Overcentre valves can be described as a type of pilot assisted relief valve, with the only difference between the two being the check valve will open fully when sufficient pressure is applied with pressure in the cylinder port being the only restrictive force, whereas the overcentre valve has to overcome force from the spring mechanism which is reduced by load pressure. There are 3 main areas of load based functions the overcentre valve provides, which are applicable to both rotary and linear load motion. These areas comprise: Controlling load – This involves the valve ensuring that the actuator doesn’t run ahead of the pump, thus reducing the risk of cavitation by controlling load induced energy and preventing loss of control. Ensuring load safety – This safety measure controls movement and ensures that load is under control when a component malfunction occurs, such as a hose failure. It also ensures that people, equipment and property remain safe when heavy machinery is used, such as a crane with a boom, which has the potential to cause substantial damage if control is lost. Holding load – Working with the directional valve when it is situated in the neutral position, the load holding function of the overcentre valve prevents any movement of the load and also prevents leakage past the directional valve while it is the closed position. Pilot ratios explained When a system is in the design stage, pilot ratios are a main factor that needs to be taken into account as different systems will benefit from different pilot ratios. For example, a system that runs stable, constant loads will normally use a high pilot pressure, while a low, unstable load will benefit from a lower pilot pressure. The pilot open pressure drop is a good measure of system performance and efficiency, as system pressure typically runs much higher than the pilot pressure needed to open the valve fully. The two-stage overcentre valve An addition to the overcentre valve family, the two-stage overcentre valve aims to tackle problems that long unstable booms suffer from, especially those with large capacity cylinders such as telescopic handlers which can suffer from instability issues. Runaway conditions are encountered in these applications when pilot valves are opened too quickly, due to heavy loads on the cylinder. The two-stage overcentre valve uses two springs with the outer spring being affected by the pilot piston with the inner used as a pressure counterbalance, thus overcoming potential instability issues. Which type of overcentre valve should you get? When looking for the correct overcentre valve, you have to ensure you cater for the pressures the hydraulic unit will need to work with. In a system with high back pressure a standard overcentre valve would struggle, as the standard spring chamber is vented to the valve port through the poppet, this increases relief pressure and systems which use a closed centre directional valve would run into difficulties. Valves are now available that help to combat this problem as the relief sections of these valves are not affected by back pressure and they are identical in every other way to a standard valve. Finally, some overcentre valves come complete with an atmospheric venting feature, which can be a beneficial feature until they are used in a corrosive type atmosphere which could cause running problems, so it is always important to check system plans and positioning when deciding on the type of valve to go for. Hydraproducts have a comprehensive selection of valves as part of their new Components Division which can be viewed here .