Introduction to Hydrostatic Transmissions

A hydrostatic transmission (HST) is simply a pump and motor connected in a circuit. Other components are added to obtain certain operating features.

The four basic configurations of hydrostatic transmissions are :

1. In-line (Fig. 6.10a)
2. U-shape (Fig. 6.10b)
3. S-shape (Fig. 6.10c)
4. Split (Fig. 6.10d)

Various pump and motor designs can be paired together for the split configuration. Manufacturers use the same designs to build the in-line, U-shape, and S-shape configurations.


Hydraulic hose is available with a working pressure rating of 6000 psi. Use of these hoses allows the split transmission to be operated at the maximum pressure rating of high-performance pumps and motors. Particularly on a vehicle, the split transmission can offer many advantages. Most often, the pump is mounted on a pump mount bolted directly to the engine, but the motors can be placed at the most convenient location, thus taking full advantage of fluid power’s ability to flow power “around a corner.”

The split transmission, because of the flexible hoses between the pump and motor(s), will absorb some deflection caused by dynamic loads applied to the frame. Consider the situation when the pump and motor are bolted rigidly together, the pump is bolted to the engine (not driven with a universal joint driveline), and the motor is bolted to the frame. Now the pumpmotor housing is subject to the dynamic loads applied to the frame. This introduction of stress into the housing is undesirable and can lead to reduced reliability.

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