the driveshaft sends a physical signal to the head to rotate as well.
    • Purpose for Connection – The rotating driveshaft transfers the desired type of energy (rotational). A head that holds the trimming line is attached to the end of the driveshaft. This line feeds out of two holes on the sides of the head. The tips of the string move perpendicularly to the direction of the growth of grasses, resulting in severed plants when the strings are moved into contact.
    • Influence of Global, Societal, Economic, and Environmental Concerns – A direct connection to the driveshaft is the most efficient way to get the head of the weed wacker to spin. It allows for the least amount of energy to be lost in translation. The head is also made out of plastic making the weed wacker lighter and easier to use.
    • Performance Influence- The connection between the driveshaft and the head is a custom designed bolt that is very finely threaded to maximize contact area to maximize holding strength. This connection also has provisions for adding grease to combat heat and friction to ensure longevity and to let the head spin freely. This connection has to be able to stand up to the constant spinning of the head and shaft without ever coming apart.


Arrangement of subsystems

Since the production of power in a weed wacker is a relatively linear process the subsystems are put together in a very specific way. They are assembled in a way that maximizes safety, performance and efficiency subject to cost constraints.

  • The air filter is connected to the carburetor.
    • These two subsystems need to be next to each other so that only clean air can mix with the gasoline providing for the best performance of the machine.
  • The carburetor is also directly attached to the fuel tank.
    • This makes it so that fuel has to travel the least amount of distance before it is mixed with air. This also cuts down on material cost because shorter lines are required for this connection.
  • The carburetor is attached to the cylinder.
    • This is necessary because the carburetor is what provides the cylinder with the gasoline/air mixture. If it were further away that would require the mixture to move further through the system before combustion. More distance means more time, resulting in inefficient control.
  • The magneto to sparkplug
    • Because they are the only two components of the engine that deal with electrical electricity these subsystems have to be connected. If it was not connected to the magneto the sparkplug could not function.
  • The piston (located inside the cylinder) has to be near the crank.
    • Because of inertia limitations the connecting arm on the piston cannot be very long, so in order for the crank to work it has to be physically close to the piston.
  • The centrifuge clutch connects to the clutch shroud.
    • If these systems were not physically connected the driveshaft would never turn and the machine would be useless.
  • The end of the driveshaft is connected to the head.
    • This is what makes the head of the weed wacker rotate. Without this connection there would be no way to make the head spin.


Subsystems that could not be adjacent

In a weed wacker there are subsystems that cannot be adjacent to each other. If they were to be adjacent it would either harm the performance of the machine or make the machine unsafe. Luckily, because of the simplicity of the machine there are relatively few of these constraints.

  • The fuel storage tank should be physically removed from the engine to reduce unwanted heat transfer (heating the fuel would increase evaporative losses and might make for hard starting).
  • Electrical leads should be isolated from the fuel system – wayward sparks might ignite fuel with disastrous consequences.
  • The cutting head cannot be adjacent to the engine – placing all of the weight at one end of the product would cause it to be unbalances and cumbersome.
  • The operator controls should be physically distant from the engine – more distance means less vibration, resulting in reduced operator fatigue.