The track drive hydraylic system is a closed-loop system called a hydrostatic transmission. Three axial piston, variable displacement motors (one motor on each track) are turned by hydraulic oil from the axial piston, variable displacement track drive pump. The hydraulic oil flows from the pump to the motors and then back to the pump in a closed loop.
The volume of oil discharged by the track drive pump is determined by the speed of the primary engine (engine B), which drives the pump drive gearbox, and by the tilt angle of the pump swashplate. The speed of engine B, of course, is set with the throttle control on the left console. The pump swashplate tilt angle is set with the pump (travel) controls located on the front console. The pump swashplate can be set at any angle up to 15 degrees either side of center.
The speed of rotation of the track motors, and thus the machine travel speed, is determined by the volume of oil discharged by the pump and by the tilt angle of the motor control plates. The motor control plates can be at maximum displacement (25 degree control plate angle) which provides maximum torque and minimum speed, or at minimum displacement (10 degree control plate angle) which provides maximum speed and minimum torque. The motors are at minimum displacement whenever the overdrive controls are at "On" and at maximum displacement when the controls are at "Off".
THE OVERDRIVE CONTROLS MUST BE AT "OFF" WHENEVER GOING UP OR DOWN GRADES OF MORE THAN 8 PERCENT. THE FRONT OVERDRIVE CONTROL MUST BE AT "ON" WHENEVER LOADING THE MACHINE ONTO A TRAILER.
Charge Pump Circuit
A gear-type charge pump is mounted on the rear of the main track drive pump. The charge pump shaft is coupled to the main pump shaft and therefore turns at the same speed. Hydraulic oil flows directly from the oil tank to the charge pump. From the charge pump the oil flows through an external 10 micron filter and then to the main pump. The filter has a 25 PSI bypass check valve that allows the charge oil to continue to flow to the pump if the filter gets clogged.
Within the body of the main pump are located three valves that control the flow of the charge oil: two valves are combination high pressure relief and makeup check valves, and the third valve is a charge pressure relief valve. See "Main Pump/Motor Circuit" for a description of the function of the high pressure relief valves.
Charge pressure oil is routed to the low pressure side of the main pump/motor circuit by one of the two makeup check valves. This charge pressure oil replaces oil lost from the main circuit because of pump and motor internal leakage and maintains a positive pressure in the low pressure side of the circuit. The second makeup check valve is held closed by the pressure in the high pressure side of the circuit.
The charge pump normally discharges more oil than is needed for makeup in the main circuit. The excess oil is released into the pump case by the charge pressure relief valve. The oil flows through the pump case, cooling the pump, and then to the hydraulic oil tank. The charge pressure relief valve maintains the charge pressure at a maximum of 320 PSI.
Charge pressure oil is also routed from the track pump to the track pump control valve in the forward console (see "Pump Displacement Control" on page 2-21).
Main Pump/Motor Circuit
The oil in the main circuit of the hydrostatic transmission flows in a continous 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 the pump swashplate is tilted from the neutral position. The volume of oil flowing in the circuit is determined by the engine speed and by the tilt angle of the pump swashplate (pump displacement).
Within the body of the pump are located two combination high pressure relief and makeup check valves. Each high pressure relief valve is connected to one side of the main pump/motor circuit. If the pressure in the high pressure side of the circuit exceeds the setting of the relief valve, the relief valve opens and releases some of the high pressure oil into the charge pump circuit. The high pressure oil flows through the other makeup check valve into the low pressure side of the main circuit, and/or through the charge pressure relief into the pump case.
Charge pressure oil from the track drive pump is sent to a control valve in the forward console. The travel control levers act through a linkage to position one of two spools in the control valve. Moving a travel control toward "Fwd" depresses one spool in the valve and moving the control toward "Rev" depresses the other spool. When a spool is depressed, the spool routes charge pressure oil out one of the valve ports to one end of the displacement control valve on the track pump.
The charge pressure oil acting on one end of the displacement control valve spool moves the spool to a position that allows charge pressure oil from within the pump to flow to one end of the pump control cylinder. The pressure in the control cylinder moves the control piston. The movement of the control piston tilts the pump swashplate. Anytime the charge pressure is removed from the control cylinder, strong springs center the control piston and thereby return the pump swashplate to neutral (0° tilt angle).
Pressure Override Control
The charge pressure oil that positions the pump control piston passes through a pressure override control valve before going to the displacement control valve and pump control cylinder. A spool in the pressure override valve is positioned by a spring to allow the charge pressure to pass on to the displacement control valve if the high pressure side of the main pump/motor circuit is not above 4500 PSI. The high pressure oil from the main pump/motor circuit is applied to the pressure override control valve spool in opposition to the spring. When the high pressure exceeds 4500 PSI, it overcomes the force of the spring to move the spool toward the spring. The movement of the spool opens a passage to the pump case, which reduces the charge oil pressure to the displacement control valve and pump control cylinder. The reduced pressure allows the pump control piston centering spring to move the swashplate toward neutral a certain amount and this reduces the pressure in the main pump/motor circuit to the override pressure.
Motor Displacement Control
Charge pressure oil from the track drive pump is sent to the two overdrive control valves in the left console. When the overdrive controls are at "Off", the charge pressure oil is stopped at the valves. When the overdrive controls are at "On", charge pressure oil from the front control valve is directed to the front track motor and charge oil pressure oil from the rear control valve is directed to both rear track motors.
Each track motor has a control plate that is positioned by a control piston. As long as no hydraulic pressure is applied to the control piston (overdrive control valve is at "Off"), a spring positions the control piston and control plate for maximum displacement. If the overdrive control valve is moved to "On", charge pressure is applied to control piston, moving the control piston and control plate all the way to the minimum displacement position.