Flow-control valves

 

Pressure-compensated, variable flow valves - This flow control is equipped with an adjustable variable orifice placed in series with a compensator. The compensator automatically adjusts to varying inlet and load pressures, maintaining an essentially constant flow rate under these operating conditions to accuracies of 3% to 5%, Figure 5. Pressure-compensated, variable flow-control valves are available with integral reverse-flow check valves (which allow fluid to flow unrestricted in the opposite direction) and integral overload relief valves (which route fluid to tank when a maximum pressure is exceeded).

Pressure- and temperature-compensated, variable flow valves - Because the viscosity of hydraulic oil varies with temperature (as do the clearances between a valve's moving parts), output of a flow-control valve may tend to drift with temperature changes. To offset the effects of such temperature variations, temperature compensators adjust the control orifice openings to correct the effects of viscosity changes caused by temperature fluctuations of the fluid, Figure 6. This is done in combination with adjustments the control orifice for pressure changes as well.

Priority valves - A priority valve, Figure 7, is essentially a flow-control valve that supplies fluid at a set flow rate to the primary circuit, thus functioning as a pressure-compensated flow-control valve. Flow in excess of that required by the primary circuit bypasses to a secondary circuit at a pressure somewhat below that of the primary circuit. Should inlet or load pressure (or both) vary, the primary circuit has priority over the secondary - as far as supplying the design flow rate is concerned.

Deceleration valves - A deceleration valve, Figure 8, is a modified 2-way, spring-offset, cam actuated valve used for decelerating a load driven by a cylinder. A cam attached to the cylinder rod or load closes the valve gradually. This provides a variable orifice that gradually increases backpressure in the cylinder as the valve closes. Some deceleration valves are pressure-compensated.

Fig. 9. Linear-type flow divider splits single input into two output flows.

Fig. 10. Flow dividers can be cascaded in series to control multiple actuator circuits.

Fig. 11. Circuit diagram for pressure-compensated flow-control valve.

Fig. 12. Cross-sectional view of proportional flow logic valve.

Other flow controls

Flow dividers - A flow-divider valve is a form of pressure-compensated flow-control valve that receives one input flow and splits it into two output flows. The valve can deliver equal flows in each stream or, if necessary, a predetermined ratio of flows. The circuit in Figure 9 shows how a flow divider could be used to roughly synchronize two cylinders in a meter-in configuration.

Like all pressure- and flow-control devices, flow dividers operate over a narrow bandwidth rather than at one set point. Thus, flow variations in the secondary branches are likely. Therefore, precise actuator synchronization cannot be achieved with a flow-divider valve alone. Flow dividers can also be used in meter-out circuit configurations or cascaded - connected in series to control multiple actuator circuits, Figure 10.

Rotary flow dividers - Another technique for dividing one input flow into proportional, multiple-branch output flows is with a rotary flow divider. It consists of several hydraulic motors connected together mechanically by a common shaft. One input fluid stream is split into as many output streams as there are motor sections in the flow divider. Because all motor sections turn at the same speed, output stream flow rates are proportional and equal to the sum of displacements of all the motor sections. Rotary flow dividers can usually handle larger flows than flow divider valves.

The pressure drop across each motor section is relatively small because no energy is delivered to an external load, as is the usual case with a hydraulic motor. However, designers should be aware of pressure intensification generated by a rotary flow divider. If, for any reason, load pressure in one or more branches drops to some lower level or to zero, full differential pressure will be applied across the motor section in each particular branch. The sections thus pressurized will act as hydraulic motors and drive the remaining section(s) as pump(s). This results in higher (intensified) pressure in these circuits branches. When specifying rotary flow dividers, system designers must be careful to minimize the potential for pressure intensification. A pressure relief valve should be placed in any actuator fluid line where this condition may occur. Rotary flow dividers can also integrate multiple branch return flows into a single return flow.

Proportional flow-control valves

Proportional flow-control valves combine state-of-the-art hydraulic valve actuation with modern, sophisticated electronic control. These valves help simplify hydraulic circuitry by reducing the number of components a system may require while, at the same time, substantially increasing system accuracy and efficiency.

An electronically controlled, proportional flow-control valve modulates fluid flow in proportion to the input current it receives. The valves can easily control cylinders or smaller hydraulic motors in applications that require precise speed control or controlled acceleration or deceleration. Most proportional flow-control valves are pressure-compensated to minimize flow variations caused by changes in inlet or outlet pressure.

An electrohydraulic proportional valve consists of three main elements:

a pilot or proportional solenoid

a metering area (where the valve spool is located), and

an electronic position-feedback device, often an LVDT (linear variable differential transformer).

Valve operation begins when it receives a signal from an outside controlling device such as a computer, programmable logic controller (PLC), traditional logic relay, or potentiometer. The control device delivers analog electrical signals to the valve driver card, which, in turn, sends a current signal to the solenoid on the valve.

The electromechanical force on the spool causes it to shift, gradually opening a flow path from the pump to the actuator port. The greater the command input signal, the greater the current to the valve solenoid, and, thus, the higher the flow from the valve. The important feature of this proportional valve is that all elements are proportional; thus, any change in input current changes force signals proportionately as well as the distance the valve spool will shift, the size of the flow path, the amount of fluid flowing through the valve, and finally the speed at which the actuator moves.

As the spool shifts, its motion is detected and monitored very accurately by an LVDT or other type of position-feedback transducer. This signal is fed back to the driver card where it is continuously compared with the input signals from the controller. If the two differ, the driver adjusts spool position until the two signals match.