In addition to failures in shafts, this article discusses failures in connecting rods, which translate rotary motion to linear motion (and conversely), and in piston rods, which translate the action of fluid power to linear motion. It begins by discussing the origins of fracture. Next, the article describes the background information about the shaft used for examination. Then, it focuses on various failures in shafts, namely bending fatigue, torsional fatigue, axial fatigue, contact fatigue, wear, brittle fracture, and ductile fracture. Further, the article discusses the effects of distortion and corrosion on shafts. Finally, it discusses the types of stress raisers and the influence of changes in shaft diameter.

The types of metal components used in lifting equipment include gears, shafts, drums and sheaves, brakes, brake wheels, couplings, bearings, wheels, electrical switchgear, chains, wire rope, and hooks. This article primarily deals with many of these metal components of lifting equipment in three categories: cranes and bridges, attachments used for direct lifting, and built-in members of lifting equipment. It first reviews the mechanisms, origins, and investigation of failures. Then the article describes the materials used for lifting equipment, followed by a section explaining the failure analysis of wire ropes and the failure of wire ropes due to corrosion, a common cause of wire-rope failure. Further, it reviews the characteristics of shock loading, abrasive wear, and stress-corrosion cracking of a wire rope. Then, the article provides information on the failure analysis of chains, hooks, shafts, and cranes and related members.

A shaft which carried the diverter sheave wheel of an electric goods lift failed, resulting in the cage failing to the bottom of the well. Failure had taken place at a reduction in diam at which no filet radius existed. Metallurgical examination did not disclose any abnormal features. The material was a mild steel in the normalized condition. The appearance of the fracture indicated failure was due to bending stresses. The absence of any fillet radius at the reduction in diam provided a region of stress concentration from which fatigue cracks developed.

A drum pinion shaft (1030 steel) which was part of the hoisting gear of a crane (capacity 18,140-kg) operating in a blooming mill failed while lifting a 9070 kg load. Chatter marks, rough-machining marks, and sharp corner radii were revealed in the keyway which extended into a shoulder at a change in diam. A circular recess below the keyway surface was revealed at each end of the keyway. A sharp corner at the end of the keyway was revealed by examination to be the origin of fracture. Beach marks were found radiating from the origin over a large portion of the fracture surface which confirmed failure of the shaft by fatigue fracture. As a corrective measure the shaft was replaced with one made of 4140 steel, quenched and tempered to a hardness of 286 to 319 HRB. The keyway was moved away from the change in section and was machined with a 1.6-mm radius in the bottom corners and a larger-radius fillet was machined at the change in section.

An 8640 steel shaft installed in a fuel-injection-pump governor that controlled the speed of a diesel engine used in trucks and tractors broke after few days of operation. The mechanism that drove the shaft was designed to include a slip clutch to protect the governor shaft from shock loading. It was revealed by visual examination that the fracture had initiated in the sharp corner at the bottom of a longitudinal hole which was part of a force feed lubricating system. Beach marks were observed on the fracture surfaces. It was revealed by further examination that the slip clutch was removed in an effort to reduce cost and hence the shaft was subjected to increased vibration and shock loading. Insufficient fatigue limit of the shaft was revealed by fatigue testing of the shafts taken from stock in a rotating-beam machine. As a corrective measure, the fatigue limit of shafts was increased to 760 MPA by nitriding for 10 h at 515 deg C.

Two vertical coal-pulverizer shafts at a coal-fired generation station failed after four to five years in service. One shaft was completely broken, and the other was unbroken but cracked at both ends. shaft material was AISI type 4340 Ni-Cr- Mo alloy steel, with a uniform hardness of approximately HRC 27. Metallographic examination of transverse sections through the surface-damaged areas adjacent to the cracks also showed additional small cracks growing at an angle of approximately 60 deg to the surface. The crack propagation mode appeared to be wholly transgranular. SEM examination revealed finely spaced striations on the crack surfaces, supporting a diagnosis of fatigue cracking. Crack initiation in the pulverizer shafts started as a result of fretting fatigue. Greater attention to lubrication was suggested, combined with asking the manufacturer to consider nitriding the splined shaft. It was suggested that the surfaces be securely clamped together and that an in-service maintenance program be initiated to ensure that the tightness of the clamping bolts was verified regularly.

After being in service for ten years the ball-and-race coal pulverizer was investigated after noises were noted in it. Its lower grinding ring was attached to the 6150 normalized steel outer main shaft while the upper grinding ring was suspended by springs from a spider attached to the shaft. A circumferential crack in the main shaft at an abrupt change in shaft diam just below the upper radial bearing was revealed by visual examination. The smaller end of the shaft was found to be slightly eccentric with the remainder when the shaft was set up in a lathe to machine out the crack for repair welding. The crack was opened by striking the small end of the shaft and the shaft was broken 1.3 cm away from the crack in the process. A previous fracture that resulted from torsional loading acting along a plane of maximum shear was revealed almost perpendicular to the axis of the shaft. Faint lines parallel to the visible crack thought to be fatigue cracks were revealed on examination of the machined surface. The shaft was repaired by welding a new section and machined to required diameters and tapers to avoid abrupt changes.

