Search Results for Agricultural equipment

The 4140 steel steering spindle on a tricycle agricultural field chemical applicator failed, causing the loss of the front wheel and overturn of the vehicle. The spindle was a solid 120 mm (4.75 in.) diam forging. It had been machined to 115 mm (4.5 in.) in diameter to fit tightly inside a collar at one point and to 90 mm (3.5 in.) for attachment to the steering mechanism at another. Visual examination showed that the spindle fractured at the fillet welds that attached it to the collar. Macrofractography and metallography revealed that the failure initiated at the root of a weld that bridged a wide gap. The most probable cause of failure was improper preheat during welding.

A ‘worn-out’ spiral bevel gear and pinion set was submitted for examination and evaluation. This was a spiral bevel drive set with the gear attached to a differential. The assembled unit was driving a new, large, experimental farm tractor in normal plowing and tilling operations. The primary failure was associated with the 4820H NiMo alloy steel pinion, and thus the gear was not examined. The mode of failure was rolling contact fatigue, and the cause of failure improper engineering design. The pattern of continual overload was restricted to a specific concentrated area situated diagonally across the profile of the loaded side, which was consistent on every tooth.

An agricultural tine, which is a relatively large double torsion spring with outer legs that are used to sweep through hay or other crops and turn them over, had failed. It was made hard-drawn carbon steel. Bending fatigue was revealed by visual examination to be almost certainly the cause of failure. The fatigue fracture origin was found on the inside surface of the legs at the point where they joined the coiled body of the spring. It was established that the tines after being wound up by loading with hay, sprung back through the neutral unloaded position and into the unwind direction. This movement into the unwind direction was concluded to be happening often enough to initiate fatigue. The stress relieving temperature was recommended to be increased to reduce the residual stresses from coiling and hence improve fatigue performance.

A support arm on a front-end loader failed in a brittle manner while lifting a load. The arm had a cross section of 50 x 200 mm (2 x 8 in.). Material used for the arm was hot-rolled ASTM A572, grade 42 (type 1), steel, which exhibited poor impact properties in the as-rolled condition and had a ductile-to-brittle transition temperature exceeding 93 deg C (200 deg F). This transition temperature was much too high for the application. It was recommended that a modified ASTM A572, grade 42 (0.15% C max), type 1 or 2, steel be used (type 1, which contains niobium, may be needed to meet strength requirements). The steel should be specified to be killed, fine-grained, and normalized, with Charpy V-notch impact-energy values of 20 J (15 ft·lbf) at -46 deg C (-50 deg F) in the longitudinal direction and 20 J (15 ft·lbf) at -29 deg C (-20 deg F) in the transverse direction.

Fractures and a crack occurred in a length of excavator boom rope. Failure took place at regions where local corrosion was evident. Microscopic examination of longitudinal sections disclosed that the majority of the cracks were broad, these being typical of corrosion-fatigue fissures. In addition, cracking took the form more typical of a fatigue crack and appeared to have originated at a stress-raiser introduced by a corrosion pit on the surface of the wire. The tendency for corrosion-fatigue cracking or the formation of pits from which fatigue cracks can develop can he reduced, if not prevented, in wire ropes by regular attention to lubrication.

The horizontal cross-travel shaft on a derrick failed after two years of service. The shaft was required to be made of 4140 steel quenched to a hardness of 302 to 352 HRB. The shaft was found to have fractured approximately 13 mm from the change in section between the splined end and the shaft proper. The cracks were found to have propagated in the longitudinal and transverse directions until failures occurred. It was showed by a transverse section through the spline that the longitudinal cracks were initiated at the sharp corners at the roots of the spline teeth. The shaft was subjected to reverse torsional loading by the operation of the derrick and the shaft fatigue fracture was caused by this. The fillets at the roots of the spline teeth were increased in size and polished to minimize stress concentrations in these areas.

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 10-cm (4-in.) chain link used in operating a large dragline bucket failed after several weeks in service. The link was made of cast low-alloy steel (similar to ASTM A487, class 10Q) that had been normalized, hardened, and tempered to give a yield strength of approximately 1034 MPa (150 ksi). A hydrogen flake approximately 5 cm (2 in.) in diam was observed at the center of the fracture surface. Beach marks indicative of fatigue encircled the hydrogen flake and covered nearly all of the remaining fracture surface. The failure of this linkways caused by an excessive hydrogen content. Two steps were taken to combat this type of failure. First, when service conditions did not require high hardness to combat wear, the links were produced of a steel having a yield strength of about 690 MPa (100 ksi) rather than 1034 M

A shaft, which carried both a worm wheel and hoist barrel, fractured at a reduction in diameter adjacent to a mating gearbox. The appearance of the fracture was characteristic of a fatigue failure of a rotating shaft resulting from excessive bending stresses. Cracks of the fatigue type broke out all around the circumference at the change of section and progressed inwards. Microscopic examination of the material showed it to be an alloy steel in the hardened and tempered condition, with no abnormal features. It was considered that the bending stresses due to the deflection of the shaft arising from misalignment were responsible for the fatigue failure, which occurred in a region of stress concentration where insignificant fillet radius had been provided.

