Self propelled lawnmower
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Contents |
Major Stakeholders and Needs
The self-propelled lawn mower has drastically improved the efficiency and quality of lawn mowing since the era of the push mower. The stakeholders involved include the customer, the manufacturer, the supplier of raw materials, and lawn owners. The customer can be anyone from a lawn owner to a commercial lawn mowing business. The customer must be ensured efficiency, quality, safety, and convenience. Therefore, it is essential that the self-propelled lawn mower is not too cumbersome, heavy, and well enough secured so that a customer of any size or strength can safely and effectively cut any lawn. This is also important when considering the lawn owner because the owner desires an attractive lawn, but wants to achieve this as quickly as possible so as to utilize the lawn. The manufacturer is responsible for producing a vast quantity of operative and safe lawn mowers, which can only be attained through the efficient and reliable shipment of raw materials from the raw material supplier.
Product Use
The operation of this particular lawn mower is fairly simple, but it is very important to be aware of the safety concerns associated with each step. As long as a few precautions are taken you can easily ensure that the mower will not malfunction, and that the user will be safe throughout the entire operation.
The first step in using the mower is to check the gasoline and oil levels. When checking the gasoline it is important to do this in a well ventilated area, away from any ignition or sparking devices. If the engine requires gas it is important to refuel the engine after it has cooled off. If the engine is hot it is a good safety practice to let the mower cool for 10 minutes before adding unleaded gasoline. One final step that must be taken during refueling is ensuring the snug fit of cap on the the tank. This is important to maintain pressure on the engine and keep dangerous fumes from escaping.
The next step in operating the lawn mower is checking the oil level. You can easily do this by removing the dipstick on the side of the engine. You then must clean off the oil and dip it in to check its level. You then can tell by the markings on the dipstick whether or not adding or removing oil is necessary. This step is important to ensure that the mower is able to properly lubricate its engine.
Now that the oil and gasoline levels have been checked, you can move onto adjusting the cut height. For this step you need to evaluate the height of the grass you intend to cut. If the grass is more than 2 or 3 inches tall it is a good idea to pivot the arms into the higher slots like slot D, or E. If the grass is not very long the lower slots like A, and B will work well. Slot C would be a good choice for grass that is of a medium height. This step is very important because if the right height isn't selected you can either kill the grass (the setting was too low) or have the engine bog out (the setting was too high).
After the wheel height has been set, you can set the throttle control. If you are planning on starting the engine cold you need to move the lever on the throttle control to the 'choke' position. It will be important to move the lever off of the choke setting after then engine warms up. Also this level can control how fast the engine is going to run. You can slide the lever to up to the rabbit to open the carburetor fully or slide it back to the turtle to close the carburetor and slow the engine down. The fully open setting would be good when your cutting thick or long grass and the slower setting would be good for shorter grass.
Part List
In order to complete our parts list we did a product dissection. Although we disassembled almost the entire mower there were some limitation. Since there are so many parts to the self propelled mower we decided to forgo removing the engine and all of its parts. We decided to make the drive train and its components the highlight of the product study. Some other limitations we faced was that in order to take certain parts completely apart, the part would have to be destroyed. This limitation was seen in the extremely tight fit of the tires on the wheels. Under close examination we determined that the rubber tires were cast with the wheel inside, and in order to remove it we would have had to cut the tires off to separate the wheel completely. An additional problem we faced was removing the rear differential (transmission). When we got to this part we also decided it would be best left intact. In the lawn mower users manual it tells you to have the transmission opened or serviced at a Toro certified service location. It warned that it would be very hard to reassemble the transmission and have it work safely if done on your own. We were able to find some exploded views of the gear sets within the differential housing in online parts catalogs. So because of the warning in the manual we decided that we would just study the outside of the housing and the parts drawings we found online.
Assembly
The following diagrams from the Toro website show how all of the dissected parts fit together. In a table following each assembly image, the parts are matched with what number they correspond to in the Parts List.
Labeled number on Assembly Image (above drawing) | Part Number (from Part List) |
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23 | 7 |
24 | 9 |
25 | 1 |
26 | 5 |
Labeled number on Assembly Image (above drawing) | Part Number (from Part List) |
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35 | 18 |
30 | 28
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Labeled number on Assembly Image (above drawing) | Part Number (from Part List) |
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1 | 11 |
23 | 10 |
28 | 12 |
24 | 15 |
29 | 4 |
30,31 | 2 |
32 | 5 |
27 | 14 |
26 | 13 |
2 | 8 |
Labeled number on Assembly Image (above drawing) | Part Number (from Part List) |
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7 | 19 |
10 | 23 |
13 | 21 |
59 | 20 |
Design for Manufacture and Assembly (DFMA)
Design for Manufacture consists of reducing costs of a product in the manufacturing stage through the proper selection of manufacturing methods. It is important, however, that the quality of the product is not reduced when attempting to reduce cost at this stage of the product life cycle. When examining and dissecting our product, we observed that several of the parts were stamped from sheet metal or plastic injection molded. Several pieces such as the drive gear shield, handle hand nut, and plastic knobs on the pivot arms can all be plastic injection molded at once, just as the tension washer and e-clip can be stamped from sheet metal at the same time. This drastically minimizes the time needed to manufacture the product. These are cheap yet effective methods of manufacture, representing a well executed design for manufacture. Another example is the manufacture of the deck, which is cast from aluminum. These are cheap yet effective methods of manufacture, representing a well executed design for manufacture.The material choice of aluminum is also an excellent demonstration of design for manufacture. The deck is responsible for protecting the user from the spinning blades so the material must be strong, yet lightweight so as not to fatigue the user. Therefore, aluminum is a great choice because even though it is high-priced, it is light and durable. Casting a part is more costly than stamping one, but the shape of the deck is too complicated to allow for stamping. Other parts such as the v-belt pulley and throttle drive control cable can be made from less expensive materials like steel because they require endurance, but not to be lightweight. Although it is rather heavy, steel does not fatigue that easily, which is essential to parts that are subjected to large amounts of stress.
