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Crankshafts
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Series: ASM Failure Analysis Case Histories
Volume: 3
Publisher: ASM International
Published: 01 December 2019
DOI: 10.31399/asm.fach.v03.c9001752
EISBN: 978-1-62708-241-9
Abstract
Results of failure analyses of two aircraft crankshafts are described. These crankshafts were forged from AMS 6414 (similar composition to AISI 4340) vacuum arc remelted steels with sulfur contents of 0.003% (low sulfur) and 0.0005% (ultra-low sulfur). A grain boundary sulfide precipitate was caused by overheat of the low sulfur steel, and an incipient melting of grain boundary junctions was caused by overheat of the ultra-low sulfur steel. The precipitates and incipient melting in these two failed crankshafts were observed during the examination. As expected, impact fractures from the low sulfur steel crankshaft contained planar dimpled facets along separated grain boundaries with a small spherical manganese sulfide precipitates within each dimple. In contrast, planar dimpled facets along separated grain boundaries of impact fractures from the ultra-low sulfur crankshaft steel contained a majority of small spherical particles consisting of nitrogen, boron, iron, carbon, and a small amount of oxygen. Some other dimples contained manganese sulfide precipitates. Fatigue samples machined from the ultra-low sulfur steel crankshaft failed internally at planar grain boundary facets. Some of the facets were covered with nitrogen, boron, iron, and carbon film, while other facets were relatively free of such coverage. Results of experimental forging studies defined the times and temperatures required to produce incipient melting overheat and facets at grain boundary junctions of ultra-low sulfur AMS 6414 steels.
Series: ASM Failure Analysis Case Histories
Volume: 3
Publisher: ASM International
Published: 01 December 2019
DOI: 10.31399/asm.fach.v03.c9001761
EISBN: 978-1-62708-241-9
Abstract
An investigation of a damaged crankshaft from a horizontal, six-cylinder, in-line diesel engine of a public bus was conducted after several failure cases were reported by the bus company. All crankshafts were made from forged and nitrided steel. Each crankshaft was sent for grinding, after a life of approximately 300,000 km of service, as requested by the engine manufacturer. After grinding and assembling in the engine, some crankshafts lasted barely 15,000 km before serious fractures took place. Few other crankshafts demonstrated higher lives. Several vital components were damaged as a result of crankshaft failures. It was then decided to send the crankshafts for laboratory investigation to determine the cause of failure. The depth of the nitrided layer near fracture locations in the crankshaft, particularly at the fillet region where cracks were initiated, was determined by scanning electron microscope (SEM) equipped with electron-dispersive X-ray analysis (EDAX). Microhardness gradient through the nitrided layer close to fracture, surface hardness, and macrohardness at the journals were all measured. Fractographic analysis indicated that fatigue was the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region, due to over-grinding, caused a decrease in the fatigue strength which, in turn, led to crack initiation and propagation, and eventually premature fracture. Signs of crankshaft misalignment during installation were also suspected as a possible cause of failure. In order to prevent fillet fatigue failure, final grinding should be done carefully and the grinding amount must be controlled to avoid substantial removal of the nitrided layer. Crankshaft alignment during assembly and proper bearing selection should be done carefully.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c0047545
EISBN: 978-1-62708-236-5
Abstract
The AISI 1080 steel crankshaft of a large-capacity double-action stamping press broke in service and was repair welded. Shortly after the crankshaft was returned to service, the repair weld fractured. The repair-weld fracture was examined ultrasonically which revealed many internal reflectors, indicating the presence of slag inclusions and porosity. A low-carbon steel flux-cored filler metal was used in repair welding the crankshaft, without any preweld or postweld heating. This resulted in the formation of martensite in the HAZ. The repair weld failed by brittle fracture, which was attributed to the combination of weld porosity, many slag inclusions and the formation of brittle martensite in the HAZ. A new repair weld was made using an E312 stainless steel electrode, which provides a weld deposit that contains considerable ferrite to prevent hot cracking. Before welding, the crankshaft was preheated to a temperature above which martensite would form. After completion, the weld was covered with an asbestos blanket, and heating was continued for 24 h. During the next 24 h, the temperature was slowly lowered. The result was a crack-free weld.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001230
EISBN: 978-1-62708-236-5
Abstract
