Thrust bearings are used to control end play in the crankshaft. End play is important because it limits the fore and aft movement of the crankshaft in the block. If an engine is assembled with too much end play in the crank, or if the thrust bearing fails, the forward movement of the crankshaft in the block can chew up the main bearing caps and block. Excessive end play can also cause connecting rods to fatigue and break, and wrist pins to work loose and score the cylinders.
For years engine and transmission rebuilders have struggled to determine the cause of crankshaft thrust bearing failures. Often, each has blamed the other for the resulting damage.
To get to the bottom of the issue, the Automotive Transmission Rebuilders Association (ATRA), the Engine Rebuilders Association (AERA), the Production Engine Remanufacturers Association (PERA), the Automotive Service Association (ASA) and bearing manufacturers got together and came up with a list of possible causes and remedies for thrust bearing failures. Their findings are published in AERA tech bulletin TB-1465R (March 1998).
What They Found
Aside from obvious causes such as dirt contamination and misassembly, there are only three things that generally cause thrust bearing failures.
- Poor crankshaft surface finish;
- Misalignment of the thrust bearing and crankshaft;
- Overloading of the thrust bearing.
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 honing marks on a cylinder wall. If these grinding marks are not completely removed by polishing, they will wipe 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.
Manufacturers of crankshaft micropolishing equipment all say polishing the thrust surface on the crank is just as important as polishing the journals. They also say machine polishing is more accurate and consistent than hand-polishing.
Doug Anderson of Grooms Engines Parts Machining in Nashville, TN, says his company uses a micropolishing machine for polishing crank journals and thrust surfaces. "The problem with polishing cranks manually is that it is totally operator dependent. It
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