For years both transmission and engine rebuilders have struggled at times to determine the cause of crankshaft thrust bearing failures.
In most instances, all of the facts concerning the situation are not revealed at the onset of the failure. As a result, This has led to each party blaming the other for the failure based only on hearsay or what some “expert” has termed the “cause”
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.
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.
So, 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:
- Poor crankshaft surface finish
- Surface finish:
When assembling thrust bearings:
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.
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
- Pump gears being installed backward
Diagnosing the problem:
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.
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?
Correct flexplate used?
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.
What condition was the throw out bearing in?
How does the torque converter exert force on the crankshaft?
The area is a little trickier. The area that is part of this equation is the difference between the area of the front half of the converter and the rear half. The oil pressure does exert a force that tries to expand the converter like a balloon. However, it is the fact that the front of the converter has more surface area than the rear (the converter neck is open) that causes the forward force on the crankshaft.
Causes for excessive torque converter pressure:
Modifications that can result in higher than normal converter pressure include using an overly-heavy pressure regulator spring, or excessive cross-drilling into the cooler charge circuit. Control problems such as a missing vacuum line or stuck modulator valve can also cause high pressure.
Every effort should be made to find the cause of distress and correct it before completing repairs, or you risk a repeat failure.
It is very important not to contact the bearing surface with the end of the file. The resulting enlarged ID chamfer will allow pressurized engine oil from the pre-existing groove to reach the loaded thrust face. As a result, This additional source of oiling will reach the loaded thrust face without passing through the bearing clearance first (direct oiling).
Special Thanks to Lance Wiggins and ATRA for Great Information