The hydrostatic drive is a fluid drive which uses fluid under pressure to transmit engine power to the drive wheels of the machine.
Hydrostatics is the use of fluids at high pressure but relatively low speeds.
In a closed loop circuit between the pump and the motor as shown, the fluid moves through the lines from the pump to the motor to transmit the power to the drive output, then returns back to the pump.
There are several different types of machinery used in industry today that are run by hydrostatics. Excavators
Skid steer Loaders
Compact rollers and Paving Equipment
Now For a Closer Look at the different types of hydrostatic drives.
Circuit 1. Fixed displacement pump driving a fixed displacement motor.
Circuit 2. Variable displacement pump driving a fixed displacement motor.
Circuit 3. Fixed displacement pump driving a variable displacement motor.
Circuit 4. Variable displacement pump driving a variable displacement motor.
Circuit No. 1
This is like a gear drive circuit, it transmits power without altering the speed or horsepower between the engine and the load. A constant input speed and torque gives a constant output horsepower. If either the speed or the torque are increased while holding the other constant, the output horsepower will increase.
Fixed pump and motor
In this circuit the output speed is variable and the torque output is constant for any pressure. This circuit gives variable speed and constant torque.
Variable pump and fixed motor
Circuit No. 3
This circuit is varied by changing the motor displacement. For constant power input if the motor displacement decreases, output speed increases but output torque drops.
Fixed pump and variable motor
Circuit No. 4
This circuit is the most flexible and expensive of all the circuits but also the most difficult to control. It is capable of operating like any of the above combinations.
Variable pump and variable motor
Reversing in variable pump-motor circuits
The direction of the output shaft rotation can be reversed in variable circuits by shifting either the pump or motor swashplate over center, thus we have neutral, forward, and reverse.