Bond Quality

Bond integrity is arguably the most important manufacturing feature of any bimetal bearing. A bond failure can be very expensive to correct, particularly considering the cost of lost production while the bearing is repaired or replaced. The bond tensile strength of a metallurgically well-bonded steel-backed babbitted bearing should exceed 60 MPa (8700 psi); bronze-backed bearings should exhibit a minimum bond strength of 40 MPa (5800 psi). In other words, the bond strength should comfortably exceed the ultimate tensile strength of the babbitt.

Bond strength is determined by destructive tests for evaluating tensile strength and ductility (Ref 3). Nondestructive ultrasonic bond testing, when conducted and evaluated by an experienced and qualified operator, does not determine strength but does reveal the location and extent of any significant unbonded areas. An unblemished surface is a very good indication that proper preparation and casting techniques have been employed throughout the babbitting process and that at least minimum bond strengths can be reasonably expected.

Defective bonds can most often be traced to poor preparation (i.e., inadequate cleaning of the base metal). The second most prevalent cause is allowing the tinned shell to cool such that the tinning is not molten when the babbitt is centrifugally or statically cast. To correct either problem, the babbitt and tinning must be removed and the shell reprocessed, starting with the basic cleaning steps and following recommended temperature guidelines.

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