Counterbalance valve simulation

Load control experiments with this counterbalance valve

 

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Last Updated
2010-06-17

 


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This module demonstrates a counterbalance valve in a load control circuit and shows how the valve is affected by different pilot ratios and external factors such as back pressure.

Experiment with this counterbalance valve

Valve Setting to allow for Hysteresis

The hysteresis between seat and reseat of the valve is about 85% so it is important to ensure that the valve is set approximately 30% above the maximum load pressure to ensure that the load is still supported when the valve reseats.

Reduce the valve setting until the poppet starts to open. This point is equivalent to the load value, which means you should then increase the setting to 30% above this value for reliable operation.

Valve Cracking Pressure (select from drop down list)

Operate the directional valve to lower the cylinder and observe the cracking pressure in line 3.

This pressure is defined by the valve's setting, pilot pressure ratio and the cylinder area as well as the load pressure.

Experiment by adjusting the valve setting and observing how the cracking pressure changes.

Use a Low Pilot Ratio - 3:1 Pilot Ratio Example

In order to obtain optimum stability use a low pilot ratio and a smaller rather than a larger capacity valve. The low pilot ratio will keep the pilot pressure higher, which helps stability and the smaller size of valve helps generate higher pressure drops which also helps provide greater stability. Both of these factors are contrary to the normal rules for producing a more efficient hydraulic circuit. The difference with a counterbalance valve is that by its very nature it is generating a pressure drop when working correctly.

10:1 Pilot Ratio Example (select from drop down list)

10:1 pilot ratio valves are normally only used on stable motor circuits. Differential cylinders, variable loads and variable speeds may all result in less stable operation compared with a 3:1 ratio valve.

Click the directional valve button start lowering the cylinder and observe the pressure in line 3, then press the '<-' key to compare this pressure when using the 3:1 pilot ratio valve.

Counterbalance Valves Provide a Crude Relief Function

A counterbalance valve will provide some relief protection but it is a slow acting valve with poor reseat compared to a conventional relief valve.

Increase the load on the cylinder and observe how the counterbalance valve provides a simple relief function.

Counterbalance Valves With Flow Control

If speed control is required with a counterbalance valve then a meter in flow control valve should be used.

Experiment with this circuit set-up to control the cylinder speed.

Typical Problems with Low Circuit Natural Frequency

All hydraulic fluid is compressible. Long pipe lengths(volumes) between the valve and the cylinder or a large load relative to the cylinder area may result in an unstable operation.

This simulation has exaggerated fluid compressibility. Observe the slow pressure rise rate and oscillations when the cylinder stops. Reduce the load and see how the system becomes stiffer and the cylinder does not oscillate as much.

Typical Problems caused by Trapped Air (select from drop down list)

Instability can be due to air trapped between the cylinder piston and the load control valve. It is vital with cylinder applications that the circuit is correctly bled of air before operation.

Typical Problems caused by Cylinder 'Stick-Slip'

Instability due to "stick-slip" on cylinder seals. If the seals used are not low friction type they will tend to cause an unstable operation which the counterbalance valve cannot correct but is likely to exaggerate.

Typical Problems cause by Low Flow Rates

Instability or noise due to low flow. The new design poppet and seat arrangement has eliminated noise in most applications but Sun also offer low flow rate cartridges as a possible solution.

Other potential problems not covered by this simulation include:-

a) High variable back pressure often associated with circuits using proportional valves can cause the counterbalance to go unstable. This is generally cured by using a vented valve where the spring chamber is vented separately back to tank.

b) Decompression shock in large flow circuits when the load control is piloted open. In this type of circuit it may be necessary to add a decompression feature as the valve is opened. Consult your local distributor to discuss possible solutions for this type of problem.

 


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