Poppet-type directional control valves
Poppet-type directional control valves are similar to pilot-to-close check valves. The cutaway view in Figure 10-15 shows the construction of a hydraulic 2-way, normally closed, poppet-type directional control valve. Fluid at the inlet port passes through the control orifice to the backside of the poppet. The tip of the spring-loaded armature closes off the outflow orifice to trap fluid behind the poppet. As the symbol shows, the valve is a check valve that stops flow from inlet to outlet in the normal condition. This design will not stop flow from outlet to inlet, although flow in this direction may be at a reduced rate. If using this valve for reverse free-flow in its normally closed condition, make sure to choose one with free-flow capability.
Energizing the solenoid coil creates a magnetic field that raises the armature to open the outflow orifice. This orifice is larger than the control orifice, so the greater flow through it causes a pressure drop behind the poppet. Now, inlet pressure pushing on the poppet’s annulus area outside the seat diameter unseats it to allow fluid flow to the outlet. De-energizing the solenoid coil lets spring force reseat the armature tip to again trap fluid behind the poppet and close it.
Unlike spool valves whose lands overlap, poppet valves open a flow path to outlet immediately. This means response time of whatever the valve controls is very fast. Also, when a spool valve shifts open it goes to the end of its stroke regardless of the amount of flow. On the other hand, a poppet only opens as much as the flow going through it needs. This means the poppet has less distance to move to stop flow, so again its response is faster.
Chapter 11 covers poppet-type slip-in cartridge valves used for directional control. These valves have the same characteristics as just explained here and they work well in circuits that require fast response. Chapter 12 covers infinitely variable spool valves that also offer very fast response.
The 4-way poppet valve in Figure 10-16 is a typical design for pneumatic service. Poppet design valves are very tolerant of contamination and many plants use them for this reason. They are also very responsive and provide a positive seal when their poppets seat. (Many poppet valves are built with resilient materials on the poppets where they contact the seats.
One drawback to this design is that air is free to go any direction as the poppets shift from one flow path to the other. In valve terminology this is called open crossover (and can be helpful with hydraulic valves as explained later). The cutaway view in Figure 10-16 shows how flow can go to both cylinder ports and to both exhausts as the poppets move to the opposite seat.
Another possible problem with poppet valves is that they usually only operate in one manner. When you purchase a 2-way, normally closed poppet valve, it cannot be changed to normally open. The port marked In is always the supply line. Air piped to the Out or Cyl port usually blows through the valve with little resistance. Spool-type valves (discussed next) overcome these problems in most cases.
The poppet valve in Figure 10-16 shifts to its second condition when the coil of the solenoid operator receives an electrical signal and pulls the armature up. This action lets supply air into the large pilot piston to move the poppets to the second valve position. Even though the small pilot piston has supply air against it all the time, it has less force. De-energizing the solenoid operator exhausts the large pilot piston and the poppets return to the normal position.
This is a very reliable design because there are no springs to rust, weaken, or break. Usually the area ratio is 2:1; so shifting force is equal in both directions. Some manufacturers also use a spring in the return end to keep the poppets in place when there is no air supply. Valves with this type of shifting arrangement usually require a minimum pressure of 25 to 40 psi or an external pilot supply at least that high.
Spool-type directional control valves
For circuits with flows less than 100 gpm, the most common hydraulic directional control valves use a spool-like internal member to direct flow. (Many air valves also use a spool, due to the advantages offered by this design.) The cutaway views in Figure 10-17 show some simplified spool arrangements and terms associated with this valve. Notice that counting the number of ports that carry working fluid on the cutaway or symbol gives the number of ways the valve has. A 2-ported valve is a 2-way valve and a 5-ported valve is a 5-way valve.
All valves in Figure 10-17 are two position as shown by two boxes in the symbol. As stated before, a 2-way valve can have only two positions because it can only stop or allow flow. All other valves are able to have three positions, while 4-way valves can have four positions in special cases. A 5-way valve is a special case mainly used in pneumatic applications where an extra exhaust port is not a problem. Notice that a 4-way valve has five ports but its tank ports are internally connected to eliminate an extra port in the body. This is important in hydraulic valves because it reduces piping and potential leak points.
Spool valve advantages
The main advantage of spool valves is that fluid entering the valve from any working port does not affect spool movement. The poppet in a poppet valve can have pressure on one side and only a light spring on the other. This can result in premature movement of the poppet when pressure enters a port. In a spool valve, pressure always is applied to two equal opposing areas or the edge of a land. Thus pressure forces that could move the spool are cancelled or non-existent. This means that a spool valve can be shifted manually, electrically, mechanically, pneumatically, or hydraulically with the same force regardless of the operating pressure. Low-force solenoids can be used because the most they need to overcome is mechanical friction and light springs.
Spool valve disadvantages
Many spool valves are designed with metal-to-metal sliding fits. As a result, some fluid may bypass these seals. If this happens, an actuator may not hold its position if outside forces are applied. It also means wasted energy and resulting heat. (Many pneumatic valves use some sort of resilient seal in the body and/or on the spool to eliminate air leaks.) To reduce bypass, spool valves have land overlap, so as they start shifting to open a flow path, there is a delay before fluid starts flowing. The delay only lasts for milliseconds and does not cause a problem -- unless the cycle is very fast and/or there are several valve shifts per cycle.
Another time delay occurs when a spool shifts to the end of its stroke. There is often more movement than required for the flow needed. When the spool shifts back to center or to the opposite flow path, it consumes more time to travel the extra distance. This slows the cycle, especially when several valves are involved. Stroke limiters that control maximum spool movement can eliminate this delay, but are seldom seen in actual practice. The common fix for these problems is to speed up traverse time by installing a larger pump. However, faster actuator movement can add shock and heat due to higher energy input.