Group 17: Gate 4: Product Explanation
The purpose of this gate is to document the reassembly of our product as well as provide further in-depth analysis of specific components. Within this analysis, we will discuss the kinematics and energy profiles of specific mechanisms that are key to the product's operations. Additionally, we will provide an overview of key system revisions we would make to the product along with the tradeoffs involved.
Project Management: Critical Product Review
Cause for Corrective Action
There were three major Issues that the Group had to overcome: finding common group time, evenly distributing the workload, and getting the assignments finished with enough time for review.
- Finding Common Group Time: Initially there was a pre-conceived notion that “group” work meant we all had to be together to work on the project simultaneously. Aligning schedules for large periods of time to work on the gates proved to be impossible. So, it was agreed that work would be evenly distributed among the members, to be worked on individually. This still posed an issue; the sections of the gates are heavily dependent on each other, and need to flow seamlessly from one section to the next. The next phase to developing a group work atmosphere without imposing strict time constraints was to set up a brief meeting time, as well as a group drop box account. The meeting time was set for the 10 minutes immediately following MAE 277 every Monday, Wednesday, and Friday. These meeting are used to divide any new work that arises, address any issues that may have come up, bounce ideas off of other groups’ members, or seek help where it is needed. The dropbox account serves as a common access point for each members work. In this way group members can refer to each other’s material across different gate sections, and tie one section to the next.
- Evenly Distributing the Workload: In the early development of the group it seemed as though there were a few members that carried the brunt of the work. All members were eager to participate. However, it was hard to integrate different sectional ideas into one flowing paper. So, one or two members would be responsible for writing the paper together and would call on other members for research purposes or ideas only. This put a heavy burden on the two members responsible for writing the entire paper. This issue was resolved by the implementation of the group dropbox, which allowed each member to work individually on his respective part, while still being able to access information developing in other sections.
- Timely Completion and Review of the Gate: Gate one was finished one hour before the due date and time. Luckily, the Wiki manager is fluent in HTML and was able to successfully upload all of the information on time. Still the gate was submitted without any form of review and received poor marks. From that point forward the group made a universal decision to complete all sections of the Gate one week in advance of the due date. This allows for sufficient time for all gates to be reviewed and peer edited by other members. Finally, all materials are turned over to the Wiki Account Manager, two days in advance, to give him ample time to load and properly format all materials to the Wiki page.
Product Archaeology: Product Explanation
- Phillips #2 Screwdriver
- T20 Torx Screwdriver
- Crescent Wrench
- Slip Joint Pliers
The same scale used for disassembly will be used for reassembly. This will give the user a comparison of how difficult each step is.
- Red will be used for the most difficult tasks. These tasks required three or more people involved, using a combination of tools and in some cases strength.
- Blue will be used for above average tasks. These required one to two people involved and using one or a combination of tools.
- Green will be used for easy tasks. These required only one person and only one tool.
- Note: Some steps might not fit exactly within the criteria of the scale but just were simply much more difficult for no obvious reason.
Engine Block Assembly:
- Insert the piston into the cylinder, the notches on the piston will properly align the piston for you.
- Insert the crankshaft into the bearing of the engine block so that the the thick end is near the crank arm on the piston. Align the notch in the crankshaft so that the crankshaft can be pressed into the bearing. A press must be used as it is a compression fitting.
- Press the piston arm onto the smaller off-center shaft that is on the crankshaft by using the press. The piston should move up and down in the cylinder freely when the crankshaft is spun. (refer to picture)
- Place the gasket for the cylinder cover around the opening so that the holes in the gasket align with the screw holes in the engine block. Then place the cover on so that the holes in the cover align with the holes in the gasket. Finally, place the six screws in the holes and tighten them using the T20 torx screwdriver to secure the cylinder cover. Note: use the 8-32 X 1 in T20 torx head screw.
- Difficulties: It was very challenging to get the piston arm to align correctly so that it smoothly slid onto the crankshaft. In addition, we had difficulty pressing the crankshaft and piston arm into place.
- This part of the assembly was most likely performed by a robot during the original assembly because of its difficulty level.
- Place the spring plate spring down inside the black holder. Make sure the notch in the spring plate fits around the notch in the black holder.
- Place the spool on top of the spring plate. The notch on the bottom of the spring plate must fit around the part of the spring that is sticking out in the middle.
