Dynamic Stress Analysis of a Multi cylinder Two-stage Compressor Crankshaft – Mechanical Project

Abstract

In this work, the dynamic stress analysis was carried out to change the crankshaft material from forged steel shaft (En19C) and steel casting (En18C) to spheroidal graphite cast iron (SG600/3) to reduce the initial investment cost for development of the new product. A dynamic simulation was conducted on a crankshaft from six cylinder two-stage oil less compressor.
Finite element analysis was performed to obtain the stress magnitude at critical locations at worst load cases. The pressure volume diagram was used to calculate the load boundary condition in dynamic simulation model, and other simulation inputs were taken from the compressor specification. The dynamic load calculation is done analytically by considering the reciprocating masses and its inertial loads in all cylinders. This load was applied to the FE model in ABAQUS, and boundary conditions were applied according to all loads acting on the crankshaft mounting .The stresses are calculated from FE model and plotted in Goodman diagram to calculate the factor of safety. Results achieved from aforementioned analysis can be used in fatigue life calculation and optimization of this component.

Introduction
In reciprocating compressor transmission system consists of crankshaft, main bearings, connecting rod and coupling connected to the prime mover. In this design, the compressor crankshaft drives the compressor as well as it is used to transmit the power to other accessories like traction motor blower and
radiator fan in diesel locomotives. Crankshaft is a large component with a complex geometry in the compressor transmission system, which converts the rotary motion of the shaft to reciprocating displacement of the piston with a four link mechanism. This analysis was conducted on a six-cylinder two stage
compressor.
Crankshaft experiences large forces from first and second stage cylinders during loading and unloading of the compressor. The power is transmitted to the crankshaft through coupling connected from locomotive main engine crankshaft. As the crank pin is rotated, the connecting rod converts the rotary motion to reciprocate the piston. The magnitude of the force depends on many factors which consists of crank radius, connecting rod dimensions, weight of the connecting rod, piston, piston rings, and pin. Gas forces in the top of the piston and inertia forces acting on the crankshaft cause two types of
loading on the crankshaft structure; torsional load and bending load.

A geometrically restricted model of a light automotive crankshaft was studied by Borges et al.2. The geometry of the crankshaft was geometrically restricted due to limitations in the computer resources available to the authors. The FEM analysis was performed in ANSYS software and a three dimensional
model made of Photo elastic material with the same boundary conditions was used to verify the results. This study was based on static load analysis and investigated loading at a specific crank angle. The FE model results showed uniform stress distribution over the crank, and the only region with high stress concentration was the fillet between the crank pin bearing and the crank web.

Boundary conditions:

Boundary conditions in the FE model were based on the compressor configuration. The mounting of this specific crankshaft is on two different bearings which
results in different constraints in the boundary conditions. Two rigid rings used to simulate the bearing constraint in all degrees of freedom. This indicates that the surface cannot move in either direction and cannot rotate. Structural coupling elements used to constraint fly end in rotational degrees of freedom and fan load applied at the non drive end as torque. It should be noted that the analysis is based on dynamic loading, though each finite element analysis step is done in static equilibrium. The main advantage of this kind of analysis is more accurate estimation of the maximum and minimum loads. Design and analyzes of the crankshaft based on static loading can lead to very conservative results. In addition, as was shown in this section, the minimum load could be achieved only if the analysis of loading is carried out during the entire cycle.

Multi cylinder Two-stage Compressor
Multi cylinder Two-stage Compressor

Conclusion
Multi cylinder two-stage compressor load pattern is established to carry out the FE analysis. Dynamic loading analysis of the crankshaft results in more realistic stresses whereas static analysis provides an overestimate results. Accurate stresses are critical input to fatigue analysis and optimization of the crankshaft. There are two different load sources in compressor crankshaft, one is the gas forces and inertia of the reciprocating masses. These two load source cause both bending and torsional load on the crankshaft. Critical locations on the crankshaft geometry are all located on the fillet areas because of high stress
gradients in these locations, which result in high stress concentration factors. The stresses are within the endurance strength and safety margin of 5.5 with SG 600/3 material. Therefore, we can conclude that, this material can used with casting process for development of crankshafts in compressor applications to bring down the initial investment cost for new product development.

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Dynamic Stress Analysis of a Multi cylinder Two-stage Compressor Crankshaft – Mechanical Project

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