Case Study 2

Venkat Rao

Abstract

The effects of tensile and compressive stresses were studied on a crankshaft. The forces that were specified were too high and led to a failure for all scenarios in which force is applied. A further analysis was done to show how the design would need to be changed to successfully meet the design criteria.

Design

The design of the crankshaft was taken directly from the course website. The design represents a standard crankshaft that might be found in engines that are currently in use.

Also, a couple of changes such as fillets were added, in an attempt to reduce stress. Since these changes did not change the outcome of the analysis, they were thrown out and the original model was used in the analysis.

Analysis Setup

Ansys Workbench was used to perform the analysis. Ansys Workbench provides important advantages over CosmosWorks. Its material database contains more data that can be used in the analysis.

The assignment required us to create four different scenarios in which to perform the analysis.

A consistent naming convention is used for the four different stress scenarios. The crank has a large and small end. When the large end is fixed and a tensile force is applied to the small end, the scenario is can Large-Fixed Tension. When the large end is fixed and a compressive force is applied to the small end, the scenario is can Large-Fixed Compression. The same goes for when the fixed and applied force ends are switched.

In Ansys, the geometry was imported from the igs file. Then a material was selected. An end was restrained based on what the case called for. Then a Force load was applied to the other end. This load was either tensile or compressive based on the scenario. Then two solutions, a equivalent stress and a factor of safety, were added to the analysis. Four different scenarios were setup in this manner.

The setup of the four different cases is showing below.

The following values were used in the Analysis for each of the materials. These values were retrieved from the Ansys database.

	Magnesium	Steel	Aluminum
Young’s Modulus(Pa)	4.5e10	2e11	7.1e10
Poisson’s Ratio	.35	.3	.33
Tensile Yield Strength(Pa)	1.93e8	2.5e8	2.8e8
Compressive Yield Strength (Pa)	1.93e8	2.5e8	2.8e8
Tensile Ultimate Strength(Pa)	2.55e8	4.6e8	3.1e8

Results

The results of the analysis are shown below. They show the range of factors of safety given the location of the stress and the location of the fixed end.

Results Summary

	Magnesium	Steel	Aluminum
Small- Fixed Tension	.11528	.14676	16603
Small- Fixed Compression	.24628	.31833	.35708
Large-Fixed Tension	.1076	.14035	.15655
Large- Fixed Compression	.25358	.32452	.36603

Results Analysis

The most important thing to note is that the factor is below one for each case. This shows us that the model will fail in each case.

Only the result of the Aluminum analysis is shown as the other materials would have similar results. The von misses stress remains the same no matter what material is used. Only the yield strengths change when the material is changed.

An important thing to notice is that the model has a higher factor of safety in compression, even though a higher compressive force is applied.

The most critical case is the Large-Fixed Tension, where the large end is fixed and a tensile force is applied to the small end. In this scenario, the factor of safety is the highest for all materials.

The boundary conditions in the case study affect the stresses in two ways. There is a slight bending in the shaft due to the restrain applied to on end. Also, tensile and compressive stress, leads to elongation and shortening of the crank shaft, respectively.

The results of this analysis are highly sensible and reinforce the basic understanding of a system under stress. Since the stress is too much, the system has a factor of below zero and will most certainly fail. When the system is placed under compression it decreased in length based on the materials Modulus of Elasticity. Under tension, the system expands to counter the load placed on it. Also stress concentrations in the crankshaft, which will be discussed in a later section, reinforce the author’s understanding of the system.

Material and Manufacturing Method Selection

Since the force applied on the system is so great, no matter what material is used this crankshaft will fail. It is also important to know that the von misses’ stress on an object does not change with the material, only the amount of elongation of an object changes based on material properties. Thus, unless the stress is lowered this crankshaft design is invalid.

If the stress was lowered and a factor of safety was met, a good material can be selected. Aluminum is the best material for this application. Aluminum provides the added benefit of low cost and weight.

Next material manufacturing method needs to be chosen. Several criteria should be applied before selecting a manufacturing method. Since the load on a crankshaft is applied in cycles. And if the crank shaft breaks the entire engine might have to be replaced. Thus, one of the most important criteria should be the durability of the material.

Cost is another consideration that must be taken into for product that will go into mass production. A low cost production method provides a nice benefit to a customer.

Based on these criteria, Die Casting would be the best means of manufacturing a aluminum crankshaft. It can also be used in large engine parts, which will need to be used since the amount of force applied is very high.

Analytical Model

To understand how much force is applied and the change in area that would be required, a simple analysis shall be performed. The goal will be to find the minimum area required for the model to hold the loads. The current Area is 278.70912 mm^2.

The only difference that this model will not be able to account for is that, the Ansys Analysis show that the model can hold more force in compression than in tension. Since the yield strengths for an aluminum alloy are the same, this model will not be able to account for this difference.

Since most of the stress is the shaft of the crank shaft, the design of the crankshaft shall be simplified. Aluminum shall be used as the material. Also since the model has a lower factor of safety in tension than in compression, tension shall be used. A factor of safety of two shall be used.

A

The total area needs to increase by a ratio of 1.3 to meet the basic design requirement. This is surprising since the factor of safety is 100 times lower.

Maximum Force

A good way to analyze this design would be to find maximum amounts of force that can be applied. The force applied needs to be reduced to meet the criteria. In the table below maximum stresses are found for each material used in the analysis. A factor of safety of 2 is used

	Steel	Aluminum	Magnesium
Tension(N)	3500	3700	2550
Compression(N)	10000	11000	7500

Stress Concentrations

Also, if the stresses were reduced, the amount of stress the crankshaft can hold will increase with the removal of stress concentrations. Major areas of stress concentrations are shown below. They are encribed within the white circles

Stress Concentration 1

Stress Concentration 2

Stress Concentration 3