200,500 & 600 SERIES
 

   The hydrostatic drive is an automatic fluid drive that uses fluid

   under pressure to transmit engine power to the drive wheels or tracks.

   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

   Technically, one difference is in the hydraulic drive system. On a Mini-Skid,

   the wheels are driven by a pair of hydraulic motors. These are powered by

   a fixed displacement pump, and controlled by a control valve. This pump

   also powers the lift arms, bucket, and auxiliary hydraulics through the

   same control valve.