Counter Balance Function - Kubota U20-3 Workshop Manual

U20-3, U25-3 WSM Hydraulic system (Mechanism section)

• Counter balance function

After the hydraulic oil discharged from the hydraulic pump is supplied to the port P1 of the counter valance
valve via the control valve, the hydraulic oil passes through the check valve L (31-2) and flows through the
port M1 into the piston motor, working to make the piston motor rotate.
The return oil from the piston motor flows through the port M2 into the counter valance valve, where the
check valve R (31-2) stop the return oil from flowing, which causes the pump discharge pressure to rise.
On the port P1 side, the further pressurized hydraulic oil passes through the orifice L and acts on the end
face of the plunger (31-1), working to move the plunger to the right with a force proportionate to the
pressure against the spring R (37) on the opposite side.
When a certain pressure is reached, the plunger (31-1) starts to move to the right. As a result, the return
oil passes through the plunger helix, flows into the port P2 while applying a back pressure to the port M2,
and returns to the tank via the control valve. When the pump discharge pressure rises further, the
opening of the plunger throttle becomes larger and the back pressure applied to the port M2 decreases.
In this manner, the opening of the plunger throttle in the return pathway is adjusted automatically so that
the piston motor rotates at a speed corresponding to the port P1-side pressure, that is, the port P1-side
flow rate.
IV-M-49