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.

A new crane failed during the overload test following erection. A test load of 5 tons at the end of the jib (rated capacity 4 tons) was in the process of being slewed at the time of this failure. Inspection revealed that the collapse had resulted from the opening out of one eye of the rimming steel tie-bar of the main jib at the lower splice. This permitted the pin to pass through and allowed the jib to fall. Examination subsequently revealed that brittle fracture of two of the corner angles of the tower head assembly had also occurred. Had the tie-bar material been of satisfactory quality and/or, if the end that failed had been flamecut instead of sheared, then the damage resulting from the excessive overload would have been limited to yielding of the material in the region of the pin-joint. Such yielding on an overload test further indicated that the scantlings of the pin-joints were inadequate. Two other crane failures showed that failure resulted from the use of rimming steel, and embrittlement of the material was evident.

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.

While a tower crane was slewing with a full load of concrete at an 80 ft radius, the operator applied the emergency brake to avert collision with another crane. One of the main square tubular uprights split over a short distance and a number of the adjacent bracings had broken away. Additional cracking had developed along one of the corner welds of the upright. Sections taken through the upright in various planes revealed a laminar defect. The through-thickness crack coincided with one end of the defect. The fracture was therefore due to separation having occurred partially in the plane of the defect. This particular tubular bracing was attached initially to virtually half of the section thickness for a little less than half of its circumference and a zone of intrinsic weakness was introduced. In service, fatigue or corrosion fatigue cracks occurred at the toes of the welds, at the corner of the box section and progressively at other positions. Final and sudden failure occurred under the excessively severe torsional stresses introduced when the emergency brake was applied.

During the lifting of a piece of machinery by means of an overhead travelling crane the hook fractured suddenly. The load was attached to the hook by means of fiber rope slings and rupture occurred in a plane which appeared to coincide with the sling loop nearest to the back of the hook. The rated capacity of the crane was 15 tons. At the time of the mishap it was being used to lift one end of a hydraulic cylinder with a total weight of about 27 tons. Fracture was of the cleavage type throughout. There was no evidence of any prior deformation of the material in the vicinity, nor was there any indication of a pre-existing crack or major discontinuity at the point of origin. A sulfur print suggested the hook had been forged from a billet cogged down from an ingot of semi-killed steel. Failure of this hook was attributed to strain-age embrittlement of the material at the surface of the intrados.

The fracture of a clevis which formed part of the derricking gear of a large crane showed well developed conchoidal markings. These were associated with a principal origin on the outer surface at about the mid-width of the section. A number of secondary origins were apparent along this same edge. Failure was initiated at the extrados which suggested that a discrepancy in the size of the pin may have contributed to failure. Microscopic examination of a section through the main origin did not reveal any material defects or the presence of weld repairs which could have led to the premature failure. Furthermore, there were no indications that corrosion had contributed to the fatigue cracking.

A 140 ft. (42.7 m) long boom on a dragline crane used in coal strip-mining operations failed. One of the principal load-bearing longitudinal beams or chords of the trussed boom had fractured adjacent to a bolt hole at a location about halfway along the length of the boom. Over the lifetime of the crane, several repairs had been made to the boom. At least a year before the failure, a reinforcing gusset plate had been bolted and welded to this chord at this location. Stereomicroscopy revealed microcracks in the weld metal. A fatigue crack 45 mm (1.8 in.) long was observed to emanate from this microcrack. Scanning electron microscopy showed an overload crack extended across the remaining cross section of the chord. It was concluded that the presence of the bolt hole used to attach the gusset plate to the chord created a stress riser adjacent to the hole. Repeated high tensile stresses on the chord during the lifting of enormous loads initiated a fatigue crack in the weld region adjacent to the bolt hole.

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.

A truck-mounted hydraulic crane had a horizontal thrust bearing with one race attached to the truck and the other to the rotating crane. The outside race of the bearing was driven by a pinion gear, and it is through this mechanism that the crane body rotated about a vertical axis. The manufacturer welded the inner race to the carrier in a single pass. After several years of service, the attachment weld between the bearing inner race and the turntable failed in the area adjacent to the heat-affected zone. The fracture zone where there was the greatest tension was heavily oxidized. In the zone where the bearing was in compression, there was a clean surface indicating recent fracture. Finally, there were areas where the weld did not meet AWS specifications for convexity or concavity. These areas were weak enough to allow fatigue cracks to initiate. Recommendations to prevent reoccurrence of the failure include the use of bolts in lieu of welding, a welding schedule that reduces the propensity of lamellar tearing, and the use of an alloy that precludes lamellar tearing. However, if abuse of the crane was the primary cause of failure, none of these recommendations would have prevented deterioration of the machine to an extent that would have rendered the failure improbable.

