One-handed weed whacker

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Executive Summary

The weed whacker is a common lawn care device used to keep a lawn trimmed in areas where a standard lawn mower cannot. A weed whacker is held with both arms and swung and maneuvered by the user. With this mobility, you can trim almost any part of your lawn that your lawn mower cannot reach, such as weeds around tree stumps, tall grass in hard to reach places, or if you just want to trim any brush to a certain height. The main factors that make a weed whacker a better choice for certain lawn care situations are its mobility, weight, and ease of use. In the market today, most of the current weed whackers are based on the same design. Most designs consist of a straight shaft connecting the wire spool to an engine. There are also electric counterparts that have the motor and spool both at the bottom of the shaft, with a battery located at the top. A user holds the weed whacker with both hands, one supporting the weight and the other controlling the movement of the weed whacker head. From our user studies of the current design, we found there was a significant issue with the weight distribution and the holding position that the user was required to have. We believe that our new weed whacker design has much potential in the market because it solves these main problems with the current weed whacker design.

Our design features a one-armed design, in which the user can hold and direct the weed whacker shaft with one arm. This is accomplished by having the weed whacker shaft and head be light weight, and the power source located on a backpack. The handle has an easy adjustable mechanism which allows the user to adjust the length and angle of the shaft. Our design utilizes a battery powered electric motor, which are both located on the backpack. This design puts more ease on the user by having most of the weight located at the small of their back, rather than the current market designs, which puts more strain on the user over time. We believe this gives our design an advantage over the current market designs. As you can see from our mechanical analysis, we found our design to require much less force input from the user's muscles, and the electric power supply method proves to be much cheaper than the gas powered engine method. These conclusions from our analysis support our design decisions.

Market Analysis

We performed some market research to find if there would be any need or potential for success in our product design. One thing we researched into was current designs in weed whackers through patents. Currently there are an exorbitant amount of weed whacker/string trimmer designs. Most of which regard attachments, blades, wires, or the wire feeding mechanism.

Regarding the entire apparatus, we found a few existing designs that focus on usability rather than novelty or alternatives. This particular patent design had the closest resemblance to what we wanted to invent:

Backpack Weed Whacker

Using a torsional cable, this design can put the majority of the weight on the users back. This is where some of our ideas were formed. However, the direct drive shaft and bevel gear casing are still required, adding a substantial weight. We thought that this concept was a great idea, but changed the method of power transfer to pneumatics for reduced weight, and focused on the way the user handles the cutting end.

  • Relieves majority of the weight from arms
  • Lowers inertia


  • Same awkward handling problems as basic weed whacker
  • Weight is shifted too far forward requiring 2 hands or a large force on user's wrist

This design still does not address the awkward handling issue that is present in current weed whacker designs. We believe our design has potential to be successful in the residential lawn care market due to its ease of use and minimization of strain on the user's arms and back.

Our design requires about the same number of parts as the current design. The main difference in production cost would be the battery and motor. The cost to produce our designed battery and motor would be approximately 20 and 40 dollars, respectively. Our design will cost approximately 130-150 dollars per unit to manufacture, which would lead to our weed whacker design having a slightly higher price to buy for the user, compared to typical high end weed whackers, which go for around 120-130 dollars. We believe the benefits our design gives to the user outweighs the extra cost and users will want to buy this.

User Study

Research and Observations


After asking many potential customers who were looking at weed whackers we found that the main concerns of users were:

  • Cost
  • Maintenance
  • Safety
  • Ease of use

Other concerns included:

  • Weight
  • Noise
  • Vibration
  • Power

We noticed that many of them assumed characteristics of the design.

Gas powered:

  • High power
  • Noisy
  • Vibrates
  • Hot

Electric powered:

  • Not enough power
  • Low battery life


After observing a few weed whacker users, we asked them what their main complaints and difficulties of using their particular weed whacker.

