Group 17: Gate 3: Product Analysis

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In the last gate, we documented the process of dissecting the weedwhacker. With the product broken up into its components, we have been able to study its inner workings and subsystems in much more detail; as well as analyze the original decisions that were made in the product’s design. Given this information, along with firsthand user experience, the group will formulate revisions we would make to the design based on one or more of the GSEE factors. In addition, we will cover the action we have taken as a group since completing the last gate to ensure we work together fairly and efficiently.

Project Management: Coordination Review

Cause for Corrective Action

There were three major Issues that the Group has 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. 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 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 impose 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 while working, 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 create a seamless flow from 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 the work developing in other sections, to allow them to incorporate ideas or points from other sections, tying the gate together as a whole.

  • 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 ample 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 Evaluation

Component Summary

The weedwacker was made up of several different components and subsystems created using a variety of manufacturing processes. The table below shows these parameters for each component, as well as the raw material(s) each part is made from.

Component Name Subsystem Material Manufacturing Process Description Component Picture
Motor Block Motor Assembly Cast Iron Sand Casting/Cylinder bore finished by machining The motor block is probably the most crucial part of the entire device. Within it are the cylinder and crankcase that provide the basis for the rotational motion of the crankshaft. The combustion chamber is formed by the cylinder walls, the head and pressure ring of the piston, and seal of the spark plug at the top. The radiator-like blades around the cylinder act as a heat sink to dissipate heat away from the cylinder walls. At the bottom, on the output side of the crankcase is the thrust bearing that supports the crankshaft and allows for free rotation. Group17 BLOCK1.jpg
Spark Plug Motor Assembly Steel/Ceramic/Copper Ceramic core is molded and fired, Steel shell is extruded and machined The spark plug acts to ignite the fuel-air mixture in the cylinder by creating a spark between two electrodes, it is powered and controlled from the ignition module.
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Crankcase Cover Motor Assembly Steel Die Cast The crankcase cover screws onto the block and acts to create a pressure seal within the crankcase, which holds the pressure in the fuel-air mixture created by the downward motion of the piston. Group17 CRANKCASE COVER1.jpg
Air box Motor Assembly Steel Die Cast The air box acts to filter the incoming air to keep contaminants out of the carburetor and cylinder block.
Air box Cover Motor Assembly Steel Die Cast The air box cover acts to hold the air box and carburetor in place on the side of the block.
Muffler Assembly Motor Assembly Steel Die Cast The muffler assembly acts to suppress the sound of the engine by dampening it at the exhaust port. Group17 MUFFLER.jpg
Fuel Cap Motor Assembly Plastic Injection Molding The fuel cap acts to preserve the vacuum created in the fuel tank and hoses by the motion of the cylinder, helping to draw fuel into the motor
Throttle Cable Motor Assembly Steel Drawing, Braiding The throttle cable connects the trigger with the throttle and opens the throttle when the trigger is pulled
Flywheel Motor Assembly Steel Investment Casting Another Crucial element in a two-stroke motor, the flywheel performs many tasks at once. Its primary function, by definition, is to smooth out the uneven rotational motion created by the motion of the motor through its high moment of inertia, and carry the motor through the unpowered intake-compression stroke. In addition, its blades act as a fan to keep air circulating across the blades of the heat sink; while the powerful magnet embedded in its sidewall acts to control the timing of the spark while generating the charge that drives the ignition module Group17 FLYWHEEL1.jpg
Connecting Rod Motor Assembly Steel Stamping, Bearings press fitted The connecting rod is attached to the cylinder head on one end, and the crankshaft on the other. It acts to convert the translational motion of the piston into torque about the crank shaft during the power-exhaust stroke, and vice-versa during the intake-compression stroke. See Piston Head
Piston Head Motor Assembly Aluminum Initial Shape Forged, Finished through Turning The piston head performs multiple tasks in a two-stroke motor. Its primary task is to translate the pressure created by the quickly burning fuel into mechanical energy , while in conjunction with the intake and exhaust ports in the motor block, it acts as a valve to control the flow through these. Also, in its downward motion, it acts to compress the fuel-air mixture in the crankcase, providing the pressure that forces the mixture into the cylinder when the piston moves below the intake port. Group17 PISTON4.jpg
