Hyundai HX140L, Manual

Page: 133 / 819

2-65
Operation
Holding operation
When the control valve is at neutral position, VA and VB ports are connected to the tank, and the
spring (33) located on both spool ends holds the spool (31) at central position.
Therefore, the passages from VA to MA and VB to MB are closed, which result in closing MA
and MB ports connected to hydraulic motor.
Since the passage to parking brake is connected to the tank line, the brake cylinder pressure is
equal to the tank pressure and the brake is applied by the springs. Thus, the rotation of the motor
is mechanically prevented.
If external torque is exerted on the motor shaft, the motor would not rotate as usual by this
negative parking brake.
In case the brake should be released for some reason, pressure is built on MA or MB port. But,
due to oil leakage inside hydraulic motor or so, high-pressure oil escapes from the closed circuit
and motor rotates a bit. So, the cavitation tends to occur in the lower pressure side of the closed
circuit. Then, the check valve, built in the spool (31), operates to avoid the cavitation and opens
the passage from VA to MA or from VB to MB. Then the oil equivalent to the leakage is sucked
from the tank line to the closed circuit.
(2)
刮
VB
VA
MB MA
MB MA
DR
VB VA
DR
125LCR2TM29