Gas Powered:

Awkward holding position: The design of the weed whacker forces the user to hold it in a position that has been reported to be very awkward to the user. The right hand holds the handle where the trigger is located and the right arm is what holds most of the weed whacker's weight. The left hand is located about ten inches down shaft of the trigger hand, and The right arm is mostly kept bent and the left arm mostly kept straight throughout the usage in order to keep the weed whacker angled downwards toward the grass. After a short time of use, the user's right arm would get tired and straighten, thus making the weed whacker hard to maintain over grass.

Weight Distribution: Users complained about the weed whacker being heavy. Also they didn't like how most of the weight was located in the back where the engine is. Users found it unnatural to maneuver, and some muscles, such as biceps and back muscles, were being over used.

Vibrations: The constant vibration of the 2 stroke engine proved to be quite an issue for users. The engine is constantly erratically vibrating while idle and while throttled, which added to the inconvenience of holding the weed whacker for a prolonged amount of time in the holding position. Also, when the trigger is pulled to throttle the engine, the sudden clockwise rotation of the axle creates an opposite reaction of the weed whacker being suddenly thrust in a counter clockwise moment. This twisting of the weed whacker seemed to be small issue with some users.


Awkward holding position: Similar to the gas powered weed whacker, the hand positions are awkward making the left arm completely extend forward and the right arm bent to the user's side. We observed this position made it difficult to sweep the device from side to side.

Weight Distribution: The electric weed whacker design had most of the weight too far forward as the motor was located at the head of the weed whacker. Although it is lighter, users experience that more effort is needed to hold the weight. Over time we observed the user resting the end of the weed whacker on the ground to rest their arms.

Power supply: The particular design used a power cord to supply power. This allowed the motor to spin at optimal speeds so we did not observe a lack of power. Vibration and noise did not seem to be a problem either. However the user constantly pulled and relocated the power cord while usage. This problem was not reported as a problem when asked. We observed that the cord could cause major injury if cut or caught on the user.

Conclusions from User Study

From our user shopping research, we found that appearance made a larger impact than the specifications listed on the box. In order to keep potential customers from overlooking our product, we must explicitly show that it is a weed whacker, and will provide the services needed. We also found that users had a preconceived notion regarding the type of engine which made them overlook the power provided.

Because of this, we decided to keep the string as the blade. The type of power source would determine the main focus of design. However, because battery power and life are extremely difficult to improve, we decided to keep an engine.

From the user study, we were able to define the major areas of improvement. Because we are continuing to use an engine as the power source, our main concerns are:

  • Awkward holding position
  • Weight
    • Weight distribution
  • Vibrations
  • Cost
  • Safety
  • Intuitive use


We decided to run the same type of user study on our improved prototype design in order to compare our new design to the current model that is on the market. For this user study, we had subjects put on the backpack and arm attachment. We then had them walk around and perform the actions of trimming a lawn. Our main focus with the physical prototype was the redistribution of weight and minimization of required inertia to swing the weed whacker. Because of this, our actual physical prototype is not fully functional- its main purpose is to simulate our estimated product weight and shape. After having the users walked around and simulated lawn trimming actions, they gave their feedback on the prototype design.

Overall, users noted how light the weed whacker was on their arm. Even though everything was weighted on their one arm, it was light enough to not cause any noticeable inconvenience to the users. Another user noted how the weed whacker prototype was very easy to swing around and point to where they wanted. This combination of light weight and easy maneuverability led to an overall better experience for the users.

In conclusion, we were quite pleased with the resulting feedback from our user study. This user study showed that our design prototype accomplished our goal of improving the main inconvenient factors of the market design:

  • Awkward holding position
  • Weight
    • Weight distribution
  • Intuitive use

By improving on these factors, we believe our design will be a good product in the market of residential lawn care.