Starter Assembly Motor Assembly Steel/Plastic/Rope Steel Spring is Drawn,plastic casing is injection molded The starter assembly allows the user to start the rotational motion of the motor by pulling the starter cord. When the cord is pulled, the plastic casing of the assembly engages the pawls on top of the flywheel, in turn causing it to rotate. Once the motor is started and the cord is released, the pawls disengage and the spring retracts the cord. Group17 Starter Assembly1.jpg
Gas Tank Motor Assembly Plastic injection Molded The gas tank acts to hold the fuel-oil mixture, while the lines give the mixture a path into the motor. Group17 GAS TANK.jpg
Carburetor Assembly Motor Assembly Steel Die Cast/Finished by machining The primary function of the carburetor assembly is to mix the fuel from the gas tank with outside air. Within the carburetor is the throttle, which controls the flow of air and fuel into the motor, and in turn its rpm. Mounted on top of the carburetor is the primer, which allows the user to draw a small amount of fuel into the motor when it isn't running to help the user start it. Group17 CARB1.jpg
Centrifugal Clutch Motor Assembly Steel Base die cast, Spring Drawn and formed by forming machines The centrifugal clutch acts to only transfer rotational motion between its input and output sides above a certain rpm. This is accomplished through one clutch surface being held apart from the other by a spring, which can only be overcome by the centrifugal force of the rotating shaft. What this effectively translates to in the case of the weed whacker is that the spool head will remain stationary while the motor is idling; but when the user pulls the trigger, which increases the motor's rpm, the clutch engages and the spool head begins to rotate. Group17 CLUTCH.jpg
Ignition module Motor Assembly Plastic/Copper/Silicon Wire Drawn, Plastic case injection molded, relay circuitry printed The ignition module consists of a relay that is charged by the magnet on the flywheel. Once enough charge has built up, the switch in the relay closes, sending the charge to the spark plug. Group17 IGNITION MODULE1.jpg
Counterweight Motor Assembly Steel Die Cast The counterweight acts to balance the crankshaft about its axis of rotation. See Crankshaft
Crankshaft Motor Assembly Steel Steel extruded into cylinder, Turned into shape of crankshaft The crankshaft converts the translational motion of the connecting rod to rotational motion about the shaft, acting as the output shaft of the motor Group17 CRANKSHAFT2.jpg
Straight Shaft Shaft Assembly Steel Outer Shaft formed from sheet metal, inner drive shaft The straight shaft acts to house the solid drive shaft, which transfers the rotational motion of the crankshaft to the curved shaft at the bottom of the weed whacker. Group17Step3.JPG
Curved Shaft Shaft Assembly Steel Outer Shaft formed from sheet metal, Flexible Drive shaft made from drawn and braided steel cable The curved shaft houses the flexible drive shaft, which both transfers the rotational motion from the straight shaft to the spool head, and redirects it at a downward angle, allowing for more ease of use. Group17Step2.JPG
Foam Grip Shaft Assembly Polyurethane Foam Outer contours and inner diameter are cut out The foam grip fits around the straight shaft and provides a cushion under the users hand to allow for more comfortable use. Group17 FOAM GRIP.jpg
Spool Housing Spool Head Assembly Plastic Injection Molded The spool casing holds the spool of cutting line that acts as the weed whackers cutting surface, and protects the joint where the output end of the curved shaft connects to the spool head. Group17 SPOOL HEAD HOUSING1.jpg
Spool head Spool Head Assembly Plastic Injection Molded The spool head holds the cutting line, and in conjunction with the Cap controls the feed of the cutting line. It is attached to the output end of the curved shaft, causing it to rotate with he motor. Group17 SPOOL HEAD1.jpg
Spool Cap Spool Head Assembly Plastic Injection Molded The spool cap acts to contain the inner spool of cutting line inside the spool head, and removes to allow the user to replace the spool of cutting line.
Deflector Shield N/A Plastic Injection Molded The deflector shield acts to block the grass and other projectiles that may be thrown in the direction of the user by the cutting line. In addition, it houses a blade at its edge that regulates the length of the cutting line as it is fed by cutting it off if too much is fed.
Handle w/trigger N/A Plastic Injection Molded The handle acts as the primary handhold for the user to support the product, in addition to housing both the control trigger and safety switch Group17 TRIGGER.jpg
Motor Casings N/A Plastic Injection Molded The motor casings act to protect the user from the heat and moving parts of the motor, as well as to improve the aesthetic qualities of the product as a whole.

Product Analysis

Trigger (w/ safety trigger)

  • The trigger of the weed whacker is being used by the controller constantly. This causes the design of it in terms of safety and ease of use to be quite important. A trigger in the system allows the user to control the speed of the spool head. This allows an adaptable product making it useful in different situations. If the string always had to go the same speed, it might not be able to be used at maximum efficiency. The safety trigger also makes the weed whacker a safer product. The safety trigger must be pushed down to engage the throttle. This ensures that the weed whacker will not be engaged by accident. This plays into the societal factors that would go into designing it for safety. Being able to move the weed whacker around while on allows a higher maneuverability.

