Wheel Drive Hydrostatic Transmission
General Description
The wheel 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 two fixed, positive displacement motors (one motor on each left wheel).
The volume of oil pumped by the variable displacement pump is determined by engine speed and by the tilt angle of the pump swashplate. Engine speed is, of course, controlled with the throttle lever and the pump swashplate tilt angle is controlled with the pump control lever (travel control). The speed and output of the motors depend only on the output of the pump and the load placed on the system.
FORWARD - TYPICAL HEAVY DUTY VARIABLE PUMP - FIXED MOTOR TRANSMISSION SCHEMATIC WITH PRESSURE OVERRIDE VALVE
Charge Pump Circuit
WHEEL DRIVE PUMP
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 both of 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 swashplate is tilted from neutral.
Oil from the charge pump is directed to the low pressure side of the main circuit 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 wheel 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.
PR-105 WHEEL DRIVE HYDRAULIC SYSTEM (504122 & SUBSEQUENT)
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 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.
Cooling Circuit
Excessive cooling fluid (charge oil) from each motor manifold charge pressure relief valve enters the motor case. On machines without an auxiliary cutter, the oil then flows through case drain lines, through the pump case, through a 33 micron filter, through the oil cooler to the oil tank. On machines with an auxiliary cutter, the oil from the front motor flows through case drain lines, through the pump case to a tee in the case drain lines where it joins the oil from the rear motor. The oil then flows through a 10 micron filter to the oil tank. When the main pump is in neutral, the shuttle valve in each manifold 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 case drain lines.
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 travel control. However, at a predetermined system pressure called the override pressure, the pressure override control valve will override the displacement control valve to prevent continued operation above the override pressure.
Accessory Hydraulic System
Pressure And Return
The accessory system pump is an axial piston, variable, positive displacement pump similar in principle to the wheel drive pump. The accessory pump, however, has no manual control; the angle of the pump swashplate is controlled by a pressure compensator that automatically changes the pump swashplate angle to maintain a constant system pressure. The pressurized hydraulic oil flows from the pump to the pressure and return manifold and then to the control valves in the consoles. The control valves route the oil to the correct port in the cylinder or other hydraulic component to perform the required function. Return oil from the hydraulic components flows back through the valves through the pressure and return manifold, to the hydraulic oil tank.
Steering Control
A steering valve and a steering cylinder for each left wheel make it possible to steer either left wheel independently or in conjunction with each other. When either control lever is moved to "Left", the steering valve routes hydraulic pressure from the pressure and return manifold to the head end of the front steering cylinder and the rod end of the rear cylinder, turning the machine to the left. When the lever is moved to "Right", the hydraulic pressure is sent to the rod end and head end of the respective cylinders. Return oil from the steering cylinders goes to the steering valve, to the pressure and return manifold, and then to the hydraulic oil tank.
PR-105 CYLINDERS HYDRAULIC SYSTEM
Machine Tilt Control
The tilt (slope) of the machine and attached cutter is set by the hydraulic (elevation) cylinder in the right leg. When the machine tilt control lever is moved to "Slope Left", the control valve routes hydraulic pressure from the pressure and return manifold to the head end of the right leg elevation cylinder, extending the cylinder rod and raising the right side of the mainframe. This tilts the mainframe and attached cutter to the left. When the control lever is moved to "Slope Right", the right leg elevation cylinder rod retracts, lowering the right side of the mainframe.
Cutter Elevation Control
The elevation of the cutter can be adjusted by moving the cutter elevation control to "Up" or "Down". When the control lever is moved to "Up", the control valve routes hydraulic pressure from the pressure and return manifold to the rod end of the cutter elevation cylinder, retracting the cylinder and raising the cutter. When the control lever is moved to "Down", the cylinder rod extends and lowers the cutter.
Auxiliary Cutter Drive Hydraulic System
The main components of the open-center auxiliary cutter drive hydraulic system are a gear-type pump; a fixed, positive displacement, piston-type motor and a pilot operated pressure relief valve. The pump is driven directly by the pump drive gearbox and therefore discharges pressurized hydraulic oil anytime the engine is running. The oil discharged by the pump flows to the pressure relief valve. If the valve is open, the oil flows through the valve, through a 33 micron filter, through the oil cooler, and then to the oil tank. If the valve is closed, the oil flows to the auxiliary cutter motor, through the motor, and then back to the downstream side of the valve.
The pressure relief valve is normally open so the hydraulic oil from the auxiliary cutter pump normally flows through the valve and back to the oil tank. When the operator moves the auxiliary cutter control to "On", the valve sends accessory oil pressure to the pressure relief valve, closing the relief valve. The relief valve then acts as a typical relief valve that will open if the pressure to the auxiliary cutter motor exceeds 3000 psi (20 685 kPa).
If the cooling system return filter gets clogged, the 65 psi (448.2 kPa) check valve will open and send the return oil to the oil cooler, bypassing the filter. If the oil cooler gets clogged or the hydraulic oil is cold, the 20 psi (137.9 kPa) check valve will open and cause the oil to bypass the oil cooler.
Water Spray System
The water spray system sprays water into the housing of the cutter being used. The water helps cool the cutter and holds down the dust created by the cutting operation. By eliminating the dust, the spray system prevents a possible health and traffic hazard, makes operation of the machine easier and safer because of better visibility, and reduces required machine servicing. It is obvious that the water spray system should be used anytime the machine is cutting.
The water pump, belt driven by the engine, draws water from the 100-gallon (378.4 L) water tank. The pump sends the water to a selector valve that routes the water to the "Main Cutter" or "Auxiliary Cutter". The pump is operating anytime the engine is running, so the hose from the pump to the selector valve will be pressurized anytime the engine is running.
Anytime the machine is parked outside for a length of time in freezing temperatures, the water should be drained from the tank, pump, and hoses to avoid damage from freezing water. See page 4-14 for procedures.
| NOTICE |
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IT IS EXTREMELY IMPORTANT THAT THE WATER SPRAY SYSTEM FUNCTION PROPERLY AT ALL TIMES. IF IT DOES NOT, LARGE AMOUNTS OF DUST WILL LEAVE THE CUTTER AREA AND BE DRAWN INTO THE RADIATOR AND OIL COOLER, CAUSING OVERHEATING OF THE ENGINE AND HYDRAULIC OIL. |
Electrical System
The PR-105 has a 12 volt, negative ground electrical system. Starting power for the engine is supplied by one or two 12 volt batteries. A 12 volt, 55 amp alternator driven by the engine provides the electrical power for the battery charging system and for machine operation while the engine is running.