Allmand Brothers TLB 535 - Manual (Page 22)

22
HYDROSTAT DUMP VALVE OPERATION
The purpose of the dump valve is to allow the movement of a disabled vehicle or if you have
a vehicle that you just want to push a short distance, without starting the engine. When a
hydrostatic driven vehicle is shut down it is virtually im-
possible to move the vehicle without opening the hydro-
static closed loop circuit. If an attempt is made to push
the vehicle the hydraulic motor becomes a pump, trying to
pump oil to the hydrostatic pump. This creates a hydraulic
lock between the motor and the pump. To overcome this
condition, a dump valve has been installed between the
high pressure relief valves in the backplate of the piston
pump. The dump valve is a plug that contains a rotating
stem which has a flat spade end that fits between the two
ends of the high pressure relief valves. When the dump
valve is in the “closed position”, the relief valves are also
in the closed position as shown in the top illustration.
When the dump valve stem is rotated 90 degrees, the flat
spade end spreads the relief valves to the “open position”
as shown in the lower illustration. This allows the oil in the
hydrostatic closed loop to “by-pass” around the high pres-
sure relief valves inside the pump backplate. The
by-passing of oil inside the pump backplate will allow the
motor to rotate freely when the vehicle is moved a short
distance. The dump valve is intended only for moving a
vehicle a very short distance and not intended for towing a
vehicle behind a truck or tractor. NOTE: serious damage
to the hydrostatic drive will result if vehicle is towed.