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WSM Minor Change II Service Engineering Section
4) Improvement of the counterbalance spool performance
The characteristics and responsiveness of
the counterbalance spool's variable orifice
are adjustable to better control the M2 port
pressure. At the start, the spool's spring
force gets weak for the counterbalance
spool to switch over at low pressure. This
makes for smooth start-up. At the stop, the
variable orifice's characteristic settings and
the counterbalance spool's return time are
adjusted to produce the M2 port pressure
smoothly.
The variable orifice characteristics refer to
changes in the counterbalance spool ori-
fice's opening area. As the spool moves, the
opening area changes, which produces a
pressure corresponding to the opening
area. The counterbalance spool's return
time is determined by the spring force and
the orifice opening. The stronger the spring
force and the larger the orifice opening, the
quicker the counterbalance spool returns. A
weaker spring force and a smaller orifice
opening, contrarily, bring back the spool
slowly. As discussed above, the counterbal-
ance spool's orifice characteristics and
return time are preset, and the actual
machine will be matching-tested to achieve
better feeling.
In case of KX91-3, two orifices are incorporated in the pilot line and counterbalance spool has
several holes for the variable orifice function as shown in Fig. 5.
With this variable orifice, spool stroke and opening area of the counterbalance spool is as
shown in Fig. 4.
Fig. 4 Spool stroke and opening
stroke
Orifice's opening area
FIg. 5 Counterbalance spool and orifices
Orifice 1 Orifice 2
Small holes for variable orifice
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