One-handed weed whacker
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The purpose of our analysis is to determine the best choice to pursue between using either a gas or electric power source. By comparing cost and weight distribution, we concluded that using the 2-stroke gas engine is the best option. We found that the gas powered option would be more expensive, however within acceptable parameters. We also found that our new design has about 36% less inertia than the original gas trimmer and about a staggering 74% less inertia than the electric trimmer. These facts led us to decide upon pursuing gas as a source of power. | The purpose of our analysis is to determine the best choice to pursue between using either a gas or electric power source. By comparing cost and weight distribution, we concluded that using the 2-stroke gas engine is the best option. We found that the gas powered option would be more expensive, however within acceptable parameters. We also found that our new design has about 36% less inertia than the original gas trimmer and about a staggering 74% less inertia than the electric trimmer. These facts led us to decide upon pursuing gas as a source of power. | ||
Revision as of 16:38, 3 December 2008
Contents |
Executive Summary
Market Analysis
Design Documentation
Product Usage
Bill of Materials
Design Analysis
Failure Mode Effects Analysis (FMEA)
Failure Mode and Effects Analysis (FMEA) is a crucial tool in determining if a particular component in a system will fail (in terms of the customer's requirements). By looking at each piece, we can rate the severity of a failure (S), the probability that it will occur (P), and the likelihood that the customer would detect the failure (D). The severity of failure is rated between 1-10, 1 being the effect is not noticed by the consumer, while 10 is hazardous. Probability of occurence is also rated from 1-10, 1 being extremely remote while 10 has an extremely high chance of occurring. Finally, detection is rated from 1-10 with 1 being almost certain to detect and 10 being no chance of detection whatsoever. With these three factors, we can multiply them together to achieve a total risk priority number (RPN) which can range from 1-1000 (1000 being the greatest risk). Below is a table outlining each part.
| Item and Function | Failure Mode | Effects of Failure | S | Causes of Failure | O | Design Controls | D | RPN | Recommended Actions | Responsibility and Deadline | Actions Taken | S | O | D | RPN
|
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Engine | Fouled spark plug | Engine won't start | 8 | Spark plug is dirty or broken | 3 | Fatigue testing | 1 | 24 | Approximate the life-span of the spark plug | Engine manufacturer | - | ||||
| Carburetor | Engine won't idle | 7 | carburetor not adjusted properly | 3 | Fatigue testing | 1 | 21 | Add more lubricant | Engine Manufacturer | - |
| ||||
| Bevel Gears | Slip | Wire doesn't rotate | 7 | gears aren't properly aligned | 2 | Unalign gears and turn on | 6 | 84 | Check how much tolerance the shafts give the rod and gears | - | |||||
| Jam | Wire doesn't rotate | 8 | Not enough lubricant | 2 | Remove lubricant and turn on | 5 | 80 | Add more lubricant | - |
| |||||
| Axle | Fracture | Wire doesn't rotate | 7 | Fatigue | 1 | Fatigue testing | 4 | 28 | Not likely, so ignore | - | |||||
| Wear down | Lower shaft axle won't spin at full potential | 6 | Lower shaft axle doesn't securely mate with upper shaft axle | 2 | Fatigue testing | 5 | 60 | Not likely, so ignore | - | ||||||
| Shaft Clamp | Dent | Shaft wont fit into clamp | 8 | Something heavy dropped onto clamp | 1 | Stress testing | 1 | 8 | Not likely, so ignore | - |
| ||||
| Handle | Slip | Reduced control when holding | 6 | Screws loosen over time | 7 | Fatigue testing | 2 | 84 | Look into improved handle design | Design Engineer | - | ||||
| Fracture | No working handle for off hand | 6 | Item is dropped onto handle | 4 | Impact stress testing | 3 | 72 | Look into improved handle design | Design Engineer | - | |||||
| Fatigue | Handle deforms over time, less convenience for user | 4 | Repeated adjustment of handle, use over time | 6 | Fatigue Testing | 3 | 72 | Look into improved handle design | Design Engineer | - | |||||
| Head | Tangled Wire | Wire inside head jams up | 8 | Improper wire loading | 3 | Field Testing | 2 | 48 | Ensure easy/intuitive wire replacement process for user | Process Engineer | - | ||||
| Fracture | Cracks or openings in head, inhibiting wire feed or even proper head fit | 8 | Slamming head into rough object while it's spinning | 2 | Field Impact Testing | 2 | 32 | Not likely, so ignore | - | ||||||
| Bearing | Jam | Bearing has friction, slows axle down | 7 | Left in rain, water can reach bearing and rust | 2 | Bearing Testing | 2 | 28 | Ensure bearings are guarded from elements | Assembly | - | ||||
| Shield | Fracture | Can't effectively guard user from debris or wires | 5 | Weed Whacker is dropped, shield takes impact | 7 | Stress Testing | 2 | 70 | Design more reliable shield | Design Engineer | - | ||||
| Grass accumulation | Grass build up under shield | 2 | Use over time without cleaning | 9 | Field Testing | 1 | 18 | Design more reliable shield | Design Engineer | - | |||||
| Trigger | Fracture | Convenient engine control diminished | 5 | Sharp force applied to trigger in wrong direction | 1 | Stress Testing | 2 | 10 | Not likely, so ignore | - |
Design for Manufacture and Assembly (DFMA)
The weed whacker is a relatively simple product with simple parts. Nevertheless, we have noticed that many of the parts can be changed to make manufacturing and assembly easier, and they include:
- Screws
- Five types of screws
- Only one tool required (same TORX wrench can be used for all screws)
- Shaft
- Same length & material
- Same spacers for the axle
- Axle
- Both are different
- Plastic parts
- Unnecessary shapes
Possible Improvements
Screws - Using five different types of screws is unnecessary in this design. It could easily be reduced down to two.
