Since a flow-control valve meters flow in one direction only, the inlet and outlet ports must be correctly connected in a circuit in relation to the flow direction to be metered. A valve’s drain connection must be piped to a tank so that a connection will not be subjected to possible pressure surges. The location of a flow-control valve with respect to workload has an affect on a circuit’s operating characteristics. The three basic types of flow-control valve installations are the meter-in, meter-out, and bleedoff circuits.
a. Meter-In Circuit (Figure 5-37). With this circuit, a flow-control valve is installed in a pressure line that leads to a work cylinder. All flow entering a work cylinder is first metered through a flow-control valve. Since this metering action involves reducing flow from a pump to a work cylinder, a pump must deliver more fluid than is required to actuate a cylinder at the desired speed. Excess fluid returns to a tank through a relief valve. To conserve power and avoid undue stress on a pump, a relief valve’s setting should be only slightly higher than a working pressure’s, which a cylinder requires.
A meter-in circuit is ideal in applications where a load always offers a positive resistance to flow during a controlled stroke. Examples would be feeding grinder tables, welding machines, milling machines, and rotary hydraulic motor drives. A flow-control-and-check valve used in this type of circuit would allow reverse free flow for the return stroke of a cylinder, but it would not provide control of return stroke speed.
b. Meter-Out Circuit (Figure 5-38). With a meter-out circuit, a flow-control valve is installed on the return side of a cylinder so that it controls a cylinder’s actuation by metering its discharge flow. A relief valve is set slightly above the operating pressure that is required by the type of work.
This type of circuit is ideal for overhauling load applications in which a workload tends to pull an operating piston faster than a pump’s delivery would warrant. Examples would be for drilling, reaming, boring, turning, threading, tapping, cutting off, and cold sawing machines. A flow-control-and-check valve used in this circuit would allow reverse free flow, but it would not provide a control of return stroke speed.
c. Bleed-Off Circuit. A typical bleed-off circuit is not installed directly in a feed line. It is Td into this line with its outlet connected to a return line. A valve regulates flow to a cylinder by diverting an adjustable portion of a pump’s flow to a tank. Since fluid delivered to a work cylinder does not have to pass through a flow-control valve, excess fluid does not have to be dumped through a relief valve. This type of circuit usually involves less heat generation because pressure on a pump equals the work resistance during a feed operation.
d. Compensated Flow. The flow-control valves previously discussed do not compensate for changes in fluid temperature or pressure and are considered noncompensating valves. Flow rate through these valves can vary at a fixed setting if either the pressure or the fluid’s temperature changes. Viscosity is the internal resistance of a fluid that can stop it from flowing. A liquid that flows easily has a high viscosity. Viscosity changes, which can result from temperature changes, can cause low variations through a valve. Such a valve can be used in liquid-powered systems where slight flow variations are not critical consideration factors.
However, some systems require extremely accurate control of an actuating device. In such a system, a compensated flow-control valve is used. This valve automatically changes the adjustment or pressure drop across a restriction to provide a constant flow at a given setting. A valve meters a constant flow regardless of variation in system pressure. A compensated flow-control valve is used mainly to meter fluid flowing into a circuit; however, it can be used to meter fluid as it leaves a circuit. For clarity, this manual will refer to this valve as a flow regulator.