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The primary purpose for using hydrostatic transmissions on turf and garden equipment is the ability to provide an infinite selection of  ground speeds. The key to varying the output speed is that the motor pistons require a positive volume of oil under pressure to react. The amount of oil displaced by the pump must be accepted by the motor. More oil will cause the motor cylinder block to rotate faster, less oil will slow rotation.

Two methods can be used to change the volume of the pump. The most obvious way is to change the speed of the engine turning the pump. Running the engine faster results in more piston strokes per minute, increasing the volume. Slowing engine speed would decrease the number of piston strokes.
Our application in a tractor however, requires that engine speed remain constant to power attachments such as mower decks, tillers, snowthrowers etc., so this method would not suit the application.

The second method is to vary the length of the piston stroke in the pump. By changing the angle of the plate (properly referred to as the swash  plate, Fig. 6) we note that the length of each piston stroke can be changed. By mounting the swash plate on a pintle or pivot, a lever can be used to 8hange the angle. When the plate is 90 to the piston bore, no piston travel occurs and no oil flows to the motor.

By moving the swash plate beyond 900, the positive pressure is created on the opposite half of the pump cylinder block, and the motor cylinder block receiving the positive pressure on the opposite side turns in reverse rotation.  This provides the tractor with infinite speed selection in reverse also.

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[Source: Simplicity publication, Hydraulic Systems Training Information, #840172, Principles and Operation of Tractor Hydraulic Systems]



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