Another failure in a turbogenerator, similar to the accidents in Toronto described in Metal Progress in July 1956, was due to the presence of fatigue cracks at ventilating holes. These acted as stress-raisers during temporary and minor overspeeding, inducing an almost instantaneous brittle failure which wrecked the machine, fortunately without human casualty.

A shaft can crack twice before it fails. A Detroit electric plant had this experience with one in a coal pulverizer. Because the first crack rewelded partially (by friction) in service, the pulverizer remained serviceable until the second crack developed.

A large fan assembly deformed and broke at multiple locations. The user wanted to know whether the bearing pillow block fracture caused the fan blade assembly to crack, or whether a fan blade assembly fracture caused the pillow block to crack. Close inspection of the entire length of the crack showed the crack probably grew quite a while before it was large enough to cause the final catastrophic event. No evidence of fatigue cracks was visible on the broken pillow blocks. In the absence of some other contradictory information, the usual conclusion would be to presume that the fatigue crack predated the single overload crack.

An intermediate shaft (3 in. diam), part of a camshaft drive on a large diesel engine, broke after two weeks of service. Failure occurred at the end of the taper portion adjacent to the screwed thread. The irregular saw-tooth form of fracture was characteristic of failure from torsional fatigue. A second shaft carried as spare gear was fitted and failure took place in a similar manner in about the same period of time. Examination revealed that the tapered portion of the Fe-0.6C carbon steel shaft had been built up by welding prior to final machining. A detailed check by the engine-builder established that the manufacture of these two shafts had been subcontracted. It was ascertained that the taper portions had been machined to an incorrect angle and then subsequently built-up and remachined to the correct taper. The reduction in fatigue endurance following welding was due to heat-affected zone cracking, residual stresses, the lower fatigue strength of the weld deposited metal, and weld defects.

The shaft of an exciter that was used with a diesel-driven electric generator broke at a fillet after ten hours of service following resurfacing of the shaft by welding. The fracture surface contained a dull off-center region of final ductile fracture surrounded by regions of fatigue that had been subjected to appreciable rubbing. The fracture appeared to be typical of rotary bending fatigue under conditions of a low nominal stress with a severe stress concentration. It appeared that the fatigue cracks initiated in the surface-weld layer. The weld deposit in the original keyway displays a lack of fusion at the bottom corner. Fatigue fracture of the shaft resulted from stresses that were created by vibration acting on a crack or cracks formed in the weld deposit because of the lack of preheating and postheating. Rebuilding of exciter shafts should be discontinued, and the support plate of the exciter should be braced to reduce the amount of transmitted vibration. Also, the fillet in the exciter shaft should be carefully machined to provide an adequate radius.

Field failures, traced to internal cracks that were initiated from gross nonmetallics, were encountered in the upset portion of forged 4118 steel shafts. Ultrasonic inspection was thought to be the best method for detection from the location of these cracks, their orientation, and the size of the shaft. A longitudinal beam was sent in from the end of the shaft. The shaft was observed to have a radially drilled oil hole 9 mm in diam. Since there was a variation in flaw orientation, testing of the shaft was desired from both the long and short end. The rejection level was set at 20% of full screen and was based on the size of flaws observed when the shafts were cut up. The inclusions were considered to be rejectable if the size was larger than 20 mm diam. Similar flaws were observed in larger shafts, but no flaws were observed once the shafts were sectioned. It was interpreted that the flaw signals were false and had happened when a portion of the beam struck the oily surface of the longitudinal oil hole. The problem was solved by removing the oil film from the longitudinal oil hole.

Shaft fracture of a 10 hp squirrel cage motor took place at the driving end just outside the roller bearing and not at an abrupt change of section behind the bearing where it might be expected to occur. A portion of shaft to the right of the fracture was deeply grooved. About a year prior to failure the inner race of the roller bearing became slack on the shaft and the seating was built up by the metal-spray process. The shaft was machined to form a rough thread to provide the requisite mechanical key for the sprayed-on metal. Part of this sprayed-on layer became detached after the fatigue failure occurred. The quality of the welding was poor. Slag inclusions were present adjacent to the sides of the keyway, which had been re-cut shorter than the original one after the welding repair. Failure at the unusual location was caused by the presence of the weld deposit.

