62 iVT International Off-Highway 2020
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Multiple pilot operations
Various valve types are suitable for the pilot
operation. In the example of a crane it is a small
pressure reducing valve, with which the control
pressure can be continuously controlled up to 20
bar. Following the control pressure the piston in
the spool valve cartridge moves against the spring
force. The more control pressure is acting on the
piston, the more the valve opens and the flow
changes. Corresponding to the volume flow and
the system pressure the winch motor rotates.
The small pressure reducing valve can be
directly operated manually with a hydraulic
joystick or, equipped with a suitable solenoid,
proportionally remote-controlled by electronics.
Thus the operator has the possibility to
hydraulically trigger a function from a distance or
remote-control it with an electronic system by a
computer or joystick. When an operator remotecontrols
the valve, he can see, what is happening
at the winch and implement corrections when
needed. The winch system then operates in a
closed, controlled control circuit. Without visual
contact of the operator the winch would operate
in an open control circuit, undesirable
movements and incidents would then be possible.
In other applications the pilot operating valve
can be replaced by an on/off valve with fixed
control pressure relief, then the spool valve
cartridge also has an on/off function.
Controlling with a pressure relief is also
possible but is not efficient because pilot oil
continuously flows to the tank so energy is lost.
For the spool valve cartridges, spools for two
switching positions (4/2-way) and three
switching positions (4/3-way) are available.
In case of two switching positions, only one pilot
oil connection is under pressure, the other having
to be connected to the tank to ensure a safe
switching of the spool. In the case of three
switching positions both connections are
supplied with pilot oil.
High volume flows
The spool valve cartridge is intended for high
volume flows, M33 up to 150 l/min, M42 up to 250
l/min. In order to master the high flow forces and to
assure a stable functioning, a high setting force of
the pilot control and a correspondingly strong
return setting force of the spring are necessary. The
spring is designed for an unlimited continual
strength in case of the foreseen operating path. The
admissible system pressure is indicated as 315 bar.
For the valve manufacturer the particularity of
the large cartridges is the long, thin-walled valve
components, which call for a high degree of
experience in their manufacture.
Thanks to the recessed installation, the risk of
leakage towards the outside is very low with the
compact block construction with screw-in
cartridges in comparison with screwed-on valves.
Because only the screw-in part projects from the
block in case of the spool valve cartridge, the
surface protection is easy to ensure. The valves are
zinc-nickel coated, they can additionally, together
with the block, be provided with a color coating. An
enhanced surface protection such as a zinc-nickel
coating or external components protected against
corrosion is also possible.
With the spool valve cartridges in particular and
with the other screw-in valve types from the
Wandfluh program, the block-builder has a
comprehensive selection of valves at his disposal,
which enables a compactness of design of the
hydraulic control blocks, which has never been
known previously. iVT
Author: Daniel Bieri, Wandfluh AG, Switzerland
Optimized design of
a control block
with M42 screw-in
of a 4/3-way function