Grass trimmer

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Figure 1: Electric Grass Shears Dissected and Analyzed [source: homelite.com]
Figure 1: Electric Grass Shears Dissected and Analyzed [source: homelite.com]

Contents

Executive Summary

Electric grass shears are a small, hand-held device which trim grass using oscillating blades. They are commonly used to trim the edge of a lawn, along a garden, or other places that a mower cannot reach. Electric grass shears offer an alternative to both string trimmers (aka weed whackers or weed eaters) and hand shears – all for cutting edges of lawns so they look nice and well-kept.

This report contains a detailed analysis of the use and design of Homelite electric grass shears, purchased for $30 from Home Depot. From our own use of the product, comments from existing users, and online reviews we found that from a user’s perspective the electric grass shears are overall very effective in trimming grass but have limited battery life (which also degrades over time), require the user to pull the trigger constantly during use as a safety feature (which can be half an hour or more), blades jam in thick grass, and blades dull over time.

After doing a DFMA analysis, we determined that the product's intended market space is the low-cost, low-end market. Also, the design lends itself to low-skilled labor on an assembly line. From our FMEA we determined that the failure modes with the highest RPNs did not cause bodily harm. The most likely failure modes are blade dulling and battery failure. Doing a DFE analysis revealed that the majority of green house gas emissions are the result of grass shear manufacturing and the electricity consumed in the charging process.

After analyzing how the product is used and problems with the current design, we have the following recommendations for improvement:

  • Use a removable battery so that the user can switch to another fully charged battery if other is completely discharged
  • Add a charge indicator to indicate battery charge status
  • Remove excess housing material on back and bottom - making an L-shape
  • A faster and more efficient rechargeable battery

Major Stakeholders

Manufacturers

  • Low cost materials, parts, and manufacturing equipment
  • Minimal assembly labor
  • Reliable supply chain

Distributors

  • Compact packaging to reduce shipping costs
  • Lightweight to reduce shipping and material handling costs
  • Durable to prevent damage during product shipping

Retailers

  • Compact packaging to reduce shelf-space costs
  • Aesthetically pleasing packaging and product to draw customer interest
  • Good quality and performance to prevent returns and improve customer loyalty

End Users

  • Function as expected – trim grass effectively and efficiently
  • Reliable and long product life-time
  • Safety features to prevent harm to user during use (very important since sharp, oscillating blades are on product)
  • Easy to use
  • Battery life to last user’s duration of grass trimming
  • Ergonomic to prevent harm and injury to user’s hand
  • Lightweight to prevent excessive strain on user’s muscles
  • Compact for portability and small storage space
  • Reasonable recharge time
  • Easy to maintain
  • Aesthetically pleasing

Use Study

Steps for Use

Initial Assembly

1. Connect charger to grass trimmer and let charge for 10 hours


2. Open the black bottom cover

Step 2a
Step 2a
Step 2b
Step 2b

3. Insert blades into latch

Step 3
Step 3

4. Close by sliding in black bottom cover

Step 4
Step 4

5. Slide the locking switch


Grass Trimming

1. Hold down the lock down button

Step 1
Step 1

2. Press the switch trigger

Step 2
Step 2

3. Bending down and holding the trimmer parallel to the ground, slide the trimmer over the grass

Step 3
Step 3

4. Let go of both the trigger and the lock down button to power off

Step 4
Step 4

Our Experience Using Product

The grass shears came in a rectangular, cardboard box. The box was plain and contained no padding. Included in the box were:

  1. Main shears assembly
  2. Attachable cutting blades
  3. User manual
  4. Charging unit.

Removing contents from the packaging was simple. The parts were all in working order; there was no visible damage.

The battery came in a very low (nearly uncharged) state. The instruction manual advised charging overnight, or at least 10 hours. The instruction manual also stated that the charger should be removed when the shears are fully charged. Unfortunately, the shears have no battery charge indicator. This is a fairly simple indicator, which is included on many consumer items. The lack of an indicator on this product is unfortunate. We charged the shears and began assembly. The only assembly required was the insertion of the cutting blades into the main assembly. We then began product testing.

