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The term cartridge valves commonly refers to pressure, directional, and flow control valves that screw into a threaded cavity. These valves are mostly rated for low flows - 40 gpm or less, although some manufacturers have units that will flow more than 100 gpm. Compact screw-in cartridges help build inexpensive circuits that are reliable and easy to maintain. Screw-in cartridges are most often part of a drilled manifold but can be purchased in individual bodies. The performance and function of screw-in cartridge valves is similar to the in-line or sub-plate-mounted valves discussed in Chapter 10.
Slip-in cartridge valves (sometime referred to as logic 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 go as high as 3000 gpm. (Several companies make screw-in logic valves in sizes as small as 15 gpm for use when the special features of logic valves are required at lower flows.) Slip-in cartridges also are compact, have low pressure drop, and operate at pressures up to 5000 psi. Slip-in cartridges can function as pressure, flow, and directional valves.
The symbols that illustrate this chapter are the preferred type as first used by the manufacturers. Chapter 4 shows cartridge valve symbols that follow the using ISO rules.
Why use slip-in cartridge valves?
The main reason for using slip-in cartridge valves in high-flow circuits is economy. Large spool valves are available with high flow capacity but few are manufactured, making them expensive with long delivery times. A better choice is a slip-in cartridge valve in a manifold body, piloted by D03- or D05-size directional control valves. (There is at least one supplier of valves that uses logic elements instead of spools that bolt directly to D08- and D10-size sub-plates.)
One important feature of slip-in cartridge valves is that there is almost zero bypass through the A port. Leakage for this port is the same as any good check valve. Flow past the B port is very low because the leak path is long and has a close fit. So when applying this type of valve, always use the A port for the connection that must block flow completely.
Another feature is fast response on opening. Because there is no overlap, flow is almost instantaneous after the valve receives a start signal. Even when controlling poppet-opening speed, there is no lag in flow response that increases cycle time. Response also is fast on closing because the poppet only opens far enough to pass the flow going through it. This means it does not have to move any extra distance to start to restrict flow and shut it off.
Logic valve circuits are also very versatile when set up with multiple control valves. Many center or crossover conditions can be duplicated by which control valves are open and how they are signaled during a cycle. This means that when designers want to try different valve configurations, they only have to change control valves or control circuitry. This versatility can be applied anytime with the right design parameters up front.