Control of a Single-Acting Hydraulic Cylinder
Figure 1.1 shows that the control of a single-acting,spring return cylinder using a three-way two-position manually actuated, spring offset direction-control valve (DCV). In the spring offset mode, full pump flow goes to the tank through the pressure-relief valve (PRV). The spring in the rod end of the cylinder retracts the piston as the oil from the blank end drains back into the tank. When the valve is manually actuated into its next position, pump flow extends the cylinder.
After full extension, pump flow goes through the relief valve. Deactivation of the DCV allows the cylinder to retract as the DCV shifts into its spring offset mode.
Bleed-Off Circuit
Compared to meter-in and meter-out circuits, a bleed-off circuit is less commonly used. Figure 1.10 shows a bleed-off circuit with extend stroke control. In this type of flow control, an additional line is run through a flow-control valve back to the tank. To slow down the actuator, some of the flow is bledoff through the flow-control valve into the tank before it reaches the actuator. This reduces the flow into the actuator, thereby reducing the speed of the extend stroke.
The main difference between a bleed-off circuit and a meter-in/meter-out circuit is that in a bleed-off circuit, opening the flow-control valve decreases the speed of the actuator, whereas in the case of a meter-in/meter-out circuit, it is the other way around.
Meter-Out Circuit
Figure 1.9 shows a meter-out circuit for flow control during the extend stroke. When the cylinder extends, the flow coming from the pump into the cylinder is not controlled directly. However, the flow out of the cylinder is controlled using the flow-control valve (metering orifice). On the other hand, when the cylinder retracts, the flow passes through the check valve unopposed, bypassing the needle valve. Thus, only the speed during the extend stroke is controlled.
Both the meter-in and meter-out circuits mentioned above perform the same operation (control the speed of the extending stroke of the piston), even though the processes are exactly opposite to one another.
Meter-In Circuit
Figure 1.8 shows a meter-in circuit with control of extend stroke. The inlet flow into the cylinder is controlled using a flow-control valve. In the return stroke, however, the fluid can bypass the needle valve and flow through the check valve and hence the return speed is not controlled. This implies that the extending speed of the cylinder is controlled whereas the retracing speed is not.
Pressure-Compensated Valves
Pressure-compensated flow-control valvesovercome the difficulty causedby non-pressure-compensated valves by changing the size of the orifice in relation to the changes in the system pressure. This is accomplished through a spring-loaded compensator spool that reduces the size of the orifice when pressure drop increases. Once the valve is set, the pressure compensator acts to keep the pressure drop nearly constant. It works on a kind of feedback mechanism from the outlet pressure. This keeps the flow through the orifice nearly constant.
Schematic diagram of a pressure compensated flow-control valve is shown in Fig. 1.5 and its graphical symbol in Fig. 1.6. A pressure-compensated flow-control valve consists of a main spool and a compensator spool. The adjustment knob controls the main spool’s position, which controls the orifice size at the outlet. The upstream pressure is delivered to the valve by the pilot line A. Similarly, the downstream pressure is ported to the right side of the compensator spool through the pilot line B. The compensator spring biases the spool so that it tends toward the fully open position. If the pressure drop across the valve increases, that is, the upstream pressure increases relative to the downstream pressure, the compensator spool moves to the right against the force of the spring. This reduces the flow that in turn reduces the pressure drop and tries to attain an equilibrium position as far as the flow is concerned.
In the static condition, the hydraulic forces hold the compensator spool in balance, but the bias spring forces it to the far right, thus holding the compensator orifice fully open. In the flow condition, any pressure drop less than the bias spring force does not affect the fully open compensator orifice,but any pressure drop greater than the bias spring force reduces the compensator orifice. Any change in pressure on either side of the control orifice, without a corresponding pressure change on the opposite side of the control orifice, moves the compensator spool. Thus, a fixed differential across the control orifice is maintained at all times. It blocks all flow in excess of the throttle setting. As a result, flow exceeding the preset amount can be used by other parts of the circuit or return to the tank via a pressure-relief valve.
Performance of flow-control valve is also affected by temperature changes which changes the viscosity of the fluid. Therefore, often flow-control valves have temperature compensation. Graphical symbol for pressure and temperature compensated flow-control valve is shown in Fig. 1.7.
Non-Pressure-Compensated Valves
Non-pressure-compensated flow-control valves are used when the system pressure is relatively constant and motoring speeds are not too critical. The operating principle behind these valves is that the flow through an orifice remains constant if the pressure drop across it remains the same. In other words, the rate of flow through an orifice depends on the pressure drop across it.
