Group 17: Gate 3: Product Analysis
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
- 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.
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.
- 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.
- 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.
- 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.
- 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.
- 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.
|Connecting Rod||Counterweight||Crankshaft||Piston Head|
|Connecting Rod Drawing||Counterweight Drawing||Crankshaft Drawing||Piston Head Drawing|
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.
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.
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.
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. This would benefit in the face of societal concerns about the aging populations in developed countries, especially as people are more likely to remain in their own home during retirement than they were in the past. It could also, make the product safer to use by allowing greater control for able bodied 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 moter 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, fitting into the same accessibility concerns as the composite shaft housing.
- Electric Motor: Exchanging the gas-powered motor out for an electric motor would make the product more environmentally friendly, by eliminating any emissions that the IC motor would produce. An electric motor would also improve the product's function in society, by appealing to the expanding base of users that consider the environment in many of their daily actions. An electric motor is much quieter than a gas engine and subsequently would be more beneficial in suburban neighborhoods, where the societal concern of noise pollution is important to many users and their neighbors. Economically, the electric motor would cause an increase in the initial cost of the product, but would reduce the costs and maintenance 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 motor with the energy required for operation. This revision is becoming much more feasible than it would've been in the past due to the development of much more lightweight and long-lasting Li-Polymer batteries, along with the decreasing cost and greatly improved Power:Weight ratio of brushless DC motors.