For low flow rates a manually operated rotary directional valve can be used, as shown in Fig. 2.26. In this case an optically flat rotary spool with pressure-loaded seats ensures either zero or near-zero leakage between ports. Two- or three-position valves are available and the flow can be throttled by adjusting to intermediate positions (i.e. partially opening the flow path).
A B
P T
T A
P B
A B
P T
T A
P B
A B
P T
T A
P B
Fig. 2.26 Rotary directional valve Fig. 2.25 Directional valve function
When electrical operation is required, the most commonly used directional valve is a solenoid-operated sliding-spool valve. In fact this is probably the most common of all hydraulic components. The valve consists of a body incorporating four main flow ports: a pressure port (P), a tank port (T) and two service ports (A and B) (Fig. 2.27). A bore in the valve body houses a closely fitting spool, which is moved by means of electrical solenoids on one or both ends. Springs can also be fitted to return the spool to a certain position when the solenoid is de-energised.
The solenoids consist of a wire coil surrounding a core tube, inside of which is a sliding armature connected to the spool via a push pin (Fig. 2.27). When an electrical current is passed through the coil it creates an electromagnetic field, which attracts the armature towards the pole piece. As the armature moves it pushes the spool across within the valve body to open flow paths between the four ports, depending on the configuration of the spool (Fig. 2.28). The magnetic field collapses when the solenoid coil is de-energised, allowing a spring on the opposite end of the spool to push it back to its initial position.
P T
A B
A B
T P
POLE PIECE
ARMATURE COIL MANUAL OVERRIDE
Fig. 2.27 Direct-acting solenoid directional valve
P T
A B
SOLENOID ENERGISED
A B
T P
Fig. 2.28 Direct-acting solenoid directional valve (energised)
Apart from the basic valve size, many different configurations and options for solenoid valves are available. These include the following:
• Valve configuration:
– single solenoid, two position, spring offset – double solenoid, three position, spring centred
– double solenoid, two position, detented (i.e. the valve stays in position when the solenoid is de-energised until the opposite solenoid is energised).
• Flow path options – many options are available, in particular in the centre condition of three-position valves. The four options illustrated in Fig. 2.29 are probably the most common, but there are several more.
• Solenoid supply voltage (and frequency for AC voltage solenoids).
• Type of electrical connector used.
• Soft-shift options – these slow down the speed of spool movement (to reduce shock in the system). This is normally achieved by orifice plugs in the solenoid itself.
• Manual override button options – these are typically recessed buttons in the ends of the solenoids which enable the valve spool to be moved manually in the event of an electrical power failure or during machine commissioning.
• Indicator lights or LEDs – these are used to signal if a solenoid is energised or de-energised.
• In-built switches: to detect the actual position of the valve spool (often required in safety circuits).
• On-board electrical power switching – this enables the valve to be connected directly to a programmable logic controller (PLC).
• Proportional solenoids – these are used instead of simple on/off solenoids to provide additional control (see Chapter 5 for further discussion of these).
SPRING CENTRED
DETENTED SPRING OFFSET A B
T P
PROPORTIONAL
Fig. 2.29 Directional valve symbols WARNING
Be very careful when using manual overrides to ensure that dangerous operation of the machine cannot occur when the valve is operated.
In some cases the centre condition of the valve could be used to unload the pump when the machine is idling (Fig. 2.30). If the centre condition connects the P port to the T port, the pump flow can flow freely back to tank at low pressure. This avoids excessive generation of heat from having to pass the flow across the relief valve at full pressure.
Fig. 2.30 Pump unloading
Solenoid directional valves intended to be mounted on a subplate or manifold block normally have a standardised interface design. The mounting specification was originally defined by CETOP (Comité Européen des Transmissions Oléohydrauliques et Pneumatiques), the European umbrella organisation for national trade associations in the hydraulics industry. Such valves are therefore often referred to as ‘CETOP valves’. Subsequently, the International Organization for Standardization (ISO) took over responsibility for the specification of direct-acting valves (ISO 4401), and there are three common sizes: ISO 02, 03 and 05. The standard simply defines the position and size of the ports, holding down bolt holes and dowel pins (where fitted), so that one manufacturer’s valve should be directly interchangeable with another’s. It does not, however, define the overall size of the valve or its flow rating.
When the required flow rate through the directional valve is higher than can be handled easily by an ISO 05 valve (approximately 120 L/min (30 gpm)), it becomes advantageous to use a two-stage valve (Fig. 2.31). In this case the main spool is pushed across in the valve body by means of hydraulic pilot pressure acting on one end of the spool. In turn, this pilot pressure is controlled by a direct-acting solenoid valve mounted on top of the main valve body. The valve is thus still electrically controlled but the main spool is moved hydraulically, hence the description electrically controlled pilot-operated valve or, more simply, two-stage or piggy- back valve.
POINT OF INTEREST In the USA, the National Fluid Power Association (nFpa) standard defines directional- valve interfaces (denoted D03, D05, etc.). It is equivalent to the ISO standard.
POINT OF INTEREST Standards are updated from time to time, and the latest version should always be followed. These can be found on the relevant organisation’s (ISO, DIN, etc.) website.
Pilot operation is indicated by the addition of solid black triangles to the solenoid symbol (see Fig. 2.31). The symbol shown in Fig. 2.31 illustrates the centre condition of the main spool only. The centre condition of the pilot spool may well be different (normally the P port is blocked with the A and B ports connected to T in the centre).
Two-stage valves also have standardised ISO interfaces in sizes ISO 05, 07, 08 and 10, and sometimes larger.