2 July, 2020

How to chose a hydraulic overcentre valve

04 May, 2020

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.

www.eaton.com




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