- Place the thick, black spring into the spool casing. The spring should slide easily into the slots as shown.
- Place the container onto the spool. The pointed end of the spring should protrude slightly.
- Difficulties: No difficulties were encountered during this portion of the reassembly
- This portion of the assembly was most likely done by hand but could have been done by robot too.
- Slide the flywheel onto the crankshaft so that the flat side of the flywheel is facing away from the engine. Make sure that the notch in the center of the flywheel is aligned with the recess in the crankshaft so that the flywheel can be pressed on properly.
- Slide the starter assembly onto the crankshaft so that the flat side is pointing away from the engine. The starter assembly should come into contact with the flat side of the flywheel as the starter assembly will be spinning the flywheel.
- Slide the fender washer onto the crankshaft so that it lies on the flat side of the starter assembly
- Screw the centrifugal clutch onto the threads that are on the end of the crankshaft until it comes into contact with the fender washer. Tighten the centrifugal clutch down with the slip joint pliers until it is snug.
- Difficulties: No difficulties were encountered during this stage of the reassembly
- During the original assembly of this product this step was most likely done by hand.
- Screw the ignition module into the engine block so that the ignition module is close to the flywheel, but has clearance so there is no contact. Note: use the T20 pan head screw and the T20 screwdriver.
- Screw the spark plug into the hole that is in the bottom of the cylinder and tighten it using the crescent wrench.
- Place the spark wire that is attached to the ignition module over the end of the spark plug that is protruding out of the engine.
- Attach the wires of the kill switch to the ignition module. Attach the wire with the circular contact to the crooked contact on the ignition module, and attach the wire with the rectangular contact to the straight contact on the ignition module.
- Feed the kill switch through the engine casing and up the shaft. The kill switch should be placed inside its assigned slot in the trigger assembly.
- Difficulties: we had a hard time positioning the ignition model so that the flywheel did not make any contact with the ignition module when the fly wheel spun.
- During the original assembly it is very likely that the ignition components were installed by hand.
- Press the rubber priming pump onto the metal valve part inside the circle.
- Place the metal gasket around the primer pump so that the holes are aligned with the primer housing.
- Place the fuel filter and the rubber gasket on the bottom of the primer assembly. Make sure the sides are flush with the primer assembly.
- Place the primer pump assembly on top of the metal carburetor opening. Make sure the notches on the bottom of the pump assembly fit into the holes on the top of the carburetor opening. Screw the primer pump assembly and the carburetor into place using the Phillips head screwdriver and bolts.
- Screw the metal plate into the bottom of the carburetor opening.
- Slide the black heat shielding over the black heat dam and then place the carburetor assembly onto the heat dam. Then slide the choke diagram onto the carburetor, screwing the whole assembly into place on the engine block.
- Slide the choke knob onto the elongated rod and screw the nut into place making sure that it is secure and spins freely.
- Difficulties: The carburetor was harder to assemble because the small parts made it very hard to handle, but this was a minor inconvenience
- Due to the number of small parts I would expect the carburetor to be assembled by robot.
- Align the gas tank as shown and screw the fasteners into the holes to attach the gas tank.
- Insert the fuel lines from the carburetor into the gas tank so that the fuel lines are in a position that they will be able to deliver fuel from the bottom of the tank to the carburetor.
- Difficulties: This was the easiest part of the reassembly by far.
- This part of the assembly could have been done by either robot or by hand due to its ease.
- Insert the black muffler into the metal housing as shown.
- Place the silver heat shield on the muffler so the curved portion of the shield wraps around the housing.
- Screw the muffler into the engine block as shown using the T20 screwdriver. The hole should be flush. Note: Use the T20 pan head screw
- Difficulties: No difficulties to report.
- This portion of the assembly may have originally been done by both human and robot. The worker may have placed the muffler components in the right order while the robot placed the muffler on the weed whacker and fastened it.
- Place the male end of the curved section of the shaft into the female end of the straight section of the shaft. Use the black coupling and yellow wing bolt to connect and secure the shaft sections. The cable drive needs to attach to the spool head at the end of the curved section.
- Insert the end of the straight section of the shaft into the completed engine and casing so that the centrifugal clutch casing spins when the shaft spins. The notch inside the hole on the red casing should match up with the slot in the shaft.
- See Trigger Assembly.
- See Spool Head Assembly.
