Calgary, Alberta, Canada dzyanis@engineer.com

Fluid Power Pro

Pilot ratio & pilot pressure of counterbalance valves

Summary:

  • Understanding the pilot ratio of the counterbalance valves
  • Calculations the pilot pressure of the counterbalance valves for different applications

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Lower pilot ratios will increase system stability and provide better motion control. Therefore, the vast majority of counterbalance applications are satisfied with a 3:1 pilot ratio.

Higher ratios will be more efficient (reduce heat generation) but at the cost of stability and smooth motion control. This is why 10:1 pilot ratio valves, generally, should be avoided. But, sometimes, on motors high pilot ratios will provide adequate dynamic control.

Two areas work to open a counterbalance valve, the relief area (port 3) and the pilot area (port 1); the pilot area divided by the relief area equals the pilot ratio:
cbv


The pilot pressure at port 3 is inversely proportional to the load pressure at port 1 and effectively reduces the relief valve setting.

Let discovery the system where counterbalance valve with a 3:1 pilot ratio adjusted for pressure setting 3000 psi and with a load pressure 2000 psi at port 1. In this case, the min. pilot pressure, required to open the relief valve is 333 psi, i.e:

[set.pressure 3000 psi] = [load pressure 2000 psi] + [pilot pressure X psi] * [ratio]

3000=2000+X*3

X=\frac{3000-2000}{3}=333\ psi

In the real systems with hydraulic actuators like cylinder or motor it is necessary to consider the actuator’s area ratio because the pressure what applied to pilot port 3 also applied to input of actuator and through actuator’s area difference – to port 1. This is why load pressure will be increased but pilot pressure will be decreased.

Let discovery the system where the counterbalance valve on the cap end of the cylinder with a load retracting the cylinder rod:

cbvratio1

For this system the formula of min. pilot pressure is:

 P_{pilot} = \frac{P_{set} - P_{load}}{CVR + \frac{1}{CR}}

where:

CVR – Counterbalance Valve Ratio. If the ratio 3:1  =>  CVR=3

CR – Cylinder’s Area Ratio.

So, if for example cylinder’a Area ratio CR = 1.64, min pilot pressure to open counterbalance valve is:

 P_{pilot} = \frac{P_{set} - P_{load}}{CVR + \frac{1}{CR}} = \frac{3000-2000}{3+\frac{1}{1.64}}= 277\ psi

Let discovery the system where the counterbalance valve on the rod end of the cylinder with a load retracting the cylinder cap:

cbvratio5

For this system the formula of min. pilot pressure is:

 P_{pilot} = \frac{P_{set}-P_{load}}{CVR+CR}=\frac{3000-2000}{3+1.64}= 215\ psi

And the latest example is the system where counterbalance valve on a motor or an equal area actuatorwith an overrunning load:

cbvratio7

For this system the formula of min. pilot pressure is:

 P_{pilot} = \frac{P_{set}-P_{load}}{CVR + 1}=\frac{3000-2000}{3+1}=250\ psi

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