D3C Series III, D4C Series III and D5C Series III HYSTAT Track-Type Tractors Hydrostatic Drive Operating Principle Caterpillar


Hydrostatic Drive Operating Principle
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1.1. General Operational Safety Tips

This machine has a hydrostatic drive system which transfers power from the engine to the tracks. The hydrostatic drive system eliminates the conventional type of transmission. The hydrostatic drive system does not use a bevel gear and a drive line. The hydrostatic drive system also eliminates the steering clutch and brake arrangement.

In the simplest form, a hydrostatic drive system is best explained as a hydraulic pump with a control system, a hydraulic motor, and high pressure lines that carry oil between the components. The pump converts mechanical power to hydraulic power in the form of oil flow and oil pressure.

To meet requirements, the machine must be able to move forward and rearward. The machine must also be able to turn. The pumps are designed to deliver varying oil flows to either the forward side or the reverse side of the hydraulic motors.

The track can be driven at varying speeds in either a forward direction or in a reverse direction.

Since there are two tracks, there are two hydraulic pumps, two sets of hydraulic lines, and two hydraulic motors.

For the forward modes or the reverse modes, the pumps and the motors work in conjunction with each other. The pumps can produce flow to either the forward side of the motors or to the reverse side of the motors. The pumps can vary the amount of flow in either direction in order to produce infinitely variable speed capabilities. To turn the machine, each pump is capable of reducing flow independently. This allows one track to be driven at a slower speed in order to cause the machine to turn.

Slowing the machine or stopping the machine is done by reducing the flow of oil to the drive motors or by stopping the flow of oil to the drive motors. Braking of the machine is caused by hydrostatic braking of the tracks.

The parking brakes are engaged while the service brake pedal is fully depressed. The parking brakes are engaged while the transmission lock lever is in the LOCKED position or while the engine is off.

The parking brakes are attached to the final drives. The parking brakes are spring engaged and oil pressure released.

The parking brakes disengage when the transmission lock lever is moved from LOCKED position. The parking brakes also disengage when the service brake pedal is released.

General Operational Safety Tips

The seat belt must be fastened at all times.

For a prompt stop, quickly depress the service brake pedal or quickly move the direction/speed control lever to the NEUTRAL position. Either movement causes an immediate stop.

For all turns that are on level ground or on side hill operations, move the direction/speed control lever toward the RIGHT position for right turns. Move the direction/speed control lever toward the LEFT position for left turns.

Brakes are applied under the following conditions.

  • The service brake pedal is fully depressed.

  • The transmission lock lever is in the NEUTRAL position.

  • The engine is off.

Information System:

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