A 60.3 mm (2.375 in.) diam drive shaft in the drive train of an overhead crane failed. The part submitted for examination was a principal drive shaft that fractured near a 90 deg fillet where the shaft had been machined down to 34.9 mm (1.375 in.) to serve as a wheel hub. A 9.5 mm (0.375 in.) wide x 3.2 mm (0.125 in.) deep keyway was machined into the entire length of the hub, ending approximately 1.6 mm (0.062 in.) away from the 90 deg fillet. A second shaft was also found to have cracked at a change in diameter, where it was machined down to serve as the motor drive hub. Investigation (visual inspection, inspection records review, optical and scanning electron microscopy, and fractography) supported the conclusion that the fracture mode for both shafts was low-cycle rotating-bending fatigue initiating and propagating by combined torsional and reverse bending stresses. Recommendations included replacing all drive shafts with new designs that eliminated the sharp 90 deg chamfers in favor of a more liberal chamfer, which would reduce the stress concentration in these areas.

The drive shaft in a marine propulsion system broke, stranding a large vessel along the Canadian seacoast. The shaft was made from quenched and tempered low-alloy steel. Fractographic investigation revealed that the shaft failed under low rotating-bending variable stress. Fatigue propagation occurred on about 95% of the total cross section of the shaft, under both low-cycle and high-cycle fatigue mechanisms. It was found that the fillet radius at the fracture’s origin was smaller than the one provisioned by design. As a result, the stresses at this location exceeded the values used in the design calculations, thus causing the initiation of the cracking. Moreover, although the shaft had been quenched and tempered, its actual hardness did not have the optimal value for long-term fatigue strength.

Failure of a case hardened steel shaft incorporated fuel pump in a turbine-powered aircraft resulted in damage to the aircraft. The disassembled pump was found to be dry and free of any contamination. Damage was exhibited on the pressure side of each spline tooth in the impeller and the relatively smooth cavities and undercutting of the flank on this side indicated that the damage was caused by an erosion or abrasion mechanism. A relatively smooth worn area was formed at the center of each tooth due to an abrasive action and an undulating outline with undercutting was observed on the damaged side. Particles of sand, paint, or plastic, fibers from the cartridge, brass, and steel were viewed in the brown residue on the filter cartridge under a low power microscope and later confirmed by chemical analysis. Large amount of iron was identified by application of a magnet. It was concluded that the combined effect of vibration and abrasive wear by sand and metal particles removed from the splines damaged the shaft. Case hardened spline teeth surface was recommended to increase resistance to wear and abrasion.

A bearing cup in a drive shaft assembly on an automobile was found to have failed. A detailed analysis was conducted using the QC story approach, which begins by proposing several possible failure scenarios then following them to determine the main root cause. A number of alternative solutions were identified and then validated based on chemical analysis, endurance and hardness tests, and microstructural examination. The investigation revealed that carbonitriding can effectively eliminate the type of failure encountered because it prevents through hardening of the bearing cup assembly.

A crane long-travel worm drive shaft was found to be chipped during unpacking after delivery. Chemical analysis showed that the steel (EN36A with a case depth of 1 mm, or 0.04 inch did not meet specifications. Magnetic particle inspection revealed a crack on the side of the shaft opposite the chip. Metallographic examination indicated that the case depth was approximately 2 mm (0.08 in.) and that a repair weld of an earlier chip had been made in the cracked area. The chipping was attributed to excessive case depth and rough handling. It was recommended that the shaft be returned to the manufacturer and a replacement requested.

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.

The drive wheel on a clutch-drive support assembly was slightly loose and caused clutch failures in service after 680,000 cycles. After failure, removal of the taper pin holding the drive wheel on the shaft was difficult, indicating that the pin was tight in the assembly. The taper pin was made of 1141 steel, the shaft 1117 steel, and the drive wheel 52100 steel. It was found that failure of the clutch-drive support assembly occurred as a result of fatigue fracture of the taper pin. A loose fit between the drive wheel and the shaft and between the drive wheel and the pin permitted movement that resulted in fatigue failure. Fretting of the pin and drive shaft was observed but did not appear to have contributed to the failure. To prevent reoccurrence, the assembly should be redesigned to include an interference fit between the shaft and the drive wheel. The drive wheel and the shaft should be taper reamed at assembly to ensure proper fit. In addition, receiving inspection should be more critical of the components and accept only those that meet specifications.

A 4340 steel shaft, the driving member of a large rotor subject to cyclic loading and frequent overloads, broke after three weeks of operation. The driving shaft contained a shear groove at which the shaft should break if a sudden high overload occurred, thus preventing damage to an expensive gear mechanism. The rotor was subjected to severe chatter, which was an abnormal condition resulting from a series of continuous small overloads occurring at a frequency of around three per second. Investigation (visual inspection, hardness testing, and hot acid etch images) supported the conclusion that the basic failure mechanism was fracture by torsional fatigue, which started at numerous surface shear cracks, both longitudinal and transverse, that developed in the periphery of the root of the shear groove. These shear cracks resulted from high peak loads caused by chatter. The shear groove in the shaft had performed its function, but at a lower overload level than intended. Recommendations included increasing the fatigue strength of the shaft by shot peening the shear groove to minimize chatter.