Shafts - The two shafts that connect to each other are different from one another when they do not have to be. By making them identical, they can share the same manufacturing process.
Axle - The axles can also be made the same to reduce the complexity of assembly.
Plastic parts - Modifying the design of the injection molded plastic parts, including the handle and the shield, could easily save materials. Both the handle and shield seem to be more complicated than they have to be. The shield for example has unnecessary ridges, and the handle is much larger than needed. Perhaps the additional complexity was added for aesthetics, but they do not have a purpose. We assume the handle was designed as it was to be gripped from the sides as well, however, using one hand to hold it from the side in this way would put a significant amount of strain on that hand in order to keep the weed whacker upright. The best place to hold it is from the top keeping the center of mass underneath the user's hands. Therefore, the handle only needs to be horizontal or perhaps triangularly shaped to maintain both user comfort as well as aesthetic appeal.
Design for Environment (DFE)
The Economic Input-Output Life Cycle Assessment (EIO-LCA) website, www.eiolca.net, contains data on the most common contributors to greenhouse gases, toxic releases, and energy usage from industries and sections of those industries. Using this tool, we can get a ball park figure of the environmental impact of the life cycle of our weed whacker.
Table 1: Economic Life Cycle Assessment
Table 2: Environmental Life Cycle Assessment
As you can see, the production of the product does not impact the environment as much as the usage of the product. Since the product is using a two stroke engine, the use of the product creates much more greenhouse gases and carbon footprint.
Usage
Since the mechanics of how a weed whacker works are quite simple, there is little that can be done to reduce waste from the usage of this product. The mechanics that go into a weed whacker are frankly quite simple. From the explanation in the functionality section, it can be seen that little can be done to improve the power transfer from the engine to the rotating wires. However, studying the engine itself shows room for possible improvement.
This particular weed whacker design uses a 2-cycle engine as its power source. This method is harmful to the environment, as it allows unused gasoline and oil to escape as exhaust. Not only is it emitting a lot of carbon dioxide as well as some hydrocarbons, it is also releasing unburnt gasoline. The gasoline vapors react with the sunlight creating even more carbon dioxide, nitrous oxides, and more volatile organic compounds. The inefficiency of the 2-cycle engine has a very large impact on green house gases and the environment.
Mechanical Analysis
Summary
The purpose of our analysis is to determine the best choice to pursue between using either a gas or electric power source. By comparing cost and weight distribution, we concluded that using the 2-stroke gas engine is the best option. We found that the gas powered option would be more expensive, however within acceptable parameters. We also found that our new design has about 36% less inertia than the original gas trimmer and about a staggering 74% less inertia than the electric trimmer. These facts led us to decide upon pursuing gas as a source of power.
Introduction
For our analysis, we decided to analyze the two different routes we can take: electric or gas powered. Both methods of power supply have their pros and cons. The main focus of our new design is to demonstrate the effectiveness of the length and angle adjustable one-handed arm attachment. If we are to use a 2 stroke engine as the power source, a flexible drive shaft will be required to transmit the power to the weed whacker head, which may lead to difficulties when trying to design a length adjustable shaft. On the other hand, using a battery with an electric motor to supply power will eliminate the need for a flexible drive shaft, as power will be transmitted via electrical wire from the battery to the motor. This can allow us more freedom in designing a length and angle adjustable shaft. On the other hand, battery powered weed whackers are known to have much less power than their gas powered counterparts. To help us decide which path we will take, we analyzed and compared the two options based on two main parameters:
• Cost
• Weight Distribution
We estimated the approximate cost of each method of power supply by calculating such things as base costs for a battery or an engine, concurring costs of recharging a battery vs purchasing additional fuel, etc. With this analysis we can then quantitatively see which method would possibly be cheaper to produce and use.
