Paper 16: Minimum Allowable Oil Film Thickness in Steadily Loaded Journal Bearings

Abstract

There are various factors limiting the load carrying capacity of a steadily loaded journal bearing operating under hydrodynamic conditions. Two of these factors are considered.

1. (1)

Surface finish criterion

Bearing and journal surfaces are slightly grooved due to machining processes, and when initial contact is made between the surfaces, it is only made at local high peaks, leaving an idealized effective hydrodynamic oil film at the minimum film thickness position in the bearing. This minimum film thickness at metal-to-metal contact conditions is dependent on the surface finish of the two mating surfaces.

(2)

White-metal wiping criterion

At high-speed operation, the oil film temperature in the region of the minimum oil film position will be high if the bearing is operating with a large eccentricity. A condition can arise where the oil film temperature is sufficiently high to cause local plastic flow of the bearing white metal. This criterion is known as ‘wiping’, and at high speeds it will predominate over the surface finish criterion.

These two criteria are used in an analysis which gives guidance to the minimum allowable oil film thickness for journals of various sizes running at various speeds. This gives the bearing designer, using conventional hydrodynamic theory (Ocvirk, Cameron and Wood, etc.), some yardstick to assess whether or not his calculated minimum oil film thickness figures are satisfactory.