Gate 3 for Group 4
1). Crank Shaft
As described in the Preliminary Project Review, our group as a whole had a very difficult time disassembling the crankshaft from the apparatus. There would be a number of ways to change this design so that it would be easier for the user to disassemble the product quicker and easier. First off, if the design were not pressure fitted, this would eliminate time and cost in making the shaft. If simple dimensions were taken accurately, then the crankshaft would be fitted properly to be taken apart faster with more ease. In addition, the Woodruff key that is installed on the crankshaft makes it harder to pull out – thus majorly increasing the amount of time it takes for the mower to be pulled apart. Instead, the Woodruff key should be completely removed. If the above precaution was taken to dimension the parts correctly to be fitted, then there would not even be a dire need for a key to keep the shaft in its place and locked in.
2). Uniform Bolts
Throughout the duration of our disassembly of the lawn mower motor, we discovered that there were numerous amounts of different types of bolts. Granted, all of these varieties of bolts do different things for the motor and need/meet different force requirements. On the other hand, though, this makes keeping track of all of the parts very hard while the user is taking apart the motor. The manufacturer needs to keep in mind that one of the main reasons why the user would be taking apart the motor is because the motor is not working properly. With this in mind, the user is probably frustrated at this situation at this point, and does not want to have to encounter more problems while taking apart the motor – disassembly should be as easy and quick as possible to help the user. If there are all different bolts, this will just add to the users’ frustration – and might even prevent the person from buying this brand of product in the future. To prevent this from happening, some of the bolts and screws should be made uniform for similar force requirements. This will be easier in the end to reassemble the model, and will leave the consumer happier overall.
3). Valve Springs – Valves
During the disassembly process, we also encountered some difficultly with taking out the valve springs for controlling the valves to move in and out. We found that the springs were very tightly wound and tightly packed in the holes around the valves. This made it very difficult to pry out with an average person’s hand – even with the help of the tip of a screw driver, the springs were lodged securely into the holes. With this, it was also even a little bit dangerous to get these springs out – since they were so tightly packed, they could have sprung out very quickly and hit someone in the face or eye, thus injuring that person. One solution to this dangerous problem would be to increase the diameter of the holes in which the valves and the respective springs are housed – this would make it easier for the user to get to the springs, thus decreasing disassembly and reassembly time.
|Part||Material||Force||Manufacturing Process||Shape||Figure #!|
|Gas Tank|| Plastic
Liquid would rust denser material, which would also add weight.
Weight of fuel.
|Casting||More depth for fuel, hollow to contain liquid.||1|
|Fan Cover|| Metal
More durable because under more stress.
|Air drag force.
|Stamping||Circular because rotates.||2|
|All internal and external forces.||Casting||Rectangular with cooling fins.||3|
Less dense metal.
|Nail shear force.
|Stamping||Circle for filtration/flow of gases.||4|
|Carburator||Plastic, Metal||Nail shear force.||Machining||Part rectangular to hold devices, part circular piping for fluid flow.||5,6|
|Piston and Arm||Steel|| Crank arm pushing/pulling.
|Flywheel||Iron||Push of engine block.
|Casting||Circular with ridges||8|
|Crank Shaft*||Stainless Steel Casting||Rotation.
|Casting||Shaft with rotational capability.||9|
|Cam Shaft||Steel Casting||Gear force.
Pushing valves --> spring force.
|Casting||Shaft to transfer force and rotate.||10|
|Oil filter||Plastic||Fluid Weight||Casting||Cylindrical||11, 12|
|Bottom of Engine Block||Iron Casting||Weight of whole engine.
Crank shaft pulling.
Nail shear forces
|Casting and Machining||Half circle holds everything in.||11|
|Machining||Circular for rotation.||13|
|Ripchord rope.||Fibers||Tension||Weaving||Chord to hold transfer tension force.||2|
|Spring for ripchord.||Metal||Spring force (F = kx).||Forging||Weaker spring for chord.||2|
Force on threads.
|Machining||Screws to hold shear force.||4|
* Equation of force for Crank Shaft:
5 HP engine
HP to RPM- 1/550(5)
T= 150 ft-lbs (found for reference on internet)
d= .875 in
For this portion of the project, we decided to analyze the rip cord attached to the main frame of the motor.
Problem Statement: What is the maximum tension that can be applied to the cord without it breaking?
Assumptions: Motor not accelerating
Motor placed on level ground
Angle at which tension acts is 45 ͦ
Friction negligible, no drag force
Gravity constant (9.8 m/s²)
Rope is of uniform shape and constant mass
Tension is constant
Tension applied at center of motor
Rope does not stretch
Governing Equations: F=ma
Fy = may
Solution Check: F=ma
In reality, there would always be a frictional force involved in any equation of motion. This was not included to ease the calculations of the problem. Also, Ff = µFn. With this, the µ for our setup and materials was not readily available – this would have to be experimentally calculated, which we did not have the means to do. If the angle the tension acted in would have been smaller, then the T-max would have been larger. In comparison, if the angle is larger, then the T-max is smaller. If the rope did in fact stretch during the applied tension, then the problem would become more complicated, and would include the additional equationδ=PL/AE, where δ= the elongation, P is the applied force (tension in this case), A is the cross-sectional are, and E is the modulus of elasticity. The motor might accelerate in the direction of the applied tension, but to keep the problem simple, this was ignored. This would also relate to the frictional force which was ignored, so it was logical to also ignore acceleration. The tension needed to be applied at the center of the motor to avoid calculating distances away from the center to where the force was acting. This also avoids having to introduce moment equations.
The mass of the motor was looked up on the internet.
With this information, an engineer designing this engine would be able to analyze the specific amount of rope needed for the tension requirements for the rip cord. Also, an engineer would be able to determine what type of string would be needed for this task – whether it be a regular cloth rope or a special blend of polyester that is stronger.
SOLID MODELED ASSEMBLY
We had used Pro-E for creating CAD Models of the engine. Pro-E is a very commonly used software for modeling and is available on campus (1019 Furnas Hall). This makes it very convenient, as we did not have to carry the engine around, which was not only somewhat large in size but was considerably heavy too. This also gave us a freedom of doing the entire engine assembly which rather than a few parts as we had all the parts in handy all the time. Also accounting for the fact that Pro-E is part of the course for MAE 377 we could always go to the professors teaching it for guidance. These points made Pro-E the right choice for solid modeling the engine.
These parts were chosen to be modeled because they are crucial to the mainframe of the engine assembly. These parts run the main mechanical functions in the motor itself. PRO-E was chosen to be the CAD modeler becuase it clearly and sufficiently demonstrates the details required for an intricate assembly model.
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