Mobile and Industrial Hydraulic Valves and Systems: Screw-in cartridges for Higher Volume flows in Hydraulics

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.