Although thrust bearings run on a thin film of oil,just like radial journal (connecting rod and main) bearing, they cannot support nearly as much load. While radial bearings can carry loads measured in thousands of pounds per square inch of projected bearing area, thrust bearings can only support loads of a few hundred pounds per square inch. Radial journal bearings develop their higher load capacity from the way the curved surfaces of the bearing and journal meet to form a wedge. Shaft rotation pulls oil into this wedge shaped area of the clearance space to create an oil film which actually supports the shaft. Thrust bearings typically consist of two flat mating surfaces with no natural wedge shape in the clearance space to promote the formation of an oil film to support the load.

Conventional thrust bearings are made by incorporating flanges, at the ends of a radial journal bearing this provides ease in assembly and has been used successfully for many years. Either teardrop or through grooves on the flange, face and wedge shaped ramps at each parting line allow oil to enter between the shaft and bearing surfaces. However, the surface of the shaft, as well the vast majority of bearing surfaces, are flat.

This flatness makes it more difficult to create and maintain an oil film. As an example, if two gauge blocks have a thin film of oil on them, and are pressed together with a twisting action,the blocks willstick together.This is similar to what happens when a thrust load is applied to the end of a crankshaft and oil squeezes out from between the shaft and bearing surfaces. If that load is excessive, the oil film collapses and the surfaces want to stick together resulting in a wiping action and bearing failure. For this reason, many heavy-duty diesel engines use separate thrust washers with a contoured face to enable them to support higher thrust loads. These thrust washers either have multiple tapered ramps and relatively small flat pads, or they have curved surfaces that follow a sine-wave contour around their circumference.

Recent developments :

In the past few years some new automotive engine designs include the use of contoured thrust bearings to enable them to carry higher thrust loads imposed by some of the newer automatic transmissions. Because it’s not practical to incorporate contoured faces on one piece flanged thrust bearings, these new engine designs use either separate thrust washers or a flanged bearing which is a three piece assembly.

Cause of failure :

Aside from the obvious causes, such as dirt contamination and misassembly, there are only three common factors which generally cause thrust bearing failures. They are :

• Poor crankshaft surface finish
• Misalignment
• Overloading