Counterbalance Valves
Counterbalance valves are one of the most widely used load controlling components in hydraulic circuits. They are used to control over-running loads in hydraulic cylinders and as brake valves for hydraulic motors. For this reason, they are commonly referred to as Counterbalance / Brake Valves. This discussion will focus on their use as load holding valves for hydraulic cylinders, how they work, and how they are set.
How do Counterbalance Valves work?
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. That is, 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.
How is a Counterbalance Valve Set?
The relief function has two important set points, the point at which the valve cracks open and the point at which the valve reseats. Although the delta between these two points varies between manufacturers, the reseat pressure is generally ~85% of the cracking pressure. For this reason, counterbalance valves are normally set at 1.3 times the maximum load pressure. This ensures that the valve will reseat under the heaviest load conditions. Some manufacturers may specify a setting of 1.5 times the maximum load pressure if that value is less than 2000 PSI.
Counterbalance valves are available in a variety of pilot ratios. Lower pilot ratios provide better motion control and stability, but are less efficient and may create excess heat. The opposite is true for higher pilot ratios. Therefore, pilot ratios of 3:1 or 4.5:1 are typically selected for load holding cylinder applications, as they provide a good balance between performance and efficiency.
Regardless of which pilot ratio is selected, calculating the pilot pressure requires consideration of the load pressure, the pilot ratio, and the cylinder ratio, as well as the location of the counterbalance valve.
For the circuit below, the load is compressive, which requires the counterbalance valve to be placed on the cap end (Extend) side of the cylinder. The formula for calculating the required pilot pressure is in this circuit is:
If the load on the cylinder is tractional, such as the circuit below, the counterbalance valve will be placed on the rod end (Retract) side of the cylinder and the formula for calculating the pilot pressure is:
P = Pilot pressure required to open valve
S = Setting of valve
L = Load induced pressure
Pr = Pilot ratio (3:1 = 3)
Cr = Bore diameter ² / Bore diameter ² – Rod diameter ²