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Flow-control valves include simple orifices to sophisticated closed-loop electrohydraulic valves that automatically adjust to variations in pressure and temperature.
the purpose of flow control in a hydraulic system is to regulate speed. All the devices discussed here control the speed of an actuator by regulating the flow rate. Flow rate also determines rate of energy transfer at any given pressure. The two are related in that the actuator force multiplied by the distance through which it moves (stroke) equals the work done on the load. The energy transferred must also equal the work done. Actuator speed determines the rate of energy transfer (i.e., horsepower), and speed is thus a function of flow rate.
Directional control, on the other hand, does not deal primarily with energy control, but rather with directing the energy transfer system to the proper place in the system at the proper time. Directional control valves can be thought of as fluid switches that make the desired "contacts." That is, they direct the high-energy input stream to the actuator inlet and provide a return path for the lower-energy oil.
It is of little consequence to control the energy transfer of the system through pressure and flow controls if the flow stream does not arrive at the right place at the right time. Thus, a secondary function of directional control devices might be defined as the timing of cycle events. Because fIuid flow often can be throttled in directionalcontrol valves, some measure of flow rate or pressure control can also be achieved with them.
Different Woes of flow measurement
Controlling flow of a fluid-power system does not necessarily mean regulating volume per unit of time from a valve. Flow rate can be specified three different ways, so it is important to be aware of how flow is to be specified or measured:
Volumetric flow rate, Q,, expressed in units of in.3/sec or min - or cc/sec or cc/min in SI metric measure - is used to calculate the linear speeds of piston rods or rotational speeds of motor shafts.
Weight flow rate, Qw, expressed in units of lb/sec or lb/min, is used to calculate power using English units of measure.
Mass flow rate, Qg, expressed in units of slugs/sec or slugs/min for English...