The constructional possibilities of modern
machine engineering lead to ever larger and higher performance machines in the
most diverse industrial fields. Movements have to be carried out more rapidly
and efficiently. Part of this is also that machines and devices should provide
more possibilities regarding functionality, while, if poossible, they should
not increase in weight. It must be the objective of the designer to achieve
functionality and nonetheless use materials economically, in order to keep the
investments within limits and to maintain the necessary drive energy as low as
possible. Energy consumption always means financial expenditure. Finally, for
the investor the „Return on Investment“ has to be right.
In the case of large
hydraulic consumers the requirement of hydraulic liquid is correspondingly high.
The valves must be of the suitable size, so that the volume can flow through
the various bores and control openings in the hydraulic control unit and in the
valves with as little energy loss as possible. For the operator of the plant energy
loss always entails costs. It is possible, that a certain energy loss is
accepted, so that a certain task can be carried out more rapidly. If the hourly
rate of the machine is higher than the costs of the energy loss, this is an
advantage for the operator. Nonetheless the energy loss has to be kept within
limits.
The requirement for a compact construction in mobile tools is increasing.
For this reason, the Wandfluh company
has complemented the screw-in valve range in the ISO-sizes M33 and M42 with a spool
valve cartridge.
The construction of valve functions in blocks made of steel or aluminium
enables an individual adaptation to the available space, without having to
accept losses in the flexibility of the functions. The well proven screw-in
technology here provides the possibility of constructing flexible solutions
adapted to the application. Because of the internal connections between the
individual valves, the block construction has the advantage of less leakage points
in comparison with the solution of aligned and flanged valve elements. This
block construction simplifies the decentralisation of the control functions, as
a result the designer can place them in the machine exactly where they are
required.
The block construction provides the designer with a lot of flexibility
in selecting the suitable valve size per function. With this, on the one hand a
technical solution precisely adapted to the requirements is achieved, and on
the other hand it is possible to optimise the space requirements and the costs.
The new spool valve
cartridges correspond to the requirements defined at the beginning, namely to
design the systems as efficient and economical. In addition, part of the valve
range of the sizes M33 and M42 in addition are pilot operated pressure relief
valves and pressure control valves with manual as well as proportional
adjustment, manual and proportional throttles as well as pressure-compensated
flow control valves in 2-way and 3-way versions, also with manual as well as
proportional adjustment.
For applications,
where a load has to be securely held, pilot operated, leakage-free poppet
valves are available. For demanding load holding and lowering functions,
tight-seating, proportionally adjustable and load–compensated flow control
valves have proved successful. The large selection of screw-in valves is
designed for operating pressures up to 420bar. Corresponding to the size of
these valves, depending on the valve type, volume flows of up to 400l/min can
be controlled. A very good controllability of the screw-in valves in the fine
range is the prerequisite for operating the system of the customer with high
precision and the demanded efficiency.
The type screw-in spool
valve does not concern an independent element, it has to be externally pilot
operated with a hydraulic pressure. Therefore additionally driving elements or
pilot operation valves are required. For the 4/3-way function (4 connections, 3
switching positions) no bores are foreseen in the ISO cavity system. For this
reason, Wandfluh has elaborated a proprietary cavity type. The size of the
valve cartridge is determined by the size of the volume flow and the required
lateral bores for the inlet – and outlet tubes.
The pilot operated
valve can be arranged directly at the spool valve cartridge, but does not have
to be. The pilot control can be located anywhere and connected to the control
block with small control lines.
Various valve types
are suitable for the pilot operation. In case of the example of a crane it is a
small pressure reducing valve, with which the control pressure can be
steplessly controlled in the range of up to 20bar. 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
hoist 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. From
a control point thus the operator can hydraulically trigger a function from a
distance or remote-control it with an electronic system by a computer or
joystick. When an operator remote-controls the valve, he can see, what is
happening at the winch and implement corrections when needed. The winch system
then operates in a closed circuit, 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.
The driving with a
pressure limitation is also possible, but is not efficient, because control oil
continuously flows to the tank and correspondingly energy is lost.
For the spool valve
cartridges, pistons for two switching positions (4/-2-way) and three switching
positions (4/3-way) are available. In case of two switching positions, only one
control oil connection is under pressure. The other control oil connection has
to be connected with the tank, otherwise the piston does not switch. In case of
three switching positions both connections are supplied with control oil.
The spool valve
cartridge is foreseen 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 force of the spring are necessary. The spring is designed for an
unlimited continual strength in case of the foreseen operating path. The
permissible system pressure is indicated as 315bar.
For the valve
manufacturer the particularity of the large cartridges is the long, thin-walled
valve components, which call for a high degree of experienced in their
manufacture.
Thanks to the embedded
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 coated and therefore satisfy the general requirements, they can
additionally, together with the block, be provided with a colour coating. Also
an enhanced surface protection such as a Zn/Ni-coating or external components
protected against rust is conceivable.
With the spool valve
cartridges in particular and with the other screw-in valve types from the
Wandfluh programme, the block-builder has acomprehensive selection of valves at
his disposal, which enables a compactness of design of the hydraulic control
blocks, which has never been known previously.
by Paul Häberli, Wandfluh AG, Frutigen, Switzerland
Paul Häberli, since 1988 design engineer at the head office of Wandfluh AG, Hydraulics + Electronics, Frutigen, Switzerland