Pressure Control Valves in Hydraulic Systems

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The article looks at the basics of pressure control valves for educational use and is presented here to refresh your memory. Hope you are able to recall the functions, circuits, and applications of pressure control valves.

Pressure control valves are used in hydraulic systems for obtaining pressure-related control functions. The control tasks include reducing the pressure in some part of a hydraulic circuit, unloading of pumps, establishing the sequence movements of actuators, the counterbalancing of overrunning loads, and the braking of hydraulic motors while running on inertia. Accordingly, pressure control valves can be categorized into: (1) pressure reducing valves, (2) unloading valves, (3) sequence valves, (4) counterbalance valves, and (5) brake valves. It may be noted that a pressure relief valve is treated as a pressure regulating valve rather than a pressure control valve as its function in a hydraulic system is to modulate the supply pressure and limit the maximum operating pressure in the system.

Symbolic Representation of a Basic Pressure Control Valve

Pressure Reducing Valve

The primary role of the pressure reducing valve in a hydraulic system is to limit the pressure in some part of the system to a value lower than that required for the rest of the system. The reduced pressure can be set by using a control spring. The valve consists of an inlet port ‘A’, an outlet port ‘B’, a spring-loaded spool, and a pressure adjusting screw. The valve is provided with an internal control passage to evaluate the outlet pressure (at port B). When the outlet pressure remains below the valve setting, the fluid flows freely from the inlet to the outlet (that is, from A to B). When the pressure at the outlet exceeds the valve setting, the spool shifts to block the outlet port, thus maintaining a reduced pressure in the regulated line.

For example, an excess force (pressure) applied to the clamping operation by the cylinder A2 may damage the workpiece being clamped. In such a situation, a pressure reducing valve can be used to limit only the clamping pressure. You may observe the pressure values in parts of the circuit below.

Unloading Valve

An unloading valve consists of an inlet port ‘A’, a tank port ‘T’, a pilot port ‘X’, and a spring-biased spool. The unloading pressure can be varied by adjusting the spring tension. The pilot port is provided to accept the external pressure signal, which acts on one end of the spring-biased valve spool. In the normal position, the valve remains closed by the spring force. When a sufficient signal is applied to the pilot spool, the spool shifts, and the pump delivery is diverted to the reservoir through the tank port (that is, from A to T) at a low-pressure. The primary function of the unloading valve is to regulate the pressure by bypassing the fluid to the system reservoir at a low energy level in response to the external pressure signal received from the load section of the system. Unloading valves can be used in accumulator circuits, hydraulic motor circuits, and two-pump ‘hi-lo’ circuits.

As for an example, consider a hydraulic system with a pump, a reservoir, a pressure relief valve, an unloading valve, and a cylinder. In contrast to the maximum pressure limiting control of the pressure relief valve, the unloading valve discharges the pump delivery back to the reservoir at a low-energy level. The use of this valve provides a way for the cost-effective operation of the system as the heat generation in the system is minimized. You may observe the circuit below for the unloading of the pump through the unloading valve, while the pilot signal, received from the load section, acts at the pilot port ‘x’.

Sequence Valve

A sequence valve consists of an inlet port ‘A’ an outlet port ‘B’, a spool and a spring. The valve remains closed by the spring force. The externally adjustable spring is provided to set the pressure to the required value. A pilot passage ‘x’ is provided in the valve to accept signals from the inlet to act on one side of the spool. When sufficient pilot pressure is applied, the spool moves against the spring force and opens the valve, thus allowing the flow through the valve. The valve is kept open until the pilot sensing pressure goes below the spring bias. They are available with built-in check valves each of that permits an unrestricted reverse flow. The spring chamber in the sequence valve is drained externally to the system reservoir to negate the effects of back pressure.

Pressure sequence valves can be used to obtain sequential operations of work processes. For example, they can be used in a hydraulic system with a clamping cylinder ‘A’ and a drilling cylinder ‘B’ to get the sequence control of these cylinders. The two critical positions of the circuit are shown in the figure below. The first part of the circuit shows the condition when the 4/3-way valve is shifted to its left-hand side envelope. When the cylinder A reaches the end of its forward stroke to clamp a workpiece, the pump pressure at the inlet of the sequence valve SV2 increases and hence the sequence valve (SV2) opens, allowing the drilling cylinder B to move forward to control the movement of the drill unit. The second part of the circuit shows the condition when the 4/3-way valve is shifted to its right-hand side envelope. When the cylinder B reaches the end of its return stroke, the pump pressure at the inlet of the sequence valve SV1 increases and hence the sequence valve (SV1) opens, allowing the cylinder A to return to its home position.

Counterbalance Valve

The counterbalance valve is a normally-closed valve with an inlet port ‘A’ and an outlet port ‘B’. It also consists of a spring-loaded spool. The externally adjustable spring is provided to set the pressure. The counterbalance must be set at a little higher pressure than that required to retain the load. A pilot passage is provided to accept a signal from the load side of the valve and the signal acts on one side of the spool. The flow path through the valve opens when the pressure at the pilot port increases beyond the pressure setting. When open, the valve discharges the fluid from the inlet port to the outlet port. The valve closes when the pressure drops below the setting of the spring. The main function of the counterbalance valve in a hydraulic circuit with a load-carrying actuator is to maintain the preset backpressure in the return line of the circuit, sufficient to balance the load held by the actuator.

Vertically-mounted cylinders connected with heavy loads or hydraulic motors on winch drives are susceptible to the dangers associated with overrunning loads. Consider a hydraulic cylinder with a heavy load attached. The load forces the cylinder to move faster than the average speed of the cylinder when the associated directional control valve shifts its position to lower the load. Therefore, the actuator with the overrunning load needs some method to retard its downward movement to obtain a smooth operation. Using a counterbalance valve is a better way to control the overrunning load, as the valve dynamically counterbalances the potential free-fall of the connected load. It prevents the actuator from running ahead of the pump due to the load induced energy.

Authored by JOJI Parambath, Founder/Director, Fluidsys Training Centre, Bangalore

Reference:

JOJI PARAMBATH, Industrial Hydraulic Systems – Theory and Practice, Universal Publishers, Boca Raton, USA, 2016.

http://www.universal-publishers.com/book.php?method=ISBN&book=1627340580

Note: A comprehensive account of the topic is given in the textbook on ‘Industrial Hydraulic Systems-Theory and Practice’ by Joji Parambath.

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