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What is a Thrust Load?

Meaning | Concept | Definition:

Thrust load is the measured amount of force directed to and from a turning mechanism. When a mechanism, such as a gear, rotates on an axis, the gear emits load in the direction it turns and against the axis it turns, either forward or backward.

In a typical gear installation, the bearing will not only provide a surface for the gear to travel on, it will also provide a surface to push on, thereby cushioning the thrust load. A typical shaft housing rotating components will commonly have a measured amount of tolerance machined to provide some degree of thrust load clearance.

In an automobile engine, a rotating assembly consisting of a crankshaft, pistons, and connecting rods works in a rotating manner.

Inside the common automobile engine, a rotating assembly consisting of a crankshaft, pistons, and connecting rods works in a rotating manner. The crankshaft is mounted on the engine block and runs inside the main bearings. The bearings not only provide a smooth surface for the crankshaft to turn, they also provide a surface to absorb and dampen the thrust load.

As the pistons drive the crankshaft around its axis, it also tries to be driven towards the rear of the engine block. A special main bearing, known as a thrust bearing, contains sides that contact a machined area on the crankshaft to damp thrust load.

A feeler gauge can be used to measure the amount of clearance between the thrust bearing side and the crankshaft.

The preset clearance in the thrust bearing is achieved by prying or tapping the crankshaft back and forth before tightening the main bearing cap. Once the cap has been properly tightened, a feeler gauge is slid between the thrust bearing side and the crankshaft and the amount of clearance is measured.

If the amount of clearance is sufficient to protect against thrust load damage, the cap can be tightened to its final specifications. Failure to properly adjust the bearing for the thrust load will result in damage to the engine and possibly the transmission. It is important to use only a dead blow, brass or lead hammer when tapping the crankshaft back and forth to avoid damaging the crankshaft.

When investigating the causes of premature thrust load bearing failure, there are a few places to start. The initial thrust setting should be checked to ensure proper bearing preload has been completed.

The main bearings should be checked to confirm that they are all tightened correctly and that the main caps are in the correct position in the main saddles. The crankshaft must be examined and checked to make sure it is straight; a bent crankshaft can affect the thrust load on any engine.

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