NB: The equations in the figures are idealised. Back pressure at port 2 is excluded from these equations, and check cracking pressure is ignored.
Where:
L = Load induced pressure
P = Pilot pressure required to open valve
PR = Pilot ratio (eg 3:1 = 3)
CR = Cylinder area ratio = Bore Dia²/(Bore dia² - Rod dia²)
The pressure calculation is dependent on the configuration. Three possibilities are considered:
* Pilot check on the blind end of the cylinder with a load retracting the cylinder rod (Figure 1).
* Pilot check on an equal area actuator (Figure 2).
* Pilot check on the rod end of the cylinder with a load extending the cylinder rod (Figure 3).
Cylinders with large area ratios (2:1) loaded to extend utilising pilot-to-open check valves may self-lock. If the cylinder area ratio is close to the check valve pilot ratio, the denominator in the above equation tends toward zero, and drives pilot pressure to open the valve toward infinity.
The resulting cylinder pressure intensification in the cylinder rod end rises faster than the check valve pilot ratio can overcome. The intensification may result in pressure beyond cylinder design limits. For these applications consider using a counterbalance valve with 10:1 pilot ratio.
Application notes
* The valves should be mounted close to the actuator, providing maximum protection in the event of an hydraulic line failure. This can be achieved by incorporating them directly into the actuator, or Sun offers a range of gasket mounted bodies that bolt directly onto the mounting face of a cylinder or motor.
* A pilot operated check is a nonmodulating device and is not suitable for smooth motion control or overrunning loads. It is a load holding valve, not a load lowering valve. When trying to lower a load, severe machine 'ratcheting' may result!
* Pilot pressure for a pilot operated check valve is directly proportional to load pressure.
* Back pressure at port 2 directly opposes the pilot pressure at port 3 and is additive to it. If port 2 is pressurised (for example in regenerative circuits) then a four port vented pilot-to-open check valve should be considered.
* Four port vented pilot operated checks should be vented back to the circuit immediately downstream of the restriction causing the back pressure.
* In the vent-to-atmosphere CK*V 3 port version, the pilot spring chamber is vented externally to atmosphere through a vent hole in the cartridge. An O-ring is fitted over the hole to minimise the ingress of dirt and moisture into the spring chamber. External leakage ('weepage') equates to one drop for every 4000 cycles. This cartridge fits into the standard 3 port cavity and is useful if the check valve needs to be made insensitive to unanticipated system back pressure in existing systems. However, the four port version is preferred for new system designs.
* Take care when utilising cylinders with large area ratios and pilot-to-open checks on the rod to hold the load. Under some conditions, 'load locking' may occur. See note and formula for Figure 3.
* Pilot-to-open check valves have very low leakage. New valves have one drop or less leakage per minute. Applications with high shock and/or contaminated oil can cause degradation of the seat resulting in increased leakage. If near zero leakage is required over time, some applications may require routine replacement of the pilot-to-open check valves.
For more information contact Fritz Kern, Axiom Hydraulics, +27 (0)11 334 3068, [email protected], www.axiom.org.za
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| www: | www.axiomsa.co.za |
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