Design for Assembly is meant to reduce the complexity of the product assembly and in doing so, minimize the ultimate cost of the product. In the product dissection, we found all of the parts to be easily removed signifying that the lawn mower was also easy to assemble. We can observe from the shape of the deck that the bolts and screws attaching the wheels, pivot arms (height adjustment mechanisms), and drive gear shield are able to be grasped effortlessly. The transmission and gear cover was also easy to be removed permitting v-belt, pulley, or transmission inspection. This simplicity of assembly reduces the time required to build the product, allowing for mass production at a minimal cost.
Design of Environment
Sector #333112: Lawn and garden equipment manufacturing
Economic Activity: $1 Million Dollars Displaying: Economic Activity
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Sector #333112: Lawn and garden equipment manufacturing Economic Activity: $1 Million Dollars Displaying: Conventional Air Pollutants |
Sector #333112: Lawn and garden equipment manufacturing Economic Activity: $1 Million Dollars Displaying: Greenhouse Gases |
Sector #333112: Lawn and garden equipment manufacturing Economic Activity: $1 Million Dollars Displaying: Energy |
Sector #333112: Lawn and garden equipment manufacturing Economic Activity: $1 Million Dollars Displaying: Toxins Released |
Carnegie Mellon University Green Design Institute. (2008) Economic Input-Output Life Cycle Assessment (EIO-LCA) model [Internet], Available from: <http://www.eiolca.net/> [Accessed 23 Sep, 2008]
FMEA
FMEA (Failure mode and effects analysis) is used to determine the potential problems with designs. It takes each component in a mechanical system and analyzes its failure modes and effects using three main criteria. These criteria are severity of the failure (S), probability of occurrence of the failure (O), and ease of detection of the failure (D). Each of these is ranked on a scale of 1 to 10. For severity, 1 is least severe and 10 is dangerous or catastrophic failure. For probability of occurrence, 1 is very unlikely and 10 is almost certain. For ease of detection, 1 is easiest to detect and 10 is impossible to detect. Once each component is assigned one number for each of the three criteria, the numbers are multiplied together to determine the RPN (risk priority number). This number can range from 1 to 1000. The higher the RPN for a specific component the greater the chance that this component needs to be looked at and improved. Along with assigning a numerical value to the risk of certain components, FMEA also attempts to determine the origin of the problem and possible solutions to improve components. It is important to note that design controls and recommended actions are intended to help the manufacturer improve the problem. The following table provides an FMEA analysis for the key components of the lawnmower.
