BOOK 2, CHAPTER 10: Flow control circuits

Meter-out pneumatic circuit with a variable load

When using a meter-out circuit on an air cylinder with a changing load, movement will not be consistent. According to the amount of force change required, movement can range from smooth, to stop, to lunging. (Note that in Figures 10-35 through 10-39 there is no allowance made for rod differential.)

Fig. 10-35. Pneumatic meter-out flow-control circuit – with loaded cylinder extending slowly and smoothly toward second equal load.

In Figure 10-35, a loaded air cylinder is stroking smoothly at a pressure difference of 30 psi between its cap and head ends. (At the bore size for this example, it takes a 30-psi difference to generate enough force to move the load). If the load remains constant, the cylinder usually will advance smoothly.

Fig. 10-36. Pneumatic meter-out flow-control circuit – as cylinder contacts second load and stops while rod-end pressure drops.

But when the load is doubled, as shown in Figure 10-36, a difference of 30 psi across the piston is not enough to keep the cylinder moving; the cylinder stops while pressure in the rod end decreases to about 20 psi. The speed of the pressure decay is in direct proportion to how quickly air discharges through the flow control. Once pressure in the cylinder rod end reaches 20 psi, Figure 10-37, the cylinder will start moving again. If the additional load stays constant, movement is smooth and steady again.

Fig. 10-37. Pneumatic meter-out flow-control circuit – with cylinder moving both loads after rod-end pressure has dropped.

When the second load is removed, as in Figure 10-38, 20 psi in the rod end is less pressure than needed to hold the piston back. At this time the cylinder will lunge forward until the pressure in the rod end increases to about 50 psi. The amount of lunge is in direct relation to the volume of air in the cylinder rod end and the piping to it. As Figure 10-39 shows, once air in the rod end again compresses to about 50 psi, the cylinder returns to normal speed.

Fig. 10-38. Pneumatic meter-out flow-control circuit – cylinder lunging forward after second load is removed. Pressure in rod end is increasing to level that will hold cylinder back.

Fig. 10-39. Pneumatic meter-out flow-control circuit – after return to original conditions, cylinder extends slowly and smoothly with single load.

If this stop-and-lunge problem is intolerable and air is the power source, add a method of oil control to the circuit. (See Chapter 3 on air-oil circuits for ways to overcome the problem.)