Design Documentation

Bill of Materials

Table 1: Individual Components of Redesigned Weed Whacker
Part Number Name Qty. Function Material Manufacturing Process Dimensioned Drawing Vendor Info
1 Sleeve 1 Attaches to main hand, controls power of motor and movement of weed whacker ABS Plastic Injection Molding N/A
2 Shaft 1 Houses flexible drive shaft for power transmission Steel Extrusion, bending N/A
3 Battery 1 Serves as a backpack and houses motor ABS plastic Injection Molding N/A
4 Head 1 Houses spool of wire ABS Plastic Injection Molding See Vendor Troy-bilt part (753-04284) for model TB10CS
5 Lower Head 1 Contains wire, which feeds out at two holes opposite of each other ABS Plastic Injection Molding See Vendor Troy-bilt part (791-610318) for model TB10CS
6 Bump Knob 1 When the user impacts this to the ground, it allows wire to feed out of the lower head ABS Plastic & Steel Injection Molding See Vendor Troy-bilt part (791-153066B) for model TB10CS
7 Shield 1 Covers the weed whacker head to prevent injury to user from kicked up debris or the spinning wire ABS Plastic Injection Molding See Vendor Troy-bilt part (753-04283) for model TB10CS 753-04283
8 Motor 1 Transmits power to the drive shaft which propels the weed whacker head T 6061 Aluminum Assembly Custom built dc motor from ARC Systems Inc.
9 Backpack Strap 2 Straps the backpack to the user Nylon Threading See Vendor
10 Arm Strap 1 Used to further secure user's arms in handle Nylon Threading See Vendor
11 Bolt 2 To hold shield and trigger in place Steel Upset forging, thread rolling See Vendor McMaster-Carr part (91440A540)
12 U-bolt 1 Used to clamp sleeve to shaft Steel Upset forging, thread rolling See Vendor McMaster-Carr part (3043T21)
13 Hose Clamp 2 Prevents sleeve from sliding off the end of the shaft, as well as holds the torsional cable into the shaft Steel Assembly & stamp See Vendor McMaster-Carr part (5415K17)
14 Nut 3 Used for U-bolt and shield Steel Upset forging See Vendor McMaster-Carr part (90494A029)
15 Lever Nut 3 Used for U-bolt Steel Upset forging See Vendor McMaster-Carr part (91044A029)
16 Washer 3 Used for U-bolt and shield Steel Stamping See Vendor McMaster-Carr part (91083A029)
17 Bearing 2 Lowers friction from rotating head and keeps it on the same axis Steel Assembly See Vendor McMaster-Carr part (6383K45)
18 Spring 1 Used for bump knob Steel Coil See Vendor Troy-bilt part (791-610317B) for model TB10CS
19 Flexible Drive Shaft & Spacer 1 Transmits the motor's power into rotation of the head at the bottom of the weed whacker Steel Extrusion See Vendor
20 Trigger 1 Controls voltage to motor Plastic (PTFE) Injection molding N/A
21 Rocker Switch 1 Kills the power to the motor upon pressing it. Plastic Assembly See Vendor G C Electronics part (35-642-BU)


The upper left picture shows the assembly of the shaft. The drive shaft will require a lubricant (un-shown).

The upper right picture shows the assembly of the sleeve that will be directly placed on the weed whacker shaft.

The lower left picture shows how the purchased assembly will be attached directly to the end of the drive-shaft, and outer shaft.

The lower right picture is the full assembly of the weed whacker barring the backpack apparatus.

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This is the assembly of the backpack, where the battery itself is strapped to the user's back, and the motor slides into the bottom of the backpack on either side.

Prototype Documentation

Our prototype is a proof of concept for our design. We built the prototype to show how our design would accomplish the task of improving weight distribution and holding position. Our prototype is not a functioning model due to the fact that we were not able to obtain the custom motor and battery specification that we prescribe in our design. Our prototype consists of a model handle which can be worn to demonstrate how a user would hold the one handed design. We also built a backpack and mounted an engine on it to simulate the weight that a user would be carrying on their back. Our prototype differs from our design in that it lacks the battery and motor, along with the appropriate torsional cable. Also, the sleeve is fastened together with nuts and bolts, but our design calls for most of the sleeve to be injection molded. Our U-bolt clamping mechanism is also represented by aluminum strip and screws to demonstrate its function. Overall, our prototype does accurately convey the simulated weight our of design and is a good proof of concept to people who would like to try it on and see exactly what it would feel like to hold our design.