A crankshaft was overloaded on a test stand and suffered an incipient crack in the crank pin. The crack run generally parallel to the longitudinal axis and branched off at the entrance into the two fillets at the transition to the crank arm. It consisted of many small cracks, all of which propagated at an angle of approximately 45 deg to the longitudinal axis, and therefore were caused by torsion stresses. Neither macroscopic nor microscopic examination determined any material or processing faults. Experience has shown that torsion vibration fractures of this kind usually appear in comparatively short journal pins at high stresses. This crankshaft fracture was an example of the damage that is caused or promoted neither by material nor heat treatment mistakes nor by defects of design or machining, but solely by overstressing.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001435
EISBN: 978-1-62708-236-5
Abstract
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.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001728
EISBN: 978-1-62708-236-5
Abstract
The propeller from a small private airplane came off in flight. The head ends of all six attachment bolts remained in the propeller hub when it was found. Two threaded shanks with nuts remained with the engine, while the remaining four shank ends with their nuts were missing. Parts available for examination, in addition to the hub and attachment bolts, were the two propeller blades and the engine crankshaft. The purpose of this examination was to determine the nature and probable cause of failure in the six attachment bolts. Indications of fatigue failure and wear were the major findings in visual and low power microscopic examination. Fracture surfaces indicated failure was initiated in the threads in four bolts and in the shanks in two. The group of four bolts failed primarily due to tensile loads, while the other two bolts failed primarily due to bending loads. It was concluded that failure was due to improper installation torqueing of the bolts.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001427
EISBN: 978-1-62708-236-5
Abstract
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.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001722
EISBN: 978-1-62708-236-5
Abstract
The crankshaft of a 37.5-hp, 3-cylinder oil engine was examined. The engine had been dismantled for the purpose of a general overhaul and in the course of this work the crankpins were chromium-plated before regrinding. The engine was returned to service and after running for 290 h the crankshaft broke at the junction of the No. 3 crankpin and the crankweb nearest to the flywheel. A typical fatigue crack had originated at a number of points in the root of the fillet to the web. In its early stages it ran slightly into the web but turned back to the pin when it encountered the oil hole. The shaft had been made from a heat-treated alloy steel. The thickness of the plating was approximately 0.025 in. and numerous cracks were visible in it, several of which had given rise to cracks in the steel below. The primary cause of the crankshaft failure was the plating of the crankpins. The presence of the grooves alone would result in considerable intensification of stress in zones which are normally highly stressed, while the crazy cracking introduced a multiplicity of stress-raisers of a type almost ideal from the point of view of initiating fatigue cracks.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001415
EISBN: 978-1-62708-236-5
Abstract
In the course of a general overhaul, the crankpins and main journals (3 in. diam) of the crankshaft of a four-cylinder oil engine were built up by metal spraying. Four weeks later, the shaft broke through the pin remote from the flywheel (driving) end. The fracture was of the fatigue type. A creeping crack originated in the fillet at the inside surface of the pin and extended parallel to the plane of the web across practically the entire section before complete rupture occurred. The sprayed metal on the fractured pin had very poor adhesion. The surfaces of the main journals had not been grooved but appeared to have been roughened by shot or grit-blasting prior to spraying and the deposit was more firmly adherent to these surfaces than in the case of the pins. It is doubtful, however, whether the adhesion of sprayed metal to a surface prepared even in this manner would always be satisfactory under severe loading conditions, such as those to which a crankpin is subjected in service.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001597
EISBN: 978-1-62708-236-5
Abstract
This case study involves two continuously cast steel crankshaft failures. Three parties performed their own failure analyses: (1) the engine manufacturer responsible for component design, specification, and application; (2) the steel supplier and forging supplier responsible for making the steel, forging the shape, and preliminary heat treatment; and (3) a supplier that provided induction hardening, finish machining, and inspection. An independent engineering firm was subsequently involved, but because each party had its own agenda, there was no agreement on the metallurgical source of the failure and thus no continued analysis to pin down and eliminate the root cause.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.process.c0089646
EISBN: 978-1-62708-235-8
Abstract