Each method of power supply will require different arrangements of components, thus changing the weight distribution and inertia of our prototype. For example, if we are to go with a battery powered design, one factor that could deter us would be the electric motor. The electric motor must be located down at the bottom of the shaft where the weed whacker head will be, since the motor will be transmitting the power to the wires. Depending on the size of said motor, the center of mass of our prototype could drastically change, thus making it too heavy for somebody to wield with one hand. With this inertia analysis we can quantitatively compare both power supply methods and see which one would be lighter on the user's arm, thus maintaining its ease of use.
Cost
Gas
To analyze the projected cost of our gas powered weed whacker model, we estimated the cost of a standard 1 Hp 31cc 2 stroke engine, and factored in the ongoing cost of fuel and oil. Since this a 2 stroke engine, oil must be mixed with gasoline, usually with a 50:1 gas/oil ratio. One gallon of gas will require approximately 2.5 ounces of oil. Oil can be usually be purchased for approximately $2 per 2.5 oz. We found the cost of a standard 31cc 2 stroke engine to be approximately 30$. For fuel costs, we found the energy in regular unleaded gasoline is 131,881,982 Joules/gallon. With the average price of regular unleaded gasoline in the U.S. currently being 2.4 $/gallon, you get 54,950,790.5 J/$. The engine is 1 horsepower which is equivalent to 745.7 Joules/second. We figured the most amount of time a person would use a weed whacker in one session would be about 3 hours. Running this engine for our chosen time of 3 hours would then utilize 8,046 kJ of work. We can calculate an estimation of how much money it would cost for this use:
(8,046,000 J) * (1/54,950,790.5 J/$) = $0.15 per 3 hour session
(2.4 $/gal) * 3 hour session / (0.15 $ per 3 hour session) = 48 hours of weedwhacker use per gallon of fuel
This is of course an ideal estimate, the actual use per gallon of gasoline would vary greatly depending on intensity of use and amount of start ups and such. One downside to using the 2 stroke engine is the fact that the user must continue purchasing gasoline and oil and mix them accordingly. This persuades the user to perhaps buy a few gallons of gas and bottles of oil and mix a batch of fuel to have stored for later use. This downside is a small one though, because the weed whacker engine can last very long on one gallon of fuel since the average user does not use a weed whacker at a very high frequency, as noted from our user studies.
Electric
To analyaze the projected cost of our electric weedwhacker model, we calculated the estimated cost of an appropriate battery to utilize as the power source. Based on our user study of the Black and Decker electric weed whacker, we know these characteristics:
•18V
•1.2 AmpHours
•~45 minutes of effective use time
For our electric model, we want to achieve an effective use time of 3 hours, therefore we chose to use an 18V battery with a charge of 4.8 AmpHours. With our known voltage and charge, we can calculate the amount of Watt-Hours used by our battery:
18V * 4.8AmpHours = 86.4 Watt-Hours
Knowing the energy use of our battery, we can calculate the cost of this battery. We found that different types of batteries have different energy costs. For example, the battery used for the Black and Decker weed whacker is Nickel Cadmium, which has an energy cost of 1.50 $/WattHour, while a Lead Acid battery, such as a car battery, only has an energy cost of 0.17 $/WattHour. Because of this, we chose to use a Lead Acid battery. It would be bigger, much like a car battery, but having the battery in a backpack much like our first prototype is no heavier than the original 2 stroke engine used in the first prototype. The cost of this Lead Acid battery would simply be:
(86.4 Watt-Hours)*(0.17 $/Watt-Hour) = 14.68 $
Of course the battery would cost a bit more than this because we would be manufacturing a custom battery with 18V and 4.8 AmpHour charge.
Given our chosen battery capacity of 4.8 AmpHours, it is also estimated that the time required to charge such a battery would be 3.6 hours. Given the average cost of residential electricity, .11 $/kWh, It can be estimated that it will cost a user approximately 4.5 cents per charge. The weed whacker battery will have to be charged after every use, so this continuous cost will increase with more use of the weed whacker, as is the case with buying fuel for the 2 stroke engine model.
Process
Timothy Andersen: Public Relations (Usability, Executive Summary, Stakeholders)
Wesley Chu: Supervisor (DFMA, DFE, Mechanical analysis)
Koji Ito: Advisor (Mechanical Analysis, Usability, References)
Michael Menchaca: Mechanic (FMEA, Dissassembly, Bill of Materials)
References
- "Mechanics of Materials" Author: Riley, Willliam F.
- "Engineering Design, 4th edition" Author: Dieter, G and L.Schmidt
- Green Design Institute. "Free Life Cycle Assessment on the Internet." Free Life Cycle Assessment on the Internet. Green Design Institute. Sept. 2008 <http://www.eiolca.net/cgi-bin/multimatrix/use.pl>.
