Group 17 - Homelite Line Trimmer (Gasoline Powered)

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Homelite Line Trimmer

As a group we will be reverse engineering a Homelite Line Trimmer weedwacker. This will be done through a set of several different gates:
Gate 1: Project Planning
Gate 2: Product Dissection
Gate 3: Product Analysis
Gate 4: N/a
Gate 5: N/a

Group Members

Ryan Edmonson
Mike Gold
Ryan Fiust-Klink
Shawn Milligan
Jeff Scott

Project Planning

Work Proposal

Proper analysis of the weedwhacker will require a full dissection. The weedwacker will be fully taken apart starting at the point where the motor meets the body. The motor and the rest of the weewacker including the body, blade and shield will then be dissected and analyzed separately. The team will be divided into two groups one group will be dissecting the motor and the other group will be dissecting the rest of the weedwacker. Dissection of the product should take approximately three hours for the group or fifteen man hours to complete. In order to provide a reference for analysis the dissection will be videotaped, clips of the video will also be used to clarify the analysis report. Tools required for dissection:

  • Nut Drivers
  • Pliers (needle nose)
  • Screwdrivers, Phillips and flat
  • Allen wrenches
  • Wrenches
  • Ratchet and socket (deep well and standard)
  • Possibly torx bits

Dissection Challenges:
The engine poses the greatest challenge for dissection. Its compact complex nature makes it challenging to maneuver tools inside of the engine to take it apart. Also, the engine’s many subsystems will make it difficult to analyze.
The body could also pose a challenge for dissection. The drive shaft is enclosed in a solid cylindrical aluminum case, which could make it difficult to analyze the mechanisms used in the drive shaft.

Group Characteristics:
The group’s main weakness is an ability to analyze the product with the intent of appropriately fulfilling the project requirements. The group has the mathematical understanding to analyze the product for mechanical function. However, analyzing the product in light of the design process responsible for the development of the weedwacker is a level of critical thinking unfamiliar to the members of the group.
Individually the members of the group possess skills that will help aid in the completion of the project. Jeff Scott has a developed understanding of engine mechanics and will be an asset in the dissection and analysis the weedwacker engine. Ryan Edmondson and Mike Gold are both strong in mechanical and physical assessment, as well as product research. Shawn Milligan is proving superior technical support by managing the group wiki website, as well as handling video editing. Ryan Fiust-Klink is responsible for developing and editing all technical documents produced by the group.

Management Proposal

Work Management:
For each gateway the work will be divided between the members as equally as possible based on time requirement. All work done will then be reviewed by the group, and then passed on to Ryan Fiust-Klink for editing, then given to Shawn Milligan to be posted to the wiki website.
The group plans to meet on Wednesday’s at 5:00pm in order to discuss assignment responsibilities and progress.

Personal Roles:
Ryan Edmonson - Technical Analyst
Mike Gold – Technical Analyst
Ryan Fiust-Klink- Report Editor
Shawn Milligan - Wiki Account Manager
Jeff Scott – Project Coordinator

Role Description:
Technical Analyst – check and review mathematical computations used in analysis
Report Editor – read and review all technical documentation for cohesion and completeness
Wiki Account Manager – upload and format all documentation to wiki
Project Coordinator – organize and communicate a plan for project completion

Preparation and Initial Assessment Questions

  • Development Profile: The weedwacker was developed with first world countries in mind. The purpose was to make it easier to care for your lawn and lighten the workload on people.

  • Usage Profile: The intended use of the product was the removal of unwanted foliage and weeds around a person’s property. Our particular product is for normal home use, as it would not last long enough to be feasible in the professional sector. The weed whacker can be used to remove weed, and to edge driveways and sidewalks.

  • Energy Profile: The system uses mechanical and thermal energy. Energy is imported into the system by way of the chemical potential energy stored in gasoline. Energy is transformed and modified through the use of heat to create compression in the cylinder, which transforms heat energy into mechanical energy by moving the piston; in turn exerting a moment on the crankshaft.

  • Complexity Profile: The three main components of the weedwacker are the motor, the control system (throttle, pull cord, etc.), and the shaft and head of the weed whacker. The motor is the most complex component of the product. The throttle, shaft, and weed whacker head are all fairly simple due to their small number of parts and lack of complexity.

