Banding wires of the rotor of an 1800 hp motor were renewed following replacement of the banding rings. After about six months of service, a breakdown occurred due to bursting of the banding wires in several places. The 0.064 in. diam wire was nonmagnetic and of the 18/8 Cr-Ni type of austenitic stainless steel. The fractures were short and partially crystalline, with no evidence of slowly developing cracks of the fatigue type. Microscopical examination of sections taken through the fractures showed the cracking to be of the multiple branching type. Because the material was in the heavily cold-worked condition, it was not possible to determine with certainty if the cracks were of the inter- or trans-granular type. It was concluded that failure was due to stress-corrosion cracking in a chloride environment. Failure of the wires was likely due to the use of a chloride-containing flux during the soldering operation.

Two examples concerning fabricated mild steel rotor spiders which failed due to lack of torsional rigidity, probably supplemented by the presence of high internal stress, are described. The machine concerned in the first case was a 3,000 hp three-phase slip-ring motor. In the second case the machine was a 200 kW alternator, direct-driven by a diesel engine running at 750 rpm. Both the foregoing failures reveal the same basic weakness, i.e., insufficient rigidity when subjected to variations or reversals of torque. In the first case, the bars welded to the arms were inadequately supported in a lateral direction, so that excessive stresses of a fluctuating nature were set up in the welds as a result of the frequent load changes that arose in service. This weakness was eliminated when designing the replacement spider. In the second example, failure also arose as a result of deficient torsional rigidity with the consequent development of excessive stresses in the welds at the junctions of the bars with the sleeve, the torque being of a fluctuating character due to the impulses imparted by the engine.

Splined rotor shafts (constructed from 1151 steel) used on small electric motors were found to miss one spline each from several shafts before the motors were put into service. Apparent peeling of splines on the induction-hardened end of each rotor shaft was revealed by visual and stereo-microscopic examination. One tooth on each shaft was found to be broken off. It was revealed by metallographic examination of an unetched section through the fractured tooth that the fracture surface was concave and had an appearance characteristic of a seam. Partial decarburization of the surface was revealed after etching with 1% nital. The presence of a crack, with typical oxides found in seams at its root, was disclosed by an unetched section through the shaft in an area unaffected by induction heating. The etched samples revealed similar decarburization as was noted on the fracture surface of the tooth. It was concluded that the seam had been present before the shaft was heat treated and these seams acted as stress raisers during induction hardening to cause the shaft failure. It was recommended that the specifications should specify that the shaft material should be free of seams and other surface imperfections.

Ball bearings made of type 440C stainless steel hardened to 60 HRC and suspected as the source of intermittent noise in an office machine were examined. A number of spots on the inner-ring raceway were revealed by scanning electron microscopy. The metal in the area around the spot was evidenced to have been melted and welded to the inner-ring raceway. It was revealed by randomly spaced welded areas on the raceways that the welding was the result of short electrical discharges between the bearing raceways and the balls. The use of an electrically nonconductive lubricant in the bearings was suspected to have caused the electric discharge by accumulation and discharge of static charge. The electrical resistance between the rotor and the motor frame lubricated with electrically conductive grease and the grease used in the current case was measured and compared to confirm the fact the currently used grease was nonconductive. It was concluded that the pits were formed by momentary welding between the ball and ring surfaces. The lubricant was replaced by electrically conductive grease as a corrective measure.

A steel bolt had been used to join the copper connecting strips between the poles of a 10-pole, series-connected, rotating field rotor of a synchronous motor. The exciting current was 155 amps. Failure of the bolt resulted in severe damage to the stator windings by the loosened ends of the strips. The bolt had fractured near the head, a location which probably coincided with the junction of the strips. A portion of the fracture surface was covered with copper that had been deposited in the molten state, while some was also present along the shank of the bolt, having apparently run in between the bolt and the hole in the strip. The bolt end adjacent to the fracture had been subjected to intense local heating. The extent of the grain-growth indicating that the temperature had been in the region of 1200 deg C (2192 deg F). When the temperature reached the melting-point of copper, 1083 deg C (1981 deg F), molten metal came into contact with the bolt, into which it penetrated along the grain boundaries, culminating in rupture.

Fusing of the switch contacts of a boiler feed pump drive motor led to the failure of a turbine. After rubbing of most of the Ni-Cr steel LP wheels had occurred, due to the admission of water carried over with the steam, a copper-rich alloy from the interstage gland rings melted, penetrated the wheel material, and gave rise to radial and circumferential cracking in four of the LP wheels. It was concluded that when the rotor moved axially and the wheels came into contact with the diaphragms there was a tendency for the former to dish, with the development of both radial and circumferential tensile stresses on the side in contact with the adjacent diaphragm. In the presence of the molten copper-rich alloy, these stresses gave rise to severe hot cracking.

Following the fusing of one of the copper leads in the choke circuit of an electric welder, a piece of the affected lead was obtained for examination. The sample had large internal cavities and surface bulges. It is remarkable that a wire containing defects of the magnitude present in this case could have been drawn without failure. Failure in service was due to overheating resulting from the inability of the conductor to carry the current where its cross section was reduced by the presence of a cavity. Another failure of a conductor occurred in one of the field coils of a direct-current motor. The mode of failure and the changes in the microstructure showed that fracture was due to a defective resistance butt-weld which had been made when the wire was in process of drawing. A further example of a conductor failure occurred in a 12 SWG copper connection between the rotor contactor and the resistance in a starter. A transverse section through the zone of failure showed an oxide layer extended almost completely across the plane of a weld, and also the grain growth that had occurred in this region.