The shaft-and-bearing assembly in a freon compressor was subjected to severe pounding and vibration after six years of service. After about one year of service, the compressor had been shut down to replace a bearing seal. One month before the shaft failed, a second seal failure occurred, requiring the collar, spacer sleeve, seal, roller bearing, and lock washer to be replaced. The shaft was made of 4140 steel, heat treated to a hardness of 20 to 26 HRC. The seal, bearing, and lock washer were commercial components. Investigation (visual inspection, 4.5x images, x-ray diffraction, hardness testing, and microscopic exam) supported the conclusion that shaft failure was initiated by fretting between the bearing race and the bearing surface on the shaft because of improper bearing installation. Once clearance was established between the bearing and the shaft, the shaft began pounding on the inner bearing race, causing final failure of the shaft surface. Recommendations included proper fitting of the shaft and bearing race to preventing movement of the bearing on the shaft. Also, the lock washer and locknut must be installed properly.

High-horsepower electric motors were utilized to drive large compressors (made of 4340 steel shafts and gear-type couplings) required in a manufacturing process. The load was transmitted by two keys 180 deg apart. Six of the eight compressor shafts were found cracked in a keyway and one of them fractured after a few months of operation. Visual examination of fractured shaft revealed that the cracks originated from one of the keyways and propagated circumferentially around the shaft. The shaft and coupling slippage was indicated by the upset keys and this type of fracture. The shaft surface both near and in the keyways indicated fretting which greatly reduced the fatigue limit of the shaft metal and initiated fatigue cracks. Fatigue marks were observed on the fractured key. Repetitive impact loading was responsible for propagation of the cracks. The high cyclic bending stresses were caused by misalignment between the electric motor and compressor and were transmitted to the shaft through the geared coupling. Flexible-disk couplings capable of transmitting the required horsepower were installed on the shafts as a corrective measure.

The crankshaft of a compressor fractured through the web remote from the driving end after about three years of service. The fracture ran diagonally across the web into the crankpin. It passed through the centers of two screwed plugs inserted into the web from opposite faces approximately in line with the crankpin center line. The fracture was of the fatigue type, slowly developing cracks having started from opposite sides of each tapped hole and crept across the section. Microstructure of the crankshaft indicated the material was a plain carbon steel, the carbon content being of the order of 0.3%. The failure resulted principally from the stress-raising effects of the screw holes combined with the cracks in the welds. If the screw holes had been left unfilled or if some form of mechanical locking had been used if plugged, failure would have been postponed if not averted.

A 3 in. diam shaft was found to have suffered excessive wear on one of the journals and was built up by welding. While it was in the lathe prior to turning down the built-up region, a crack was discovered in the root of the oil-seal groove and subsequently the end of the shaft was broken off with hammer blows. The fracture surface was duplex in nature, there being an annular region surrounding a central zone, which suggests that the fracture developed in two stages. Microscopic examination confirmed that the fracture was of the brittle type. The shaft material showed a microstructure typical of a medium-carbon steel (carbon approximately 0.4%) in the normalized condition, a material not weldable by ordinary methods. It was concluded that the post-welding crack arose primarily from the thermal contraction which developed in the weld metal on cooling. It is probable that if the built-up zone had extended beyond the oil seal groove, failure in the manner would not have occurred. Experience indicated however, that failure from fatigue cracking would still have been likely to occur.

Routine magnetic-particle inspection revealed crack indications in a number of shafts produced from hot-rolled 4130 steel bar. A pronounced indication of this size is cause for rejection if the defect is not eliminated during subsequent machining. A microstructural analysis of the shaft cross section revealed that the crack was approximately 0.5 mm (0.020 in.) deep and oriented in a radial direction. Furthermore, no stringer-type nonmetallic inclusions were observed in the vicinity of the flaw, which did not display the intergranular characteristics of a quench crack. The defect did, however, contain substantial amounts of oxide, which evidently resulted from the hot-working operation. This evidence supports the conclusion that the appearance of this discontinuity, with the long axis parallel to the working direction and radial orientation with regard to depth, strongly suggests a seam produced during rolling. Use of components with surface-defect indications as small as 0.5 mm (0.02 in.) can be risky in certain circumstances. Depending on the orientation of the flaw with respect to applied loads, the nature of the applied forces (for example, cyclic), and the operating environment, such a surface flaw can become the initiating site for a fatigue crack or a corrosion-related failure.

An amusement ride failed when a component in the ride parted, permitting it to fly apart. The ride consisted of a central shaft supporting a spider of three arms, each of which was equipped with an AISI 1040 steel secondary shaft about which a circular platform rotated. The main shaft rotated at about 12 rpm and the platforms at a speed of 20 rpm. The accident occurred when one of the secondary shafts on the amusement ride broke. The point of fracture was adjacent to a weld that attached the shaft to a 16 mm thick plate, which in turn bore the platform support arms. Investigation (visual inspection, 0.4x magnification, and stress analysis) supported the conclusion that a likely cause for the fatigue failure was the combination of residual stresses generated in welding and centrifugal service stresses from operation that were accentuated by areas of stress concentration at the undercut locations. Without the excessive residual stress, the shaft dimensions appeared ample for the service load. Recommendations included applying the fillet weld with more care to avoid undercutting. The residual stresses could be minimized by pre-weld and post-weld heat application.