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News Releases : Hysteresis... Counterbalance Valve’s Worst Enemy

For Immediate Release
Elgin, Illinois (November 2000)

Webster’s Definition (circa 1881) for "Hysteresis"

Main Entry: hys·ter·e·sis
Pronunciation: "his-t&-'rE-s&s
Etymology: New Latin, from Greek hysterEsis shortcoming, from hysterein to be late, fall short, from hysteros later.

Definition: a retardation of an effect when the forces acting upon a body are changed (as if from viscosity or internal friction); especially : a lagging in the values of resulting magnetization in a magnetic material (as iron) due to a changing magnetizing force - hys·ter·et·ic /-'re-tik/ adjective.

When an actuator is supporting a vertically mounted load and its directional valve shifts to lower the load, the load may fall uncontrollably as its weight pushes fluid out of the actuator. To prevent this, a counterbalance valve typically is installed downstream of the actuator. A counterbalance valve’s function, also known as an over-center valve, is to maintain a set pressure opposite of the load, to keep it from free-falling. This valve also serves as a thermal relief valve but for very low flow. The counterbalance valve uses a pilot signal, usually from the inlet of the actuator, to assist in opening the valve poppet. A pilot assist makes the counterbalance more efficient and reduces the horse-power requirement and heat generation within the system.

The majority of the counterbalance valves on the market are based on the same design principle as the differential area relief valve. Differential area relief valves must have seals to prevent leakage. Most counter-balance valves have one to three dynamic seals in them, which of course is friction and that means hysteresis. The greater the number of dynamic seals in the counterbalance valve the greater the hysteresis will be. Friction caused by the dynamic seals makes the valve stick, thus causing an overshoot in pressure and then the counterbalance valve tries to correct itself and undershoots. This phenomenon is called "hysteresis", resulting in a bounce, a loud chatter or a high pitch squeal.

Many counterbalance valves offer a few options to band-aid this phenomenon. Most counterbalances come with wide range of pilot ratios anywhere from 1.75:1 to 12.0:1. This selective pilot ratio matching to particular equipment works some times. Another attempt to stabilize this situation is to have a short stroke valve or an adjustable stroke valve to restrict the flow until it becomes stable, but at the high cost of energy (horsepower). A small orifice (dash-pot) or an accumulator in the pilot line to slow down the response of the valve can make the valve quiet, but it is very sensitive to fluid viscosity and becomes very sluggish in cold temperatures. All these band-aids do help to stabilize the valves, but are not cures.

New advancements in counterbalance valve technology are making controls easier and smoother. To reduce hysteresis in the counterbalance valve we must eliminate all the dynamic seals in it.

Command Controls Corp. took a different approach – designing a counterbalance valve with NO dynamic seals, making it virtually hysteresis free. The valve is so fast that it can follow any signal supplied to it thus eliminating hysteresis, resulting in no bounce, loud chatter, or high pitch squeal. This counterbalance valve is a high flow valve (saving horse-power) but it is extremely controlable at very low flow. This valve also serves as a full flow relief valve.

These valves have no seals on the pilot piston, thus allowing a small amount of hydraulic fluid to leak past the pilot piston purging the air out of the pilot lines automatically and providing warm oil to be circulated in to the pilot piston, making the valve very responsive. A small amount of leakage around the pilot piston has nothing to do with load holding capability at port #1 (cylinder). The best position for the valve to be mounted on the actuator is with the adjusting screw pointing down; this will allow faster air purging from pilot line and the spring chamber. Screw in (CW) the adjusting screw to increase the pressure, screw out (CCW) to decrease the pressure same as all other valves in the Industry. Experience has shown that a counterbalance valve, with a 4:1 pilot ratio will satisfiy all applications without experimenting with a number of different pilot ratios. Another problem of hysteresis is in the cylinder. A counterbalance and the cylinder operate as a team. If one is faulty, the other can-not correct the other’s fault.

This counterbalance valve has a built in free reverse flow check (port #2 to port #1). Valves are also available with seals on the pilot piston for master/slave cylinder ap-plications and with spring chamber vented to atmosphere (insensitive to back pressure at port #2) for servo valve or regenerative circuits.

Written By: Constantine (Connie) Kosarzecki, President

About Command Controls Corp.
Command Controls Corp. is a manufacturer of high performance hydraulic cartridge valves and manifold systems for mobile and industrial equipment. Details about the company are available at www.commandcontrols.com.

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