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The thrust load is the amount of measured force directed at and from a turning mechanism. As the mechanism such as a gear turns on a shaft, there is both load emitted from the gear in the direction it is turning as well as against the shaft it is turning on, either forward or to the rear. In a typical gear installation, the bearing will not only provide a surface for the gear to ride on, it will also provide a surface to push against, thus cushioning the thrust load. A typical shaft that houses rotating components will commonly have a measured amount of tolerance machined in to provide some degree of thrust load clearance.
Within the common automobile engine, a rotating assembly consisting of a crankshaft, pistons and connecting rods works in a spinning manner. The crankshaft is mounted in the engine block and rides inside of the main bearings. The bearings not only provide a smooth surface for the crankshaft to spin on, but they also provide a surface to absorb and cushion the thrust load. As the pistons drive the crankshaft around its axis, it also attempts to get driven towards the rear of the engine block. A special main bearing, known as the thrust bearing, contains sides which contact an area machined on the crankshaft to cushion the thrust load.
The clearance pre-set into the thrust bearing is achieved by gently prying or tapping the crankshaft in both forward and rearward directions before tightening the main bearing cap down. Once the cap has been properly tightened, a feeler gauge is slid between the side of the thrust bearing and the crankshaft and the amount of clearance is measured. If the amount of clearance is sufficient to protect against damage from the thrust load, the cap can be torqued to its final specifications. Failure to properly set the bearing for the thrust load will result in engine and possibly transmission damage. It is important to use only a dead blow, brass or lead hammer when tapping the crankshaft forward and back to prevent damaging the crankshaft.
When investigating the causes for premature thrust load bearing failure, there are a few places to start. The initial thrust setting should be verified to ensure proper bearing pre-load was completed. The main bearings should be checked to confirm that all are torqued properly and the main caps are in the correct position on the main saddles. The crankshaft should be examined and checked to make certain it is straight; a bent crankshaft can affect the thrust load in any engine.