The disadvantage of these valves is discussed below. The inlet pressure is the pressure from the pump that remains constant. Therefore, the variation in pressure occurs at the outlet that is defined by the work load. This implies that the flow rate depends on the work load. Hence, the speed of the piston cannot be defined accurately using non-pressure-compensated flow-control valves when the working load varies. This is an extremely important problem to be addressed in hydraulic circuits where the load and pressure vary constantly.
Schematic diagram of non-pressure-compensated needle-type flow-control valve is shown in Fig. 1.3. It is the simplest type of flow-control valve. It consists of a screw (and needle) inside a tube-like structure. It has an adjustable orifice that can be used to reduce the flow in a circuit. The size of the orifice is adjusted by turning the adjustment screw that raises or lowers the needle. For a given opening position, a needle valve behaves as an orifice. Usually, charts are available that allow quick determination of the controlled flow rate for given valve settings and pressure drops.
Sometimes needle valves come with an integrated check valve for controlling the flow in one direction only. The check valve permits easy flow in the opposite direction without any restrictions. As shown in Fig. 1.4, only the flow from A to B is controlled using the needle. In the other direction (B to A), the check valve permits unrestricted fluid flow.
Pilot-Operated Unloading Valve
A pilot-operated unloading valve has less pressure override than its direct-acting counterpart.So it does not dump part of the flow prematurely.
A pilot-operated unloading relief valve is the same as a pilot-operated relief valve with the addition of an unloading spool. Without the unloading spool, this valve would function just like any pilot-operated relief valve. Pressure buildup in the pilot section would open some flow to the tank and unbalance the poppet, allowing it to open and relieve excess pump flow.
Schematic diagram of unloading valve is shown in Fig. 1.10.In a pilot-operated unloading valve; the unloading spool receives a signal through the remote-pilot port when pressure in the working circuit goes above its setting. At the same time, pressure on the spring-loaded ball in the pilot section starts to open it. Pressure drop on the front side of the unloading spool lowers back force and pilot pressure from the high-pressure circuit forces the spring-loaded ball completely off its seat. Now there is more flow going to the tank than what the control orifice can keep up with. The main poppet opens at approximately 20 psi. Now, all high-volume pump flow can go to the tank at little or no pressure drop and all horsepower can go to the low-volume pump to do the work. When pressure falls approximately 15% below the pressure set in the pilot section, the spring-loaded ball closes and pushes the unloading spool back for the next cycle.
An unloading valve requires no electric signals. This eliminates the need for extra persons when troubleshooting. These valves are very reliable and seldom require maintenance, adjustment or replacement.An unloading valve unloads the pump when the desired pressure is reached.It allows rapid discharge of pressurized oil near atmospheric pressure.As soon as the system pressure reaches the setting pressure that is available at the pilot port, it lifts the spool against the spring force.When the spool is held by the pilot pressure, the delivery from the pump goes to the tank.An unloading valve is used to perform operations such as stamping, coining, punching, piercing, etc.
Figure1.11shows the application of unloading valve in a punching press. It is a circuit that uses a high-pressure, low-flow pump in conjunction with a low-pressure, high-flow pump. In a punching press, the hydraulic cylinder must extend rapidly over a great distance with low-pressure, high-flow requirements. This rapid extension of cylinder occurs under no external load (when the punching tool approaches the sheet metal).But during punching operation for short motion, the pressure requirements are high due to punching load. During this cylinder travel, high-pressure, low-flow requirements are needed.
When punching operation begins, the increased pressure opens the unloading valve to unload the low-pressure pump. The purpose of relief is to protect the high-pressure pump from over pressure at the end of the cylinder stroke and when direction control valve (DCV) is in its spring centered mode. The check valve protects the low-pressure pump from high pressure, which occurs during punching operation that occurs at the end of cylinder extension and when the DCV is in its spring centered mode.
The above circuit given in Fig. 1.11 eliminates the necessity of having a very expensive high-pressure, high-flow pump.
Direct-Acting Unloading Valve
A direct-acting unloading valve consists of a spool held in the closed position by a spring. The spool blocks flow from the inlet to the tank port under normal conditions. When a high-pressure fluid from the pump enters at the external-pilot port, it exerts force against the pilot piston. (The small-diameter pilot piston allows the use of a long, low-force spring.) When the system pressure increases to the spring setting, the fluid bypasses to the tank (as a relief valve would function). When the pressure goes above the spring setting, the spool opens fully to dump the excess fluid to the tank at little or no pressure.
