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 or 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-in flow control of a running-away load
Figures 10-15 and 10-17 show a running-away load controlled by a meter-in circuit and counterbalance valves. The meter-in flow control works exactly as explained previously, while a counterbalance valve makes cylinder movement resistive. (See Chapter 5 for an explanation of counterbalance valves.) Figure 10-16 pictures a bleed-off flow control circuit that gives the same results as a meter-in circuit -- without most of the heat generation.
Fig. 10-15: Meter-in flow-control circuit for vertical cylinder extending with over-running load. Counterbalance valve prevents load from falling.
A meter-in flow control circuit for an over-running load is not the normal design but it may be necessary when the circuit has a pressure switch or a sequence valve.
In any meter-in circuit with a fixed-volume pump, the wasted energy will heat the fluid. In the circuit in Figure 10-15, almost 95% of the power used by the system becomes heat. In this circuit, fluid from the pump enters the cylinder as fast as the meter-in flow control allows. A counterbalance valve at the rod-end port keeps the cylinder from running away as it extends.
Fig. 10-16: Bleed-off or bypass flow-control circuit for vertical cylinder extending with overrunning load. Counterbalance valve prevents load from falling..
To save energy while using a fixed-volume pump, the circuit in Figure 10-16 works well. A bleed-off or bypass flow control greatly reduces the amount of wasted energy. With a bleed-off circuit, excess pump flow goes to tank at the pressure required to move the cylinder. In the circuit in Figure 10-16, pressure would be approximately 50 psi as the cylinder extends. The extension-stroke speed is still infinitely variable, while pressure in the cylinder cap end line never goes higher than that caused by load resistance.
Fig. 10-17: Meter-in flow-control circuit with load-sensing pressure-compensated pump and vertical cylinder extending with over-running load. Counterbalance valve prevents load from falling.
Figure 10-17 shows another way to reduce energy loss and heat generation – using as load-sensing, pressure-compensated pump in conjunction with a meter-in flow-control circuit. A sensing line, teed into the cylinder cap-end line after the meter-in flow control, transmits pressure information to the pump. With a load-sensing, pressure-compensated pump, pressure at the pump outlet stays 150 to 200 psi higher than the load until it tries to go above the compensator’s pressure setting. The only energy loss here is the 150- to 200-psi pressure drop across the flow control at the volume set. (Heat generation within a load-sensing pump circuit is explained later in this Chapter.)