The same vehicle shown in Fig. 6.1 is shown in Fig. 6.11 with the mechanical transmission replaced with a hydrostatic transmission; all other components remain the same. To provide a specific example, we will begin by specifying a variable displacement axial piston pump and a fixed displacement axial piston motor. Pump output is increased by stroking the pump, thereby increasing the speed of the motor. The vehicle can be speeded up and slowed down by moving the hand control that strokes the pump. The rotation of the motor shaft can be reversed by moving the swashplate control through the neutral position and displacing it in the opposite direction. The reverse position of the swashplate causes fluid to flow in the opposite direction, which causes the motor to turn in the opposite direction, thus reversing the vehicle. Vehicle motion can be changed from forward to reverse with a simple hand movement. This maneuverability is often the justification for installing a hydrostatic drive on a vehicle. A good example is a skid-steer loader, cycling back and forth, unloading a boxcar. It is tiring and time-consuming when an operator has to shift a mechanical transmission each time the direction of motion is changed. Vehicle productivity is increased with a hydrostatic transmission.
A hydrostatic transmission, like an automatic shift transmission, connects the engine and load with a fluid connection. (Remember that an automatic shift transmission has a torque converter that is a fluid power device.) Some of the same advantages are achieved with both types of transmission. The key disadvantage, as with all fluid devices, is some decrease in efficiency.