Item | 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 |
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Wheels | Translate power from drive shaft to horizontal motion | Plastic dries out and cracks | Mower can get stuck and won't move | 8 | Use of mower in dry weather too often | 5 | Use plastic that is less likely to dry out | 2 | 80 | Fatigue testing | Wheel manufacturer | 8 | 5 | 2 | 80 | |
Wheels | Translate power from drive shaft to horizontal motion | Plastic dries out and cracks | Mower can get stuck and won't move | 8 | Use of mower in dry weather too often | 5 | Use plastic that is less likely to dry out | 2 | 80 | Fatigue testing | Wheel manufacturer | 8 | 5 | 2 | 80 | |
Blade | Cut grass | Dulls | Grass cutting in ineffective | 8 | Natural wear with time | 7 | Use harder material that is less likely to dull | 2 | 112 | Fatigue testing | Design engineers | 8 | 7 | 2 | 112 | |
Blade | Cut grass | Rusts | Grass cutting is uneven | 8 | Natural wear with time | 7 | Use material that does not rust | 3 | 168 | Waterproof coating | Design engineers | 8 | 7 | 3 | 168 | |
Belt | Transfer power from engine to transmission | Rubber dries out and breaks | Rear wheel drive won't work | 6 | Use of mower in dry weather too often | 5 | Provide extra belts with mower | 5 | 150 | Analyze tension in belt | Design engineers | 6 | 5 | 5 | 150 | |
Tires | Provide traction on wheels | Tread wears with time | Mower will lose traction and ability to climb hills | 4 | Natural wear with time | 7 | Consider different tread pattern that takes longer to wear | 1 | 28 | None, unlikely occurrence | 4 | 7 | 1 | 28 | ||
Rear Wheel Gears | Gears down power from engine to rear wheels for increased torque | Teeth break | Rear wheels will not turn evenly or consistently | 1 | Too much strain on rear wheels | 2 | Consider different material for gears | 9 | 18 | None, unlikely occurrence | 1 | 2 | 9 | 18 | ||
Cables | Attach to mounting bracket to engage transmission | Stretch | It will be difficult for the user to engage drive mechanism | 4 | Overuse of drive system | 5 | Improve connection points | 7 | 140 | Failure tests of connection points | Design engineers | 4 | 5 | 7 | 140 | |
Cables | Attach to mounting bracket to engage transmission | Break | User cannot engage drive mechanism | 1 | Overuse of drive system | 2 | Lower required tension in cable by adding a system that provides mechanical advantage | 1 | 2 | None, unlikely occurrence | Design engineers | 1 | 2 | 1 | 2 | |
Gas Tank | Holds fuel for engine | Leak | Engine will not be able to operate | 6 | Puncture from debris | 4 | Locate critical puncture points and reinforce them | 6 | 144 | Puncture testing | Design engineers | 6 | 4 | 6 | 144 | |
Wheel Brackets | Allows user to change ride height and attaches wheels to chassis | Knobs for changing height break | Height cannot be changed | 8 | Constant Pressure on weld seam | 7 | Replace with removable piece so that it can be easily replaced if it does break | 1 | 56 | Knob redesign | Design engineers | 8 | 7 | 1 | 56 | |
Wheel Brackets | Allows user to change ride height and attaches wheels to chassis | Constant bending can cause plastic deformation | Bracket is at risk of breaking | 7 | User bends bracket too much when changing height | 4 | Use material with a higher elastic modulus | 1 | 28 | None, unlikely occurrence | 7 | 4 | 1 | 28 | ||
Driveshafts | Transfer power from transmission to rear wheels | Shear | Rear wheel drive won't work | 1 | Engaging rear drive while rear wheels are stuck | 1 | Reanalyze the key points where the driveshaft is attached | 2 | 2 | None, unlikely occurrence | 1 | 1 | 2 | 2
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Mechanical Analysis
For the mechanical analysis we decided to examine a key bracket on the transmission casing. This bracket (Pulley Tightening Bracket, part number 17 on the parts list) is mounted to the top of the transmission casing by two quarter inch bolts. When the user pulls the lever to engage the drive mechanism a cable tightens and pulls on this bracket. This forces the entire transmission to rotate upwards about the axis of the driveshaft, engaging the transmission via a pulley that is connected to the engine. The purpose of this exercise is to estimate the amount of force required by the user to keep the drive system engaged. This will involve a static analysis of the pulley tightening bracket. Once the tension in the cable is determined, we can employ FEA to locate the maximum stresses in the bracket and possibly optimize it.
The above figure is a side view of the pulley tightening bracket. ‘T’ is the tension in the cable; ‘W’ is the weight of the transmission; ‘O’ is the axis of the driveshaft; ‘D1’ is the distance from the point on the bracket where the tension is applied to the axis of the driveshaft; ‘D2’ is the distance from the bolt holes on the bracket (where the weight of the transmission is focused) to the axis of the driveshaft; ‘α’ is the angle between the direction the tension is applied and D1; ‘β’ is the angle between the direction that the weight of the transmission is acting and D2. Since the system is in static equilibrium we can sum moments about point ‘O’ and set it equal to zero (taking counter-clockwise to be positive).
∑ Mo = 0 = TD1sinα – WD2sinβ
Solving for T,
T = WD2sinβ/D1sinα
Plugging in all known values we are able to closely estimate that the tension in the cable, ‘T’, is equal to 13 pounds.
The following image is a Solidworks model of the pulley tightening bracket.
The following image is a stress plot of the bracket with a tension force of 13 pounds applied to the connection point of the bracket and the cable. This plot shows that the maximum stress in the bracket is 6,540 psi and the yield stress of the steel is 89,980 psi. This gives a factor of safety of 13.76 which is well within the acceptable range and will be the basis for maintaining during optimization.
As it is now, the bracket has a weight of 0.23 pounds and a volume of 0.83 cubic inches. Optimization was done to remove excess material in areas of low stress concentrations. The following image shows a stress plot of the bracket after the optimization.
After optimization, the bracket has a mass of 0.20 pounds and a volume of 0.70 cubic inches. The new value for the maximum stress is 7,148 psi. This maintains a factor of safety of 12.59 which is still within the acceptable range. With this new design, each unit will save 0.03 pounds of material. When produced on a large scale this may have a significant effect on material costs if the excess steel is recycled. It then becomes an issue of whether or not it will be cost effective to change the dies in the stamping machine to cut out the new pattern.