The product is operated using a two-button mechanism- meaning, two buttons must be depressed, simultaneously, in order to actuate the cutting blades. The buttons are located at the top and bottom of the shear handle. This is a safety feature, used to avoid accidental operation of the cutting mechanism.

The product was used to cut moderately high grass, bushes and thin branches. The product cut through all three media, with ease. The largest branch tested was about 1/4” in diameter. Cutting of thicker branches may be feasible, but it not tested in this study. When cutting taller grass, it was harder to see the work that the shears were doing, because the cut grass fell on top of the shears and impeded view.

The battery life was tested by running continuously (not cutting any grass) and was found to be 25 minutes. The manufacturers claimed that the device would last for 1 to 2 hours which may be correct depending on how often the device is switched on an off.

Overall, the product accomplishes the job for which it was built- cutting grass and light brush, with precision. The price ($30) seems reasonable for the level of performance provided.

Other Customers' Experience Using Product

After discussing the product with customers who have used the product and reading online reviews, we have found that customers are generally satisfied with the performance of electric grass shears (in general, not restricted to the specific product analyzed in this report). They thought it functioned well and found it easy to use. However, were dissatisfied with its battery life, having to pull the trigger constantly during use, occasional jamming, blade dulling, and limited ability to cut thick and wet grass. Users consistently compared the product to its alternatives because the product is not a must-have - it is a nice-to-have. Compared to string trimmers, electric grass shears are more precise, less expensive, quieter, more compact, lighter, easier to use, and do not throw grass into gardens, but cannot cut as thick grass. Compared to hand shears, they are less strenuous for the user's hands and easier to use, but more expensive and require more storage space.

Electric grass shears also come in two other variants: Some are sold with long poles so that the user can stand while using the device (user gets less precision but is more comfortable). Others are sold with an additional attachment to replace the grass blades with hedge blades for trimming hedges and bushes.

Mechanical Function

The device uses a rechargeable battery to provide power to a motor. Recharging the device is accomplished by plugging the device into a wall outlet via a power adapter. When the user deactivates the safety and pulls the trigger, a switch supplies power to the motor. The motor in turn rotates a gear set. The gear shaft connects to a link mechanism that rotates one of the blades back and forth relative to the other (using a rotating cam in a slot).

Figure 2: Flow Chart of Grass Shears Mechanical Function
Figure 2: Flow Chart of Grass Shears Mechanical Function

Assembly

Figure 3: Complete Assembly Exploded View
Figure 3: Complete Assembly Exploded View
Figure 4: Main Body Assembly
Figure 4: Main Body Assembly
Figure 5: Powertrain Assembly
Figure 5: Powertrain Assembly
Figure 6: Blade Assembly Exploded View
Figure 6: Blade Assembly Exploded View
Figure 7: Blade Assembly
Figure 7: Blade Assembly
Figure 8: Partial Shears Assembly - Blades with Slot
Figure 8: Partial Shears Assembly - Blades with Slot
Figure 9: Blade Gear and Cam Assembly
Figure 9: Blade Gear and Cam Assembly
Figure 10: Trigger Switch Assembly
Figure 10: Trigger Switch Assembly