Nodular cast iron crankshafts and their main-bearing inserts were causing premature failures in engines within the first 1600 km (1000 mi) of operation. The failures were indicated by internal noise, operation at low pressure, and total seizing. Concurrent with the incidence of engine field failures was a manufacturing problem: the inability to maintain a similar microfinish on the cope and drag sides of a cast main-bearing journal. Investigation supported the conclusion that the root cause of the failure was carbon flotation due to the crankshafts involved in the failures showing a higher-than-normal carbon content and/or carbon equivalent. Larger and more numerous cope side graphite nodules broke open, causing ferrite caps or burrs. They then became the mechanism of failure by breaking down the oil film and eroding the beating material. A byproduct was heat, which assisted the failure. Recommendations included establishing closer control of chemical composition and foundry casting practices to alleviate the carbon-flotation form of segregation. Additionally, some nonmetallurgical practices in journal-finishing techniques were suggested to ensure optimal surface finish.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.process.c0047830
EISBN: 978-1-62708-235-8
Abstract
Textile-machine crankshafts forged from 4140 steel fractured transversely on one cheek during one to three years of service. The cause of failure for two forgings (one complete fractured forging and second a section that contained the shorter shaft fracture cheek) was determined. Indication of fatigue failure was revealed by visual examination of the fracture surfaces. Rough grooves from hot trimming of the flash were visible on the surface of the cheeks. The outer face of one cheek of the throw on the forging contained shallow surface folds. Slightly decarburized forged surface was identified around one of the folds and a fatigue crack initiated in the fold and propagated across the cheek. Properties representative of 4140 steel, quenched and tempered to a hardness of 20 to 22 HRC, were observed. Tempered bainite was revealed in the general microstructure. As a corrective measure, the forgings were normalized, hardened and tempered to 28 to 32 HRC before being machined to increase fatigue strength and extremely rough surfaces were removed by careful grinding.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.process.c9001253
EISBN: 978-1-62708-235-8
Abstract
Octagonal cast ingots weighing 6.5 tons and made of unalloyed heat treated steel CK 45 according to DIN 17200, and crankshafts forged from these ingots showed internal separations during ultrasonic testing. To determine the cause of defect, an ingot slice and a crank arm were examined metallographically. Investigation showed this was a case where flaky forgings were made from cast ingots with primary grain boundary cracks. This parallelity supports the often expressed opinion that both occurrences have the same origin, i.e. that hydrogen precipitation was the driving force in the formation of primary grain boundary cracks in cast ingots.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.process.c0046210
EISBN: 978-1-62708-235-8
Abstract
Several crankshaft failures occurred in equipment that was being used in logging operations in subzero temperatures. Failure usually initiated at a cracked pin oil hole, and the failure origin was approximately 7.6 mm (0.3 in.) from the shaft surface. The holes were produced by gun drilling, giving rise to surface defects. The fracture surface was characteristic of fatigue in that it was flat, relatively shiny, and exhibited beach marks. The crack surface was at a 45 deg angle to the axis of the shaft, indicating dominant tensile stresses. The material was the French designation AFNOR 38CD4 (similar to AISI type 4140H) and was in the quenched-and-tempered condition, with a yield strength of about 760 MPa (110 ksi). It was treated to have compressive surface stresses, and the prior-austenite grain size was ASTM 8. Analysis (visual inspection, stress analyses, and macrographs) supported the conclusion that failure was caused by fatigue stress caused by surface defects in the oil holes. Recommendation includes drilling the oil holes by a technique that essentially eliminates surface defects.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.rail.c9001418
EISBN: 978-1-62708-231-0
Abstract
The crankshaft of a six cylinder, 225-hp diesel engine driving a small locomotive was examined. About nine months after installation a fall in oil pressure was traced to damage to No. 5 crank pin bearing. A small lip present on one side of the discontinuity apparently served to scrape the bearing material. The defect was stoned smooth, a new bearing fitted, and the engine returned to service. The engine performed satisfactorily for a further twelve months until fracture of the crankshaft through the No. 5 crank pin supervened. The fracture revealed a complex torsional fatigue failure. Microscopic examination revealed that the pin had been hard chromium plated and that the plating followed the curved edge of the outer extremity of the defect. This crank pin contained an inherent defect in the form of a slag inclusion or crack situated at the surface. That the crack only showed itself after a period of service suggests that initially it may have been slightly below the surface of the machined pin and some slight extension outwards took place in service.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.design.c9001232
