BOOK 2, CHAPTER 10: Flow control circuits

When meter-in circuits are necessary

In some cases a meter-in circuit is the only way to control the speed of an actuator -- even for pneumatic devices. Figures 10-18 through 10-21 show several instances requiring meter-in circuits.

Fig. 10-18: Meter-in flow-control circuit with pressure switch for end-of-stroke indication.

Many machine circuits use pressure switches to indicate when an actuator meets resistance. If pressure in the actuator builds prematurely, the machine cycle gets out of phase. With the meter-in circuit shown in Figure 10-18, pressure in the cylinder will be just enough to move the cylinder and its attachments until it reaches the load. With a meter-out circuit, pressure in the cylinder cap end would build as soon as the directional valve shifts, tripping the pressure switch long before the cylinder contacts the load.

Fig. 10-19: Meter-out flow-control circuit with pressure switch for end-of-stroke indication.

However, it is possible to use a pressure switch with a meter-out flow control circuit. In Figure 10-19, notice that the pressure switch is on the cap-end line to the cylinder. It passes a signal when the cylinder cannot extend farther. Notice also that the pressure switch setting is very low (50 psi). While the cylinder is moving, oil flowing from the cylinder head-end port remains pressurized by the meter-out flow control. When the cylinder contacts the load, pressure in the head-end port drops, actuating the pressure switch and sending a signal. (Use a normally open, 3-way, pilot operated, spring-return directional valve in place of the pressure switch to produce an air or hydraulic pilot signal.)

The pressure-decaying circuit in Figure 10-19 works well in pneumatic circuits because metering air flow out provides good control. Several companies furnish air logic elements designed specifically for this type of circuit.

Fig. 10-20. Meter-in flow-control circuit with sequence valve for end-of-stroke indication.

Sequence valves often are used to start a second actuator after a cylinder meets resistance and builds tonnage. With the meter-in flow control shown in Figure 10-20, pressure in the cylinder cap end increases when -- but not before -- the cylinder contacts the work. Pressure at the sequence valve’s inlet stays lower than its spring setting while the actuator is moving. (With a meter-out circuit, pressure in the cylinder’s cap end would go to system pressure when the directional valve shifts. Because of this, flow to the secondary circuit would take place prematurely.

The vertical single-acting, weight-return cylinder shown in Figure 10-21 needs meter-in control as it extends. This will be the case even if it is a pneumatic cylinder where meter-out control works best. For a different retraction speed, use a second meter-out flow control (as shown in Figure 10-21).

Fig. 10-21. Meter-in flow-control circuit for a single-acting cylinder extending.

Note: A pressure switch or a sequence valve will only indicate that pressure has reached a certain level. What caused the pressure build-up is unimportant. If the actuator positively has to be at a certain position before the next function starts, do not use a pressure-sensing device. Always use a limit switch. (Use a pressure switch or sequence valve in series with the limit switch if cylinder position and force are both important.)