Directional-control valves also control flow direction. However, they vary considerably in physical characteristics and operation. The valves may be a
• Poppet type, in which a piston or ball moves on and off a seat.
• Rotary-spool type, in which a spool rotates about its axis.
• Sliding-spool type, in which a spool slides axially in a bore. In this type, a spool is often classified according to the flow conditions created when it is in the normal or neutral position. A closed-center spool blocks all valve ports from each other when in the normal position. In an open-center spool, all valve ports are open to each other when the spool is in the normal position.
Directional-control valves may also be classified according to the method used to actuate the valve element. A poppet-type valve is usually hydraulically operated. A rotary-spool type may be manually (lever or plunger action), mechanically (cam or trip action), or electrically (solenoid action) operated. A sliding-spool type may be manually, mechanically, electrically, or hydraulically operated, or it may be operated in combination.
Directional-control valves may also be classified according to the number of positions of the valve elements or the total number of flow paths provided in the extreme position. For example, a three-position, four-way valve has two extreme positions and a center or neutral position. In each of the two extreme positions, there are two flow paths, making a total of four flow paths.
Spool valves (see Figure 5-11) are popular on modern hydraulic systems because they—
• Can be precision-ground for fine-oil metering.
• Can be made to handle flows in many directions by adding extra lands and oil ports.
• Stack easily into one compact control package, which is important on mobile systems.
Spool valves, however, require good maintenance. Dirty oil will damage the mating surfaces of the valve lands, causing them to lose their accuracy. Dirt will cause these valves to stick or work erratically. Also, spool valves must be accurately machined and fitted to their bores.