Search Results for Torsion

High failure rates in the drive shafts of 40 newly acquired articulated buses was investigated. The drive shafts were fabricated from a low-carbon (0.45%) steel similar to AISI 5046. Investigators examined all 40 buses, discovering six different drive shaft designs across the fleet. All of the failures, a total of 14, were of the same type of design, which according to finite-element analysis, produces a significantly higher level of stress. SEM examination of the fracture surface of one of the failed drive shafts revealed fatigue striations near the OD and ductile dimpling near the ID, evidence of high-cycle fatigue. Based on the failure rate and fatigue life predictions, it was recommended to discontinue the use of drive shafts with the inferior design.

A crankshaft was overloaded on a test stand and suffered an incipient crack in the crank pin. The crack run generally parallel to the longitudinal axis and branched off at the entrance into the two fillets at the transition to the crank arm. It consisted of many small cracks, all of which propagated at an angle of approximately 45 deg to the longitudinal axis, and therefore were caused by torsion stresses. Neither macroscopic nor microscopic examination determined any material or processing faults. Experience has shown that torsion vibration fractures of this kind usually appear in comparatively short journal pins at high stresses. This crankshaft fracture was an example of the damage that is caused or promoted neither by material nor heat treatment mistakes nor by defects of design or machining, but solely by overstressing.

Several torsion bars had failed in a projectile weaving machine and were analyzed to determine the cause. Specimens prepared from the damaged components were subjected to visual inspection, hardness testing, chemical analysis, and metallurgical evaluations. The failed torsion bars had been fabricated from spring steel which, according to stress calculations, did not have sufficient torsional strength. Examination of the damaged parts confirmed the finding, revealing that all fractures started at a shoulder radius in an area of high stress concentration. Based on the investigation, the shoulder radius should be increased to alleviate stress and the working torsion angle of the bar should be decreased to improve safety factors.

An axle shaft in an open-pit mining truck hauling overburden failed after operating for 27,000 h. Previous failures had resulted from longitudinal shear, but this had not, bringing material quality into question. Chemical analysis verified that the part was SAE4340 Ni-Cr-Mo alloy steel and thus met material specification. The failure was a result of torsional fatigue in the tensile plane, originating from one of several gouges around the splined radius of the shaft. The fatigue crack progressed for a large number of cycles before final fracture. The shaft met metallurgical requirements and should have withstood normal operating conditions. The spacing of the gouge marks coincided with the spacing of the splines, indicative of careless assembly with the mating wheel gear.

In a shaft subjected to reversed torsional stresses, failure resulted from the gradual development of fatigue cracks from opposite sides of the shaft. These broke out from origins located adjacent to the fillets at the start of the square section. The remaining uncracked material which fractured at the time of the mishap was in the form of a narrow strip, situated slightly to one side of the center of the shaft. The material was a mild steel in the normalized or annealed condition, having a carbon content of approximately 0.3%. The cracking was characteristic of that resulting from torsional fatigue. Because it occurred on two different planes at 45 deg to the axis of the shaft it was due to reversals of torsional stress rather than fluctuations of unidirectional torque. Following this failure, the shafts of six other similar cranes were tested ultrasonically. Cracks to varying degree were found in all the shafts. Timely replacement was possible and the likelihood of serious accidents removed.