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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
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.
Click for a closer view
[Source: Simplicity publication, Hydraulic
Systems Training Information, #840172, Principles and Operation of Tractor