BOOK 2, CHAPTER 10: Flow control circuits

Action of a meter-in air circuit with a varying load

When using a meter-in circuit on an air cylinder with a variable load, movement will not be consistent. Depending on the required range of forces, movement may be smooth, the cylinder may over-speed, or it may even stop. In Figure 10-22, the cylinder is moving smoothly at a pressure difference of 30 psi. (It takes 30 psi in the bore size being used to generate the force to move the load.) If the load remains constant, the cylinder can -- and probably will -- advance smoothly.

 Fig. 10-22. Meter-in pneumatic flow-control circuit for loaded cylinder extending slowly and smoothly toward a second load. Fig. 10-23. Meter-in pneumatic flow-control circuit as cylinder contacts second equal load. Cylinder stops while cap-end pressure builds to produce force required.

When the load doubles, as in Figure 10-23, 30 psi is not enough to keep the cylinder moving. At this point the cylinder will stop until pressure in the cap end reaches 60 psi. (The meter-in flow setting determines how long this takes.)

 Fig. 10-24. Meter-in pneumatic flow-control circuit moving both loads after cap-end pressure reaches load requirement.

Once pressure in the cylinder cap end reaches 60 psi, Figure 10-24, the cylinder starts moving again. If the higher load stays constant, movement is steady.

When the second load is reduced, as diagrammed in Figure 10-25, 60 psi in the cap end is more pressure than needed. This high pressure will cause the cylinder to lunge forward and, as a result, pressure in the cap end will start to decay. The amount of lunge is in direct proportion to the total volume of air in the cylinder’s cap end and the piping leading to it. Next, as Figure 10-26 shows, once decompression reaches 30 psi, the cylinder slows to its original speed.

 Fig. 10-25. Meter-in pneumatic flow-control circuit as second equal load drops off. Cylinder lunges forward as pressure in cap end decreases.

If this stop/lunge/over-speed problem is intolerable and air is the required power source, add some method of oil control to the circuit. (See Chapter 3 on air-oil circuits.)

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