A crane hook was stamped S.W.L. 3 tons and, while its main dimensions were in approximate accordance with those specified in B.S. 482 for a hook of this capacity, its shape in some respects was not exactly in conformity with that recommended. At the time of fracture, the load being lifted was slightly under 10 cwts. Fracture occurred away from the normal wearing surface where the hook makes contact with the lifting slings. There was no evidence that fracture was preceded by any appreciable deformation locally or in the region of the failure. A sulphur print, taken on a cross section of the hook adjacent to the plane of fracture, showed the hook was made from a killed steel free from major segregation. Microscopic examination showed the material to be a mild steel in the normalized condition, the carbon content being of the order of 0.25%. Bend tests showed the material at the intrados of the hook would deform in a ductile manner both under slow and impact-loading conditions if in the form of an unnotched test piece, but if notched, it failed in a brittle manner under impact, though not under slow loading.

The failure of a 45 Mg (50 ton) rail crane bolster was investigated. Spectrochemical analysis indicated that the material was a 0.25C-1.24Mn-0.62Cr-0.24Mo cast steel. SEM examination revealed the presence of fatigue, as well as intergranular and ductile fractures. Microstructural analysis focused on an area where an antisway device had been welded to the structure and revealed the presence of coarse, untempered martensite that had resulted from faulty weld repair techniques. It was suggested that the use of proper welding procedures, including preheating and postheating, would have prevented the failure.

A bridge wheel on a crane, forged from 1055 steel, fractured after one year of service. The wheel fractured in the web between the hub and the rim. A small area containing beach marks that originated in a heavily burned area on the web surface was revealed by visual examination of the fracture surface. Surface burning to a depth of approximately 0.8 mm was disclosed by metallographic examination of a section taken through the region that contained the beach marks. A forging defect was indicated by the degree of decarburization and oxide dispersion that were visible. The failure was concluded to have been caused by surface burning during the forging operation. As a preventive measure more closely controlled heating practice during forging to eliminate surface burning was recommended. The burnt region was suggested to be removed in case burning occurs.

The crane hook (rated for 13000 kg) failed in the threaded shank while lifting a load of 9072 kg. The metal in the hook was revealed by chemical analysis to be killed 1020 steel. It was disclosed by visual examination that the fracture had at the last thread on the shank and rough machining and chatter marks were evident on the threads. Beach marks that emanated from the thread-root locations on opposite sides of the fracture surface identified these locations to be the origins of the fracture. A medium-coarse slightly acicular structure was revealed by metallographic examination which indicated that the material was in the as-forged condition (which meant lower fatigue strength). The fracture was concluded to have occurred due to stress concentration in the root of the last thread. Normalizing of the crane hook after forging was suggested as a corrective measure. A stress-relief groove with a diam slightly smaller than the root diam was placed at the end of the thread and a large-radius fillet was machined at the change in diameter of the shank.

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.

During the pre-test inspection following the stress calculation check on a 7-ton capacity Scotch derrick crane, it was noted that threads on the back stay anchorage bolts were of unusually fine pitch (11 tpi) and that the machined faces of the nuts showed irregular pits or depressions disposed in an annular manner. When sectioned, the nuts showed a surprising method of construction. The nuts for the bolts had been made by using conventional pipe couplings inserted into sleeves made from hexagonal bar and the coupling secured to the sleeve by welding at each outer face. The ends of the sleeve bore were chamfered to form a weld preparation. After welding, the faces were machined which resulted in the removal of most of the weld metal and revealed a pronounced lack of penetration. All bolts used to anchor derrick crane back stays should be designed in accordance with the recommendations of British Standard 327:1964 (Clauses 10 and 18).

The sudden collapse of a tower crane on a building site resulted in severe injuries to the driver. Failure took place at the upper portion of the foundation or lowermost section. The mast sections were constructed from four main corner angles welded to end frames also made from angle sections which were gusseted and fitted with additional doubling plates in the corners where the jointing bolts were fitted. It was evident that the collapse was due to failure of the welds attaching the corner angles to an end frame. Many of the welds at the locations where failure occurred were of poor quality. The corner angles appeared to have been cut slightly shorter than the required dimensions. This was compensated in one case by the use of a weld build-up and in the other three by make-up pieces attached by welds of insignificant dimensions.

JIS SCM435 steel bolts that connected the slewing ring to the base carrier on a truck crane failed during the lifting of steel piles. The bolts were double-ended stud types and had been in operation for 5600 h. Failure occurred in the root of the external thread that was in contact with the first internal thread in the slewing ring. Examination of plastic carbon replicas indicated that failure was the result of fatigue action. Failure was attributed to overloading during service and increased stress concentration on a few bolts due to nonuniform separations around the slewing ring. A design change to achieve equal separation between bolt holes was recommended.

Failure analysis of a mobile harbor crane wheel hub that included SEM and EDS analyses demonstrated that the mechanism of failure was fatigue. The wheel hub was a ductile cast iron component that had been subjected to cyclic loading during a ten-year service period. The fracture surface of the fatigue failure also contained corrosion deposit, suggesting that cracking occurred over a period of time sufficient to allow corrosion of the cracked surfaces. Replacement and alignment of the failed wheel hub was recommended along with inspection of the nonfailed wheel hubs that remained on the crane.

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.