An LNG tanker experienced a fracture of the solid tail shaft, which is a section of the main drive shaft. The tail shaft was made of a forged low-carbon steel. In spite of two ultrasonic inspections, a large defect the size of a football in the center of the shaft was missed. During heat treating following forging, it was surmised that the defect led to the propagation of an internal brittle crack, or clink. A fatigue crack propagated from this origin to the outer surface of the shaft after about a year of service. Finally a last ligament of a few square inches held the shaft together and broke, leading to the separation of the shaft. The cause of failure was fatigue crack initiation and crack growth under reverse bending cyclic stresses. There was no indication that misalignment existed because there was no indication of fretting at the bolt holes in the flange at the end of the shaft. In the case of this shaft, a solution would have been to machine the core of the shaft to remove the brittle material or to use a tubular shaft.

The drive shaft on a centrifugal pump fractured after five months of operation. The fracture occurred at a packing gland inside a sleeve against which the packing material sealed. The shaft and sleeve were of duplex stainless steel. In contrast to a previous conclusion that heating had caused property deterioration resulting in embrittlement and fracture, it was concluded that the shaft must have fractured (most probably by fatigue cracking originating at the change of section) and that heating had then taken place from friction between the rotating input shaft and the remaining part attached to the pump. High temperature was thus a result, not the cause, of the failure.

A steel eyebolt which attached a rear lift strut to the right wing of a helicopter failed by fatigue. As a contributing factor, thread cutting produced sharp notches at thread roots, reducing fatigue life. Also, design fatigue life may have been exceeded as the part was in use about 10,000 h. Cumulative damage resulting from a previous accident could have occurred too. Because of this accident, inspectors were instructed to examine threaded zones of eyebolts by magnetic particle inspection after every 100 h in service. A maraging steel drive shaft of a helicopter also failed because of corrosion (pits), and continuous abnormal misalignment as well. Corrosion probably developed from moisture and water droplets on shaft diaphragm profiles. Improved diaphragm pack seals and coatings made by an electron-coat process (such as a Sermetal finish) are now used in new shafts.

Crane collapse due to bolt fatigue and fatigue failure of a crane support column, crane tower, overhead yard crane, hoist rope, and overhead crane drive shaft are described. The first four examples relate to the structural integrity of cranes. However, equipment such as drive and hoist-train components are often subject to severe fatigue loading and are perhaps even more prone to fatigue failure. In all instances, the presence of fatigue cracks at least contributed to the failure. In most instances, fatigue was the sole cause. Further, in each case, with regular inspection, fatigue cracks probably would have been detected well before final failure.

The drive-shaft hanger bearings failed after 300 to 400 h in service. The shaft, supported by labyrinth-sealed single row radial ball bearings of ABEC-1 tolerances, was made of aluminum 2024-T3 tubing (2.5 cm diam and 1.2 mm wall thickness). The bearings were lubricated with a paste-type mineral-oil lubricant (containing molybdenum disulfide and polytetrafluoroethylene particles) or grease conforming to MIL-G-81322 (containing thickening agent and synthetic hydrocarbons) and had two-piece spot-welded retainers. On visual examination, the balls were observed to be embedded in the inner-ring raceway which had been softened by the elevated temperatures reached during the failure. Broken retainers and worn and bent out of shape seals were found. Penetration of gritty particles, water and other corrosive agents and leakage of lubricant out of the bearing permitted by the worn seals was observed. It was concluded that overheating was caused by lubricant flow was permitted by wear of the labyrinth seals. Positive rubbing seals and MIL-G-81322 grease lubricant were found to have longer life than those with the labyrinth seals and mineral-oil-paste lubricant on testing under simulated environmental conditions and were installed as a corrective measure. Importance of dirt free supply and drainage of oil was discussed.

The flanged bearing bush carrying the drive shaft of a feed pump suddenly fractured after about two years of service. The chemical composition was normal for high chromium ledeburitic cast steel, which was corrosion and wear resistant as well as refractory. For unknown reasons the rotating shaft came into direct contact with the flange. Mechanical friction caused a rise in temperature on both contact surfaces. This mutual contact lasted long enough for the temperature in the contact zone to exceed 1200 deg C, at which the flange material became softened or molten. As a result, considerable structural changes took place on the inner wall of the flange. Thermal stresses and excessive mechanical loads due to smearing of the flange material then led to fracture of the flange.

Replacement sprockets installed on chain drive shafts for winding fibers exhibited excessive wear. Metallographic and chemical analyses conducted on the original and replacement sprockets showed that the material of the replacement sprocket was 1020 low-carbon steel, whereas the original (and specified) material was medium-carbon 1045 steel. The low-carbon steel also had lower hardness because of a lower pearlite fraction in the microstructure. It was recommended that replacement sprockets be made of normalized 1045 steel. It was further suggested that wear resistance could be improved by through hardening or induction surface hardening of the teeth.

A carbon steel ball-peen hammer ejected a chip that struck the user's eye. Failure occurred when two hammers were struck together during an attempt to free a universal joint from an automotive drive shaft. Two samples were cut from the face of the hammer one through the chipped area on the chamfer and the other from the undamaged area on the chamfer. The shape and texture of the fracture surfaces were typical of spalling. The fracture was conchoidal and exhibited a complete lack of plastic deformation. White etching bands that intersected the face and chamfer were revealed during metallographic examination. Fracture occurred through a white band. Failure was attributed to formation of envelopes of untempered martensite under the chamfer that ruptured explosively during service.