OR element symbols and a cross-sectional view are shown in Figure 19-3. (A shuttle valve serves the same purpose as an OR element.) Either input to an OR element produces an output. Pilot signals from two different sources can pass through to start the next function. Another way of saying this is that a signal at A or B produces an output at C. An OR element differs from an in-line tee because an OR passes either input to the output but does not allow the inputs to pass to each other.

From the cross-sectional view, it is easy to see how a signal entering the A or B ports can only pass out the C port. A slight puff of air may escape as the blocking wafer moves from seat to seat after an input signal, but this is usually not strong enough to start another function. Although an OR is a passive element, that is not a problem because it always passes the higher of the two signals it receives.

Stack OR elements to allow for more than two inputs. Use an extra OR for each new input after the first two signals -- with one input from the preceding OR and the other from the new signal.

NOT element

The NOT element symbols and cross-sectional view in Figure 19-4 designate an active, normally open logic element. A NOT logic element is a normally open 3-way valve. An input signal or pressure to the supply port passes through the valve until there is a pilot signal at port A. Pressurizing port A blocks supply and exhausts the output signal to atmosphere. Without a pilot signal, a NOT always returns to the normally open condition.

From the cross-sectional view you can see that supply is free to flow to output in the normal condition. A signal at the A port pushes the dual-seat poppet down to block supply and exhaust the output air. This element will block a signal as well as a supply when necessary. A diaphragm above the poppet prevents air entering port A from exhausting or passing to output. The area on top of the dual-seat poppet is approximately 10:1 in size over the shutoff seat at the supply port.

For some applications, a NOT element can replace a limit switch to indicate that a cylinder is at the end of stroke. Pressure from a cylinder port goes to port A of the NOT, holding it closed. As the cylinder strokes to the work, pressure stays the same when using meter-out flow controls. When the cylinder contacts the work, the signal on port A drops, the NOT then opens and sends a signal to start the next operation. (For a full explanation of this circuit see the author's book "Fluid Power Circuits Explained." Also see Figure 19-12 for a NOT as an end-of-stroke indicator.) Using a NOT element as a stroke-position indicator is not positive because an output signal is generated any place the cylinder stops for any reason. This always happens, whether the cylinder stopped where it should or stalled for some other reason. Because this is the case, take care using a NOT to replace a limit valve. Conversely, this feature can be advantageous when clamping different sized parts. Use a NOT element for applications where different work locations stop the cylinder and there is no safety hazard or possible part damage when the cylinder does not complete its full stroke.

Most manufacturers supply a different pilot ratio for a NOT element to be used as a limit switch. The valve function is the same but the pressure at which it shifts is lower. Some manufacturers make a special NOT element that mounts directly to a cylinder port. A port-mounted, meter-out flow control used in conjunction with this special NOT makes a compact installation.

Flip-flop element

The cross-sectional view and symbols in Figure 19-5 are for a flip-flop element. A flip-flop is a double-piloted 5-way valve that sends supply air to either the outlet port with a signal at pilot ports S or R. The S signal stands for set and the R signal stands for reset. The S signal shifts the flip-flop for a function and whether the signal continues or not, the element stays shifted. The R signal puts the flip-flop back to its original position for the next cycle. Supply can be system pressure or air from another logic element. Manual Overrides make it possible to check the valve function and operate the circuit manually. Flip-flops (sometimes called memory elements) stay in the last shifted position even with no air supply. Whether the signal is maintained or drops out, the output port of a flip-flop stays the same.

The main use for a flip-flop is to eliminate dual pilot signals to a directional control valve powering an actuator. Applying two pilot signals to a directional control valve leaves the valve in the position it was when the second signal arrived. (In Figure 19-12 there is a circuit using a flip-flop to perform this blocking function.)

Another use for a flip-flop is to start a new cycle by allowing the operator to momentarily push the start buttons. This same flip-flop can then eliminate unwanted signals and set up the circuit for cycle completion as required.

A normally closed, double-pilot-operated 3-way element with only one output is also available to perform the same function as a flip-flop.

The symbol in Figure 19-6 shows how a flip-flop is represented schematically and the cross-sectional view shows one manufacturer's configuration. Note that this type uses differential pilot areas so the valve can be reset when there is a set signal present.