General
The wheel and cutter drive systems are closed-loop hydraulic systems called hydrostatic transmissions. The main components of the cutter system are a variable positive displacement pump and a fixed positive displacement motor. The main components of the wheel drive system are a variable positive displacement pump and four variable positive displacement motors, one on each wheel assembly.
MAXIMUM DISPLACEMENT
PARTIAL DISPLACEMENT
MINIMUM DISPLACEMENT
PR-275-RT CUTTER JOG & AUXILIARY HYDRAULIC SYSTEM
PR-275-RT WHEEL DRIVE
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 individual pump control lever on the control console. The speed and torque output of the cutter motor depend only on the output of the pump and the load placed on the system. The speed and torque output of the wheel motors depend on the tilt angle of the motor swash plate as well as on the output of the wheel drive pump. The motor swash plates are at partial displacement for travel and at maximum displacement for work (cutting operation).
Cutter And Wheel Drive Hydrostatic Transmissions
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 motor 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 connected across the main circuit 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.
The 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. Springs center 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, or when the pump is at zero displacement (in neutral).
Cooling Circuit
Excessive cooling fluid (hydraulic oil) from the cutter manifold charge pressure relief valve enters the motor case and then flows through the case drain line to the return system. The excess cooling fluid from the wheel pump and cutter pump charge relief valves enters the respective pump case and then flows through the case drain lines to the return system. A shuttle valve assembly mounted near each wheel motor, ports at least 1 gallon of hydraulic oil per minute from the main pump/motor circuit to the motor case, to cool the motor whenever the pump swash plate is not in the neutral position. The hydraulic oil flows through the motor case, through the case drain line to the return system.
When the cutter or wheel pump is in neutral, the shuttle valve is centered and the excess flow from the charge pump is directed to the pump case by the charge relief valve in the charge pump. From the pump case, the oil flows through the case drain line to the return system. There is no cooling flow through the motors when the pumps are in neutral
Displacement Control
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.
NOTE: THE OVERRIDE PRESSURES FOR THE CUTTER AND WHEEL PUMPS ARE FACTORY-SET AT 4,500 PSI. DO NOT ADJUST ANY HIGHER, OR OVERHEATING AND DAMAGE TO THE SYSTEM WILL OCCUR.