Directional Control Valve

CHECK VALVE

A check valve is the simplest type of directional control valve used in hydraulic systems. Check valves stop the flow of fluid in one direction and allow free flow in the opposite direction. They are also known as non-return valves. Check valves may be used as:

  • Prefill valves (anti-cavitation)

  • Bypass valves (e.g. throttling points or return-line filters)

  • Stops for flow in one direction

  • Pre-tensioning by creating a certain backpressure

  • Protection of hydraulic components against pressure surges

Most check valves are spring-loaded and use a ball or plate to seal the flow in one direction. Check valves are designed with seats and thus are able to isolate circuits with no leakage. Balls, plates, poppets or poppets with soft seals are used as isolating elements.

There is a special type of check valve that prevents pistons or cylinder plungers from coming down and causing accidents. This is called a line rupture valve. When the line ruptures, the flow through the line rupture valve increases substantially, causing an increased pressure drop. This in turn creates a stronger force on the ball, which will close immediately.

Other special types of check valves are pilot-operated check valves and shuttle valves. A pilot-operated check valve allows flow in either direction by application of an external pilot pressure signal. A shuttle valve permits free flow at the highest operating pressure.

DIRECTIONAL SPOOL VALVE

Directional spool valves comprise a moving spool situated in the valve housing. When an actuating force moves the control spool, the annular channels in the housing are connected or separated. Directional spool valves have several unique features, such as:

  • Low cost due to simple design

  • Low actuating force (due to good pressure compensation)

  • High switching power

  • Low losses (even though oil leakage flows continuously from the high pressure to the low pressure side)

  • Wide variety of control functions

Directional spool valves may be direct-operated or pilot-operated. A direct-operated spool valve is either electrically controlled with solenoids, mechanically (e.g. manually) controlled with levers or rollers, or controlled with hydraulics or pneumatics. Whether a directional spool valve is direct- or pilot-operated depends on the actuating force needed to move the spool. Thus, this is dependent on the flow, i.e. nominal size of the directional spool valve.

When valves are operated at higher hydraulic system operating pressures, leakage losses around the spool and the housing should be taken into account, especially at system pressures over 350 bar. The leakage loss is determined by the size of the gap between the spool and the housing, the viscosity of the hydraulic fluid and the hydraulic system pressure.

Typical parameters of spool valves are:

  • Nominal sizes: NG6 (Cetop 3) and NG10 (Cetop 5) (up to NG120 for pilot-operated valves)

  • Maximum flow: up to 150 l/min (up to 7,000 l/min for pilot-operated valves)

  • Maximum pressure: up to 350 bar

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