The sequence valve is used to ensure that a certain pressure level is achieved in one branch of the circuit before a second branch is activated. Consider a machining operation where the workpiece must be clamped with a certain force before it is extended to make contact with the cutting tool. If the piece is not securely fastened, it can slip and damage both the tool and the piece.
In the circuit shown in Fig. 3.21, the sequence valve is set on 600 psi, meaning that pressure must build to 600 psi before the valve opens. This setting ensures that the clamp cylinder exerts a 600-psi clamp force before the extend cylinder moves. When the directional control valve is shifted for reverse flow, the check valve provides free flow, and there is no sequencing of the cylinders. Either one can retract before the other, depending on the pressure required for retraction. The cylinder with the lowest pressure requirement always retracts first.
Proper sizing of the cylinders will minimize energy loss in a sequence valve circuit. Suppose the maximum pressure to extend the workpiece in Fig. 3.21 is 400 psi. Pressure drop across the sequence valve is 600 ? 400 = 200 psi. If a larger clamp cylinder is used, such that the pressure required to achieve the clamp force is only 500 psi, the sequence valve can be set at 500 psi. Pressure drop across the valve is 500 ? 400 = 100 psi, and energy loss is reduced.
A functional diagram of a sequence valve with flow through to a primary circuit is shown in Fig. 3.22. When pressure at the inlet creates a hydraulic force large enough to offset the spring force, the spool shifts to open a passage to the secondary circuit and close the primary circuit.
There is a type of unloading valve identified as a differential unloading valve. This valve is designed to allow the accumulator to partially discharge before the valve is unvented. Generally, the valve is designed to unvent when the pressure drops 15%. Differential unloading valves are also available that unvent when pressure drops 30%. Unventing means that the piston moves enough to allow the dart to reseat. Once this occurs, the unloading valve functions like a pilot-operated relief valve.
Referencing Fig. 3.17, consider the moment when pressure just reaches 975 psi, and the dart just begins to unseat. Once it cracks open, the pressure in the top chamber cannot increase above 975 psi, so the pressure on one side of the piston (right side in Fig. 3.17) cannot increase above 975 psi. The pressure on the left side can continue to increase up to 1000 psi. This pressure difference causes a force imbalance, so the piston moves to the right, the rod unseats the dart, and the valve is vented.
In a differential unloading valve, the area of the piston is 15% greater than the projected area of the dart. This means that the hydraulic force holding the dart unseated is 15% greater than the hydraulic force that initially unseated the dart. Pressure on the accumulator side must drop 15% below 975 psi before the dart spring can reseat the dart. The force balance is
Pressure on the accumulator side must drop to 848 psi before the valve is unvented.
The symbol used for a differential unloading valve is shown in Fig. 3.19. The vent line connected downstream from the check valve denotes the function of the piston. An accumulator is shown with the symbol in Fig. 3.19 to clarify that the differential unloading valve works with an accumulator.
In the circuit in Fig. 3.15, the pump builds pressure in the accumulator until the setting of the unloading valve is reached. At this point, the unloading valve opens, and flow bypasses to the reservoir. The pressurized fluid is trapped in the accumulator by the check valve and the closed-center directional control valve.
A functional diagram of an unloading valve is shown in Fig. 3.16. Two features are added to a pilot-operated relief valve to create the unloading valve. A check valve is built in, and a small piston is included in the top section in line with the dart and pilot spring. When the unloading valve is closed, fluid flows through the check valve to charge the accumulator.
As with the pilot-operated relief valve, it is helpful to assign values to the springs. For our discussion, we assume the accumulator has a 1000 psi rating. The pilot spring is assigned a value of 975 psi, and the spool spring is assigned a value of 25 psi. When pressure reaches 975 psi, the dart is unseated, allowing fluid to flow through the internal drain to the reservoir. Pressure in the upper chamber cannot increase above 975 psi. The spool is held in place by the 25-psi spring. The small piston has balanced hydraulic forces, because the same pressure acts on both sides. The projected areas of both sides are equal, thus the hydraulic forces are equal.
