The track drive hydrostatic transmission is a closed-loop hydraulic system (hydraulic oil flows from the pump to the motors and then back to the pump in a closed loop). The main components of the system are a variable positive displacement pump and three fixed positive displacement motors.
The volume of oil pumped by the variable displacement pump is determined by engine speed and by the tilt angle of the pump swash plate. Engine speed is, of course, controlled with the throttle lever and the pump swash plate tilt angle is controlled with the pump control lever (travel control) on the control console. The speed and output of the motors depend only on the output of the pump and the load placed on the system.
Charge Pump Circuit
Hydraulic oil flows from the oil tank, through a filter, to a charge pump mounted on the main pump. The charge pump provides the flow of oil needed for cooling purposes, to maintain a positive pressure on the low pressure side of the main pump/motor circuit, for pump control purposes and for internal leakage makeup.
Main Pump And Motor Circuit
The oil in the main circuit of the hydrostatic transmission flows in a continuous closed loop from the pump to the motors and back to the pump. The direction of oil flow in the circuit, and thus which side of the circuit is the high pressure side, is determined by the direction in which the pump swash plate is tilted from the neutral position.
Oil from the charge pump is directed to the low pressure side by one of two check valves. The second check valve is held closed by the oil in the high pressure side of the circuit.
A manifold valve assembly on each track motor is connected across the main circuit and contains two pilot-operated pressure relief valves. The pressure relief valves serve to prevent high pressure surges in either side of the circuit by dumping oil from the high to the low pressure side. High pressure surges may occur during rapid acceleration or braking, or with sudden application of a load.
Each manifold valve assembly also contains a shuttle valve and a charge pressure relief valve. The shuttle valve establishes a circuit between the low pressure side of the main circuit and the charge pressure relief valve, so that the relief valve can dump excess cooling oil added by the charge pump and control the charge oil pressure. A spring centers the shuttle valve in the closed position so that no high pressure oil is lost from the main circuit during the transition period when the pressures are being reversed.
Excess cooling fluid from the motor manifold charge pressure relief valve enters the motor case and then flows through the case drain lines, through the pump case, to the return manifold. When the main pump is in neutral, the shuttle valve is centered and the excess flow from the charge pump is directed to the cooling circuit by the charge relief valve in the charge pump. The cooling oil flows through the pump case to the return manifold.
During normal operation the displacement of the hydrostatic transmission pump is controlled by the displacement control valve which is in turn controlled by the control lever. However, at a predetermined system pressure, called the override pressure, the pressure override control valve will override the displacement control valve to reduce the pump displacement and thus prevent continued operation above the override pressure.