The term cartridge valves commonly refers to screw-in types of pressure, directional, and flow control valves. Screw-in type cartridge valves are mostly low-flow valves -- 20 gpm or less, although some manufacturers’ valves can handle more than 100 gpm. Screw-in cartridges are very compact, develop low-pressure drop, have little leakage, and produce inexpensive circuits that are reliable and easy to maintain. Screw-in cartridges are most often part of a drilled manifold but also are available in individual bodies. The function and performance of screw-in cartridge valves are the same as in-line or subplate-mounted valves.
Slip-in cartridge valves are different because -- except for pressure controls -- they are simply 2-way, bi-directional, pilot-to-close check valves. Most circuits using slip-in cartridge valves flow at least 60 gpm and can go as high as 3000 gpm. Slip-in cartridges are compact, develop low-pressure drop, and operate at pressures to 5000 psi. Slip-in cartridges can function as pressure, flow, and directional control valves.
Figure 4-1 shows a cutaway view and symbol of a 1:1 area ratio, poppet-type cartridge valve. Pressure relief, sequence, unloading, and counterbalance functions normally use a 1:1 area ratio poppet. The area ratio is the relation of the pilot area to the A port area. The 1:1 area valve stays closed when pilot pressure is equal to or greater than the A port pressure.
Figure 4-2 shows a cutaway view and symbol for a 1:1.1 area ratio valve. Here the pilot area is 1.1 times the A port area. Use this 1:1.1 ratio for special directional controls where system pressure at the pilot area must hold against excess pressure at the B port. Some pressure control applications also use this area ratio. Flow is possible from A to B, or B to A with low or no pilot pressure.
Figure 4-3 shows a cutaway and symbol for a 1:2 area ratio cartridge valve. Most directional-valve functions use this area ratio. Here, pilot area is twice the A or B port area. The 1:2 ratio valve allows flow from A to B or B to A with the same pressure drop. When the pilot area sees the same pressure as the A and/or B, all flow stops.
Slip-in cartridge pressure-relief valves
The schematic symbol and cutaway in Figure 4-4 are for a slip-in cartridge relief valve. The symbol for a cartridge is more pictorial than for spool valves, though the pressure-adjusting section uses a conventional ISO symbol.
Pressure relief cartridges can only flow from port A to port B. Port A is always connected to the pump while port B is always connected to tank. The spring that holds the poppet in place allows it to open at about 30 psi. This internal spring seats the poppet regardless of valve mounting position.
A slip-in cartridge valve has a cover that contains porting relative to the function the valve will perform and an adjustable spring-loaded poppet (the adjustable relief). This cover also holds the slip-in cartridge in place. The slip-in cartridge has a bushing with seals to prevent leakage to the outside or across the ports. This bushing fits in a machined cavity and contains the poppet that moves to allow fluid to pass. The poppet on a relief valve has a ratio of 1:1, which means the areas at the working fluid side, at the A port, and at the pilot side are equal.
Drilled pilot passages allow fluid to flow through control orifices to the pilot area of the poppet and to the adjustable relief in the cover. As system pressure increases, the poppet sees the same pressure on both sides and stays closed . . . held by the 30-psi spring. When system pressure reaches the relief setting, the adjustable relief opens a small amount, allowing pilot flow to tank. When pilot flow to tank is greater than control orifice flow from the A port, pressure on top of the poppet lowers. Then the poppet unseats to pass excess pump flow to tank.
Figure 4-5 shows the same cartridge relief valve with a single-solenoid directional valve — or venting valve — mounted on the cover. This solenoid-operated relief holds maximum pressure with the solenoid energized and unloads the pump to tank at approximately 30 psi when the solenoid is de-energized. Reversing the solenoid coil and spring keeps the pump loaded until the venting valve is energized.
Figure 4-6 shows the symbol for a dual-pressure relief valve with pump unloading. Pressures are set at the two manually adjustable relief covers and the solenoids select which relief to use. When both solenoids are de-energized, the pump unloads.