Bill of Materials

Bill of Materials
Part Number Name Quantity Material Mass (g) Function Image
1 Female Housing 1 Plastic 151 Houses components and provides handle for user
2 Male Housing 1 Plastic 154 Houses components and provides handle for user
3 Housing Screw 7 Aluminum 1 Fastens housing halves together
4 Upper Blade 1 Steel 28 Moving blade to cut grass
5 Lower Blade 1 Steel 95 Stationary blade to cut grass
6 Blade Rotation Pin 1 Aluminum 2 Provides pivot pin on which the upper blade rotates
7 Blade Rotation Gear 1 Plastic 4 Transfer rotational motion from gear train to blade assembly
8 Blade Rotation Plate 1 Aluminum 3 Prevents blade rotation gear and disc from going through blade casing
9 Blade Rotation Disc 1 Plastic 3 Off-center rotation to cause oscillating blade motion by moving inside blade rotation guide
10 Blade Rotation Guide 1 Plastic 1 Guide for rotating blade rotation disc
11 Blade Connection Pin 2 Aluminum 1 Connects blade rotation gear, plate, and disc
12 Blade Casing 1 Plastic 14 Cover blade assembly and guides blade assembly while being attached to grass shears body
13 Blade Casing Screw 4 Aluminum < 1 Fastens together blade assembly
15 Blade Cover 1 Plastic 14 Safety covering over blades
16 Blade Housing 1 Plastic 28 Attaches and cover blade assembly at bottom of grass shears body
17 Blade Housing Latch 1 Plastic 2 Helps attaching and detaching blade housing from grass shears body and also acts as safety (device will not turn on when latch is not in place
18 Trigger 1 Plastic 7 User pushes trigger to turn on device
19 Safety Button 1 Plastic 5 Safety button that must be pressed in order for device to turn on (in addition to trigger)
20 Safety Lock 1 Plastic 3 Link to blade housing latch which must be in place to turn on device
21 Trigger Spring 1 Aluminum < 1 Provides resistance for trigger
22 Safety Button Spring 1 Aluminum < 1 Provides resistance for safety button
23 Safety Lock Spring 1 Aluminum < 1 Provides resistance for safety lock
24 Battery Half 2 88 Provides power to device
25 Battery Sheath 1 Plastic < 1 Holds battery assembly together
26 Battery Wire 2 4 Electrically connects battery to other electrical components
27 Motor Housing Shell 1 Aluminum 21 Support for motor's main components
28 Motor Housing Shell 1 Aluminum 43 Support for motor's main components and has permanent magnets on inside of shell
29 Magnetic Coil 1 Aluminum 85 Coiled wire through which current is run to create a magnetic field which interacts with the permanent magnets, creating rotational motion
30 Motor Housing Screw 2 Aluminum < 1 Fastens motor platform to motor
31 Motor Housing Clasp 1 < 1 Improves connection between motor housing cap and motor housing washer
32 Motor Housing Washer 1 Aluminum < 1 Spacer between motor housing cap and magnetic brush
33 Motor Housing Cap 1 Aluminum 19 Protects end of motor with magnetic brush and provides structural support
34 Magnetic Brush 1 13 Transfer electrical current to wires in magnetic coil
35 Motor Platform 1 Aluminum 20 Structural part to connect motor to grass shears body housing
36 Motor Platform Pin 2 Steel 5 Connects motor platform to main body housing
37 Large Motor Gear 1 Plastic 20 Large gear to increase torque and decrease angular velocity
38 Medium Motor Gear 1 Plastic 4 Transfer rotational motion from small gear to large gear
39 Small Motor Gear 1 Plastic 4 Attaches to motor output shaft
40 Motor Gear Washer 2 Aluminum < 1 Spacer for gear assembly
41 Charger Connection 1 Plastic 5 Connects wires from battery to external charger for recharging battery from wall outlet
42 Charger Connection Wire 2 2 Electrical connection between charger and battery
43 Charger Connection Wire Sheath 4 Plastic < 1 Protection over electrical connector
44 Charger Connection Wire Clasp 2 Aluminum < 1 Connects wires to electrical components
45 Trigger Switch Housing 1 Plastic 2 Houses electrical/mechanical switch mechanism
46 Trigger Switch Cover 1 Plastic 1 Covers trigger switch housing
47 Trigger Housing Wire 1 2 Electrical connection between electrical switch and motor
48 Trigger Switch Components 4 Aluminum < 1 Components in electrical/mechanical switch
50 Charger Cable/Adapter 1 76 Power adapter between wall outlet and grass shears battery

Design for Manufacturing and Assembly (DFMA)

Tables 1-A and 2-A, found in the appendix below, show our findings of the DFMA guidelines of the grass trimmer. The main objective in this design seemed to be to make an affordable grass trimmer for the lower end of the market. This was done by using a simple design while allowing for lower quality manufacturing processes and materials. The assembly process most likely included low skill workers in an assembly line. Not many opportunities exist to make competing product cheaper as it is all ready a base model but some savings could come in reducing the size of the housing.

