Counterbalance Valves Part I
Jun, 25 2009
In this edition of Newsletters that Teach we will cover the basic function of a counterbalance valve. Counterbalance valves are also often referred to as motion control valves and over-center valves. All three names address the important function of these valves.They are deployed in applications where hydraulic cylinders or motors are working against gravity in one direction, and with gravity (sometimes called a negative load) in the other direction.
The application that is easiest to relate to is a crane or hoist boom. Sending fluid into the the base (cap end port or blind end port) of the cylinder will cause the boom to rise against gravity. But what happens when fluid is applied to the rod end port? When the valve is opened, gravity will allow the boom to drop faster than the flow entering at the rod end.
Think for a moment of a winch cable operated elevator in an office tower. Is it easier to control the starting and stopping of the elevator car if the car is the only load on the winch, or is it easier to control the car if there is a counterweight on the opposite end of the winch cable? I'm sure you've guessed right. Having a counterweight with a mass approximately equivalent to the weight of the car plus passengers makes motion control easier. When the elevator car is traveling upwards, the same winch cable is lowering the counterweight. When the car is descending the counterweight is being lifted. When the car is descending, the draw works on the roof of the building do not have to fight against gravity to halt the car if an equivalent weight is being hoisted up.
By now you've probably got a good idea about what the counterbalance valves are doing on the crane boom. They are there to help prevent a run away situation and bring the boom to a halt when the operator centers the valve handle.
Lets have a look at the symbol for a counterbalance valve.
The external pilot line (3) is connected to the opposing actuator line. This means that the counterbalance valve poppet is closed. In this state it will hold the raised cylinder in position (to a preset pressure maximum). But when the opposing actuator line is pressurized, the pilot (3) on the counterbalance is also pressurized. This pre-compresses the poppet against the internal spring, bringing the poppet to a near open or cracked condition. This makes it possible for oil to flow through the counterbalance from (1) to (2).
It is important to note here that when the counterbalance valve receives pilot pressure on port (3), the flow from inlet to outlet is not a free, unrestricted flow. The flow is still variably restricted by the valve's spring and the opposing pilot pressure which constantly vary the poppet opening. This keeps the pressure differential from port (2) to port (1) steady, to control any runaway tendencies. This action provides smooth consistent motion at the actuator while it is being lowered.
In Counterbalance Valves Pt. 2 we'll look at troubleshooting and the serious safety issues surrounding counterbalance valve adjustments. We'll also look at the built in load holding function more closely.
Until next time...............