Motion-Control Valves Solve Equipment-Instability Problems

Cartridge valves offer the same control options as traditional hydraulic valves but are generally smaller, lighter, and more tolerant of vibration and fluid contamination.

Because they eliminate the need for many of the hoses, tubes, and fittings in a circuit, there are fewer potential leak points. And combining several cartridge valves in a common manifold, which creates a hydraulic integrated circuit, results in a dedicated package to control specific machine functions, often with considerable cost savings.

One important type of cartridge valve is the overcenter or motion-control valve. Variations abound, but these valves perform three basic functions for both linear and rotary motion, including:

Load holding. An overcenter valve prevents a load from moving when the directional valve is in the neutral position. This lets engineers use open-center directional valves and prevents leakage past the spool of closed-center directional valves.

Load control. Overcenter valves prevent actuators from running ahead of the pump due to load-induced motion. (This occurs, in essence, when gravity pulls the load faster than the pump supplies fluid to move the cylinder rod.) This eliminates cavitation in the actuator and loss of control.

Load safety. In the case of a line break, an overcenter valve on an actuator prevents uncontrolled load movement. On a crane boom, for instance, hose-failure protection is vital as the loss of load control could endanger people and property.

Standard valves
The standard overcenter valve is a pilot-assisted relief valve with an integral free-flow check. Pilot pressure must overcome the spring force, which is counteracted by load pressure. This ensures a gradual opening and metering of flow past the poppet. It differs from a pilot check, where the check valve opens fully as soon as pilot pressure overcomes resistance from pressure in the cylinder port.

For example, Eaton’s Integrated Hydraulics overcenter valves have a poppet that seals flow from an actuator; a check element that permits free flow to the actuator; and a pilot section that opens the poppet, permitting flow from the actuator at a controlled rate.

Two basic designs each have several variants. The direct-acting version, where actuator pressure acts on the full area of the poppet nose, is ideal for flows up to 200 lpm. The differential-area design, where pressure acts on an annular area, is suitable for flows up to 300 lpm. Being poppet valves, both have excellent sealing characteristics. Maximum leakage is 0.5 ml/min for valves up to 200 lpm capacity and 4 ml/min for 300-lpm valves.

The cartridge has three ports: cylinder, valve, and pilot. Pressure exceeding the valve setting applied to the cylinder port opens it as a relief. Pressure applied to the valve port will open a low-pressure check, permitting free flow into the cylinder port. Pressure on the pilot port acts over a larger area on the poppet than the area facing the cylinder port, so the valve will open at a low pressure.