As pressure continues to build and reaches 975 + 25 = 1000 psi, the unloading valve opens and flow bypasses to the reservoir. The pressure drops, and the check valve closes. Pressure on the accumulator side of the piston pushes it to the right (Fig. 3.17), where the rod pushes the dart off its seat. As long as the dart is held off its seat, the unloading valve is vented, and the pump is unloaded at 25 psi, the pressure required to compress the main spool spring.
When the directional control valve is shifted, fluid drains from the accumulator and the pressure drops. The hydraulic force on the piston drops and, when the pilot spring force becomes greater, the dart reseats. At this point, pressure equalizes on both sides of the spool skirt. The spool spring reseats the spool and the pump begins to build pressure.
Pressure drops at the directional control valve as the accumulator empties. Simultaneously, the pump is building pressure. The resulting pressure vs. time curve has a shape as shown in Fig. 3.18. The minimum pressure is a function of the load, the characteristics of the accumulator, and the characteristics of the pump.
The symbol for an unloading valve is similar to the symbol for a relief valve except that the pilot line is not connected to sense pressure at the valve inlet. The two symbols are compared in Fig. 3.14.
A circuit with an unloading valve is shown in Fig. 3.15. It is necessary to first discuss the operation of this circuit and understand the function of the unloading valve before studying the operation of the valve.
The accumulator is a key component in the Fig. 3.15 circuit. There are three types of accumulators: bladder, diaphragm, and piston. The diaphragm accumulator is a pressure vessel divided into two compartments by a flexible diaphragm. The top half is precharged with a gas, generally nitrogen, and sealed. The bottom half is connected to the hydraulic circuit. Fluid is pumped through the check valve into the bottom of the accumulator. The bottom half is filled, and extra fluid is pumped in as the diaphragm bulges upward. Pressure increases as the gas is compressed. The accumulator is designed for some rated pressure, and pressure must be controlled to ensure that it does not exceed this rating. The unloading valve accomplishes this task.
An accumulator provides pressure to the actuator (cylinder in Fig. 3.15) at the moment the directional control valve is shifted. Pressure does not have to build from a low pressure as it does in a circuit with a open-center directional control valve . Often, acceleration of the load is a significant issue in circuit design. In most manufacturing applications, profitability is increased when the number of cycles per unit time is increased. The cylinder must extend and retract as quickly as possible. If a large load is being moved, pressure must build to achieve enough force to overcome static friction and provide the inertial force to accelerate the mass. It takes an interval of time to build this pressure. This time delay can be eliminated if pressure is already available at the moment the directional control valve is shifted.
An accumulator also provides another important feature. The extra fluid stored in the accumulator allows the desired actuator speed to be achieved with a smaller displacement and, therefore, a lower-cost pump. An example will illustrate how this can be an advantage.
The actuator cycle has a 5-second active part and 20-second passive part, meaning that flow is needed for 5 seconds, and there is a 20-second interval before flow is needed again. The accumulator can supply 924 in3 of oil. If it supplies this fluid during the active part of the cycle, it must be recharged during the passive part. Pump flow to accomplish the recharge is
The pump and prime mover must be four times larger if the accumulator is not included in the circuit.
The analysis done for this example does not include all factors. Design of a circuit with an accumulator requires knowledge of the pressure vs. volume curve as the accumulator is being filled.
A pilot-operated relief valve can be used to unload the pump at low pressure during periods between work cycles. The schematic in Fig. 3.10 shows a solenoid-actuated directional control valve connected to the remote pilot port on the side of the valve. In the position shown, the port is connected to the reservoir. The pilot spring cavity is vented, and the main relief valve opens at the main spring setting. In the shifted position, the directional control valve blocks the port, and the pilot-operated valve operates as previously described. Repeating the description, when the control valve is shifted to the left (port blocked), the integral pilot relief valve is operational, and the main relief valve acts like a pilot-operated relief valve. In the unactuated position, the port is connected to the reservoir, and the main valve is held closed by the 75 psi spring only. When pressure increases above 75 psi, the valve opens, thus the pump builds only 75 psi pressure during the periods between work cycles.