EISBN: 978-1-62708-233-4
Abstract
A bolt breaks along a change in cross section well below its rated capacity. An anchoring screw spins freely in place, having snapped at its first supporting thread. A motor unexpectedly disengages its load, its driveshaft having fractured near a keyway. Such failures – involving axles, leaf springs, engine rods, wing struts, bearings, gears, and more – can occur, seemingly without cause, due to vibrational fracture. Vibrational fractures begin as cracks that form under cyclic loading at nominal stresses which may be considerably lower than the yield point of the material. The fracture is proceeded by local gliding and the development of cracks along lattice planes favorably orientated with respect to the principal stress. This non-reversible process is often misleadingly called “fatigue” and presents significant challenges to engineering teams that ill-advisedly take to searching for material faults. Several examples of notch-induced vibrational fractures are presented along with guidelines for investigating their cause.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.modes.c9001152
EISBN: 978-1-62708-234-1
Abstract
Thermal and transformation stresses, resulting from welding, adding up with operational stresses can result in failure. Examples involving the crankshaft of a shaft-drive to produce artificial waves in a swimming pool, the joint bar of a dredger cast out of a running non-alloyed steel with 39 kg/sq mm tensile strength, which had been strengthened by welding plate strips on both sides had fractured in service; an axle tube out of 40 Mn 4 after DIN 17 200 from a paper fabrication machine, which had three short longitudinal slits distributed uniformly over its surface; welding to repair worn out bearing or fits, and a broken rear axle tube of a bus are described.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.modes.c9001192
EISBN: 978-1-62708-234-1
Abstract
The fracture cause had to be determined in a three-cylinder crankshaft made of chrome steel 34Cr4 (Material No. 1.7033) according to DIN 17200. The fracture occurred after only 150 h of operation. The fracture was of the bend fatigue type which originated in the fillet of the main bearing and ran across the jaw almost to the opposite fillet of the adjoining connecting rod bearing. The fillet was well rounded and smoothly machined. Thus, no reason for the fracture of the crankshaft could be found externally. No material defects were discernible in the origin or anywhere else. No cause for the crank fracture could be established from material testing. Probably the load was too high for the strength of the crank. Tensile strength could have been increased for the same material by tempering at lower temperature. Additionally, the resistance against high bend fatigue stresses or torsion fatigue stresses could have been increased substantially by including the fillet in the case hardening process.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.process.c9001197
EISBN: 978-1-62708-235-8
Abstract
The front wall of a cast iron crankcase cracked at the transition from the comparatively minor wall thickness to the thick bosses for the drilling of the bolt holes. Metallographic examination showed the case was aggravated by the fact that the casting had a ferritic basic structure and the graphite in part showed a granular formation, so that strength of the material was low. In a second crankcase with the same crack formation the structure in the thick-wailed part was better. But it also showed granular graphite in the ferritic matrix in the thin-walled part between the dendrites of the primary solid solution precipitated in the residual melt. A third crankcase had fractures in two places, first at the frontal end wall and second at the thinnest point between two bore holes. In all three cases casting stresses caused by unfavorable construction and rapid cooling were responsible for the crack formation. A fourth crankcase had cracked in the bore-hole of the frontal face. In this case the cause of the fracture was the low strength of a region that was caused by a bad microstructure further weakened by the bore hole.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.marine.c9001429
EISBN: 978-1-62708-227-3
Abstract
Two diesel engine crankshafts of similar dimensions, the journal diam being approximately 7 in., failed due to cracking originating in the fillet at the junction between the crankpin and the web nearest to the flywheel. The cracks were discovered before rupture occurred. Several small cracks originated in the fillet, ran together and developed as two main crack fronts that ultimately merged into one, a typical example of a fatigue failure. Electromagnetic crack detection revealed the presence of a number of discontinuities which were located at a position that would correspond to the vertical axis of the original ingot. The crankshaft had not been stress-relieved after a welding operation had been carried out. The only satisfactory course to follow when dealing with a highly stressed part in which defects of the type in question are revealed during machining is to scrap the forging.
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