Search Results for Bending fatigue

Two A6 tool steel (free machining grade) shafts, parts of a clamping device used for bending 5.7 cm OD tubing on an 8.6 cm radius, failed simultaneously under a maximum clamping force of 54,430 kg. The shaft was imposed with cyclic tensile stresses due to the clamping force and unidirectional bending stresses resulting from the nature of operation. Nonmetallic oxide-sulfide segregation was indicated by microscopic examination of the edge of the fracture surface. Both smooth and granular areas were revealed on visual examination of the fracture. The shaft was subjected to a low overstress as the smooth-textured fatigue zone was relatively large compared with the crystalline textured coarse final-fracture zone. The fatigue crack was nucleated by the nonmetallic inclusion that intersected the surface and initiated in the 0.25 mm radius fillet at a change in section due to stress concentration. To minimize this stress concentration, a larger radius fillet shaft at the critical change in section was suggested as corrective measure.

A pushrod made by inertia welding two rough bored pieces of bar stock installed in a mud pump fractured after two weeks in service. The flange portion was made of 94B17 steel, and the shaft was made of 8620 steel. It was disclosed by visual examination that the fracture occurred in the shaft portion at the intersection of a 1.3 cm thick wall and a tapered surface at the bottom of the hole. The fatigue crack was influenced by one-way bending stresses initiated at the inner surface and progressed around the entire inner circumference. A heavily decarburized layer was detected on the inner surface of the flange portion and sharp corner was found at the intersection of the sidewall and bottom of the hole. It was concluded that the stress raiser due to the abrupt section change was accentuated by decarburized layer. As a corrective measure, the design of the pushrod was changed to a one-piece forging and circulation of atmosphere during heat treatment was permitted through a hole drilled in the flange end of the rod to avoid decarburization.

A failed spiral gear and pinion set made from 4320H Ni-Cr-Mo alloy steel operating in a high-speed electric traction motor gear unit driving a rapid transit train were submitted for analysis. The pinion was intact, but the gear had broken into two sections that resulted when two fractured areas went through the body of the gear. Wheel mileage of the assembly was 34,000 miles at the time of failure. All physical and metallurgical characteristics were well within specified standards, and both parts should have withstood normal loading conditions. The primary mode of failure was tooth bending fatigue of the gear from the reverse direction near the toe end. The cause of failure was a crossed-over tooth bearing condition that placed loads at the heel end when going forward and at the toe end when going in reverse. The condition was too consistent to be a deflection under load; therefore, it most likely was permanent misalignment within the assembly.

A portion of a 19 mm (0.75 in.) diam structural steel bolt was found on the floor of a manufacturing shop. This shop contained an overhead crane system that ran on rails supported by girders and columns. Inspection of the crane system revealed that the bolt had come from a joint in the supporting girders and could be considered one of the principal fasteners in the track system. Analysis (visual inspection, metallographic exam, and hardness testing) supported the conclusions that fatigue induced by the overhead movement of the crane produced failure of the bolt. The bolt was deficient in strength for the cyclic applied loads in this case and probably was not tightened sufficiently. Recommendations included removing the remaining bolts in the crane support assembly and replacing them with a higher-strength, more fatigue-resistant bolt, for example, SAE grade F, 104 to 108 HRB. The bolts should be tightened according to the specifications of the manufacturer, and the system should be periodically inspected for correct tightness.

A 13 mm diam 18 x 7 fiber-core improved plow steel nonrotating wire rope, brought into service as a replacement for 6 x 37 improved plow steel ropes, failed after 14 months of service on a stacker crane. The change was reported to have been caused by difficulties twisting of the 6 x 37 rope. The hoist arrangement for this crane was found to consist of one rope with each end attached to a separate drum and the rope was wound around two 30-cm diam sheaves in the block and back up around an equalizer sheave. The rope section that had been in contact with the sheaves was deduced by measurement checks. The presence of broken wire ends, which indicated that the rope failed by fatigue, was revealed by reverse bending of the section of the rope which was normally subjected to this flexing. It was found that minimum sheave diam for a 13-mm 18 x 7 wire rope was 43 cm and hence the currently used smaller sheaves caused excessive bending stresses in the rope. The 18 x 7 rope was replaced by two 6 x 37 side-by-side counter-stranded steel-core ropes as a corrective measure.

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.

A spiral bevel gear set in the differential housing of a large front-end loader moving coal in a storage area failed in service. The machine had operated approximately 1500 h. Although the failure involved only the pinion teeth, magnetic particle inspection was performed on each part. The 4817 NiMo alloy steel pinion showed no indication of additional cracking, nor did the 4820 NiMo alloy steel gear. The mode of failure was tooth bending fatigue with the origin at the designed position: root radius at midsection of tooth. The load was well centered, and progression occurred for a long period of time. The cause of failure was a suddenly applied peak overload, which initiated a crack at the root radius. Progression continued by relatively low overstress from the crack, which was now a stress-concentration point. This was a classic tooth bending fatigue failure.