How to chose a hydraulic overcentre valve

04 May, 2020

Overcentre valves play an integral role in safely controlling an actuator that is holding a load or a person in the air. However, there are various types of overcentre valve available on the market to suit very different applications.

Selection criteria

When selecting overcentre valves, many options exist, and few people really understand them all. This fact has led engineers to select valves from familiar suppliers without really questioning or understanding their choice. Eaton can help solve this problem with the industry’s broadest portfolio of overcentre valves.

Overcentre valves protect cylinders from unwanted downward drift. This function is referred to as load holding and is important in aerial work platforms. Users can also deploy overcentre valves to control loads by preventing actuators from running ahead of the pump due to energy created by the load. This function prevents loss of control and damage to the actuator caused by cavitation.

Additionally, in cases where a hydraulic line breaks, a load-holding valve mounted to the actuator will prevent uncontrolled and unsafe load motion.

Basic overcentre application

For most applications, the most costeffective solution is to use a basic loadholding valve, such as the Eaton 1CE, which is suitable for use with static or dynamic loads when trying to stop a load from running away.

A common example is a cherry picker, where the load is often a person. If instability occurs while using a standard overcentre valve, consider changing the valve’s pilot ratio. Most overcentre valves are available with a variety of pilot ratios and are interchangeable. Handling induced pressure

Some machines, such as wheel loaders, are designed using closed-centre directional valves, while also requiring the use of overcentre valves. This configuration brings additional challenges if the machine bucket is driven into a pile of rock, soil or manure. Not only can pressure be trapped between the directional valve and overcentre valve but a high induced pressure will result, and the cylinder will need to give in order to prevent mechanical damage.

If a standard overcentre valve is used in this situation, a tremendous amount of pilot pressure would be needed to open it, while the port reliefs on the directional valve would also be rendered inoperative. A common solution is to use a fully balanced valve with an atmospheric drain, but these are susceptible to contamination and subsequent leakage. Instead, a partbalanced valve, such as Eaton’s 1CER, can overcome this issue.

When pressure builds between the closed-centre directional valve and the overcentre valve, it is applied to the valve port of the latter. The 1CER references the added pressure on the valve port to the spring chamber so that it acts on both ends of the poppet and becomes balanced, significantly reducing the pilot pressure needed to open it.

Managing high or variable back pressure

Many types of equipment with proportional systems create back pressure that is often constantly changing, thus requiring a different approach. Cranes, for example, frequently use meter-out proportional valves, creating a constantly varying back pressure between the directional control and overcentre valve. If using a standard or part-balanced valve, the variable back pressure can cause the system to become unstable by effectively changing the pilot pressure required to open the poppet.

In these scenarios, Eaton’s 1CEB and 1CEBD overcentre valves provide fully balanced relief. Like part-balanced valves, fully balanced valves reference pressure from the valve to the spring chamber. However, with fully balanced valves, the pressure in the spring chamber is vented Tolu Oluwatudimu: “Overcentre valves perform critical functions to help keep loads safe and stable.” to either the atmosphere or a separate drain port. By venting the spring chamber, the pilot pressure required to open the valve is no longer variable and a stable counterbalance results.

Compensating for high system instability

Another common challenge relates to vehicles with a high degree of load dynamics. Using the example of a telescopic boom, the long cylinder can act as a capacitor and store energy when fully extended. The pressure within the cylinder will rise to system pressure at the end of the stroke. Here, the overcentre valve will reset and lock system pressure in the cylinder irrespective of the loadinduced pressure. When the operator begins to lower the load, this stored energy gives the overcentre valve the message that a heavy load is on the boom, and less pilot pressure is required to open the overcentre valve. The valve opens very quickly and allows the stored energy to dissipate, causing a momentary runaway condition or prompting the valve to overreact and cause some initial instability as the boom is retracted.

In this situation, many vehicle engineers use restrictive and semirestrictive valves, which although reduce instability by limiting flow, are inherently inefficient and generate heat. A more efficient alternative is a two-stage valve, such as Eaton’s 1CEL, which maintains an initial first stage overcentre pressure when the valve is opened to prevent a total decay of the stored energy within the cylinder.

Providing hose burst protection

Finally, while all overcentre valves provide a level of hose-burst protection when applied correctly, BoomLoc hose rupture valves (HRVs) offer added protection to meet ISO8643 requirements. These valves, such as Eaton’s 1CPB series, are designed to work with a directional valve to control motion, especially in cases where a hose failure has a significant impact. HRVs are found commonly on excavators and cranes.

Conclusion

Overcentre valves perform critical functions to help keep loads safe and stable. Eaton offers options that can help hydraulics engineers tackle system design, requirement and cost challenges, while providing high levels of stability and performance.

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