  • Material Profile: Plastic coverings, steel, and rubber are clearly visible, but we would assume that there are some other types of metals within the weedwacker, such as copper in the ignition wire, and platinum or iridium in the spark plug. In addition, the crankcase and cylinder body are most likely made of aluminum or cast iron, while the air filter is made of some kind of cloth or paper.

  • User Interaction Profile: The user interfaces with the product through the use of the throttle, pull cord, choke, primer, and kill switch. The starting procedure may not be very intuitive, but there is a step by step set of instructions on the weed whacker that tell you how to start it efficiently. The throttle system is very intuitive because you just squeeze the trigger to increase the speed of the motor. The only maintenance required is to change the spark plug once a year so that it runs correctly, keep the choke and throttle body clean, change the air filter regularly, and to refill the weedwacker string when it runs out. No oil changes are required because it is a two stroke motor, and the oil is mixed in with the gas. All required maintenance is very simple and virtually anyone would be capable of performing it.

  • Product Alternative Profile: Alternatives to the gas weedwacker include hedge trimmers and lawn shears. The advantages of these products are that they are cheap and require no gasoline. Other alternatives include electric weedwackers, with the closest competition coming from those powered by wet-cell batteries. The only maintenance necessary to hedge trimmers and lawn shears are is to sharpen them so that they cut correctly. The disadvantage to these products is that they are many times slower than the weed whacker that we plan to dissect. The difference in cost may only be about 40 to 50 dollars considering how cheap a weed whacker can be bought in 2012. Considering the small cost difference and the performance difference, the weedwacker is definitely the best choice for the job with all things considered.

Product Dissection

Challenges Thus Far

Lab Availability

  • The first challenge in dissecting the weed whacker was gaining access to space in the dissection lab. There was a limited amount of time that the group was available to work together. When the group finally assembled in the lab, it was full with zero table space available. So, we found space outside of the lab in the hallway and went to work on the floor.

Limited Tools

  • Snap rings were not expected to be used in the production of the weed whacker, so the need for snap ring pliers was also unexpected, and thus they were inaccessible. In order to remove the snap rings a flat head screw driver was used to pry the snap ring out of its housing. The snap rings were bent and destroyed, but their removal was necessary for complete dissection of the weed whacker.


  • In Gate 1, there was a miscommunication as to who would be doing the final draft. This caused the group to be rushing through that part of the project. We fixed this for Gate 2 by giving each person a specific part to be working on along with dates to have them completed by. The group was able to get much more accomplished through this method.

Parts Not Intended for Removal

  • Fuel Lines
  • The fuel lines are not intended to be separated from the carburetor, or fuel tank, without being cut or ripped. The reasoning being that if they need to be replaced they can be cut or ripped off. As a result, in order to remove the gas tank and carburetor completely from the rest of the engine, the fuel lines needed to be cut using a pair of wire cutters.
  • The Piston and Crank Shaft
  • The piston arm is attached to the bottom of the piston head on one end, and was attached to the drive shaft on the other. The piston arm was attached to the drive shaft using a pressed fitting, preventing the removal of both the piston from the chamber, and crank shaft from it housing. Being that a press was not readily available in the dissection lab, the piston arm needed to be bent off of the crank shaft with a hammer allowing for the crank shaft to be removed, and subsequently allowing for the piston to be removed.
  • The Fly Wheel
  • The fly wheel is fit tightly to the crank shaft in order to prevent it from falling off during use. A press was required for proper removal. However a hammer was used instead to force the fly wheel free from the crank shaft.

Ease of Dis-assembly

The overall dissection of the weed whacker was relatively simple. There were a few times when multiple tools were required to take it apart. In some instances the screws were extremely tight and took patience to remove. The difficulty of removing each part of the weed whacker is measurable in accordance with the scale detailed below:

  • 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.

The fact that the weed whacker was primarily assembled using Torx screws and Philip head screws indicates it was meant to be able to be taken apart. However, many parts were not intended to be able to be taken apart. Based on the difficulty of removing the fuel lines, piston, crank shaft, and the fly wheel were not intended to be removed. here

Step by Step Dis-assembly

Required Tools

  • T15 Torx screwdriver
  • Phillips #2 screwdriver
  • Pliers
  • ¼” Socket wrench
  • Snap ring pliers
  • Press
  • Note: if a press is not available and the weedwacker does not need to be reassembled, the parts can be removed by leverage and brute force.

Dis-assembly Instructions

Step 1 – Remove shaft assembly from motor casing by removing 2 Torx screws and pulling them apart.


