| Popis: |
Diesel engine crankshafts serve the function of transmitting torque generated by the power cylinders to the driven machinery. During the re-qualification of the crankshaft at overhaul, there may be small cracks found in the crankshaft. These cracks, known as “heat checks” in the industry, may render the crankshaft unfit for replacement in an engine, thereby representing a financial loss to the owner of the engine. The focus of this work is on the crankshafts, bearings and lubricating oils used in the diesel engines made by GE Rail, a manufacturer of locomotives. A metallurgical analysis of the thermal cracks in a typical crankshaft was made. Bearing rig tests were configured and executed to measure the relative tendency of different shaft materials (heat treatment) to seize and crack when running with different bearing materials and lubricating oils. Transient thermal and elastic-plastic analyses were used to verify the hypothesis of thermal shock. A one-dimensional generalized plane strain model of a long cylinder subjected to a transient heat flux on the outside diameter proved useful in understanding the sensitivity of different geometry and material parameters to the thermal shock mechanism. The finite element method and bearing back temperature measurements were used to estimate the heat flux from actual bearing seizure events. It was determined that the thermal cracks in the surface of the crankshaft journals are the result of a thermal shock mechanism. The rapid heating of the journal due to contact with the bearing will cause a temperature gradient that may be sufficient to cause constrained plasticity near the surface of the shaft. Subsequent cooling of the shaft may result in “reverse yielding” of the shaft surface and the generation of tensile hoop stresses that are sufficient to crack the material. Prior experience and some experimental evidence obtained in this work suggest that case hardening of the crankshaft may exacerbate the tendency to crack under thermal shock. |
