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Pressure controls (other than relief and unloading valves)
There are some parts of fluid power circuits that need pressure control. (Chapter 9 covered relief and unloading valves that control pressure in pump circuits.) Other types of pressure controls include sequence valves, counterbalance valves, and reducing valves. Though the internal works (and the symbols) are similar, these three pressure controls perform entirely different functions. Sequence valves and counterbalance valves are normally closed — like relief valves and unloading valves — but they usually allow bi-directional flow, so they need a bypass check valve in their bodies. Sequence valves always have an external drain connected directly to tank. Counterbalance valves are internally drained, except when used in some regeneration circuits.
Reducing valves are normally open and respond to outlet pressure to keep outlet flow from going above their set pressure. They also can have a bypass check valve. Reducing valves always have an external drain connected directly to tank. Any backpressure in this drain line adds to the valve’s spring setting.
Relief valves, unloading valves, sequence valves, counterbalance valves, and reducing valves are the most difficult to discern on a schematic drawing because their symbols are so similar. Take extra care when diagnosing a problem to make sure these valves are correctly identified and their function understood.
There are times when two or more actuators, operating in a parallel circuit, must move in sequence. The only positive way to do this is with separate directional control valves and limit switches or limit valves. This setup assures the first actuator has reached a specific location before the next operation commences. If there is no safety concern or possibility of product damage if the first actuator does not complete its cycle before the second starts, a sequence valve can be a simple way to control the actuators’ actions.
The symbols and cutaways in Figure 14-1 are for hydraulic and pneumatic sequence valves. The main difference between these valves is that most hydraulic sequence valves are single purpose and must be used in series with a directional control valve, while many air sequence valves are pilot-operated directional control valves with an adjustable spring return. In either case, a preset pressure must be reached before the valves allow fluid to pass or change flow paths. Many manufacturers offer a direct-acting internally piloted hydraulic sequence valve like the design shown in Figure 14-1. This valve can be changed to external pilot in the field if required.
Several manufacturers offer pilot-operated sequence valves also. Pilot-operated sequence valves stay closed to within 50 psi or less of their set pressure. Direct-acting sequence valves may partially open at pressures that are 100 to150 psi below set pressure — and thus allow premature actuator creep.
A balanced spool — held in place by an adjustable-force spring — blocks fluid at the hydraulic sequence valve’s inlet. When pressure at the inlet reaches the spring setting, pressure in the internal pilot line pushes the spool up to allow enough flow to the outlet to keep pressure from going higher. Pressure at the inlet never drops below set pressure when there is flow to the outlet. When outlet pressure exceeds set pressure, the valve opens fully and pressure at both ports equalizes. Notice that the drain port hooked to tank must be at no pressure or constant pressure because any pressure in this line adds to spring setting. (Remember that a sequence valve must always have an external drain.)
A bypass check valve allows reverse flow when the valve is used in a line with bi-directional flow. In some applications a sequence valve may be externally piloted from another operation. Most valves can be converted in the field. (The designer should always change the part number to reflect the conversion.)
Pneumatic sequence valves typically are 5-way directional control valves with adjustable springs to set their shifting pressure. They are used to start a second operation after the preceding one finishes. Some older machines have one solenoid valve to start the cycle and several sequence valves to extend and retract all other actuators. Some precautions:
• A sequence valve shifts on a pressure build-up and may start a second operation prematurely if an actuator stalls or is stopped for any reason. If personnel safety or product damage can occur due to an incomplete stroke, don’t use sequence valves. Instead, use limit switches or limit valves and directional control valves for each operation sequence.
• When flow controls are required they must be meter-in types. Take the signal to the sequence valve from the line downstream from the flow control because pressure at this point will be whatever is required to move the actuator and its load.
The circuit in Figure 14-2 is typical for air-powered machines. Cyl. 1 extends to clamp a part when an electrical input signal shifts the solenoid pilot-operated valve. As Cyl. 1 extends, pressure beyond the meter-in flow control at its cap end becomes as high as necessary to move the cylinder and its load. With the sequence valve set to shift at 70 psi, Cyl. 2 should not move until Cyl. 1 has extended and securely clamped the part. If the clamp does not make a full stroke for any reason, the Cyl. 2 extending prematurely will not damage the part or be unsafe. When the clamp is at 70 psi or higher, the sequence valve shifts to extend Cyl. 2. Both cylinders can return simultaneously without causing any problems.
One great feature of a sequence-operated circuit is it does not matter how far the first cylinder must move before the next operation takes place. Thick or thin parts are clamped at the same force before the next operation starts because pressure must build to the same level to trigger the next sequence.
Cyl. 2 has meter-out flow controls to retard its movement and hold pressure on Cyl. 1 during the stamping operation. De-energizing the solenoid pilot-operated valve allows both cylinders to return home at the same time.
The hydraulic sequence circuit in Figure 14-3 is typical for a machine that must clamp and hold pressure while a second operation takes place. Sequence valve 1 is set at 550 psi; pressure at clamp Cyl. 1 must be at least 550 psi before punch Cyl. 2 can extend. While punch Cyl. 2 is extending, pressure in the circuit never drops below 550 psi. If the punching operation requires more than 550 psi, the pressure in the whole circuit increases -- up to the relief valve setting.
Sequence valve 2 (set at 450 psi) keeps Cyl. 1 from getting a retract signal until Cyl. 2 has returned and pressure increases. A pilot-operated check valve maintains clamp force while the punch cylinder retracts. The signal to open the pilot-operated check valve comes from the line between Sequence Valve 2 and Cyl. 1, so there is no signal until Cyl. 2 fully retracts. (This circuit is not safe if pressure buildup comes from some source other than clamp contact or the end of stroke so that the punch cylinder operates prematurely.)
Sequence valves often generate a great deal of heat because the first actuator to move takes higher pressure than the subsequent actuators. This means there is usually a high pressure drop across a sequence valve that results in wasted energy. In some circuits, a kick-down sequence valve can reduce the energy loss. The cutaway view and symbol in Figure 14-4 show the inner workings of a kick-down sequence valve to explain how it controls opening pressure and then unloads it.
Fluid from the inlet flows through the control orifice and up to the adjustable poppet where it is blocked. The resulting pressure tries to open the poppet while equal pressure and a light spring acting on the opposite side hold it shut. When pressure increases enough to unseat the adjustable poppet and more flow starts passing the poppet than going through the control orifice, the pressure imbalance lets the poppet raise. When the poppet moves enough to let trapped fluid go through the bypass orifice, pressure on top of the poppet drops off -- because the bypass orifice is larger than the control orifice. At this point, the only force acting to hold the poppet shut is spring force and backpressure at the outlet port. When flow stops, the poppet closes again due to pressure equalization and spring force on the poppet.
The circuit in Figure 14-5 is the same as in 14-3 except it incorporates kick-down sequence valves in place of standard sequence valves. Cyl. 2 will not extend in this circuit until pressure on Cyl. 1 has reached 750 psi. The difference is when a kick-down sequence valve opens at its pressure setting, it allows fluid to pass at 50 psi plus whatever it takes to overcome downstream resistance. This means the whole circuit from the pump to all actuators is 50 psi plus Cyl. 2’s resistance. The pilot-operated check valve at Cyl. 1’s cap-end port keeps it pressurized at near full force, while Cyl. 2 extends at low force. Energy waste is very low so heat buildup is minimal. (Other sequence valve circuits can be found in the e-book Fluid Power Circuits Explained by the author of this manual, which will be launched in the next few months.)