Whether smaller or larger the hydraulic system, there must be at least one valve in it. Unlike the size and design, hydraulic valves play a vital role in controlling the pressure, flow and direction of fluid present in the system. Also, valves provide security while handling heavy loads. We have already discussed the important types of hydraulic valves. This section will deal with the counterbalance valves, an important valve used for holding heavy loads.
Counterbalance valves are also known as load holding valves because of its ability to safely hold heavy loads. The load holding capacity is achieved by allowing free flow into the cylinder/actuator and by blocking the reverse flow until a pilot pressure inversely proportional to the load is applied. In short, the counterbalance valve permits flow in one direction and block the flow opposite to it. Vertical presses, lift trucks, loaders and other machine tools that positions or hold suspended loads are some counterbalance valve applications. Some important benefits of the counterbalance valve are listed below.
- Prevent unintended or uncontrolled movement of static or dynamic loads
- Protect the equipment from damages by handling induced pressure
- Offers greater stability to the machinery
- Control the load speed in case of hydraulic hose failure
- Stabilize the system when it experiences high or variable backpressure.
- Suitable for safe and stable load handling
Counterbalance valves are well suited for clamping applications and it avoids falling of negative loads when any failures occur. These valves can be used with paired cylinders. In a paired cylinder, the pilot pressure will open the valve of the heavily loaded cylinder and this load will be transferred to the next cylinder and its corresponding closed valve and will open. The counterbalance valve is either flange mounted or connected with a metallic pipe and is fitted next to the actuator to offer safety.
The counterbalance valve functions are similar to that of flow control valves and are used for high-speed safe lowering/elevation of loads. Load holding, load control and load safety are the three important counterbalance valve functions. The counterbalance valve prevents unintended motion of actuators/cylinders holding heavy loads. These valves will help to stabilize the load, avoid damage of equipment and eliminate dangers caused as a result of the hose failures. For aerial work platforms, it is necessary to prevent sudden or unwanted downward drifts. Even under the inoperable conditions, these valves will hold the load firmly against the gravitational force by applying a back pressure against the load. Under operating conditions, the load can be elevated by passing fluid through the check valve. Also, this valve will prevent uncontrolled or unsafe motion of loads as a result of line breaks or overloads.
Now consider counterbalance valve working. In the circuit, you will find a check valve for free fluid flow, a relief valve for reverse flow and a pilot piston to lower the relief settings. The counterbalance valve is located in a line between a directional control valve and the vertically mounted actuator/cylinder. The counterbalance valve working principle states that the fluid will be trapped inside the spool under pressure until the pilot pressure overcomes the pre-set value. These valves will reduce the power consumption, improve the life span of pumps and decrease the fluid temperature.
The diagram above shows the internal construction of counterbalance valves. In the closed position, the discharge port(8) will be blocked by the spool valve(4) and the fluid will enter freely through the inlet port(5). Direct flow from inlet port(5) to the discharge port(8) is not possible because of the spool valve(4). So, the fluid will pass through the pilot passage(6) towards the small pilot piston. The force applied by the pilot piston will move the spool upwards and the fluid will pass from inlet port(5) to the discharge port(8) directly as indicated in the figure. This is the mechanism in the counterbalance valve for lowering of the weight or extension of the cylinder.
For retracting the hydraulic cylinder, the reverse operation is performed. That is, the fluid will flow from the discharge port(8) towards the inlet port(5). But, direct flow is not possible because of the blocked spool valve(4) under the action of spring force(3). Here, the fluid will flow through the check valve(7) and will lift the spool valve(4) as a result of piston pressure.