Rotary unions: Some applications require fluid to flow into or out of rotating parts of a machine. The rotation may be continuous or only part of a turn; the application may have one or many flow paths. Many manufacturers make rotary unions that do this for fluids at pressures as high as 5000 psi, with as many as 20 flow paths. . (Some rotary unions pass electricity as well as fluids if required.) The cross-sectional view in Figure 18-7 is a simplified drawing of a single-path rotary union. The symbol is a circle on a flow line; in this case, the energy triangle indicates hydraulic fluid. Multiple flow paths are shown by multiple lines of whatever type the flow is. (Some rotary unions pass electricity as well as fluids if required.)

Quick disconnects: When all or any part of a pneumatic or hydraulic circuit must be removed or changed frequently, a fast way to do so is with quick-disconnect couplings. Quick disconnects usually require a worker to connect and disconnect them manually. However, there are some styles that break away when pulled by mechanical force. Other types only stay connected while held in place by an external force.

The cross-sectional view in Figure 18-8 illustrates the socket-and-plug pair that make up a typical quick disconnect. Sliding the lock-unlock ring to the left allows the detent balls to move out of the way so the plug can be inserted. Inserting the plug all the way into the socket stops leakage as it passes the O-ring seal, opens both check valves, and allows the detent balls to lock in the detent notch to hold the connection together. Sliding the lock-unlock ring to the left again releases the plug as the detent balls lose their backing. The three symbols in the figure show quick disconnects disconnected with dual check valves, connected with dual check valves, and disconnected in a typical air line configuration.

It may be necessary to install oversized quick disconnects because their construction can cause high backpressure. Always check pressure drop in the manufacturer's catalog to assure proper flow capabilities. There are designs that have full flow porting in air and low-pressure hydraulic styles.

Pressure switches: Some fluid power circuits require electrical control signals when pressure reaches specific levels - such as the pressure buildup when a part is clamped or a certain weight is met - or if overpressure may cause damage or is a safety hazard. (Sequence valves -- discussed in Chapter 14 -- can cycle from a pressure buildup, but will not produce a signal to an electrical control circuit when a pressure requirement is satisfied.)

The cross-sectional view and symbols in Fig 18-9 show electrical pressure switches that are set to monitor maximum or minimum pressure and then send a signal to the electric control circuit. (Another electrical output device that reads pressure and sends a signal is a pressure transducer. Pressure transducers are more responsive and have better repeatability, but require additional electronics to read their input.)

The depicted pressure switch includes a plunger that reacts to system pressure by moving. An adjusting screw sets spring pressures against the plunger and allows different settings. When system pressure is high enough to push the plunger upward against spring tension, the plunger closes a limit switch to signal that the set pressure has been reached. When pressure falls, the plunger drops and the limit switch opens again.

Never depend on a pressure switch to indicate actuator position when the actuator positively has to be in a certain position to prevent machine or product damage or to avoid a safety hazard.