Manufacturing

Many components were injection molded plastics and it seems like most of the components which were not injection molded were outsourced. There were also many standardized components. The main flaw with this design is the overall shape of the design. Right now the housing is very bulky with a high mass. The main body of housing is in the shape of a rectangle with most of the components in the upper (handle) and left (motor/blades) of the housing. This could be reduced in both mass and size by moving all components to the upper and left of the housing and removing the housing material from the other two sides making an “L” shape.

Assembly

Assembly of this product could be done fairly easy by attaching most of the components directly to the housing and connecting the housing. This would most likely be done in an assembly line as mentioned above. Connection of the housing looks to be the most difficult part of the assembly however as it requires fitting two housing pieces together, each with multiple components attached, and then tightening several screws to complete assembly. This could be fixed by having all components attached to one side of the housing and by having the housing component attach to one another via snap fits.

Design for the Environment (DFE)

When designing for the environment, the main focus is to design a product that is environmentally friendly. The environmentally friendliness of a product is measured by the amount of greenhouse gases emitted during production and during the use of the product.


Looking at this product closely, it can be seen that environmental friendliness wasn’t priority during the design process. By putting environmental friendliness as the priority during a redesign, large strides can be made to make this product have less of an impact, such as using lithium ion batteries rather than the nickel-cadmium ones currently being used, and to use less plastic when making the shell/casing.

Sources for images (left to right): [1] http://www.homelite.com/catalog/hedge-trimmers/UT44174 [2] http://www.frconline.co.uk/wp/frc-fire-academy/frc-fire-academyfire-science-made-simple/fs7-electricity/ [3] http://www.ccwater.org.uk/ [4] http://baby-dresses.org/?tag=boys-smocked-clothes [5] http://aegisoil.co.nz/our-products/other-oils/coolcut-soluble-threading-and-machining-oil/

Looking at the above table it can be seen that the largest impact is made by the electrical power generation. This is most likely due to the needed 10 hour long charge after every use of the product.

Production

Looking at the Figures 2-A and 3-A in the Appendix below, it’s seen that the manufacturing of the grass shear and the electrical power consumed by the grass shear are two major factors in the product’s greenhouse gas emissions. In total, if there were to be a tax on GHG emissions, the product would incur a tax of $ 2.57 which is 8.6% of its total cost.

In order to reduce the emissions during use and production, a change that can be made would be to use a battery that requires less time charging since the current battery requires 10 hours of charging after every use. Reducing this would greatly reduce the GHG emissions since a majority comes from the use of electricity.

The uncertainty in the above table comes from the cutting fluid. The cutting fluid is used to lubricate the blades after each use. Trouble was had finding the appropriate sector to use for the cutting fluid. It can be approximated that most users with not, or very rarely lubricate the blades since it requires an additional purpose, and because of this the environmental impact of the cutting fluid manufacturing with be negligible.

Figure 1-A: "DFE Guidlines and Ideas for Improvement" goes into further detail of the DFE analysis which was completed. It can be seen in the Appendix below.

Failure Modes & Effects Analysis (FMEA)

Failure modes and effects analysis (FMEA) is a design tool- used to predict possible ways that a product may fail, consider the effects of a failure and prescribe preventative action against serious, negative outcomes.

Electric grass trimmers have a number of mechanical and electrical components that can fail. These include, the battery assembly, blades assembly, trigger assembly and outer shell (case). In the FMEA process, we analyzed the various failure modes of the product, both injurious and non-injurious. A few key ideas were attained. First, it was discovered that the severity of a failure mode does not necessarily indicate the overall RPN value. The failure mode, with the greatest potential for bodily harm, is "breaking and shattering of blades", but the low probability of thick steel blades shattering makes the overall RPN score relatively moderate. The highest scoring failure modes- in terms of RPN- do not pose high injury risk. Instead, they result in loss of product functionality. These modes are "dulling of blades" and "inability of battery to hold charge." In both cases, the failure mode causes loss of product usability, moderate detectability and high probability of occurrence. In summary, the presence of exposed, moving blades results in the possibility for serious injury, but the most highest scoring failure modes, for the product itself, come from less injurious sources.