Shaft misalignment and rotor unbalance contribute to the premature failure of many machine components. To understand how these failures occur and quantify the effects, investigators developed a model of a rotating assembly, including a motor, flexible coupling, driveshaft, and bearings. Equations of motion accounting for misalignment and unbalance were then derived using finite elements. A spectral method for resolving these equations was also developed, making it possible to obtain and analyze dynamic system response and identify misalignment and unbalance conditions.

Macrofractographs of the fracture surface from a multibladed fan showed that cracks started at the corner where bending stress was concentrated and propagated through the blade by fatigue. Peak stress at the monitoring position was less than 10 MPa. To simulate crack growth, the rotor was repeatedly deformed by a hydraulic fatigue tester. Comparison of striations of the failed blade with that of the tested one revealed the failed blade was loaded with more than 30 MPa of stress. These tests confirmed that the rotor and blades had sufficient strength to withstand up to 3x the stress of normal operation. The casing of the fan was vibrated at 10 to 60 Hz. Peak stress easily overcame 30 MPa, which was enough to initiate cracking. The fracture surfaces and starting position were the same as those on the failed fan. It was concluded that the exciting force from an air compressor caused blade failure.

Vibration analysis can be used in solving both rotating and nonrotating equipment problems. This paper presents case histories that, over a span of approximately 25 years, used vibration analysis to troubleshoot a wide range of problems.

An accidental overspeed condition during wind tunnel testing resulted in the destruction of a propeller rotor The occurrence was initially attributed to malfunction in the collective pitch control system. All fractured parts in the system were inspected. Highly suspect parts, including the pitch control thrust bearing set, head bolts, hub fork, and actuator rod end, were examined in more detail The thrust bearing set (52100 steel) was identified as the probable source of the uncommanded pitch angle change. A complete failure analysis of the bearing indicated that failure was precipitated by excessive heating, causing cage disintegration, plastic flow of the races and balls, and eventual separation of inner and outer races. It was recommended that the bearing set be resized to accommodate the large thrust as and that a thermocouple be added to monitor the condition of the bearing during testing.

A heat transport pump in a heavy water reactor failed (exhibiting excessive vibration) during a restart following a brief interruption in coolant flow due to a faulty valve. The pump had developed a large crack across the entire length of a bearing journal. An investigation to establish the root cause of the failure included chemical and metallurgical analysis, scanning electron fractography, mechanical property testing, finite element analysis of the shrink fitted journal, and a design review of the assembly fits. The journal failure was attributed to corrosion fatigue. Corrective actions to make the journals less susceptible to future failures were implemented and the process by which they were developed is described.

A combination of adverse factors was present in the disruption of a turbo-alternator gearbox. The major cause was the imposition of a gross overload far in excess of that for which the gearbox was designed. The contributory factors were a rim material (EN9 steel) that was inherently notch-sensitive and liable to rupture in a brittle manner. Discontinuities were present in the rims formed by the drain holes drilled in their abutting faces, and possibly enhanced by the stress-raising effect of microcracks in the smeared metal at their surfaces It is probable that the load reached a value in excess of the yield point within the delay time of the material so when the fracture was initiated, it was preceded by several microcracks giving rise to the propagation of a brittle fracture.

An air blower in an electric power plant failed unexpectedly when a roller bearing in the drive motor fractured along its outer ring. Both rings, as well as the 18 rolling elements, were made from GCr15 bearing steel. The bearing also included a machined brass (MA/C3) cage and was packed with molybdenum disulfide (MoS 2 ) lithium grease. Metallurgical structures and chemical compositions of the bearing’s matrix materials were inspected using a microscope and photoelectric direct reading spectrometer. SEM/EDS was used to examine the local morphology and composition of fracture and contact surfaces. Chemical and thermal properties of the bearing grease were also examined. The investigation revealed that the failure was caused by wear due to dry friction and impact, both of which worsened as a result of high-temperature degradation of the bearing grease. Fatigue cracks initiated in the corners of the outer ring and grew large enough for a fracture to occur.

This article provides assistance to a failure analyst in broadening the initial scope of the investigation of a physical engineering failure in order to identify the root cause of a problem. The engineering design process, including task clarification, conceptual design, embodiment design, and detail design, is reviewed. The article discusses the design process at the personal and project levels but takes into consideration the effects of some higher level influences and interfaces often found to contribute to engineering failures.

Hydrogen embrittlement is the brittleness affecting copper and copper alloys containing oxygen which develops during heat treatment at temperatures of about 400 deg C (752 deg F) and above in an atmosphere containing hydrogen. The phenomenon of hydrogen embrittlement of copper and its alloys is illustrated by examples from practice and reference is made to data from recent publications on the subject. Embrittlement due to this cause can only be identified by microscopic examination because other modes of failure in copper; e.g., from heat cracking, mechanical overload, the formation of low melting point eutectics or corrosion; show a similar appearance when investigated on a macroscopic scale.

The intent of this article is to assist the failure analyst in understanding the underlying engineering design process embodied in a failed component or system. It begins with a description of the mode of failure. This is followed by a section providing information on the root cause of failure. Next, the article discusses the steps involved in the engineering design process and explains the importance of considering the engineering design process. Information on failure modes and effects analysis is also provided. The article ends with a discussion on the consequence of management actions on failures.

Rolling-element bearings use rolling elements interposed between two raceways, and relative motion is permitted by the rotation of these elements. This article presents an overview of bearing materials, bearing-load ratings, and an examination of failed bearings. Rolling-element bearings are designed on the principle of rolling contact rather than sliding contact; frictional effects, although low, are not negligible, and lubrication is essential. The article lists the typical characteristics and causes of several types of failures. It describes failure by wear, failure by fretting, failure by corrosion, failure by plastic flow, failure by rolling-contact fatigue, and failure by damage. The article discusses the effects of fabrication practices, heat treatment and hardness of bearing components, and lubrication of rolling-element bearings with a few examples.

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.