Figures 2-17 through 2-20 show a two cylinder circuit with CYLA extending (A+, CYLB extending (B+), CYLB retracting (B-), and CYLA retracting (A-).

Notice CYLB retracts immediately after extending, which means there would be an extend signal opposing a retract signal if the circuit only has limit valves for control. Using a "one shot" valve to stop the opposing signal works, but is less reliable than the "flip flop," FF circuit shown here.

Figure 2-17 shows both palm buttons depressed, causing the output of the anti-tie down circuit (see the previous explanation of an anti-tie down circuit) to shift FF. The output of the top port of FF sends a signal to shift a doubled pilot valve and extend CYLA. The FF output will also supply the normally closed port of limit valve LVA1. Shifting the FF also drops the signal from limit valve LVB0 that retracted CYLA. CYLA extends until it contacts limit valve LVA1.

When CYLA contacts LVA1, Figure 2-18, air from the top port of FF passes through it and shifts a double piloted valve making CYLB extend. The signal to retract CYLB came from the bottom port of FF that is now exhausting to atmosphere. CYLB continues to extend until it contacts limit valve LVB1.

The normally closed inlet port of LVB1 has a constant air supply, so when CYLB contacts it, Figure 2-19, it shifts FF back to starting position. A signal from the bottom port of the FF shifts a double-piloted valve to retract CYLB and supplies air to the normally closed port of limit valve LVB0. After FF shifts back to the starting condition, it drops the extend signals to both double piloted directional valves. This makes it possible to shift the double-piloted valves to retract the cylinders. CYLB continues to retract until it contacts LVB0.

A signal from LVB0 shifts the double-piloted valve to retract CYLA as shown in Figure 2-20. This cylinder can retract since its extend signal dropped out when FF shifted from LVB0. CYLA retracts to home position and ends the cycle.

The nonrepeat feature is possible because when the circuit is in the at rest position, there is a supply to the left palm button from the rod end port of CYLA. After the cycle starts and CYLA reaches the end of its stroke, the left palm button loses its supply. Whether the operator lets off the palm buttons or not, loss of air to the left palm button disables the anti-tie down circuit.

With both palm buttons supplied with direct shop air, if the operator kept the palm buttons shifted all during the cycle, the machine would probably stall after CYLB extended. The nonrepeat feature adds little cost, but may save lost production.

Using modified "not" elements as limit valves

The circuit in Figure 2-21 operates the same as Figures 2-17 through 2-20 on the preceding page. The only difference is pressure controlled "not" elements replace limit valves.

"Not" elements can replace limit valves when the movement they are detecting is not critical. "Not" limits operate any time the cylinder has a pressure drop. The pressure drop could be end of stroke or any place the cylinder stops for any reason. If actuator position is critical, always use limit valves.

Using a standard "not" to replace limit valves works, but the special low pressure "not" is best. Some manufacturers call this an "inhibitor", others, a "pressure trip release." Whatever the name, the modification causes the valve to shift at a lower differential pressure. This keeps a reduced backpressure at the cylinder port from giving a premature signal.

Using "not" elements in place of limit valves makes installation and plumbing easier, but can make troubleshooting more difficult. Placing the "not" elements in the control box works, but cylinder port mounting is best. No matter the location, they must read the air between the cylinder port and a meter-out flow control. This location ensures they see backpressure when the cylinder is moving.



















Because a "not" is normally open, pressure holding the cylinder in position and backpressure from a meter-out flow control when the cylinder is moving give the signal to hold it shut. When the cylinder stops, pressure drops, allowing the "not" to open and send a signal to continue the cycle.

The circuit in Figure 2-21 uses "not3" to tell CYLB to extend, "not5" to tell CYLB to retract, and "not4" to tell CYLA to retract.

Since the "not" works on loss of pressure, a cylinder with leaking seals can keep it from shifting. After a slowly moving cylinder stops, slow deterioration of pressure may delay the output signal.

Loss pressure valves have many benefits. For example, it does not matter when the cylinder contacts the part. Whether the part is 1 or 20 in. thick, when the cylinder makes contact, there will be an indication. In addition, air pressure changes have little or no effect on them as the "not" only reads minimum pressure. Finally, maximum pressure setting does not affect a "not" like it does a sequence valve. "Not" elements are a preferred choice over sequence valves because sequence valves only work with meter-in flow controls. Any air cylinder has better control with a meter-out circuit and overrunning loads require meter-out flow control.

Always use loss of pressure controls with caution since they can operate any time cylinder pressure drops below their minimum shifting pressure.