A diagram of a circuit that uses a pilot-operated relief valve to unload the pump at low pressure is shown in Fig. 3.11. The relief valve symbol designated with the letter “A” refers to the main spool of the pilot-operated relief valve. The orifice in the skirt is orifice B, and the symbol designated with letter “C” is a symbol that shows that the valve is held closed with spring pressure and a pilot pressure. The relief valve symbol designated with a letter “D” refers to the pilot stage of the valve (dart held in place with the pilot spring). The circuit operates in the following manner. When the four-way, three-position directional control valve is shifted to extend (or retract) the cylinder, the three-way, two-position directional control valve is simultaneously shifted. The pilot-operated relief valve is thus set to open at the pilot spring setting plus the main spring setting, 1925 + 75 = 2000 psi. When the three-position directional control valve shifts back to the center position, the two-position directional control valve shifts to connect the pilot pressure line to the reservoir. The main relief valve now opens at 75 psi.
When drawing circuit diagrams, a designer will often use a simplified symbol to designate a pilot-operated relief valve. The complete symbol (components A, B, C, and D in Fig. 3.11) is used only when it is needed to explain circuit operation.
A second method for using the pilot-operated relief valve to unload the pump between work cycles is shown in Fig. 3.12. Here, a special directional control valve is used with a fifth port. This port provides a pathway for the pilot line to be connected to the reservoir when the directional control valve is centered. When the directional control valve is shifted, the pilot line is blocked, and the pilot-operated relief valve will not open until the pressure equals the pilot spring pressure plus the main spring pressure.
The circuit in Fig. 3.13 is designed to provide high-pressure relief during extension and low-pressure relief during retraction. (The functional requirements of the circuit are unexplained at this point. The low-pressure relief may be needed to prevent damage or injury, if the workpiece strikes an obstruction during retraction.) Use of the pilot-operated relief valve is similar to the use in Fig. 3.11. The check valve is held in place by high pressure, thus blocking the remote pilot connection on the pilot-operated relief valve. The valve then opens at the high-pressure setting (pilot spring + main spring pressure). During return, the pilot line is connected to the reservoir (as shown in Fig. 3.10); therefore, the valve opens at the low-pressure setting (main spring pressure). It is understood that the low-pressure setting must be high enough to provide the force needed for normal retraction.
A pilot-operated relief valve has the same function as a direct-acting relief valve; however, it has a different pressure vs. flow curve. The performance curves for the two types of relief valve are given in Fig. 3.7. The pilot-operated valve opens completely over a narrow pressure range. This allows the circuit to operate over a wider pressure range without loss of fluid over the relief valve.
A functional diagram of a pilot-operated relief valve is shown in Fig. 3.8. The main spool has a small hole (orifice) drilled in the skirt. Because of this hole, pressure is the same on the top and bottom of the skirt. As long as there is negligible flow through the orifice, there is no pressure drop across the orifice.
The pilot section of the valve is the top section. A dart is held in place by the pilot spring. When the hydraulic force on this dart becomes greater than the pilot spring force, the dart is unseated, and fluid flows from the cavity above the skirt, through an internal drain to the valve outlet. Flow through the orifice replaces the fluid lost from the cavity above the skirt. The spool is still held in position by the main spool spring.
At this point, discussion is facilitated if the two springs are assigned values. Suppose the pilot spring is a 1425-psi spring, and the main spool spring is a 75-psi spring. When pressure at the valve inlet reaches 1425 psi, the dart is unseated. Pressure in the upper cavity cannot increase above 1425 psi. The hydraulic force on the top and bottom of the skirt is equal, and these two forces balance. The main spool is held in position only by the spring force produced by the 75 psi spring.
What happens when the pressure at the valve inlet reaches 1425 psi? The relief valve stays closed. If pressure continues to increase and reaches 1500 psi, the spool lifts, and fluid is bypassed to the reservoir. As pressure continues to increase above 1500 psi, the main spool opens further until it is completely open. Only a small pressure increase is needed to completely compress the 75 psi spring. (In Fig. 3.7, the pilot-operated valve is fully open at 1585 psi.) In other words, the valve goes from cracking to full open with a very small increase in pressure. When the load is changing quickly, and sharp pressure spikes are created, the quick opening feature of a pilot-operated relief valve is needed to protect the circuit.