Design Analysis

Using the same analyzing techniques that we performed on the old model weed whacker in the first report, we analyzed the design of our new weed whacker design. These techniques include Failure Mode Effects Analysis, Design for Manufacturing and Assembly and Design for Environment. These analysis methods help us evaluate the effectiveness of our new design.

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 (O), 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

Battery Low/no voltage Motor does not run at optimal speeds 1 Undercharged 4 Increase Amp-Hr capacity of battery 2 8 Recharge or replace battery -
Leak Decrease lifespan 8 Corrosion 2 Corrosion-resistant leads 3 48 Maintenance User -
Drastically Decrease lifespan 8 Electrical short 1 More clearance between leads 5 40 Insulation/housing Design Engineer -

Axle/Torsion cable Fracture Wire doesn't rotate 5 Fatigue 1 Fatigue testing 3 15 Not likely -
Wear down Wire doesn't spin at full potential / High heat generation 5 Axle does not have enough lubrication 1 Fatigue testing 7 35 Maintenance User -

Handle/Clamp 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 3 Item is dropped onto handle 4 Impact stress testing 2 24 Stronger Material Design Engineer -
Fatigue Handle deforms over time, less convenience for user 4 Repeated adjustment of handle, use over time 3 Fatigue Testing 2 24 Replace material / improve handle design User/Design Engineer -
Strap Wear Decrease ability to function 3 Excessive re-adjusting/use 3 Fatigue testing 1 9 Replace User -
Backpack Failure Strap breaks 4 Improper use of strap 2 Stress Testing 2 16 Stronger material Design Engineer -
Wear Increases risk of failure 2 Corrosion 2 Fatigue Testing 2 8 Inspection/maintenance User -

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 -

Conclusions from FMEA

From this analysis, we found the failure modes to be quite similar to the previous market weed whacker design. There were differences of course, for example, instead of potential failures in an engine, there are potentials failures in a battery and motor. We found that the failure area with the highest RPN, or basically the failure area to pay more attention to, was again the shaft clamp area of the handle. This is because that component is the most complex component of the handle sleeve and requires a bit tighter tolerance in manufacturing than the other components, which would lead to a higher potential of failure. The failure is not severe, but more of an inconvenience to the user. The slipping of the clamp would lead to an increased inconvenience to the user.

Design for Manufacture and Assembly (DFMA)

The new design is greatly improved over the original design. By removing and merging many parts, and redesigning pieces to have multiple functions we reduced the complexity of the design and lowered the amount of fasteners required for assembly.


Screws - Instead of using five different types of screws in the original design, our design only requires 1.

Shafts - The two shafts from the original design is consolidated into one for the new design, removing the need for the shaft clamps and the multiple axles.

Plastic parts - Our design uses more plastic, however it still reduces the part count by one, and eliminates the need for many of the screws used in the original design.


Torsional cable - By moving the power supply and source to the backpack, we added another unique part into the design.

Custom battery - With the power supply needs, a custom battery must be created. Having it supply a common voltage may allow it to be used in other applications to offset this disadvantage.


Overall, we believe our product design is an improvement to the previous market design due to its ease of manufacturing. It utilizes less complex components that are able to serve more than one function. For example- the shaft friction clamp, when opened, allows the user to adjust the shaft's length as well as its angular placement. When this clamp is closed, it holds the shaft firmly in place and supports the weight. We felt it was important to have this component control angular and translational adjustment in order to minimize the number of parts needed for the adjustment function. Our main handle sleeve is to be produced via injection molding. This process is good for high volume production since you get more bang for your buck if you produce more units per mold.