- Attach the deflector shield to the shaft near the spool head in such a position that it will protect the user from flying debris. This position can be adjusted to the user’s preference. Thread the bolt through the deflector shield and tighten the deflector shield using the wing nut.
- Place the handle in the appropriate position on the shaft. Thread a bolt through the handle and secure the handle using the wingnut.
- Attach the throttle cable onto the valve in the carburetor assembly
- Difficulties: this was a fairly simple step of the reassembly due to the fact that it doesn’t require any tools other than the hands.
- This step was probably done by hand in the factory because it could be completed quickly and easily by a worker.
Spool Head Assembly
- Place the yellow housing onto the end of the shaft.
- Slide the spool onto the end of the shaft. Make sure the spring is in place. Press down and spin the spool so the string is protruding from the eyelets.
- Insert the bolt into the spool retainer. Then line up the bolt with the hole in the end of the shaft and screw it into place to tighten the spool head assembly.
- Difficulties: The spool is tricky to get in place correctly at first, but other than this minor issue there were no difficulties.
- This was probably assembled by hand in the factory because several parts need to be stacked and fit into each other. A complex robot would be needed to stack the parts and assemble them quickly and it would probably be more effective to use a worker.
- Place the kill switch in the slot as shown. Also feed the cords through the bottom bracket as shown.
- The bottom piece of the trigger lines up on the shaft by the given hole and tab. Place the bottom part onto the shaft.
- Place the safety trigger into the correct slot in the backing as shown.
- Thread the throttle cable into the backing.
- Slide the spring onto the slot as shown. The spring must be bent and pushed down into place.
- Attach the trigger. The spring must go into the small hole and the throttle cable hammer must go into the large hole.
- Place top half onto the casing and screw into place with 1” coarse thread screws using the T20 screwdriver and T20 screws.
- Difficulties: Putting the trigger in with the spring and hammer in place is quite difficult as the spring will cause it to pop out if not held in place.
- This part of the assembly is also probably done by human in the factory.
In the weedwacker, the cylinder assembly works in the form of a crank slider. The purpose of the crank slider is to convert the kinetic energy of the piston’s translational motion into rotational kinetic energy, which is stored in the crankshaft, during the power-exhaust stroke. This process works in reverse during the intake-compression stroke to reciprocate the piston by means of the crankshaft’s angular momentum.
In the weedwacker’s motor, chemical potential energy from the fuel is converted to thermal energy via the combustion process. This thermal energy expands the gases inside the combustion chamber, converting the thermal energy to translational kinetic energy in the piston, which is in turn converted to rotational kinetic energy stored in the crankshaft. The First Law of Thermodynamics leads to the conclusion that the total energy injected into the system through the chemical potential energy in the fuel is equal to the total energy removed from the system, which equals the resultant of the thermal energy lost due to friction and general inefficiency added to the resultant work done by the weedwacker.
Chemical Energy stored in fuel => Translational kinetic energy of the piston => Kinetic energy of connecting rod => Rotational kinetic energy of the crankshaft => Resultant work done + Resultant thermal energy lost + Remaining unused Chemical potential energy
This energy flow leads us to the equation:
- Chemical potential energy in fuel = Resultant work done + Resultant thermal energy lost + Chemical potential energy of exhaust
There are many sets of equations that govern the behavior of a crank slider, depending on conditions. For the purpose of our analysis, we are assuming that the motor runs at a constant angular velocity. Due to the high rotational inertia of the flywheel, the angular velocity of the crankshaft remains relatively constant once the motor reaches its normal operating parameters (temperature, etc.), so for the equations we are using, we are assuming that the angular velocity remains constant within the running speeds of the motor (throttle idle, throttle open)
In order to reach these equations, we must simplify the mechanism to a set of members and parameters:
As shown in the picture or reviewed in equations:
- L = Length of the connecting rod
- R = Radius of crank
- Φ = Angle formed between the connecting rod and the slider’s axis of displacement
- Θ = Angle formed between the crank and the slider’s axis of displacement
- X = Position of the slider along its axis of displacement
- ω = Angular velocity of the crankshaft
- τ = Torque
- F = Force on connecting rod
- Position of the Slider
- X=R-R cosθ+L-L cos∅
- Translational Velocity of the slider along its axis of displacement
- V_slider=cos∅ ωR
- Kinetic Energy of Rotation:
- KE=1/2 mω^2 R^2 cos(_^2)∅
- Torque exerted on crank by slider
- τ=F cos∅ R
In the weedwacker, a centrifugal clutch is used to interrupt the transmission of torque and energy between the motor and the driveshaft while the motor is at idle, while providing a connection for this motion while the motor is at speed.
