Rather than using gears to transmit power, as in a typical transmission, a hydrostatic transmission transfers power by pressurizing hydraulic fluid using a small motor connected to the engine's crankshaft. Hydrostatic drives, or hydrostatic transmissions, are often used on lawn mowers and garden tractors.
Generally, rotary power from the engine is used to turn a pump. This pump pressurizes hydraulic fluid, which is then used to turn the wheels. This system can be used to reduce the complexities associated with traditional transmissions. Since the power can be started, stopped, and run in forward or reverse, a mechanical clutch is unnecessary.
Additionally, hydrostatic transmissions are "continuously variable" in that they can create an infinite number of torque reductions or mechanical advantages. This is typically accomplished by controlling the displacement of the pump and or motor. This is in stark contrast to traditional transmissions, in which the number of torque reductions is discrete and limited. Hydrostatic transmissions are not only used to transmit power to the drive wheels. They can be used to power the mower blade, power steering or auxiliary mower attachments.
Some disadvantages of the hydrostatic transmission do exist. The transmissions may be costlier than the traditional manual or automatic transmissions. Additionally, there is a tendency for these transmissions to transmit too much power to the wheels in situations in which the ground is soft, occasionally resulting in loss of traction.