What is in this article?:
- BOOK 2, CHAPTER 4: Slip-In Cartridge Valves (part 1)
- Slip-in cartridge pressure-reducing valves
Slip-in cartridge pressure-reducing valves
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.
Slip-in cartridge directional control valves
Slip-in cartridge check valves
The simplest directional control valve is a check valve. Figure 4-12 shows the symbol and cutaway for a cartridge check valve. A check valve has a cover with a control orifice to control pilot fluid. The control orifice dampens the poppet movement. It is available in several diameters. The cover also holds the cartridge in place and seals it with an O-ring. The cartridge has a bushing with seals to prevent leakage to the outside or across the ports. A machined cavity holds the bushing that contains the poppet that will open when fluid flows in the right direction. The poppet on a check valve has a 1:2 ratio, which means the area at the two working ports (A or B) is one half of the pilot area. A 1:2 ratio poppet allows flow in either direction as long as pilot pressure is off or slightly less than half the working pressure.
There are several spring forces available -- from as low as 5 psi to more than 70 psi. The lowest spring pressure possible is best for normal check valve operation.
In Figure 4-12, a drilled pilot passage senses the pressure at the A port. Flow from the B port to the A port passes with a slight pressure drop caused by the volume of flow plus the spring force. When flow tries to reverse (from the A port to the B port) as pressure on the A port half area increases, it goes through the pilot passage to the main pilot area. Because the A port area is only half the pilot area, the poppet stays closed and blocks reverse flow.
Figure 4-13 shows the same valve with the pilot passage drilled to the B port. With this valve, flow is free to go from A to B, but not from B to A.
The symbol and cutaway in Figure 4-14 are for a cartridge pilot-operated check valve. The cartridge is the same as a standard check valve, but with a different cover. The cutaway shows the works of the cartridge pilot-operated check valve cover. On the left of the cover, a pilot piston pushes a simple ball check from the left seat to the right seat. The ball check stays to the left -- its normal position -- held by a light spring.
If oil tries to pass from the B port to the A port, the same pressure that is trying to open the check on the half area also is applied to the pilot area, keeping the poppet closed.
Flow from the B port to the A port requires a pilot pressure equal to at least 30% of pressure at the B port to shift the pilot piston. When there is sufficient pressure on the pilot piston, it will move the ball check off the left seat, opening a path to the drain. At the same time, closed flow at the right seat blocks flow from the B port. With little or no pressure at the pilot area, the 1:2 poppet opens, allowing flow from the B port to the A port. If pilot pressure drops while oil is reverse flowing, the poppet shuts due to pressure on the pilot area like any check valve. Various sizes of dampening control orifices control shifting speed of the poppet to help reduce system shock.
Slip-in cartridge directional control valves
The symbol and cutaway in Figure 4-15 are for a simple 2-way cartridge valve. Most cartridge directional valves have a 1:2 pilot ratio, although a 1:1.1 ratio works better in certain circuits. In either case, pilot pressure equal to the working pressure at port A and/or port B closes the poppet.
From the cutaway view in Figure 4-15, it is plain to see that fluid pressure at port A will push the poppet off its seat and allow flow to port B. Although it is less obvious, fluid pressure at port B will also open the poppet and allow flow to port A. To stop flow in either direction, apply pilot pressure to the pilot area opposite port A. If any pilot pressure generates a closing force equal to the opening force at the A and/or the B ports, the spring bias closes the poppet.
Although slip-in cartridge directional valves appear to be normally closed, they open easily without pilot pressure. A vertically mounted cylinder controlled by slip-in cartridge valves can free-fall when the pump stops and pilot pressure drops. This problem is easy to fix, as will be shown in some later circuits.
The cutaway view in Figure 4-15 shows a plain cover with a pilot passage and a control orifice. (Pilot pressure in this type of valve would come from another solenoid valve or control valve in the circuit.) Control orifices come in a variety of sizes to provide smooth, non-shock movement of the poppet. To control shock even more, add a skirt with V notches to the poppet. Figure 4-23 shows the symbol and cutaway for a cartridge poppet with a V-notched skirt for flow control or dampening function. Different manufacturers have other ways to achieve this dampening effect.
Figure 4-16 shows the symbol for a 1:2 slip-in cartridge with an interface for a solenoid-operated directional valve on the cover. This solenoid valve directly operates the cartridge beneath it. It also can pilot other cartridge valves through drilled passages in the manifold. The single solenoid pilot valve can keep the poppet normally closed or normally open. Figure 4-16 shows a normally closed configuration.
Figure 4-17 shows a double-solenoid, detented pilot operator. The cartridge poppet stays in its last position even with both solenoids de-energized. With this type of solenoid operator there is no need to maintain current on the solenoid after the valve shifts.
Figure 4-18 shows a double-solenoid, spring-centered pilot operator. The center condition of the pilot operator allows the cartridges it pilots to open when both solenoids are de-energized. This could allow a cylinder to relax in case of power failure or when activating the emergency stop.
Conversely, Figure 4-19 has a double-solenoid pilot operator that closes all cartridges when both solenoids are de-energized. The actuator would stop suddenly and be locked in place. This type of pilot operator could cause system shock without some means of decelerating the cylinder.
Slip-in cartridge valves with 1:2 area ratios appear to be normally closed because of the spring inside the poppet. However, one half the pilot area is connected to port A or port B, and pressure at these ports can open the poppet. The only way to keep a slip-in cartridge valve closed is to keep pilot pressure on the pilot area at all times.
Anytime the pump is running, there should be enough pilot pressure to keep a poppet closed. However, when the pump stops or pilot pressure drops for any reason, the poppet may open, allowing an actuator to move. This might cause a safety hazard or machine damage.
The symbol and cutaway in Figure 4-20 are for a cover with an integral shuttle valve. A shuttle valve will take signals from two sources and send the higher pressure signal to the pilot area. At the same time, the shuttle valve will not let either signal pass through to the other signal passage.
The cutaway of the shuttle operator shows that a pilot signal to pilot passage 1 or pilot passage 2 goes to the pilot area, but not out the line with little or no pilot signal. This happens because the shuttle poppet closes the inactive or low-pressure opening and only allows pilot oil from the active or higher-pressure side to flow to the pilot area. Because the area of the shuttle ball is equal to that of both pilot passage ports, the strongest signal always goes to the pilot area. In most applications, this is an important feature.
The vertically mounted, rod-down cylinder shown in Figure 4-21 is holding a heavy weight. This is an example of an over-running load. With standard externally piloted slip-in cartridges, the weight will fall when the pump stops or anytime pilot pressure drops below approximately 275 psi. This is because the 23,000-lb weight, acting on the 40.06 square inches of rod end area, produces a static pressure of 574 psi (23,000/40.06 = 574 psi). This 574 psi would act against half the area of the poppets to push them open. It takes approximately 275 psi on the pilot area plus the spring force to hold the poppets shut. For safety’s sake, change this circuit to one with shuttle-valve covers.
The circuit in Figure 4-22 is the same as above except for a shuttle valve in the cover. One pilot supply is from the pump, while the second pilot supply is from the cylinder’s rod end. While the pump is on and the system is at pressure, pilot supply is from the pump. In case of low or no system pressure, pilot oil comes from the cylinder’s rod end. With the cylinder’s rod end as the pilot source, pressure that is trying to open the poppets on the half area of port A also acts on the pilot areas. Because the pilot areas are twice the A port area, the poppets stay closed. The shuttle valve cover assures there is always pilot pressure on the pilot area when the cylinder is not fully extended.