Traction Circuit: Transport (2WD)
The traction circuit piston pump is a variable displace- ment pump that is directly coupled to the engine fly- wheel. Pushing the traction pedal engages a hydraulic servo valve which controls the variable displacement piston pump swash plate to create a flow of oil. This oil is directed to the front wheel and rear axle motors. Oper- ating pressure on the high pressure side of the closed traction circuit loop is determined by the amount of load developed at the fixed displacement wheel and axle mo- tors. As the load increases, circuit pressure can in- crease to relief valve settings: 4000 PSI (274 bar) in forward and 5000 PSI (343 bar) in reverse. If pressure exceeds the relief setting, oil flows through the relief valve to the low pressure side of the closed loop traction circuit. The traction circuit provides operation in either 4WD (mow) or transport (2WD).
Traction circuit pressure (forward and reverse) can be measured at test ports on the sides of the machine.
The traction circuit pump and motors use a small amount of hydraulic fluid for internal lubrication. Fluid is designed to leak across traction pump and motor com- ponents into the case drain. This leakage results in the loss of hydraulic fluid from the closed loop traction circuit that must be replaced. The charge circuit is designed to replace this traction circuit leakage.
The gear pump section that supplies oil to the steering and lift/lower circuits also provides charge oil for the traction circuit. This gear pump is driven directly off the traction pump. It provides a constant supply of charge oil to the traction circuit to make up for oil that is lost due to internal leakage in the traction pump and motors.
Charge pump flow is directed through the oil filter and to the low pressure side of the closed loop traction circuit. A filter bypass valve allows charge oil flow to the closed loop if the filter becomes plugged. Charge pressure is li- mited to 250 PSI (17 bar) by a relief valve located in the oil filter manifold. Charge pressure can be measured at the charge circuit pressure test port on the oil filter man- ifold.
Forward Direction
With the transport/4WD switch in the transport position, solenoid valve (SV) in the 4WD control manifold is ener- gized. The solenoid valve spool shifts to direct charge pressure that shifts the PD1 and PD2 control valve spools. The shifted PD1 valve prevents hydraulic flow from the piston pump to the rear axle motor. With flow blocked to the rear axle motor, all pump flow is directed to the front wheel motors to allow a higher transport speed in the forward direction.
Without flow to the rear axle motor, the rotating rear wheels drive the axle motor so it acts like a pump. Inlet oil to the axle motor is provided by a check valve that al- lows charge oil into the rear axle motor circuit. Oil leav- ing the axle motor enters the 4WD control manifold at port M2 and is directed back to the axle motor through the shifted PD1 cartridge and manifold port M1. To allow for rear wheel loop cooling when in forward transport op- eration, a small amount of oil exits through the shifted PD1 and PD2 cartridges that returns to the reservoir.
Reverse Direction
The traction circuit operates essentially the same in re- verse transport (2WD) as it does in the forward direction. However, the flow through the circuit is reversed. The shifted solenoid valve (SV) and directional valves PD1 and PD2 in the 4WD manifold prevent oil flow from the rear axle motor. Oil flow from the hydrostat is therefore directed to only the front wheel motors. This oil drives the front wheel motors in the reverse direction and then returns to the hydrostat. Oil circulation through the rear axle motor loop is the same as in the transport (2WD) forward direction.
Hydraulic System
Groundsmaster | Page 4 | Hydraulic System |