Design for Environment (DFE)

Another way to analyze our product design is to analyze its effects on our environment. Throughout the product's life cycle,it will have an impact on the environment; the goal is to try to minimize this impact in our design. The lifetime of our product design consists of these major stages:

• Raw Material Acquisition- Raw materials required for the production of the design such as plastic, sheet metal, etc.
• Transport- Materials are transported via diesel-traction rail or truck
• Production- This involves any injection molding, steel bending and extruding, and any other process used to assemble the product
• Distribution- The finished product is also transported via diesel traction rail or truck
• Use- The product is being used by a consumer
• Disposal- The product is at the end of its life and is disposed of. An example would be in a landfill.

Our design's impact on the environment is much better than the current design in the Use stage of product life. Our design utilizes an electric motor for power transmission. This requires the user to input electricity to the weed whacker, which is much less impactful on the environment than the gas-powered 2 stroke engine that is usually used in weed whackers. Our design uses a rechargeable battery, which the user provides electricity to in order to recharge the battery for an additional use. A user owning an engine powered weed whacker will need to refuel it with gasoline for additional uses. The exhaust emissions of gasoline are known to impact the environment negatively, and this is compounded by the fact that it is being used in a 2-stroke engine, which is known to be a very smoky engine due to its simplicity.

Quality Function Deployment (QFD)


This house of quality shows that weight is the most important aspect of our new weed whacker design, followed by part count then inertia. This reinforces our confidence in our one-handed weed whacker, since the weight and inertia were the aspects that were significantly changed for the better. Although part count did increase slightly, the increase is minimal, and manufacturing and assembly have been improved by our design, as stated in DFMA above. The competing products graph also shows that our design is generally better than the originial weed whackers.

We found that there were quite a few customer needs that needed to be considered. We ranked ease of use and cost amongst the highest since the trimmer is a function based product, and cost is inevitably important in a competitive market. We ranked adjustability high, since the user would be highly frustrated with the product if they weren't able to use it comfortably. For example, if the trimmer was too long, the user would have to hold it higher, which in the case of the one-handed design would put more strain on the arm and would also create an awkward angle between the trimmer and grass. Environmental impact was ranked low because customers generally do not think about weed whackers being environmentally unfriendly. Aesthetics may influence the way a customer sees the trimmer, but in the end is quite unimportant.

Mechanical Analysis


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.

Mass/Inertia Comparison


Black and Decker Model
Electric motor


Weight: ~7.0 lbs
Center of Mass: ~35 inches from the center of trigger handle
Force on dominate hand: ~5 lbs downwards
Force on non-dominate hand:~12 lbs upwards
Inertia about trigger handle: ~6500 lb in^2

Compared to the other models, the inertia is significantly higher. It is over twice the inertia than the gas powered design. The benefit to this design is that overall, it is 60% of the weight of the gas powered design. However, the position of the center of mass is place at a point where the user is required to apply a moment to the weed whacker. This moment makes the work required by the user negating the weight benefit over the gas model.


Troybilt Model
2-cycle gas motor


Weight: ~11.5 lbs
Center of Mass: ~0 inches from center of trigger handle
Force on dominate hand: ~11 lbs upwards
Force on non-dominate hand:~.5 lbs upwards
Inertia about trigger handle: ~2679 lbs in^2

The benefit of this model over the electric design is that the mass is better balanced, making it easier to handle. The weight of this gas model is 50% more than the electric design, however the placement of the center of mass is between the users hands so that additional forces are not needed.


Redesigned Troybilt Model
Power and Transmission Source located on Backpack

Our design was meant to eliminate the trade-off between weight and handling. With the new placement of the handles and removing excess weight, our design improves both the weight as well as the inertia to reduce the work needed from the user to operate efficiently.


• Point masses
• Weed whacker is held at approximately a 45° angle

With these assumptions, we can sum the inertia about a decided point. Using the elbow plate as this point, the inertia can be calculated as the sum of:

I = mr2

where m is the mass and r is the horizontal distance from the elbow plate.