The symbol in Figure 4-7 is for an infinitely variable cartridge relief valve. A proportional solenoid valve is mounted on the cover of this 1:1 cartridge. The proportional solenoid valve controls vent flow, which in turn controls pressure. An electronic signal sets infinitely variable pressure to protect the system in varying conditions. The manually adjusted relief cover under the proportional solenoid sets maximum system pressure regardless of electrical input.
Figure 4-8 shows the symbol for a relief valve with a low-pressure unloading port. Set the relief cover for maximum pressure as before. Then, when it reaches maximum pressure, the relief cartridge opens to unload the pump at approximately 30 psi. Venting pressure comes from piping the unloading port downstream of a check valve that holds fluid in the accumulator. Until there is about a 15% pressure drop in the accumulator holding circuit, the pump will stay unloaded. When pressure drops about 15%, the relief cartridge closes until system pressure reaches maximum setting again.
The schematic symbol and cutaway in Figure 4-9 are for a cartridge pressure-reducing valve. The ISO symbols for the cartridge and the pressure-reducing section are conventional. Pressure-reducing cartridges only flow from port B to port A. Port B always sees inlet or system pressure, while port A is the reduced-pressure outlet. If reverse flow is necessary, add a bypass check valve to allow return flow around the reducing valve. The spring directly holding the spool in place keeps it open regardless of valve mounting position when pressure is below the adjustable relief setting.
The slip-in cartridge reducing valve has a cover that contains porting relative to the function to be performed. An adjustable spring-loaded poppet (the adjustable relief) in the cover sets outlet pressure. This cover also holds the slip-in cartridge in place. The cartridge has a bushing with seals to prevent leakage to the outside or across the ports. This bushing fits in a machined cavity and contains the spool that closes as pressure increases. The spool on a reducing valve has a ratio of 1:1 -- which means that the A port area and pilot area are equal.
A drilled pilot passage allows fluid to flow through a pressure-compensated control orifice to the adjustable relief in the cover, as well as to the top of the spool. As pressure builds, the spool stays open because of the spring and the equal pressures on equal areas, thus letting flow continue through the valve. When the A port reaches the reduced pressure setting, the adjustable relief opens and pilot fluid flows to tank through the drain. When pilot flow is greater than control orifice flow, lower pressure on top of the spool allows it to rise, blocking flow from the B port to the A port. Pressure at the A port will not exceed that set on the adjustable relief unless a load-induced pressure tries to force flow back through the closed spool. There will be pilot flow out the drain port whenever the reducing valve is at reduced pressure. Blocking or closing the drain port causes the spool to fully open and allow outlet pressure to reach system pressure.
A pressure-reducing valve will not allow reverse flow after it has reached its set pressure. For example, if the reduced pressure is 500 psi at a cylinder and some outside force starts pushing against the cylinder, there is no place for most of the fluid to go. About 50 to 100 in.3/min of excess fluid passes through the pilot circuit and out the drain port while the valve is reducing. Fluid in excess of drain flow becomes trapped and pressure builds, possibly to dangerous levels. If there is a chance of outside forces that can increase outlet pressure, add a relief valve bypass at the outlet. A bypass relief valve relieves trapped fluid before excessive pressure can damage the valve or machine.
Figure 4-10 shows a cartridge reducing valve with dual-pressure capabilities. A solenoid-operated selector valve and a second adjustable relief mounted on the cover give the option of two pressures. Always use the first adjustable relief above the spool for maximum pressure setting. A single-solenoid directional valve (as shown) allows default to maximum pressure. Using a 2-position detented directional valve maintains the last pressure selected.
Figure 4-11 shows a proportional solenoid valve mounted on the adjustable relief. Such a valve allows selection of infinitely variable pressures via an electrical command. Allowing pilot flow to bypass the adjustable relief gives a reduced pressure of anything lower than the adjustable relief setting. An electronic signal to the proportional solenoid varies pilot flow that controls pressure on top of the spool.