Step 2 – Disassemble the shaft assembly into two separate parts, the straight shaft and the curved shaft. This is held together with a single Torx screw. The foam grip can then slide off of the shaft.


Step 3 – Remove the handle on the top part of the shaft by unscrewing a single wing nut.


Step 4- Remove the deflector shield from the shaft using a ¼” socket wrench.

Step 5 – The spool head of the weedwacker on the bottom of the shaft has a cap that unscrews. Unscrewing this will allow 4 parts to separate, the shaft, spool casing, spool, and cap.


Step 6- The motor casing has two parts, one red and one black. Separate them by removing the torque screws around their edge.


Step 7 – Once the cases are off, the throttle cable can be removed my hand.


Step 8 – The gas tank is held in by one Torx screw. Once unscrewed the gas lines must be detached so that the tank can separate from the motor.


Step 9 – The crankcase cover has 6 Torx screws holding it in place. Once these are removed, lift it away to reveal the crankshaft, counterweight and piston connecting rod.
  • Note: The screws were on extremely tight making this much more difficult then usual.


Step 10 – The muffler assembly can be taken off with two #2 Phillips screws. This removes the muffler, muffler gasket, and heat shield from the motor.


Step 11 – The air box and air box cover is held in by one Torx screw. Unscrewing this will allow for the removal of the part.


Step 12 – A gasket is located on the opposite side of the primer bulb on the carburetor that can be pried off.


Step 13 – The carburetor assembly can be removed by unscrewing two Torx screws and lifted off the rest of the motor.


Step 14- Remove the ignition module by removing the single Torx screw holding it in place and pulling it away. Also, be sure to disconnect the spark boot from the spark plug.


Step 15- Remove the spark plug using a ¼” socket wrench

Step 16- Remove the centrifugal clutch assembly by twisting it off using vice grips, while bracing the motor block.


Step 17- Remove starter assembly by removing snap rings on crankshaft and lifting off.

Motor Assembly1.jpg

Step 18- Remove flywheel
  • Note: a press would be required to properly remove flywheel, however we were able to remove by means of leverage and brute force


Step 19- Disassemble starter assembly into five parts. They can be taken apart by hand.

Starter Assembly2.jpg

Step 20- Remove piston and crankshaft
  • Note: piston and crankshaft were only possible to remove using brute force and would not be removed during a normal dis-assembly

Dis-assembly Notes:

  • The Driveshaft is sealed inside shaft assembly, and removing it would require cutting through the entire assembly. We assumed it is a cable drive due to the uniform diameter of the shaft assembly, leaving no space for a universal joint. Also, because the shaft operates at a constant angle, there would be no reason to justify the cost of a CV joint.
  • All Torx screw heads are T15

Parts List

  • Motor Assembly
  1. Motor Block
  2. Spark plug
  3. Crankcase cover
  4. Air box
  5. Air box cover
  6. Fuel cap
  7. Throttle cable
  8. Flywheel
  9. Piston
  10. Starter Assembly
  11. Gas Tank
  12. Carburetor Assembly
  13. Centrifugal Clutch
  14. Ignition Module
  15. Crankshaft and counterweight
  • Shaft assembly
  1. Straight Shaft
  2. Curved Shaft
  3. Foam Grip
  • Spool head assembly
  1. Spool casing
  2. Spool
  3. Cap
  • Deflector Shield
  • Handle w/ trigger
  • Motor Casings

Subsystems of the Weedwacker

Connections between the subsystems

  • The safety system is connected to the fuel system
  • The safety system is physically connected to the fuel system by way of a safety trigger that must be pressed in order to open the throttle. The safety trigger and throttle are both located in a housing on the shaft of the weed whacker. There is a hook that is attached to the safety trigger that does not allow the throttle trigger to be pulled. In order to move this hook out of the way the safety trigger must be pressed in order to engage the throttle. The housing holds the triggers in place. The housing is held together by standard bolts with TORX heads on them.

  • The fuel system is connected to the motor
  • The fuel system is connected to the motor by cables that are connected to the throttle trigger. There is a cable that runs from the housing to the carburetor, which is located on the motor. When the trigger is pulled the throttle is opened and fuel flows from the fuel tank into the carburetor (mass). The fuel then flows into the cylinder (mass) of the motor where it is ignited by the spark plug, where compression is created (energy). As a result of the compression, the piston moves inside the cylinder and causes the crankshaft to spin.