Centrifugal Clutch

  • A user will not be engaging the head of the weed whacker the entire time it is being used because there are generally patches of high grass, weeds, etc. that do not grow in the same area. This means the weed whacker will be moving around quite often. This clutch allows the engine to be running without the head being engaged. This allows for a higher maneuverability and safety. If the head was engaged the entire time the weed whacker was running, it could be dangerous for the person carrying it. Turning the weed whacker off when moving from one point to the next would be a tiresome and inconvenient task. The additional safety features of the product contribute to the societal factor.


  • The piston has to be very precise in measurements to ensure a smooth fitting within the chamber. If the shape or dimensions are off by a slight amount, the engine will not be nearly as efficient. The engineer would need to calculate the tolerances that could be the most cost effective and maintain a high level of efficiency. This allows them to sell their product at the lowest cost possible. This is driven by economic factors, as selling a product at a lower cost can maximize sales and profit. The size of the piston must be optimized so that it can provide maximum torque and horsepower. In different places, different amounts of power could be needed, so the amount of power needed is affected by the global factor.

Cable Drive

  • The cable drive is what transfers the power from the engine to the spool head. The cable drive is exposed to relatively high amounts of torsional force. The material needs to be strong enough to withstand the amount of torque that would be generated by the engine. If the material is not strong enough, the cable will shear due to the torsional force. However, the use of a shaft that is too thick would cause the manufacturing cost to go up, which would shrink the profit margin. Steel is the best material due to how cheap and readily available it is. The cable is also much cheaper than a CV joint system, which would use gears and other mechanisms that would drive the manufacturing cost up.

Spool Head

  • Changing the string on a weed whacker is part of the machines everyday use. The user will burn through string on a weed whacker very fast when working near fences or stone walls. As a result, the string refilling process should be relatively simple. The design of the spool head allows the user to simply remove the head and refill the string. The parts within the spool have to withstand the abuse and wear and tear that every day use causes. The wear and tear can occur when accidental contact with an unforgiving surface (concrete, chain link fence, brick, ect.) is made, or just from the thick foliage that the weed whacker comes into contact with. A main societal demand is ease of use, so the ease of use effects the sales and reputation of the company that manufactures the product.

Deflector Shield

  • When the user operates the weed whacker, the head will most likely come into contact with materials such as pebbles, twigs, and grass. The rotating spool head and string often send pieces of the foliage flying in many different directions. The deflector shield protects the user from the majority of the flying foliage, which could cause the user to be injured. Without the shield behind the spool head, the user would be pelted constantly, which would make the user’s experience much less enjoyable. This would make the product unsafe and much less popular, meaning fewer consumers would purchase the product. In addition, the flying foliage could be a huge liability for the company if it were to injure the user seriously. The deflector shield also has limitations due to the performance expectations. The deflector shield could not be too big because it would hinder the main function of the product by getting in the way of the spool head. This could make actual use uncomfortable and undesirable. It would also cost more due to the fact that more material would be needed to make a bigger deflector shield. The design of the deflector shield has societal and economic factors involved due to the cost of the deflector shield and the safety it provides.

Shaft Assembly

  • In today's day and age, the ergonomics of a product are a key factor to consider in any design. The shaft being curved allows for a more comfortable and ergonomic use. This feature makes it for taller people to operate because it allows them to remain upright instead of in a bent over position. The foam grip and handle also allows for a more comfortable feel. This plays into societal factors since every user enjoys a product that is easy to operate. If the user enjoys the feel and comfort of the product, then a higher quantity of the product will be sold.

Solid Modeled Assembly

We created a solid modeled of the piston and crankshaft moving as they would within the engine block using Autodesk Inventor. We chose these components because they form the key mechanism for transferring the chemical energy stored in the fuel to mechanical energy that can be transmitted through the driveshaft. Moving Piston.gif

Connecting Rod Counterweight Crankshaft Piston Head
Group17 Connecting Rod.jpg Group17 Counterweight.jpg Group17 Crankshaft.jpg Group17 Piston Head.jpg
Connecting Rod Drawing Counterweight Drawing Crankshaft Drawing Piston Head Drawing

Engineering Analysis

Component: Cable Drive

Identified Need:
Need to design a component that can transfer rotational energy from the motor down the shaft of the weed whacker in order to turn the blade. It must be able to fit inside of the hollow tubing that composes the shaft, and endure the torsion applied by the motor and foliage. Additionally, the solution must come at minimal cost to satisfy market demands.