A centrifugal works on the principal that as the crankshaft follows centripetal motion, its parts will tend to resist the centripetal force and move towards the outside of the rotational curve. By placing springs between the clutch surfaces, you can allow their position to vary directly with the magnitude of the centripetal force, and in turn with the angular velocity.
The equations required for the analysis of a centrifugal clutch depend greatly on the specific layout of the components. For the purpose of our analysis, we are assuming a component layout as shown below.
To simplify the analysis, we are able to analyze one side of the clutch as sectioned through the springs due to the symmetry of the design. The resultant spring force on each shoe can be resolved to a single force passing through the center of the boss, while the same goes for the distributed normal force across each shoe. Through symmetry we can also simplify the 2 springs extending from each side of the output shaft having constant k to a single spring along the radius having constant 2k as shown below:
In the illustration above:
- C = Center axis of clutch
- K = Spring constant
- R = Radius from center axis to clutch shoes
For the purpose of our analysis, we are neglecting any change in rotational inertia that may be caused by our simplifications of the system, in addition to only examining the clutch at the two principal speeds of the engine (throttle idle, throttle open), neglecting angular acceleration; leaving the simplified system in static equilibrium. We will also be analyzing the spring within a reference frame the rotates about c with the input shaft, allowing us to treat any deficiency of centripetal force as a force in itself, often given the misnomer centrifugal force. Under these assumptions, we can analyze the spring as a two force member, with tensile force exerted by the spring resolved to the endpoint C.
This simplification leaves us with the parameters:
- Fs = Force exerted by the spring
- Fc = Deficiency of centripetal force (centrifugal force)
- FN = Resultant normal force exerted on the shoes by the output drum
- K = Resolved Spring constant (2k in illustration)
- X = Displacement of spring, i.e. the radial distance the shoes move when the motor accelerates from idle to running speed
- m = Resolved mass of both shoes
- ω = Angular velocity of crankshaft
- R = Radius from center axis to shoes at idle
When the motor is at idle, and the clutch is disengaged, there is no normal force exerted on the shoes by the drum, and thus for the spring-shoe system to be in static equilibrium:
- FC = mRω2 = FS = KX
On the other hand, while the motor is at running speed and the clutch is engaged, there is a normal force on the shoes to take into account, leaving us with:
- FC = m(R+ΔX)ω2 = FS + FN = KX + FN
Note: Equations are given in terms of magnitudes
The following design revisions are recommended in place of the current subsystem. They are intended to enhance the performance of the product in the domain of one or more of the GSEE factors.
- Interchangeable Heads: Interchangeable heads would allow the user to adjust the blade type based on the kind of debris the user is going to be cutting. A mechanism for easily exchanging the blades would need to be implemented increasing the products complexity, and thus increasing its cost of production. The cost of extra blades can be overcome by selling different types of blades separately. Adjustable blade types would better suit the weed whacker for use in many regions with varying foliage; which would increase the available market for the product.
- Electric Starter Subsystem: An electric starter for a gas powered mower would improve the overall user experience. It would facilitate the arduous engine start up process by removing the physical strain of starting the engine via the pull cord. Use of an electric starter requires a power source. A plug that can be connected to an extension cord, and then connected to the wall to provide a power source for the electric starter without significantly increasing the weight of the overall product. The electrical start can be considered a societal consideration due to its influence on the intended marketable audience.
- Electric Motor and Fueling System: Exchanging the gas engine out for an electric motor would make the product more environmentally friendly, by eliminating any emissions that the gas engine would produce. An electric motor would also improve the products function in society. An electric motor is much quieter than a gas engine and subsequently would be more accommodating in suburban neighborhoods. Economically the electric motor would cause an increase in the initial cost of the product, but would reduce the costs associated with product operation. Along with the electric motor two other components would need to be replaced. In order to power the new engine the gas tank would need to be replaced with a battery in order to supply the engine with the appropriate form of energy. The cable driven throttle would need to be replaced with an on/off trigger to signal the engine.