Principles of Engineering Design
The first project of the semester is the design and analysis of a pair of pliers, these pliers are to have the ability to tighten/untighten nuts or bolts, and cut through wires. Group 10 is to make a design for these pliers, make a 3D cad model on a design software package. The group is then to perform a series of analysis that include: load analysis, stress analysis, and deflection analysis. All is to be validated with a design software package. Background:
The analysis of these pliers is going to depend heavily on the choice and use of materials. A basic understanding of material selection is in order. Most structural engineering materials in use today are metal or metal alloys. Metals are an incredibly versatile material because their properties range so far in many aspects. For example when high strength is needed there are some metals that have a huge load bearing capability, and when deflection is desired in a design there are some metals with a high modulus of elasticity. Of course all is a balance; there is not one single material that is lightweight, high modulus of elasticity, high strength, etc. But it is an engineers’ job to choose the material that will suit his design the best. For example in this project the team was looking for a material that had a very high tensile strength but a high level of toughness, in order to have toughness one needs a material that has some deformation. A very hard and brittle material is not ideal in a design like this one. Obviously a material with a high modulus of elasticity would help with the brittle problem, but it would deflect and yield far too much under loading. For this reason the team decided to use the material L2 Low-Alloy Tool Steel. It has a high enough level of tensile strength to withstand the loading a human can exert on the plier handles, it will not be brittle, and has a high endurance limit meaning it will have a lifespan worthy of its’ design.
Another import design characteristic is how one will analyze the pliers for loading, stress, and deflection. The team decided to make an assumption and treat the pliers as a cantilever beam. Where the two parts of the pliers to each other at the pin is where the team decided to make a fixed point. This makes the analysis far simpler when one of the arms of the pliers undergoes loading one side will be in compression while the other side in tension. Here is where the real design comes into play, with the area moment of inertia. The moment of inertia relies on where the mass is located in reference to the neutral axis, the further away from the neutral axis the higher of a load the material can withstand perpendicular to that axis.
The application also has a large role in the design of the pliers. Pliers have handle on them for the reason of giving the human operator a mechanical advantage using a simple lever principle. But with that also comes an amplified force applied to the handles, thus inducing large loading on the material. The team did some research and realized that an average human being can only apply about 125 lbf with the use of one hand. The team used the force of 160lbf due to the fact that some humans can exert more, and sometimes operator may put pipes on the ends of the handles to give a bigger mechanical advantage, by doing this the team is basically implying a factor of safety to the pliers.
The team also had to take into consideration when the pliers would fail, now there are several ways the pliers could fail. The two main ways are through cyclic loading or through overloading. Overloading was taken into account with the load and stress analysis, and played a key role in the material choice. But the group was torn between two choices stainless 304 and L2 tool steel. The team chose L2 tool steel because the endurance limit was...
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