Feasibility Analysis:
In order to consider the feasibility of using a cable drive to transfer rotational energy down the shaft from the motor to the blade. The governing equations listed below are used to determine the maximum demands of the cable drive, as well as the ability of the drive to meet those demands under the necessary spatial constraint.

The blade will come in contact with some objects able to produce enough force to stop it completely (i.e. a fence or rock). This means that the shaft would need to withstand the maximum torque produced by the engine without shearing. The cable drive is composed of ASTM-A36 steel. The cable drive is treated as a straight, solid steel rod in order to simplify the calculations. This assumption will have minimal effect on the analytical report, as the difference in results will not have an effect on the products overall performance or user safety.

Governing Equations:

Known Values:
Max torque produced by the 26cc weed whacker motor = 7.3 N*m Yield property of ASTM-A36 steel for shear = 145 MPa = 1.45*108 Pa

Calculations for Minimum Cable Radius:
3.17mm< Inside radius of the Shaft Housing
The minimum radius of the steel rod in order to withstand the maximum torque without shearing is 3.17 mm. The use of a steel Cable Drive is thus feasible because it is able to fit inside the spatial constraint of the shaft housing.

Final Dimensions:
Using the minimal shaft radius available without taking into account error in calculations or potential misuse of the product could potentially result in damage to the product, or put the operator at risk. A safety factor must be taken into consideration. An acceptable margin of safety defined by an increased maximum sheer torsion must be determined. Correspondingly a new cable drive radius must be calculated in order to test the feasibility of the increased radius. After determining the margin of safety in sheer torsion, repeat the calculations above to determine the radius able to handle the torsion.
The shaft in the Homelite Weed Whacker has a radius of approximately 4 mm. Using the measured radius it is possible to carry out the following calculation.
As a result of using a cable shaft diameter of 4 mm the shaft is able to withstand approximately 14.58 N*m of torque, about 2 times the amount of torque that the 26 cc engine motor is able to produce.
Using a safety factor of 2 does not have impact the design of the product much as it is still able to fit within the necessary spatial constraints. Also, it has little effect on the manufacturing cost, as the extra steel necessary to meet the safety requirement is relatively inexpensive. If the manufacturer wished to do so, they could use a lower grade steel that has a shear yield of about 100 MPa in order to offset some of the cost, while still producing a viable product.

Testing and Validation:
Test protocol for the cable drive should simulate a worst case scenario in which the blade is abruptly stopped from its highest momentum, in varying climate conditions. Assuming the product will be used in the most extreme temperatures ranging from 10˚F to 110˚F degrees the blade should be stopped instantaneously and the cable drive subjected to its maximum force 100 consecutive times in both 10˚F temperatures as well as 110˚F degree temperatures. If in all 200 test trials the cable drive is able to withstand breaking or warping, the cable drive is safe for use. The weed whacker should also be put through a test where it is run for an extended period of time in a normal use setting to ensure that the rod will endure extended use, as well as determine probable damages to other components of the weed whacker under the forces experienced by the cable.

Design Revisions

The following design revisions are recommended in place of the current component, or subsystem. They are intended to enhance the performance of the product in the domain of one or more of the GSEE factors.

  • Composite Drive Shaft Housing: Utilization of a composite light weight material would reduce the overall weight of the product. Reduced product weight would make it accessible to a broader range of users. Permitting a feebler user to properly operate the weed whacker. It could also, make the product safer to use by allowing greater control of the product for able bodies users. Finally, a lighter product could appeal to a user intending to operate the product over a longer range of time. Making it more feasible for the user to endure, sustained use of the product.

  • Electric Start for A Gas Motor: An electric starter for a gas powered mower would improve the overall user experience. It would facilitate the arduous engine start up process. The starter would take the physical strain out of starting the engine via the pull cord. This feature would need to be coupled with a few other minor design adjustments. An electric starter would require a power source. A battery could be used to supply the necessary energy. However, a battery may be too large to reasonably mount to the weed whacker. Another more practical option for an electric starter would be a plug that can be connected to an extension cord, and then connected to the wall. A plug would not significantly increase the weight of nor would nor would it be difficult to find a convenient place for it. The electrical start can be considered a societal consideration due to its influence on the intended marketable audience.

  • Electric Motor: 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 beneficial 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. The cable driven throttle would need to be replaced with an on/off trigger to signal the engine. Finally, the gas tank would need to be replaced with a battery in order to supply the engine with the appropriate form of energy.


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