Weight: ~4.6 lbs (est.) with ~6.5 lbs on back
Center of Mass: ~ 32 inches from elbow plate
Force on elbow: ~4.9 lbs downwards
Force on dominate hand:~9 lbs upwards
Inertia about elbow plate: ~788 lbs in^2

This mechanical analysis shows the significant change in both mass and inertia our new design has in relation to the original weed whacker. Specifically, the new design’s inertia is 70% less than the original. This also means that the energy required to sweep the trimmer one foot is 70% of the original as well. The ease of use is a major concern in our design process, and these numbers show that our new model takes a big step in the right direction. These numbers are also important in considering the new way the trimmer will be held. Although the trimmer will now only be held with one hand, the majority of the weight will be held up by the straps around the forearm. The trimmer originally weighed 11 lbs. Now it weighs just over 4 lbs. The combination of weighing less than half the original weight and the additional support given by the forearm gives us confidence that the user will feel much less strain from our new design. The combination of the lower inertia and the forearm support shows how much easier it will be to use our product. The user will now have significantly more control when moving the trimmer from side to side. The old design required coordination between both hands, and its weight and inertia made it awkward and clumsy. The one-handed design allows the user to control the trimmer with the very simple movements of his arm. This improvement in ease of use allows the user to both trim more efficiently as well as more accurately.



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 is 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/$) = $0.75 per 3 hour session

(2.4 $/gal) * 3 hour session / (0.75 $ per 3 hour session) = 9.6 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.


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:
•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 1 cent 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.


This analysis serves as a justification of our new design using a battery powered motor for our power transmission. Before, even after debating within our team about the two major choices of using a gas engine or electric motor, we were unable to find a clear choice. We decided to weigh two major factors when making a final decision – weight distribution and cost.

In measuring ease of use through weight distribution, our train of thought led us to calculate the weight and inertia associated with each design. These values pointed us strongly towards our new design using the gas powered engine. The motor at the head of the electric weed whacker proved to be a significant factor in its weight distribution, giving it an inertia greater than that of even the original bulky gas engine trimmer. In order to decrease this inertia, a smaller, more compact motor with the same power capacity is required. Another solution we thought of was to have the motor located on the backpack along with the battery and have power transmitted from the motor to the weed whacker head via flexible drive shaft and torsion cable. This would keep most of the weight on the user's back, leaving the shaft and head to maintain as little weight as possible.

Cost is another clear consideration in our design plans. Our calculations show that the cost of gasoline for using the gas trimmer is small – about 75 cents for 3 hours. The difference in costs of a large battery and a 2-stroke engine is cause for consideration. We approximate that the engine alone will cost about $10-20 per unit more than the lead-acid battery. Research of existing products shows that gas powered trimmers are significantly more expensive than their electric counterparts.

Ultimately, we concluded that the electric power supply method is clearly much cheaper to the consumer than the internal combustion engine counterpart. Also, a design involving the motor being located in the backpack would take the weight off the weed whacker shaft and drastically lower the inertia required to move the shaft. This would be a great improvement over the hypothetical design of having the motor at the weed whacker head, much like the Black and Decker electric weed whacker we used in our analysis. Therefore, for our design, we decided to go with a battery powered motor for our power transmission.

Next Steps

Now that we have reached this point, we believe we would still need some more work on our design before further pursuing its introduction into the market. From our user studies, we have much confidence in our design's improved handling for the user. One aspect of the design we were not fortunate enough to physically test was the power supply feasibility. Further testing on our motor and flexible torsional cable power transmission would greatly help us evaluate our design's capabilities. Building a completely functional prototype would allow us to do more in depth testing on the power transmission. Also, a fully functional prototype would be useful in providing us with new insight in updated user studies. From this we can further evaluate our design.


Timothy Andersen: Prototype Construction, Mechanical Analysis, QFD
Wesley Chu: Prototype Construction, User studies, Market Analysis, FMEA, DFMA, Bill Of Materials, Mechanical Analysis, CAD drawings
Michael Menchaca: Prototype construction, User studies, Market Analysis, Bill of Materials, FMEA, DFE, Cost analysis


  • "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 <>.
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