The direction of output shaft rotation can be reversed in variable setups by shifting either the pump or the swash plate of the motor over center.

Remember three factors control the operation of a hydrostatic drive. These factors are as follows:

RATE of oil flow - gives the speed

DIRECTION of oil flow - gives the direction

PRESSURE of the oil - gives the power

The pump is driven by the engine of the machine and is linked to the speed set by the operator. It pumps a constant stream of high-pressure oil to the motor. Since the motor is linked to the drive wheels or tracks of the machine, it gives the machine its travel speed.

The advantages ofhydrostatic drive are as follows:

Infinite speeds and torque

Easy one-lever control

Smooth shifting

Shifts "on the go"

High torque available for starting up

Flexible location - no drive lines

Low maintenance and service

Reduces shock loads

Compact size

Eliminates clutches and large gear trains

Hydrostatic Drive Operation

For you to understand how a hydrostatic drive operates, we will explain the operation of a typical system. The system we will use has an axial piston pump and motor which is the 'most common hydrostatic drive system. The pump has a variable displacement, while the motor has a fixed displacement. Now look at the complete system in operation - forward, neutral, and reverse.

FORWARD (fig. 6-15). - When the operator moves the speed control lever forward, the spool in the displacement control valve, also known as the FNR valve (Forward, Neutral, and Reverse), moves from its NEUTRAL position. This action allows pressure oil to flow into the upper servo cylinder forcing the swash

Figure 6-15. - Forward operation.