Toro 2020, 5020, 3020 Hydraulic Flow Diagrams, Traction Forward, Hydraulic System Rev. A, Page 4

Hydraulic Flow Diagrams

Traction Forward

The traction circuit of the hydraulic system consists of a hydrostat connected in a closed loop circuit to three or- bital vane wheel motors. Hydraulic fluid losses are de- signed to occur from case drain leakage of the traction pump (P1) and bleed off from the right wheel motor (M1). These losses are replenished by the charge pump (P2), which is integral to the hydrostat.

The engine drives traction pump (P1) directly through a coupling. The traction pump is a variable displacement piston pump. The traction pedal connects through a link- age to the trunnion shaft and swash plate of the pump. With the engine running and the traction pedal in the neutral position, P2 supplies no flow to the wheel mo- tors. When the traction pedal is pressed to the forward position, the linkage from the pedal positions the swash plate in the traction pump so oil flows out port B. Oil flow out of port B goes to the wheel motors and turns them in the forward direction. On the SP 5020, the oil flow goes through the front motor first and then through the left and right rear wheel motors. Oil flowing out of the wheel motors returns to port A of the hydrostat and is continuously pumped out of port B.

The hydrostat uses a small amount of hydraulic fluid for internal lubrication. Fluid is designed to leak across pump parts into the case drain. This leakage results in the loss of hydraulic fluid from the closed loop circuit that must be replenished.

The charge pump (P2) is a fixed displacement gerotor pump. It is driven directly off the traction pump. The pump replenishes the closed loop circuit with fluid from the tank. The charge relief valve (R2) supplies sufficient head so that charge pump flow is guided to the low pres- sure side of the traction circuit through one of two check valves. Pump flow in excess of replenishment require- ments is relieved through the charge relief valve back to the tank.

The right wheel motor bleeds off a small amount of hy- draulic fluid for cooling of the closed loop circuit. This bleed off happens in the forward direction only. The high pressure side of the motor forces a shuttle spool to shift against a spring. The pressure drop across the motor causes a small amount of fluid to bleed off through a fixed orifice on the low pressure side of the motor and then through the shuttle spool. This bleed off returns to the tank through the oil cooler.

The acceleration valves reduce the rate of change in ac- celeration (jerkiness) when hydrostat output is in- creased by the action of the operator. An increase of pressure on the output side of the hydrostat will by–pass some pump flow to the low pressure side of the pump. The valve on the high pressure side closes at a predeter- mined rate as pressure increases. This gives the hy- drostat a smooth acceleration rate when the swashplate is stroked rapidly.

Hydraulic System (Rev. A) Page 4 – 8 Sand Pro 2020/3020/5020