Counterbalance Valves vs. Pilot Check Valves

The two most common types of load-holding valves used in hydraulic cylinder applications are counterbalance valves and pilot operated check valves. When selecting a load holding valve for a new cylinder design it is important to understand the difference between these two valves and how they perform under load. It is also important to understand whether the load is static or dynamic and whether the working environment is constant or variable.

Counterbalance Valve

A counterbalance valve is a combination of a pilot-operated relief and a reverse free flow check valve. When utilized as a load holding valve in a hydraulic cylinder application, the free flow check allows oil to flow unrestricted into the cylinder to raise the load. When the control valve is shifted to the neutral position, the load induced pressure forces the check valve closed. As long as this pressure is below the relief function setting, the load will remain in place until the control valve is shifted to lower it. Once this occurs, the pressure on the opposite side of the cylinder acts on the pilot port and works together with the load-induced pressure to overcome the relief setting. When the control valve is shifted back to neutral, the pilot pressure is lost and the relief valve closes again, holding the load in position.

A counterbalance valve has a modulating characteristic that is a function of both the load pressure and pilot pressure, which creates an inverse pilot ratio; light loads require more pilot pressure, and heavy loads require less pilot pressure. During normal operation, the adjustable spring force of the relief function is overcome by a combination of load-induced pressure and pilot pressure. The higher the load-induced pressure the less pilot pressure is required – and vice versa. This modulation, gives the counterbalance valve smooth operational control throughout a dynamic load curve.

Pilot Operated Check Valve

A pilot operated (PO) check valve is designed to allow free flow in one direction and block flow in the opposite direction until commanded to open. The command is a pilot signal from the opposite side of the cylinder, which forces the check ball off the seat and allows flow to pass through the valve. This works fine in a static application where the load is constant and so is the signal. If a pilot to open check valve were used in a dynamic application where the load was changing the pilot signal would fluctuate and become unstable. This is because the load pressure is working to close the check and the pilot pressure is working to open the check. This back-and-forth pressure battle results in the check valve chattering rapidly and the load control becoming jerky and unstable.

Load Type

Loads on hydraulic cylinders may be static or dynamic. Examples of static loads are a stabilizer cylinder on a mobile crane or a press cylinder in an industrial plant. These cylinders see a fairly constant load and are required to hold position without any drift for an extended period of time.  As PO checks are considered “Zero Leak” valves and perform well under static load, they are often used in these circuits. Examples of dynamic loads are a boom cylinder on a mobile crane or a bucket cylinder on a wheel loader. These cylinders see a variable load that tries to “run away” when being lowered. Because counterbalance valves have an inversely proportional metering characteristic, they allow these types of loads to be lowered smoothly with good control.

Operating Conditions

Operating conditions may be constant or variable. A cylinder working a press application in an industrial plant will experience a relatively constant environment and temperature, while a cylinder working in a stabilizer application on a mobile crane will experience a variable environment and temperature. This is important because hydraulic cylinders are often required to hold loads in position for a period of time. In the case of a bucket loader or a crane, that period of time may be a few minutes or a few hours. In the case of an outrigger or stabilizer, that period of time may be days or even weeks. If the cylinder is under static load in a variable environment, temperature changes may cause thermal expansion inside the cylinder. This may cause a PO check valve to hydraulically lock or could cause outright failure of the cylinder.

PO check valves are available in a variety of pilot ratios. Thermal expansion may cause the pressure inside the cylinder to more than double. If a 2:1 pilot ratio was selected for the PO check, the cylinder would become hydraulically locked in place, as the system pressure would not be sufficient to release the load.

The formula for calculating thermal expansion is:

ΔP = ΔT x K

P = PSI, T = Deg F, K = 90

P = BAR, T = Deg C, K = 11

 

Example: the outrigger is pressured to 3000 PSI at 50 degrees F. The temperature rises to 85 degrees F.

85-50=35. 35 x 90 = 3150. 3000 + 3150 = 6150 PSI

A PO check valve with a 2:1 pilot ratio would be hydraulically locked if the system pressure was 3000 PSI.

If a counterbalance valve was used in this application and was set at 1.3 times the working load (3000 x 1.3 = 3900 PSI) the valve would open and relieve the pressure in the cylinder once the pressure reached 3900 PSI. This would protect the cylinder – but would also release the load. It should also be noted that counterbalance valves are not considered “Zero Leak” valves and will pass as much as 3 drops per minute under load. This may cause slow cylinder drift over a long period of time in a static position.

It is important to discuss the application and working environment details with the cylinder designer as early in the process as possible. This will aid in selecting the correct materials for the work environment – including the correct load holding valve. It is also important to integrate the load holding valve within the hydraulic cylinder, if possible, to optimize performance and reduce external plumbing. RHK Hydraulics are experts in hydraulic cylinder manufacturing and hydraulic cylinder repair. Contact us today.

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