II - 86
WSM Minor Change II Service Engineering Section
2) Better feeling at start and stop
To improve the start-up feeling, the counter-
balance spool must be activated quickly to
keep the start-up pressure at port M2 low.
To enhance the stop feeling, on the other
hand, the brake pressure at port M2 should
rise smoothly. In other words, it is important
how well the pressure at port M2 can be
controlled in starting and stopping the
machine.
3) Conventional pressure control method
The pressure at port M2 has been so
far controlled by giving better counter-
balance mechanism response and
mounting a relief valve with shockless
function. As illustrated in Fig. 6, the
shockless relief valve is activated uti-
lizing the difference between the pres-
sure receiving area (S1) and spring
chamber area (S2). While the free
piston provided around the relief
housing is moving, the M2 port pres-
sure is held at low level. When the
free piston has completely moved, the
relief valve gets activated at a speci-
fied pressure. The M2 port pressure is
thus raised in two stages, which
eases possible violent jolts.
Fig. 7 shows the brake pressure waveform of the travel motor with shockless relief valve. As
clear in this figure, the M2 port pressure is different at a first-speed stop and a second-speed
stop. If the stop feeling is adjusted in matching the shockless action pressure with the M2 port
pressure for second-speed stop, the feeling will not be just right at the M2 port pressure for first-
speed stop. If adjusted for the first-speed stop, to the contrary, the second-speed stop feels
unsmooth. In the existing construction, it is difficult to have both the first-speed and the second-
speed stop feelings equally good.
(1) Pump
(2) Check valve
(3) Counterbalance spool
(4) Control valve
(5) Tank
Fig.2 At motor stop
(2)
(3)
(1)
(4)
(5)
Braking started
Pressure [MPa]
M1 port pressure
M2 port pressure
Shockless relief activated
Fig. 3 Brake pressure waveform
(with shockless relief valve)
First speed
Second speed
0
9.8
9.8
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