How to choose a counterbalance valve
09 October 2017
Thanks to some very specific functions, counterbalance valves play an integral role in safely controlling an actuator which may be holding a load or a person in the air.
Eaton counterbalance valves
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But what are the important elements to consider when selecting a counterbalance valve? Todd McIntyre of Eaton Hydraulics takes an in-depth look:
The functions of load-holding valves
To understand how to apply a counterbalance valve, it’s important to know how they work. Counterbalance valves, when placed between a directional control and the head or rod end of a cylinder, work in three distinct ways:
1. Load holding: Counterbalance valves prevent cylinders from unwanted downward drift. Loads are raised through an integral free-flow check that allows flow from the directional valves to the cylinder, but prevents flow in the reverse direction.
2. Load control: Counterbalance valves prevent actuators from running ahead of the pump due to energy created by the load. This function prevents loss of control and damage to the actuator caused by cavitation.
3. Load safety: In cases where a line breaks, a load-holding valve directly mounted to the actuator will prevent uncontrolled and unsafe load motion.
Basic counterbalance application
Standard counterbalance valve (Eaton 1CE)
For most applications, the most cost-effective solution is to use a basic load-holding valve such as the Eaton 1CE, which is suitable for use with static or dynamic loads when trying to stop a load from running away. A common example is a cherry picker, where the load is often a person. If instability occurs while using a standard counterbalance valve, consider changing the valve’s pilot ratio. Most counterbalance valves are available with a variety of pilot ratios and are interchangeable.
In general, higher pilot ratios work well for consistent, stable loads, while lower pilot ratios suit unstable or variable loads. The pilot ratio does not necessarily have a substantial impact on the working pressure, as the normal working pressure for a system is often higher than the pilot pressure required to fully open the valve.
Two types of relief function in counterbalance valves
There are two main types of relief valve construction: direct acting and differential area. Direct-acting valves, such as the Eaton 1CE, is based on the simplest form of relief valve design and provides stability through a range of pressures and flows.
In direct-acting valves, load pressure acts on the full area of the poppet. As a result, greater spring force is required to meter and reseat the poppet. This spring force plays an important role in the metering of flow, or what is referred to as the relief characteristic of the counterbalance valve.
In contrast to direct-acting valves, differential area designs work by applying load pressure to a differential area between the poppet and the seat, creating a smaller area for the load pressure to act upon. As the poppet area is smaller for a given load pressure, the poppet will require less spring force to meter and close the valve. This results in a valve that can meter large amounts of flow very rapidly.
Direct-acting valves are more stable because the heavier spring makes the poppet less reactive to small fluctuations in load pressure.
Handling induced pressure
Closed-centre directional control valve
Use an OCV with part-balanced relief (1CER)
Some machines, such as wheel loaders, are designed using closed-centre directional valves, yet also require the use of counterbalance valves. This brings additional challenges if the machine bucket is driven into a pile of rock or soil. Here, pressure can be captured between the directional and counterbalance valve. In addition, the resulting high induced pressure means the cylinder will have to give in order to prevent mechanical damage.
A common industry solution is to use fully balanced valves with an atmospheric drain. While these valves help relieve pressure, they are subject to contamination ingress and subsequent external leakage. However, a part-balanced valve, such as Eaton’s 1CER, overcomes this issue.
When pressure builds between the closed-centre directional valve and counterbalance valve, it is applied to the valve port of the latter. To overcome this effect, the part-balanced 1CER references the added pressure to the spring chamber. Pressure is now acting on both ends of the poppet and becomes balanced, or negates the effect of the back pressure on the valve port.
Managing high or variable back pressure
Regenerative or proportional systems
Use a fully balanced relief valve (1CEB, 1CEBD)
Many types of equipment featuring proportional systems create back pressure with a closed-centre directional valve system, including cranes and regenerative systems. If using a standard or part-balanced valve in one of these applications, the variable back pressure can cause instability by effectively changing the pilot pressure required to open the poppet. In these situations, a fully balanced relief valve is recommended to manage pressure fluctuations.
Eaton’s 1CEB and 1CEBD counterbalance valves provide fully balanced relief, referencing pressure from the valve to the spring chamber. However, different to part-balanced valves, the pressure in the spring chamber is vented to atmosphere or a separate drain port. As a result, the pilot pressure required to open the valve is no longer variable, giving a stable counterbalance.
Compensating for high system instability
Two-stage valves (1CEL)
Another common challenge in the application of counterbalance valves is vehicles with a high degree of load dynamics. The long cylinder can act as a capacitor and store energy when fully extended. The pressure within the cylinder will rise to system pressure at the end of the stroke. Here, a counterbalance valve will reset and lock system pressure into the cylinder irrespective of the load-induced pressure. When the operator begins to lower the load, this stored energy gives the message that a heavy load is on the boom, and less pilot pressure is required to open the counterbalance valve. The valve opens very quickly and allows stored energy to dissipate, causing a momentary runaway condition or prompting the valve to overreact. The consequence is an initial instability as the boom is retracted.
Many vehicle engineers use restrictive and semi-restrictive valves for dynamic vehicle applications, but these are inherently inefficient and will generate heat. A more efficient alternative is a two-stage valve, such as Eaton’s 1CEL. Two-stage valves maintain an initial counterbalance pressure when opened to prevent total decay of stored energy within the cylinder. This is done by maintaining the counterbalance pressure through the centre poppet and inner spring.
Providing hose burst protection
BoomLoc hose rupture valves
While all counterbalance valves provide a level of hose burst protection when applied correctly, BoomLoc hose rupture valves (HRVs) provide extra security to meet ISO8643 requirements. Valves such as Eaton’s 1CPB are designed to work with a directional valve to control motion, especially in cases where a hose fails.
Just as with a standard counterbalance valve, a HRV allows free flow into the cylinder with minimal pressure loss when the load is being lifted. When controls are in neutral, HRVs close to lock pressure in the cylinder, securing the load in place. While the load is being held, a separate relief valve provides overload relief by returning flow to the tank via the main control valve port relief.
The most important feature of a HRV, which is found commonly on excavators, is how it helps mitigate hose failure issues during lowering by limiting boom acceleration. This is accomplished by hydraulically balancing the poppet without providing any relief function. Any over-pressure caused by shock or temperature changes is handled by an additional relief element.
Counterbalance valves perform critical functions that help keep loads safe and stable. In all application examples, Eaton offers options that can assist engineers in tackling design, requirement and cost challenges, while providing superior stability and performance for load-holding applications.
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