Hydrostatic transmission (HST) is a term referring to a closed-circuit pump and motor system where the pump is generally of variable-displacement, axial-piston design. The motor, or motors, can be fixed or variable displacement design. Machine propulsion is the traditional application for HSTs, but various work functions also benefit equally from the drive advantages of an HST.

The primary function of the HST is to accept rotary power from the prime mover (usually an internal combustion engine), and transmit that energy to a load via a hydraulic motor. In the process, the HST generally regulates speed, torque, power, and direction of movement. The typical HST drives a load from full speed in one direction to full speed in the opposite direction, with infinite variation of speed between the two maximums. Input speed to the pump is fixed for most off-road machinery, but sometimes is controlled via the engine throttle.

HSTs offer some important advantages over other forms of power transmission. Depending on its configuration, a hydrostatic transmission is capable of:

  • transmitting high power in a compact package
  • exhibiting low inertia
  • operating efficiently over a wide range of torque-to-speed ratios
  • maintaining controlled speed regardless of load, within design limits
  • accurately maintaining preset speed against driving or running loads
  • transmitting power to multiple locations, even if position and orientation of the location changes
  • providing faster response than mechanical or electromechanical transmissions of comparable rating, and
  • providing dynamic braking.

Whatever its task, the HST must be sized, designed, and controlled for an optimum match between the engine and the load. This allows the engine to operate at its most efficient speed and the HST to make adjustments to operating conditions. The better the match between input and output characteristics, the more efficient the entire power system.