The key advantage of a pilot-operated valve is that it allows the designer to use pressure to within 100 psi of the valve setting to meet the functional objective of the circuit. In comparison, the direct-acting valve cracks open at 1500 psi, and pressure must increase to 2000 psi before it is fully open.
A pilot-operated relief valve can be used with a remote pilot as shown in Fig. 3.9. The remote pilot functions like the pilot built into the top of the main relief valve. It allows the designer to set two pressure levels with one main relief valve.
Suppose the pilot on top of the main valve has a 1925 psi spring, and the remote pilot has a 925 psi spring. If the pressure at the remote pilot reaches 925 psi, the dart unseats, and the pressure in the cavity above the skirt is limited to 925 psi. The main valve cracks open at 925 + 75 = 1000 psi. If pressure at the remote pilot location stays below 925 psi, then circuit pressure can continue to build until it reaches 1925 psi, the setting of the pilot built into the housing of the main valve. The main valve now cracks open at 1925 + 75 = 2000 psi. In this illustration, no information is given on where the remote pilot is connected in the circuit. We only know that it is someplace other than the main pressure line from the pump.
A schematic of a direct-acting relief valve is shown in Fig. 3.4. Pressure acts on the annular area of the valve spool. The hydraulic force is given by
The notation Fs will be used for the spring force. When Fh equals Fs , the valve cracks open, meaning that the spool lifts off its seat and allows fluid to flow to the reservoir. As pressure increases, the spool lifts higher, allowing more flow to bypass to the reservoir. At some pressure level, the total flow bypasses to the reservoir.
A typical flow vs. pressure curve for a direct-acting relief valve is shown in Fig. 3.5. The valve is set to open at 1500 psi. This pressure is known as the cracking pressure. When pressure reaches 2000 psi, the valve is fully open, and all flow is bypassed to the reservoir; no flow goes to the remainder of the circuit. The 500 psi differential between cracking and full bypass is needed for a direct-acting valve when it has a functional role in flow control in addition to its pressure limiting function. Pilot-operated relief valves have a much lower differential and are used when the sole function of the relief valve is overpressure protection for the circuit.
The characteristics of a direct-acting relief valve can be used in a simple circuit to control the speed of the actuator. In the circuit shown in Fig. 3.6, the flow control valve is simply an adjustable orifice in the circuit. When the flow control valve is partly closed, a pressure drop is created across the valve. Pressure at the relief valve is the sum of the pressure drop across the flow control valve plus the pressure drop across the motor. (For this simple example, pressure drops in the lines are neglected.) To slow the motor, the flow control valve is closed to create enough pressure at the relief valve to cause it to crack open. Part of the pump output now bypasses to the reservoir; thus, flow to the motor is reduced, and the speed decreases.
A simple analysis will illustrate the performance of the circuit in Fig. 3.6. Suppose the relief valve has the characteristics shown in Fig. 3.5. The fixed displacement pump is delivering 10 GPM to the motor. The flow control valve is fully open, and the pressure at the relief valve is 1000 psi. To reduce the motor speed to one-half its current value, what pressure drop must be created at the flow control valve?
Flow to the motor must be reduced to 5 GPM to cut the speed by half, which means that 5 GPM must flow across the relief valve. As shown in Fig. 3.5, pressure must rise to 1750 psi before 5 GPM bypasses through the relief valve. Pressure drop across the motor is only 1000 psi; therefore, the required pressure drop across the flow control valve must be 1750 ? 1000 = 750 psi.
No mechanical energy is output at the relief valve; consequently, all the hydraulic energy in the flow across the valve is converted to heat energy. The circuit in Fig. 3.6 is simple but not energy efficient.
