Although thrust bearings run on a thin film of oil, just like radial journal (connecting rod and main) bearings, 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.
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. Grooves on the flange, face, and wedge shaped ramps at each parting line allow oil to enter between the shaft and bearing surfaces.
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
Surface finish:
Crankshaft thrust faces are difficult to grind because they are done using the side of the grinding wheel. Grinding marks left on the crankshaft face produce a visual swirl or sunburst pattern with scratches - sometimes crisscrossing - one another in a cross-hatch pattern similar to hone marks on a cylinder wall. If these grinding marks are not completely removed by polishing, they will remove the oil film from the surface of the thrust bearing much like multiple windshield wiper blades. A properly finished crankshaft thrust face should only have very fine polishing marks that go around the thrust surface in a circumferential pattern.
Alignment:
The grinding wheel side face must be dressed periodically to provide a clean, sharp cutting surface. A grinding wheel that does not cut cleanly may create hot spots on the work piece leading to a wavy, out-of-flat surface. The side of the wheel must also be dressed at exactly 90° to its outside diameter, to produce a thrust face that is square to the axis of the main bearing journal. The crankshaft grinding wheel must be fed into the thrust face very slowly and also allowed to "spark out" completely. The machinist should be very careful to only remove minimal stock for a "clean-up" of the crankshaft surface.
In most instances a remanufactured crankshaft does not require grinding of the thrust face(s), so the grinding wheel will not even contact them. Oversize thrust bearings do exist. Some main bearing sets are supplied only with an additional thickness thrust bearing. In most of those instances, additional stock removal from the crankshaft thrust face surface may be required. Crankshaft end float should be calculated and determined before grinding additional material from the thrust face.
Crankshaft grinding wheels are not specifically designed for use of the wheel side for metal removal. Grinding crankshaft thrust faces requires detailed attention during the procedure and repeated wheel dressings may be required. Maintaining sufficient coolant between the grinding wheel and thrust surface must be attained to prevent stone loading and "burn" spots on the thrust surface. All thrust surface grinding should end in a complete "spark out" before the grinding wheel is moved away from the area being ground. Following the above procedures with care should also maintain a thrust surface that is 90° to the crankshaft centerline.
When assembling thrust bearings:
Tighten main cap bolts to approximately 10 to 15 ft.lb. to seat bearings, then loosen.
Tap main cap toward rear of engine with a soft faced hammer.
Tighten main cap bolts, finger tight.
Using a bar, force the crankshaft as far forward in the block as possible to align the bearing rear thrust faces.
While holding shaft in forward position, tighten main cap bolts to 10 to 15 ft.lbs.
Complete tightening main cap bolts to specifications in 2 or 3 equal steps.
The above procedure should align the bearing thrust faces with the crankshaft to maximize the amount of bearing area in contact for load carrying.
Loading:
A number of factors may contribute to wear and overloading of a thrust bearing, such as:
1. Poor crankshaft surface finish.
2. Poor crankshaft surface geometry.
3. External overloading due to.
a) Excessive Torque converter pressure.
b) Improper throw out bearing adjustment.
c) Riding the clutch pedal.
d) Excessive rearward crankshaft load pressure due to a malfunctioning front mounted accessory drive.
Note: There are other, commonly-thought issues such as torque converter ballooning, the wrong flexplate bolts, the wrong torque converter, the pump gears being installed backward or the torque converter not installed completely. Although all of these problems will cause undo force on the crankshaft thrust surface, it will also cause the same undo force on the pump gears since all of these problems result in the pump gear pressing on the crankshaft via the torque converter. The result is serious pump damage, in a very short period of time (within minutes or hours).
Diagnosing the problem:
By the time a thrust bearing failure becomes evident, the parts have usually been so severely damaged that there is little if any evidence of the cause. The bearing is generally worn into the steel backing which has severely worn the crankshaft thrust face as well. So how do you tell what happened? Start by looking for the most obvious internal sources.
Engine related problems:
Is there evidence of distress anywhere else in the engine that would indicate a lubrication problem or foreign particle contamination?
Were the correct bearing shells installed, and were they installed correctly?
If the thrust bearing is in an end position, was the adjacent oil seal correctly installed? An incorrectly installed rope seal can cause sufficient heat to disrupt bearing lubrication.
Examine the front thrust face on the crankshaft for surface finish and geometry. This may give an indication of the original quality of the failed face.
Once you are satisfied that all potential internal sources have been eliminated, ask about potential external sources of either over loading or misalignment.
Transmission related problems:
Did the engine have a prior thrust bearing failure?
What external parts were replaced?
Were there any performance modifications made to the transmission?
Was an additional cooler for the transmission installed?
Was the correct flexplate used? At installation there should be a minimum of 1/16" (1/8" preferred, 3/16" maximum) clearance between the flex plate and converter to allow for converter expansion.
Was the transmission property aligned to the engine?
Were all dowel pins in place?
Was the transmission-to-cooler pressure checked and found to be excessive? If the return line has very low pressure compared to the transmission-to-cooler pressure line, check for a restricted cooler or cooler lines.
If a manual transmission was installed, was the throw out bearing properly adjusted?
What condition was the throw out bearing in? A properly adjusted throw out bearing that is worn or overheated may indicate the operator was "Riding The Clutch".