A number of options are available to deal with these failure modes. Each option, however, brings its own set of tradeoffs. In order to deal with "blade dulling", the blades could be sharpened to a steeper angle. This would make the blades more resistant to dulling; but, the reduced dulling does not come for free. The initial sharpness of the blades is reduced with this solution, in order to reduce long term blade dulling. Another solution could be to replace the steel blades with ceramic blades, as seen in kitchen knives. This reduces blade dulling, but it also increases the price of the blades. In order to deal with the battery life problem, a few options are available. First, an indicator could be included so that the user knows the health and charge percentage of the battery. The downside to this addition is the added cost of a sensor and LED light. In order to deal with the possibility of blades breaking and injuring the user, a shield could be added to trimmers, which would cover the moving blades assembly. While this would add safety, it would also obstruct the view of the user and make the device less agile. In order to deal with a "runaway blades" situation, caused by a stuck trigger switch, an emergency kill switch could be added. The tradeoff is that adding this kill switch results in another possible failure point and added system complexity. For every solution, a tradeoff must be made.

Overall, the design has been executed fairly well to reduce serious injury. Examples of good practice are evident, including, the addition of a double trigger safety mechanism (to prevent accidental actuation), the choice of thick steel blades and the use of the "chomping" blade motion (to reduce bending). There are areas for improvement, some of which were touched upon in the previous paragraph. The most valuable additions would be: a charging indicator, an emergency kill switch and a retractable safety shield.

For a more thorough view of the FMEA analysis, please see Figure 4-A and Table 3-A, in the Appendix.

Team Roles

Humberto Gonzalez: DFE, Secretary

Alex Fry: FMEA, Product Testing

Jordan Zile: DFMA, Treasurer

Brian DeVincentis: Use Study

Cameron Hall: BOM

Appendix

Table 1-A: Design for Manufacturing
Design Objectives Strengths Weakness
Minimize Part Count
  • Overall simple design leads to fewer parts
  • Outer housing is made of only two parts
  • Could use snap fits on housing to reduce screw count
  • Could potentially use fewer gears and other parts in scissor assembly
  • Trigger assembly could use fewer parts by moving location
Standardize Components
  • Standard screws, washers, wire, electrical connectors, gears, and springs were used
  • Instead of using multiple screw types, one type could be used
  • Charger is not a commonly used type
Commonize Product Line
  • Injection molded parts lead to low marginal costs
  • Large investment for injection molding tooling
Standardize Design Features
  • Two housing parts are very similar in dimensions and could both be made with slight tooling alterations
  • Both blades could be made from same sheet metal
  • Could have potentially made the two triggers into single part type
Keep Designs Simple
  • Simple overall design with no complicated features added (settings, etc…)
  • Outer housing design is overly complicated; mass shape could be reduced
  • Scissor assembly could be made simpler
Multifunctional Parts
  • Housing protects inner components, provides support and placement for components, and is a handle for the user
  • Bottom cover lock functions as both a clamp and a safety lock
  • Could eliminate switch housing and use overall housing to also support inner switch components
Ease of Fabrication
  • Used an easily injected molded plastic for many of the parts
  • Could have used only one types of plastic for all plastic parts but instead used 3 types
Avoid Tight Tolerances
  • Tolerances for component support in housings seemed to be adequate without having tight tolerances
  • Tolerances of where the triggers moved within the housing could have been better
Minimize Secondary & Finishing Operations
  • By injection molding housing, finish could be done when molded and not as separate process
  • Finish on brand lettering could be removed and placed on already used adhesive paper
Take advantage of Special Process Properties
  • Used grittiness of inside of housing mold tooling to allow for a better surface for the adhesive paper
  • N/A