Pressure-Reducing Valve
The second type of valve is a pressure-reducing valve. This type of valve (which is normally open) is used to maintain reduced pressures in specified locations of hydraulic systems. It is actuated by downstream pressure and tends to close as this pressure reaches the valve setting. Schematic diagram of pressure reducing valve is shown in Fig. 1.6, symbolic representation is shown in Fig. 1.7 and three-dimensional view is shown in Fig. 1.8.
A pressure-reducing valve uses a spring-loaded spool to control the downstream pressure. If the downstream pressure is below the valve setting, the fluid flows freely from the inlet to the outlet. Note that there is an internal passageway from the outlet which transmits outlet pressure to the spool end opposite the spring. When the outlet (downstream) pressure increases to the valve setting, the spool moves to the right to partially block the outlet port. Just enough flow is passed to the outlet to maintain its preset pressure level. If the valve closes completely, leakage past the spool causes downstream pressure to build up above the valve setting. This is prevented from occurring because a continuous bleed to the tank is permitted via a separate drain line to the tank.
Reverse free flow through the valve is only possible if the pressure exceeds the valve setting.The valve then closes, thus making reverse flow impossible. Therefore, pressure-reducing valves are often equipped with a check valve for reverse free flow.
External forces acting onto a linear actuator increase the pressure between the pressure-reducing valve and the actuator. In some systems, it is therefore desirable to relieve excess fluid from the secondary system to the tank in order to maintain a constant downstream pressure, regardless of such external forces.
A reducing valve is normally open. It reads the downstream pressure. It has an externaldrain.This is represented by a line connected from the valve drain port to the tank. The symbol shows that the spring cavity has a drain to the tank.
Figure 1.9 shows an application for a pressure-reducing valve. Here two cylinders are connected in parallel. The circuit is designed to operate at a maximum pressure p1, which is determined by the relief valve setting. This is the maximum pressure at which cylinder 1 operates. By the function of the machine, cylinder2 is limited to pressurep2 (p2<p1).This can be accomplished by placing a pressure-reducing valve in the circuit in the location shown in Fig. 1.9.If the pressure in the cylinder2 circuit rises above p2, the pressure-reducing valve closes partially to create a pressure drop across the valve. The valve then maintains the pressure drop so that the outlet pressure is not allowed to rise above p2 setting.
The disadvantage of this method is that the pressure drop across the reducing valve represents the lost energy that is being converted into heat. If the pressure setting of the reducing valve is set very low relative to the pressure in the rest of the system, the pressure drop is very high, resulting in excessive heating of the fluid. When the hydraulic oil becomes too hot, its viscosity reduces causing increased component wear.
Compound Pressure Relief Valve
A pilot-operated pressure-relief valve consists of a small pilot relief valve and main relief valve as shown in Fig. 1.4. It operates in a two-stage process:
1. The pilot relief valve opens when a preset maximum pressure is reached.
2. When the pilot relief valve opens, it makes the main relief valve open.
The pilot-operated pressure-relief valve has a pressure port that is connected to the pump line and the tank port is connected to the tank. The pilot relief valve is a poppet type. The main relief valve consists of a piston and a stem. The main relief piston has an orifice drilled through it. The piston has equal areas exposed to pressure on top and bottom and is in a balanced condition due to equal force acting on both the sides. It remains stationary in the closed position. The piston has a light bias spring to ensure that it stays closed. When the pressure is less than that of relief valve setting, the pump flow goes to the system. If the pressure in the system becomes high enough, it moves the pilot poppet off its seat. A small amount of flow begins to go through the pilot line back to the tank. Once flow begins through the piston orifice and pilot line, a pressure drop is induced across the piston due to the restriction of the piston orifice. This pressure drop then causes the piston and stem to lift off their seats and the flow goes directly from the pressure port to the tank.
The advantages of pilot-operated pressure-relief valves over direct-acting pressure-relief valves are as follows:
1. Pilot-operated pressure-relief valves are usually smaller than direct-acting pressure-relief valves for the same flow and pressure settings.
2. They have a wider range for the maximum pressure settings than direct-acting pressure-relief valves.
3. They can be operated using a remote while direct-acting pressure-relief valves cannot.
Graphic symbol of a pressure-relief valve is shown in Fig. 1.5. The symbol shows that the valve is normally closed (the arrow is offline). On one side of the valve, pressure is fed in (the dashed line) to try to open the valve, while on the other side, the spring tries to keep it adjustable, allowing the adjustment of pressure level at which the relief valve opens. The arrow through the spring signifies that it is adjustable, allowing the adjustment of pressure level at which the relief valve opens.
