Traction Circuit: 4WD (Mow)
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 oil for the charge cir- cuit. This gear pump is driven directly off the traction pump. It provides a constant supply of charge oil to make up for oil that is lost due to internal leakage in the traction pump and motors.
Pump flow for the charge circuit 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 limited 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 manifold.
Forward Direction
When the transport/4WD switch is in the 4WD (mow) position and the traction pedal is pushed in the forward direction, oil from the piston pump is directed to the front wheel motors and 4WD manifold. Oil flow to the front wheel motors drives the motors in the forward direction and then returns to the hydrostat. Oil flow to the 4WD manifold enters the P1 port and then is directed to the PD1 cartridge and out of the manifold M1 port to drive the rear axle motor in the forward direction. Oil returning from the rear motor
When going down a hill, the tractor becomes an
Reverse Direction
The traction circuit operates essentially the same in re- verse 4WD (mow) as it does in the forward direction. However, the flow through the circuit is reversed. Oil flow from the hydrostat is directed to the front wheel motors and also to the 4WD manifold. The oil to the front wheel motors drives them in the reverse direction and then re- turns to the hydrostat. The oil to the 4WD manifold en- ters the manifold at port P2 and flows through pressure reducing valve (PR) which limits the down stream pres- sure to the rear axle motor to 650 PSI (45 bar) so the rear wheels will not scuff the turf during reverse operation. This reduced pressure flows through the PD2 cartridge and out port M2 to the rear axle motor. Return oil from the rear motor
Hydraulic System
Groundsmaster | Page 4 | Hydraulic System |
