Expected Cycle Life of Crankshaft

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Expected Cycle Life of Crankshaft

Description (target value, direction of improvement) and related User Requirements

The crankshaft's cycle life will be calculated with a target value of around 10 years of use. The mower would be used on average for an hour at a time once a week. Since the mower may be in a warmer climate, it could potentially be used 52 weeks a year. The direction of improvement is up and relates to the user requirements of durability and long lasting.

Strategy for Analyzing Engineering Specification

To test the cycle life of the crankshaft the calculated torques from the first analysis will be applied to the shaft. Although, the crankshaft will need to be simplified to allow for finite element modeling in ANSYS. These simplifications will include the removal of the balancing weights along with the material that supports the flywheel. The necessary forces will then be applied to the appropriate locations on the shaft as pressures spread across the nodes. The crankshaft will need to be constrained at the cut in all degrees of freedom. In actuality the frame would supply an f1 joint constraint on the shaft. Once a solution is obtained, a result plot will be provided displaying the nodal Von Mises stresses. Comparing the maximum stresses to a fatigue life curve for aluminum will result in the cycle life of the crankshaft.

Design Decisions/Parameters Affected

The manufacturer designed the crank to be thick enough and strong enough to resist all torques and forces applied to it throughout the combustion cycle.

Key Geometric, Inertia, and Material Properties

The material of the shaft was found to be 3 series aluminum as given by the manufacturer. The important dimensional properties are the length vs. width while the pertinent material properties are density at 0.0986 lb/in3, modulus of elasticity at 10000 ksi, poisons ratio of 0.33, and tensile yield strength of 18000 psi.

Type of Analysis for Obtaining Results

The crankshaft was first simplified. The balancing weights were removed along with the material that extends to the fly wheel as the forces that are applied by them are negligible compared to the piston and grass. ANSYS was used to analyze the stress within the crank due to the torque supplied by the grass, and force from the piston. A simple back of the envelope calculation was performed using solid mechanics methods to give a ball park stress number for the stress due to the torque. This equation was shear stress = Torque * radius / polar moment of inertia.

Boundary Conditions and Loading

Ansys1 mjd.JPG

Quantitative Results

The following figure displays the simulated deformation that the crankshaft would undergo during loading. The crank was constrained at the right end due to difficulties in simulating a bearing boundary condition near the blade on the left side. Consequently, the right side would be bowing in real life.

Ansys2 mjd.JPG

Below are the Von-mises stress contour plots for the crankshaft under the above loading conditions. A torque of 2.18 ft-lbs came from the blade acting on the crankshaft’s key slot and 70 lbs from the piston. These numbers were taken from the ADAMS view model. These plots show the nodal solution as the elemental solution was slightly discontinuous due to the complex geometry. The maximum stresses can be seen at the ridges and sharp corners which are to be expected. The maximum stress was found to be 4254 psi which is well under the maximum 18000 psi yielding stress of the 3 series aluminum. This gives a factor of safety of 4.23 thus verifying that the design was satisfactory.

Thumbnails: Click to Enlarge
Ansys3 mjd.JPG
Ansys4 mjd.JPG

When the piston’s load is not applied the only forces present on the crank is the effect of the grass on the blade and the inertia. These forces mainly slow the crankshaft down and do not produce much stress. Therefore when the piston is not firing the stress on the shaft is near zero.

A simple mechanics of materials analysis on the shaft for the torque applied by the blade was done for verification. τmax = Тρ/Ј. The torque was found to be 2.19 ft-lbs, ρ being 0.4375 in and Ј equaling 0.05645 in4. This gave a maximum shear of around 200 psi. By looking at the contour plots, the area around the key at the end of the shaft has stresses ranging from 2 to 475 psi. This ball park estimate gives validation weight to the ANSYS analysis.

Suggested Changes to Improve the Quality of this Design

Since the design fell under a factor of safety of over 4, not much can be done to improve the design. However, the above plots show that the majority of the stress resides along the sharp edges on the crankshaft. If these edges could be removed the factor of safety would increase even further.

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