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.