Table 2-A: Design for Assembly
Design Objectives Strengths Weakness
Minimize Part Count
  • Housing is in just two parts
  • Could eliminate switch housing
  • Could eliminate a gear in the motor/scissor assembly
Minimize Assembly Surfaces
  • N/A
  • Two housing assembly surfaces could be reduced to one
Use Sub-assemblies
  • Most parts and sub-assemblies are outsourced
  • N/A
Mistake-Proof
  • Grooves in housing ensures proper placement of most components
  • Trigger assembly is not mistake proof and could be assembled backwards
Minimize Fasteners
  • N/A
  • Could use snap fits instead of screws to connect housings
Minimize Handling
  • N/A
  • Components placed on two housing surfaces which makes it hard to put two surfaces together without parts misaligning
Minimize Assembly Direction
  • N/A
  • Wiring makes for harder assembly
Provide Unobstructed Access
  • Unobstructed access granted by having two housing components instead of one
  • N/A
Maximize Assembly Compliance
  • Chamfers and radii on inner housing surfaces allow for easier placement of components
  • N/A
Assembly Support Features
  • N/A
  • N/A


Figure 1-A: DFE Guidlines and Ideas for Improvement
Figure 1-A: DFE Guidlines and Ideas for Improvement


Figure 2-A: GHG Emissions during manufacturing of grass shear
Figure 2-A: GHG Emissions during manufacturing of grass shear


Figure 3-A: GHG emissions during power generation
Figure 3-A: GHG emissions during power generation


We used the following criteria, from Dieter and Schmidt's Engineering Design, in performing our FMEA.


Figure 4-A *Criteria used in assigning severity, occurrence and detectability scores; [from Dieter & Schmidt, Engineering Design]
Figure 4-A *Criteria used in assigning severity, occurrence and detectability scores; [from Dieter & Schmidt, Engineering Design]


Table 3-A: Failure Modes and Effects Analysis - Grass Trimmer
Item & Function Failure Mode Effects of Failure S Causes of Failure O Current Controls D RPN Recommended Actions
Blade AssemblyBlades jamTemporary or permanent loss of function5Particulate trapped between blades; Bending of blades due to misuse7Lubrication of blades; "chomping" blade motion270Easier method to clean and lubricate blades; cleaning/charging port, like those seen in electric razors
Blades breakPossible serious injury; permanent loss of function8Blades hit hard surface; misuse3Blades made of relatively thick steel; Cutting motion is really blades "chomping together", which prevents bending moments and lessens shear stress in blades248Increase blade thickness; Provide shield around blades, to block flying debris
Blades dullInability to cut grass5Extended use; Cutting unintended materials7Use of steel blades4140The company could sharpen the blades to prevent rapid dulling (steeper sharpening angle), but this would reduce initial sharpness; include sharpening instructions for user (unlikely they would be followed)
BatteryBattery does not retain chargeProduct cannot be used for extended periods of time5 large number of recharge cycles; Improper charging methods7Two batteries provided4140Add a charging indicator to enable proper charging techniques; Use higher energy density batteries
Battery leaksLoss of function; possibility of minor injury6Dropping shears; Impact to battery2Contain battery within housing560Cover battery assembly in protective coating
Outer CoverCover cracksAesthetic degradation; loss of functionality if seriously cracked2Misuse; Direct, hard impact3Case is a single piece of molded plastic212Stronger case material
Trigger mechanismTrigger becomes stuckDevice runs uncontrolled; possible serious injury7Debris in mechanism; Misuse by fixing trigger shut2Two trigger mechanism prevents malfunction of one trigger228Include kill switch, in case of malfunction
Trigger mechanism breaksTemporary or permanent loss of functionality4Repeated, hard use; shocking impacts, which dislodge components3Secure trigger components in crevices of molded outer case336Change trigger component materials

References

Dieter, George Ellwood, and Linda C. Schmidt. Engineering design. McGraw-Hill Higher Education, 2009


Images

http://www.frconline.co.uk/wp/frc-fire-academy/frc-fire-academyfire-science-made-simple/fs7-electricity/

http://www.ccwater.org.uk/

http://aegisoil.co.nz/our-products/other-oils/coolcut-soluble-threading-and-machining-oil/

http://baby-dresses.org/?tag=boys-smocked-clothes

http://www.homelite.com/catalog/hedge-trimmers/UT44174

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