Spool-valve transition conditions
When a spool valve shifts from one flow condition to another, it can pass through different flow conditions than those indicated by straight or crossing arrows in the symbol. These transition or crossover conditions are unimportant in most circuits, but can cause shock, drifting, or cylinder regeneration under certain circumstances. Figure 10-29 shows typical transition conditions for open- and closed-center, 2- and 3-position valves. For transition conditions of other valve center types, see Chapter 4.
An example of how an open transition or crossover can eliminate shock is the case of a fast-moving actuator that must reverse direction before the end of its stroke. With a closed-crossover valve in this circuit, all flow to and from the actuator is blocked for a brief period as the spool shifts from extend to retract. The pump side of the valve always has a relief valve (or a pressure-compensated pump) to protect it from over pressure. However, the actuator side has no protection of any kind. When flow from the extending actuator is blocked, pressure can build to levels well above the pressure ratings of pipes, seals, and hardware. This pressure spike is very brief, but it happens every cycle -- and soon shows up as cracked fittings, blown hoses, leaking seals, or broken parts. Changing to an open-crossover spool in this application would connect the impending spike to the relief valve or pressure-compensated pump. There is still a pressure spike but its intensity is now below the damage level of the components.
An example of where a closed transition spool helps is when a vertical cylinder with a heavy platen must be reversed in mid stroke. With an open transition, the cylinder will continue its forward travel after the valve receives the reverse signal. With a closed transition, the cylinder will stop almost immediately and start reversing shortly thereafter. Also, when the platen must retract from a stop position, such as during set up, it can drop before being powered up with an open-crossover valve in the circuit.
Crossover problems usually show up when a 3-position valve shifts to or from center condition. Cylinders may move in the opposite direction or move when signaled to stop for no apparent reason. Most suppliers show crossover flow in their catalog so check it out if the problem is an unusual movement.
Mounting pneumatic valves
Pneumatic valves can be line-, subplate-, or manifold-mounted. Inline or bar-type manifolds make it convenient to stack valves with a common inlet and/or exhaust. Pneumatic circuits seldom, if ever, require custom manifolds as hydraulic circuits do. The graphics in Figure 10-30 depict some pneumatic-valve mounting styles. In-line mounting types have the whole valve assembly in a body with ports out the sides. This style is usually less expensive, but is more trouble if it has to be replaced.
The typical pneumatic subplate assembly is often a subplate with end covers bolted to it. Fasteners hold the parts together and molded seals eliminate leaks. Valves with seals mount to the subplate and all piping connects to it. Some manufacturers have wiring troughs in the subplate and use plug-in connectors on solenoid-operated valves.
The typical pneumatic manifold assembly consists of two or more subplates connected to make a valve stack with a common inlet and exhaust. These assemblies eliminate many connections and make valve installation replacement easy. These units also are available with wiring troughs and plug-in valves for solenoid operation. Most air valves use unique mountings and port arrangements that are not inter-changeable. However, there is an ISO standard subplate mounting that several companies offer. The valves match each other’s mounting patterns but otherwise do not have interchangeable parts. This assembly is physically large and thus more expensive, but it makes it easy to combine valves from several suppliers.