  • The motor is connected to the power transfer system (the cable in the shaft that makes the head turn)
  • The motor is connected to the power transfer system because the crank shaft and cable both meet in a housing located at the end of the shaft that is connected to the motor. The crankshaft then spins the cable which transfers power (energy) down the shaft of the weed whacker.

  • The power transfer system is connected to the head of the weed whacker
  • The power transfer system is connected to the head of the weed whacker by way of a male/female connection. The square male end of the cable fits into the square female end of the head which allows energy to be transferred to the head of the weed whacker, causing it to spin and cut weeds.

These subsystems are all connected so that energy can be created by the ignition of gasoline, then energy can be transferred from the motor to the head of the weed whacker, and then the head of the weed whacker can spin. Basically, the subsystems are connected so that the weed whacker can function correctly.

GSEE Factors


  • This whole system is meant to use gasoline and other types of fuels to decrease the amount of labor that has to be done. This would make it as a focus of first world countries, where the use of gasoline to lower labor is used quite often.


  • The safety system was influenced by society because it would appeal to the consumer more if they knew they were buying a safer product. It is most likely a marketing strategy that is playing on what society wants.


  • The subsystems are all relatively simple and made with cheap materials so that the product can be sold at a cheaper price. Plastic and steel can all be used at a relatively cheap price. There is also no complicated assembly work involved with this weed whacker. A weed whacker of this type can be bought for 50 dollars on sale for this reason. In addition, the weed whacker subsystems are all efficient enough that a small amount of fuel is used. This allows for cheaper operation because fuel is the main cost of operation.


  • The motor subsystem uses such a small amount of fuel that it is not really of environmental concern when people consider the output of greenhouse gases that cars and power plants cause. In addition, none of the subsystems contain any

Performance and Connection

  • The connections need to be as efficient at transferring energy as possible so there is less fuel consumption

  • The connections also need to be made in a way that failure will not be a common occurrence. This is the reason for simple, mechanical wires. If these were to fail commonly, performance could be hindered.

  • The connection between the engine and the head is a steel chord going through the metal shaft. This chord has to be strong enough to have the amount of resistant torque the head would endure in usage.

Arrangement of Subsystems

  • The safety system and throttle trigger are located near the center of gravity of the weed whacker for ease of use. The fuel tank and system are located in such a manner that gravity will allow gasoline to flow through the carburetor into the engine. The motor is located in such a fashion that the crankshaft is pointing in the same direction as the cable that transfers energy to the head of the weed whacker so that the cable and crank shaft can meet inside of the housing described previously. The cable is curved for ease of use so that the head can point downward and be used in an ergonomic way.

  • The reason for the arrangement of the subsystems is such that the weed whacker is easy to use and balanced. The subsystems are also arranged in such a manner that allows each subsystem to sequentially do its job in order to transfer energy from gasoline, to the motor, down the cable/shaft, and into the head of the weed whacker.

  • All of the subsystems could be adjacent, but if some of the subsystems were near each other than the weed whacker would be unbalanced and unpleasant to use. The way that the subsystems are arranged now is most likely the best arrangement. It would not be very smart if the throttle trigger and safety trigger were near the head because that would be placing the users hand in danger. Other than this the subsystems could theoretically be placed near each other

Product Analysis

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 Archeology: Product Evaluation

Component Summary

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

  • If someone is uncomfortable while using a product, it places a black mark on that particular company’s reputation. 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 designed a model of the piston and crankshaft moving as they would within the engine block. 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:

τmax = Tc/J
T = Torque
c = radius of steel rod
J = (½)*π*c4 for a steel rod

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:

τmax = Tc/(½)*π*c4 -> τmax = T/(½)*π*c3 -> c = (T/( τmax *(½)* π ))(1/3)
c = ((7.3 N*m)/(1.45*108 Pa*0.5*3.1416))1/3 = 3.17*10-3 m = 3.17 mm

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.

τmax = Tc/(½)*π*c4 -> T = (τmax*π*c4)/2c -> T = (τmax*π*c3)/2
T = ((145*106 Pa)*3.1416*(0.0043))/2 = 14.58 N*m

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.


"Homelite UT-08520 26cc Parts." Homelite UT-08520 Parts List and Diagram : N.p., n.d. Web. 14 Nov. 2012. <>
"Lecture Notes." Lecture Notes. N.p., n.d. Web. 14 Nov. 2012.