Stopping (Braking); Runaway Prevention Function - Kubota KX080-3 Workshop Manual

WSM KX080-3 Mechanism
(323)
(328)
(323)Spool
(327)Valve
(328)Spring
(323)
(327)

2. Stopping (braking)

When the pressure oil is interrupted, the hydraulic motor
gets stopped. Here are the details.
When the pressure oil to the port A is blocked, the pres-
sure at the port A and in the chamber b drops. Accord-
ingly the spool (323) starts sliding from left to the neutral
position by the force of spring (328). Then the oil in the
chamber b is expected to flow through the passage a
of spool (323) toward the port A. The throttling effect of
passage a , however, produces a back pressure and
controls the return speed of spool (323).
The hydraulic motor, even after the block of the pressure
oil at port A, is expected to keep running by its inertial
force. The hydraulic oil from the hydraulic motor is thus
expected to pass through the port D, the spool's groove,
the rear flange (301) back to the port B.
When the spool (323) has returned exactly to the neutral
position, the return passage of the hydraulic motor is
completely blocked and the hydraulic motor stops itself.
In doing so, the brake valve narrows down the return
passage of the hydraulic motor under the sliding speed
control of spool (323) as well as by the spool groove's
(327)
shape. The resulting back pressure brings the hydraulic
motor, still under its inertial force, to a smooth halt.
The valve (327) gets activated by a very small negative
pressure. The oil passage between the circuit of port A
and the motor's suction port C gets open, which pre-
vents cavitation in the hydraulic motor.

3. Runaway prevention function

If an external force affects the hydraulic motor during
its steady run, a runaway trouble may occur. This may
happen on the travel motor of a power shovel, for
example, which is running down a steep slope. Here is
the mechanism to prevent such trouble.
(328)
Let's suppose an external force applies upon the
steady-running hydraulic motor. The hydraulic oil (sup-
plied at port A or B) drops in pressure. Before the pres-
sure nears a runaway level, the spool (323) starts
sliding to the neutral (center) position. According to the
spool's shift, the hydraulic oil passage from the
hydraulic motor's outlet port D to the port B becomes
narrower. The hydraulic oil flow rate from port D to B is
thus limited. At the same time, the back pressure at
port D rises.
In this manner, the flow rate and pressure at the
hydraulic motor's outlet port are controlled, preventing
the runaway problem.
(323)Spool
(327)Valve
(328)Spring
(405)Parking brake
(328)
IV-M-101
IV Hydraulic System