Quality Assurance Testing of a Slewing Bearing
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CASE HISTORY—PEER-REVIEWED
Quality Assurance Testing of a Slewing Bearing Joseph Maciejewski
Submitted: 11 June 2008 / in revised form: 10 July 2008 / Published online: 29 July 2008 Ó ASM International 2008
Abstract Quality assurance testing conducted on an exemplar slewing bearing prevented out-of-specification bearings from entering the marketplace with high potential for liability. Load testing, chemical analysis, and microstructural analysis were performed. Keywords Bearing failure Induction hardening Quality control
Background A distributor of slewing bearings, a supplier for marine on-board cranes, submitted an exemplar bearing from a new, foreign supplier. The single row ball bearing was approximately 13 inches in diameter and had an overturning-moment rating of 37,500 ft-lbs (Fig. 1). The bearing was ordered to be of 42CrMo4 alloy steel material. The detailed specifications for hardness and case depth were not available during the analysis. Several exemplar bearings, claimed to be manufactured to the specifications, were delivered at a significantly lower cost than domestic suppliers familiar to the company. In the interest of quality assurance, one bearing was proof loaded and post-test evaluated.
then loaded in an overturning-moment mode to the 37,500 ft-lb rating and held at that load for 1 min. No catastrophic failure was observed at that time. The bearing rings were then sectioned to reveal the races. The races exhibited plastic deformation at the rolling element locations in the form of Brinelling (Fig. 2). The bearing would not have operated correctly after moment loading in service. Cross sections of the inner and outer races were prepared using standard metallographic techniques. The specimens were chemically etched with Nital reagent. Fig. 3 shows that the microstructural changes associated with case hardening of the races extended into the part only slightly at the shoulders of the raceways. The 45o points in the raceways (i.e., the points of greatest contact pressure) exhibited little or no hardening. The case profile suggested an induction hardening technique. Measurement of the case depth and hardness was not performed considering the large variations along the profiles.
Testing The inner ring of the bearing was bolted securely in a universal tension/compression machine. The outer ring was J. Maciejewski (&) Applied Technical Services, Inc., Marietta, GA, USA e-mail: [email protected]
Fig. 1 As-received condition of the bearing
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J Fail. Anal. and Preven. (2008) 8:431–433
A typical case hardened area for the outer ring (Fig. 4) consisted of mixtures of hardened martensite and original fine pearlite structures. The inner ring was similar. The time and temperature achieved during the induction hardening cycle were clearly insufficient to fully austenitize even the outer surface layers. The core microstructures of the rings consisted of ferrite and fine pearlite (Fig. 5), indicating the rings were not through-hardened before induction case hardening. Chemical analysis was conduc
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