Group 17 - Homelite Line Trimmer (Gasoline Powered)
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
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
- Ratchet and socket (deep well and standard)
- Possibly torx bits
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.
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.
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.
Ryan Edmonson - Technical Analyst
Mike Gold – Technical Analyst
Ryan Fiust-Klink- Report Editor
Shawn Milligan - Wiki Account Manager
Jeff Scott – Project Coordinator
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.
Challenges Thus Far
- 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.
- 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
- T15 Torx screwdriver
- Phillips #2 screwdriver
- ¼” Socket wrench
- Snap ring pliers
- 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.
- 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.
- 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.
- 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
- 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
- Motor Assembly
- Motor Block
- Spark plug
- Crankcase cover
- Air box
- Air box cover
- Fuel cap
- Throttle cable
- Starter Assembly
- Gas Tank
- Carburetor Assembly
- Centrifugal Clutch
- Ignition Module
- Crankshaft and counterweight
- Shaft assembly
- Straight Shaft
- Curved Shaft
- Foam Grip
- Spool head assembly
- Spool casing
- 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.
- 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
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
Solid Modeled Assembly
Component: Cable Drive
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.
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.
τmax = Tc/J T = Torque c = radius of steel rod J = (½)*π*c4 for a steel rod
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.
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.
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.