Flow control valves are devices in the collection of hydraulic components that are manufactured with an intention to modify the rate of fluid flow in a portion of a hydraulic circuit. Flow variability mainly happens through the adjustment of the effective orifice size inside the valve and this is obtained via several techniques that are based on the type of valve and its manufacturer. The basic flow control is just an orifice which is a disc or plug or component having a simple hole drilled through the middle of it for limiting the rate at which flow can pass. By both the size of the hole and the applied pressure for pushing fluid through the hole, the flow rate is dictated. The difference of pressure between downstream and upstream sides of the orifice is referred to as the pressure drop and with certain considerations to the orifices, the pressure drop can be a predictable split between the flow and pressure.
For applications in which modifications are required for the actuator speed while the machine is still in operation, a variable orifice is beneficial. Variability can be achieved in several ways but the most common method is via a needle valve. An inline body will have a through passage with a cone-shaped pocket machined into the flow path. A threaded needle when installed into the valve will close and open via its threaded adjustment, closing and opening the gap in between the needle and cone and vary the flow through the valve. By most of the definitions, a true flow control valve is a needle valve with a reverse flow check valve. The check valve can supply free flow in one direction and restrict the flow in the returning direction. Flow control valves are installed into the work ports of a cylinder port. When the flow entry is restricted, it is called meter in control and when the exit is restricted, it is known as meter-out control.