Mechanical power from the engine is converted to hydraulic power by a pump-motor team. This power is then converted back to mechanical power for the drive wheels or tracks.

The pump-motor team is the heart of the hydrostatic drive system. Basically, the pump and motor are joined in a closed hydraulic loop; the return line from the motor is joined directly to the intake of the pump, rather than to the reservoir (fig. 6-11). A charge pump maintains system pressure, using supply oil from the reservoir.

The hydrostatic drive functions as both a clutch and transmission. The final gear train then can be simplified with the hydrostatic unit supplying infinite speed and torque ranges as well as reverses speeds.

To understand hydrostatic drive, you must understand two principles of hydraulics:

Liquids have no shape of their own.
Liquids are not compressible.
The basic hydrostatic principle is as follows (fig. 6-12):

Two cylinders connected by a line both filled with oil. Each cylinder contains a piston.

When a force is applied to one of the pistons, the piston moves against the oil. Since the oil will not compress, it acts as a solid connection and moves the other piston.

In a hydrostatic drive, several pistons are used to transmit power- one group in the PUMP sending power to another group. in the MOTOR. The pistons are in a cylinder block and revolve around a shaft. The pistons also move in and out of the block parallel to the shaft.

Figure 6-11.- Pump and motor form a closed hydraulic loop.

Figure 6-12.- Basic hydrostatic principle.

To provide a pumping action for the pistons, a plate, called a SWASH PLATE, is located in both the pump and motor (fig. 6-13). The pistons ride against the swash plates. The angle of the swash plates can be varied, so the volume and pressure of oil pumped by the pistons can be changed or direction of the oil reversed. A pump or motor with a movable swash plate is called a variable-displacement unit. A pump or motor with a fixed swash plate is called a fixed displacement unit. There are four pump-motor combinations, which are as follows (fig. 6-14):

Fixed displacement pump driving a fixed displacement motor (fig. 6-14, view A). This setup will give you constant horsepower and

Figure 6-13.- Connected cylinders with swash plates.

torque at the output with a steady input speed. If input speed varied, horsepower and speed will vary but torque will remain constant. Because both the pump and motor are fixed displacement, this system is like a gear drive; it transmits power without altering the speed or horsepower between the engine and the load.

Variable displacement pump driving a fixed displacement motor (fig. 6-14, view B). Since the pump is variable, output speed is variable and torque output is constant for any given pressure. This setup provides variable speed and constant torque.

Fixed displacement pump driving a variable displacement motor (fig. 6-14, view C). In this setup changing the motor displacement varies output speed. When motor displacement decreases, output speed increases, but output torque drops. When the setup is balanced, it gives a constant horsepower output.

Variable displacement pump driving a variable displacement motor (fig. 6-14, view D). This setup gives an output of both constant torque and constant horsepower. It is the most flexible of all the setups, but it is also the most difficult to control.

Figure 6-14.- Pump and motor combinations for hydrostatic drives.

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 of hydrostatic drive are as follows:
Infinite speeds and torque
Easy one-lever control
Smooth shifting
Shifts "on the go"
High torque available for starting up

Compact size
Reduces shock loads
Low maintenance and service
Flexible location- no drive lines

Eliminates clutches and large gear trains