BOOK 2, CHAPTER 10: Flow control circuits

Different locations for flow controls

Figures 10-48 through 10-51 suggest other possible locations for flow-control devices in circuits. Figure 10-48 shows a meter-in needle valve in the pump line. At first, it appears this arrangement results in equal speed in both directions of travel. While it does slow the stroke in both directions, the speeds are not identical with a single-rod cylinder. As the cylinder advances, metered oil flows to the cap-end area (4.91-in.2 for a 2.50-in. bore). With a 1.375-in. diameter rod, the head-end area would only be 3.43 in.2 Thus, with the same flow entering the different areas, retraction speed would be faster.

Fig. 10.48. Meter-in flow-control circuit with needle valve in pump line.

Note that all of the same precautions given previously for meter-in circuits apply to this single meter-in needle valve as well. In addition, some solenoid valves cannot stand backpressure on the tank line, so use caution when applying this circuit.

The circuit in Figure 10-49 locates a single meter-out needle valve in the tank line. Again extension and retraction speeds appear to be identical. But as with the tank-line meter-in circuit, piston area differences when extending and retracting result in different speeds. The single meter-out flow-control circuit is faster on the extension stroke with a single-rod cylinder.

Fig. 10.49. Meter-in flow-control circuit with needle valve in tank line.

The pump-line bleed-off circuit diagrammed in Figure 10-50 is identical in action to the meter-in circuit in Figure10-48. If a fixed-volume pump supplied both circuits, the bleed-off circuit would generate less heat.

Fig. 10-50. Bleed-off flow control circuit with pressure-compensated needle valve teed into pump line.

To get identical speed in both directions of cylinder travel, use the circuit in Figure 10-51. Locating a needle valve in the rod- or cap-end line controls the same cylinder area extending and retracting. This circuit meters in flow during one direction of travel and meters flow out in the opposite direction.

Single-rod cylinders have different areas on the two faces of the piston so pressure drop across the needle valve is different when extending and retracting. The circuit in Figure 10-51 has a higher pressure drop across the needle valve orifice when the cylinder is extending. The higher pressure drop has no effect on the pressure-compensated needle valve in this circuit due to the rectifier check valve shown. The rectifier check valve allows oil to flow through the needle valve in the same direction when the cylinder extends or retracts. Cylinder speed will stay the same with this setup, even though pressure drop differs during extension and retraction.

Fig. 10-51. Meter-in/meter-out flow-control circuit with pressure-compensated needle valve and rectifier on rod-end cylinder port.