It is instructive to calculate the energy flow in this simple circuit. The pump is a fixed-displacement unit; consequently, the delivered flow is constant. (At this time, we neglect that pump leakage increases as pressure increases, and therefore, pump output decreases as pressure increases.) Total hydraulic power delivered by the pump is
Only 28.5% of the hydraulic power is delivered as mechanical power by the motor. The remainder is converted to heat. Operating temperature of this circuit will be high. Obviously, it is a poor design; however, the analysis does reinforce an important concept in pressure control. Any time there is a pressure drop across a valve and no mechanical power is output, heat is generated and circuit efficiency is reduced. Simple circuits may have a lower initial cost, but the higher operating costs over their design life often offset this advantage.
ANSI symbols are introduced here as they are needed. The directional control valve symbol is the most intuitive and selfexplanatory of the symbols. Some experience with the directional control valve symbol has already been gained. At this point, it is necessary to review the three most common center configurations (Fig. 3.1) for spool-type directional control valves.
An open-center (float) valve allows flow between all four ports when the valve is in the center (nonactuated) position. The actuator (downstream from the valve) is not held in position but is free to float.
The open-center valve also allows free flow from the inlet port to the return (or tank) port, but it blocks the actuator ports. The actuator cannot move (neglecting leakage) when the open-center valve is in the center position.
The closed-center valve has all four ports blocked when it is in the center position. There is no pathway through the valve between any of the four ports.
There are many configurations for directional control valves. Two are shown in Figs. 3.2 and 3.3. The check valves ensure that flow can go only from the pump to the circuit. Thus, the pump is isolated from pressure spikes that may occur due to load dynamics. Both figures show two directional control valves stacked in the same housing. Ten or more valves can be stacked in this manner. (The reader may have observed a bank of handles on a machine for manual actuation of individual valves stacked in this manner.) As is often the case, the figures show a relief valve built into the valve housing, a feature which simplifies circuit assembly.
Figure 3.2 shows a valve stack where the flow passes directly through the valve and returns to the reservoir when neither spool is actuated. The bottom half of this figure shows both spools activated simultaneously. If the pressure required by both cylinders is approximately equal, the pump flow will divide, and some flow will go to each cylinder. However, if one cylinder requires more pressure, the flow will always take the path of least resistance and goes to the lower-pressure cylinder first. When this cylinder is fully extended or hits a stop, the pressure will rise to the level required to extend the second cylinder.
The valves in Fig. 3.3 are still open-center valves, but they are configured differently. Both spools are shown in the actuated position in the bottom half of the figure. Flow goes to Cylinder 1 and no flow to Cylinder 2. If Cylinder 1 is returned to the center position, then flow will go to Cylinder 2. This spool design ensures that only one circuit can be actuated at any time.
Pressure control is a key element in the design of any circuit. Not only is it the key to achieving a given functional objective, it is also the key to safe operation.
Components are designed to operate at a given maximum pressure and will withstand pressure peaks up to some burst pressure. Failure of a component can be dangerous to nearby workers. They can be injured by shrapnel, or they may be injured when they are hit by a stream of high-pressure, high temperature oil. Injuries received when oil penetrates the skin are very difficult to treat and require specialized medical knowledge. Often, there is also potential for worker injury by losing control of a load held against gravity.
The fundamental pressure control problem in circuit design is the limiting of pressure to a level below the working pressure of the lowest-rated component in the circuit. If a piece of hose, rated at 1500 psi working pressure, is used in a circuit where all other components are rated at 3000 psi, then maximum pressure in this circuit must be limited to 1500 psi. Pressure can build to the relief valve setting at all points between the pump and the load.
Six pressure-control valves will be discussed in this section, These valves are:
1. Relief valves
2. Unloading valves
3. Sequence valves
4. Pressure-reducing valves
5. Counterbalance valves
6. Brake valves
Each of these valves works on the same principle; a spring force balances a hydraulic force. The hydraulic force is produced by pressure acting on a given area. When the hydraulic force becomes greater than the spring force, the valve spool moves. There are many different ways in which this principle is used in valve design. The construction of some valves is intricate, but the principle of operation is simple. It is appropriate to re-emphasize the principle; a spring force opposes a hydraulic force.