What is in this article?:
- Book 2, Chapter 10: Flow Control Circuits
- 3-speed meter-in circuit
- Meter-in flow control of a running-away load
- When meter-in circuits are necessary
- Action of a meter-in air circuit with a varying load
- Meter-out flow controls
- Three speed meter-out circuit
- Meter-out pneumatic circuit with a variable load
- Bleed-off (bypass) flow controls
- Three-speed bleed-off circuit
- Different locations for flow controls
- Heat generation in hydraulic flow-control circuits
- Motor-type flow-divider speed control
- Another motor-type flow-divider speed control
- Controlling speed of hydraulic motors
- Three-port flow control
Meter-out flow controls
Meter-out controls restrict fluid leaving the cylinder to retard the cylinder’s movement. This type of flow-control circuit works for any type of load -- and works best with air-operated devices. Figure 10-27 shows a meter-out flow-control circuit in the at rest condition.
In Figure 10-28, the directional valve has shifted and the cylinder starts to extend. Fluid in the cap end of the cylinder is at system pressure and the relief valve is dumping excess pump flow to tank. Pressure at the head end of the cylinder will be at system pressure or higher according to the rod size and force required to move the load. The action of meter-out flow controls is smooth and steady in hydraulic circuits.
Fig. 10-28. Meter-out flow-control circuit – with cylinder extending.
Figure 10-29 shows the pressure pattern of an air cylinder while it is extending. By restricting flow out of the cylinder, the action will be smooth when the load remains constant. (Figures 10-35 through 10-39 show the action of an air cylinder that is moving a changing load.)
In Figure 10-30, meter-out flow controls are controlling the load on a down-acting vertical cylinder. This over-running load moves steadily because fluid flow leaving the cylinder is restricted. The meter-out circuit keeps the load from running away, but depending on the load and the rod size, there could be excessive pressure in the cylinder’s head end. Notice that the rod-end pressure is 3000 psi when extending. This is because the rod is oversize (2:1) and the load is heavy. At a relief valve setting of 3000 psi, this head end pressure could be as high as 7000 psi.
Fig. 10-29. Pneumatic meter-out flow-control circuit – with cylinder extending.
When using a meter-out system with a running-away load, check load-induced pressure and hydraulic-force-induced pressure at the rod end. This pressure can be much higher than components’ rating -- even when the relief valve setting is well below maximum rated pressure.
Fig. 10-30. Meter-out flow-control circuit – cylinder extending with over-running load.