Both halves of a gray cast iron transmission housing from a 50-ton dump truck were found to contain numerous cracks. The housing material was possibly G3000 grade designation for automotive gray cast iron. No service duration or material specifications were provided. Investigation (visual inspection, tensile testing, 2% nital etched 59x cross sections, and metallographic analysis) supported the conclusion that failure was due to applied stresses sufficient to fracture the castings which exhibited brittle overload cracks at highly stressed locations. No recommendations were made.

Two type 420 martensitic stainless steel load cell bodies, which had been installed under two of the four legs of a milk storage tank failed in service. The failure occurred near a change in section and involved fracture of the entire cross section. Examination showed a brittle fracture that was preceded by a small fatigue region. Pitting corrosion was evident at the fracture origin. The areas around the load cells had been subjected to regular washdowns using high-pressure hot water, and the pitting was attributed to crevice corrosion between the load cell and the holddown bolts. Prevention of such corrosion by the use of a flexible sealant to eliminate the crevice was recommended.

The 14-cm diam main hoist shaft of a mobile shovel was found to have multiple crack indications when ultrasonically inspected in the field. A crack around the entire circumference at the change in section was revealed by magnetic-particle inspection of the shaft. The crack was found to coincide with the junction of the fillet and the smaller diam at this change in section. A slight step in the continuity of the fillet and some machining marks were noted at this junction. A fine crack extending 2.5 mm from the surface and originating at the machining marks was revealed by microscopic examination. The shaft was identified by chemical analysis to be 1040 steel (hardness 170 HRB) which was concluded to have insufficient fatigue strength. The step at the base of the fillet was revealed as the point of initiation of the fatigue crack. Shaft material was changed to 4140 steel oil-quenched and tempered to a hardness of 302 to 352 HRB and all machining discontinuities were removed.

Failure occurred in the teeth of a case-hardening Ni-Cr-Mo alloy steel spur gear in the transmission system of heavy duty tracked vehicles. The defects were in the nature of seizure on the involute profile. Scrutiny of the transmission system showed there might be choking in the lubricating oil line. Such would cause seizure of the gears and damage. The incidence of such defects stopped after corrective measures were taken.

A cast dragline bucket tooth failed by fracturing after a short time in service. The tooth was made of medium-carbon low-alloy steel heat treated to a hardness of 555 HRB. The fracture surface was covered with chevron marks. These converged at several sites on the surface of the tooth. A hardfacing deposit was located at each of these sites. Visual inspection of the hardfacing deposits revealed numerous transverse cracks, characteristic of many types of hardfacing. This failure was caused by cracks present in hardfacing deposits that had been applied to the ultrahigh-strength steel tooth. Given the small critical crack sizes characteristic of ultrahigh-strength materials, it is generally unwise to weld them. It is particularly inadvisable to hardface ultrahigh-strength steel parts with hard, brittle, crack-prone materials when high service stresses will be encountered. The operators of the dragline bucket were warned against further hardfacing of these teeth.

A steering knuckle used on an earthmover failed in service. The component fractured into a flange portion and a shaft portion. The flange was 27.9 cm (11 in.) in diam around which there were 12 evenly spaced 16 mm diam bolt holes. The shaft was hollow with a 10.5 cm (4 in.) OD and a wall thickness of 17 mm. The steering knuckle was made of 4340 steel and heat treated to a hardness of about 415 HRB (yield strength of about 1069 MPa, or 155 ksi). The vehicle had been involved in a field accident six months before the steering knuckle failed. Several components, including portions of the frame, had been damaged and replaced, but there was no observed damage to the steering. Analysis supported the conclusion that the fracture was the result of the prior accident, the most likely explanation being that the shaft was bent and that continued use caused a crack to initiate and propagate to fracture. No evidence of a defective design, improper microstructure, high inclusion count, or other stress-raising condition was observed. No recommendations were made.

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.

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.

A riveted 0.25% carbon steel oil-storage tank in Oklahoma was dismantled and reassembled in Minnesota by welding to form a storage tank for soybean oil. An opening was cut in the side of the tank to admit a front-end loader. A frame of heavy angle iron was welded to the tank and drilled for bolting on a heavy steel plate. The tank was filled to a record height. In mid-Jan the temperature dropped to -31 deg C (-23 deg F), with high winds. The tank split open and collapsed. The welding used the shielded metal arc process with E6010 electrodes, which could lead to weld porosity, hydrogen embrittlement, or both. At subzero temperatures, the steel was below its ductile-to-brittle transition temperature. These circumstances suggest a brittle condition. Steps to avoid this type of failure: For cold conditions, the steel plate should have a low carbon content and a high manganese-to-sulfur ratio and be in a normalized condition, low-hydrogen electrodes and welding practices should be used, all corners should be generously radiused, the welds should be inspected and ground or dressed to minimize stress concentrations, postweld heating is advisable, and radiographic and penetrant inspection tests should be performed.