BOOK 2, CHAPTER 10: Flow control circuits

Types of flow-control circuits

There are three types of flow control circuits from which to choose. They are: meter-in, meter-out, and bleed-off (or bypass). Air and hydraulic systems use meter-in and meter-out circuits, while only hydraulic circuits use bleed-off types. Each control has certain advantages in particular situations.

Figure 10-7 shows a meter-in flow-control circuit for a cylinder. Notice that a bypass check valve forces fluid through an adjustable orifice just before it enters the actuator. Figure 10-8 shows the circuit while the cylinder is extending – with the pressures and flows indicated. With a meter-in circuit, fluid enters the actuator at a controlled rate. If the actuator has a resistive load, movement will be smooth and steady. This is because hydraulic fluid is almost incompressible.

Fig. 10-7: Meter-in flow-control circuit – at rest.

In pneumatic systems, cylinder movement may be jerky because air is compressible. As air flows into a cylinder, as depicted in Figure 10-9, pressure increases slowly until it generates the breakaway force needed to start the load moving. Because the subsequent force needed to keep the load moving is always less than the breakaway force, the air in the cylinder actually expands. The expanding air increases the cylinder speed, causing it to lunge forward. The piston moves faster than the incoming air can fill the cylinder, pressure drops to less than it takes to keep the cylinder moving and it stops. Then pressure starts to build again to overcome breakaway force and the process repeats. This lunging movement can continue to the end of the stroke. A meter-out circuit is the best control to avoid air-cylinder lunging.

Fig. 10-8: Meter-in flow-control circuit – with cylinder extending.

Figure 10-7 shows the components in a meter-in flow-control circuit. Notice that a bypass check valve forces fluid through an adjustable orifice just before it enters the actuator.

Figure 10-8 shows an extending hydraulic cylinder and indicates the pressures and flows in various parts of the circuit. With a meter-in circuit, fluid enters the actuator at a controlled rate. If the actuator has a resistive load, movement will be smooth and steady with a hydraulic circuit. This is because oil is almost non-compressible.

Fig. 10-9: Pneumatic meter-in flow-control circuit – with cylinder extending

In the case of an air system, pressure builds slowly and cylinder movement may be jerky. This jerky movement comes from compressibility of the air. As air enters the cylinder, Figure 10-9, pressure builds slowly until it generates the breakaway force to start the piston moving. Because moving force is always less than breakaway force, air in the cylinder expands. The expanding air speeds up cylinder movement, causing it to lunge forward. This increased speed moves the piston faster than the incoming air can fill the space behind it, so pressure drops to less than it takes to keep it moving and the cylinder stops. After the cylinder stops, pressure starts to build again to develop breakaway force and the process repeats. This lunging movement can continue to the end of the stroke. A meter-out circuit is the best control for an air cylinder.

Fig. 10-10: Meter-in flow-control circuit for overrunning load – with cylinder extending.

If the actuator has an overrunning load, such as in Figure 10-10, a meter-in flow control will not work. When the directional valve shifts, the vertical load on the cylinder rod makes it extend. Because fluid cannot enter the cylinder’s cap end fast enough, a vacuum void forms there. The cylinder then free falls, regardless of the setting of the meter-in flow adjustment. The pump will continue to supply metered fluid to the cap end of the cylinder and will eventually fill the vacuum void. After the vacuum void fills, the cylinder can produce full force.