What are flow control valves in hydraulic circuits?

In hydraulic circuits, the flow control valves are used to either split the flow of hydraulic oil between multiple circuits or control the flow and speed of an actuator. Control of flow means that it is either increasing, decreasing or maintaining the flow of oil. Speed of the actuator is how fast it is doing its movements. The flow control valves are basically of two types plus flow dividers.

•  Non compensated or fixed orifice flow control
•  Pressure compensated or bleed off flow control
•  Flow dividers

Flow control valves that are used to regulate the flow of hydraulic oil to an actuator function by defining a flow area for the oil to pass through. It can be classified as adjustable and non-adjustable. They are rated by operating pressure and capacity, and can be used in meter-in and meter-out applications. Meter-in means the flow that is required to enter the actuator and meter-out means the flow which is required not to enter the actuator.

 

Non compensated or fixed orifice flow control

 

                    Fig. Fixed orifice flow control valve        Fig. Single direction flow control valve

When a meter-in fixed orifice flow control valve is used to control the flow of the oil, the valve is installed between the control valve and the actuator. The fixed orifice flow control valve has a restricted passage a shown in the figure above. This kind of flow control valve is used in such cases when the action of an actuator always resists the pump flow.

In many cases, the flow control is required in only one direction, not resisting the pump flow. If a dump box of a haul truck raising its dump box ram, the flow control valve can be bypassed to utilize the pump pressure to maximum. But when it is coming down, the oil will pass through the restricted orifice limiting the oil flow and reducing the speed of the actuator. We can see in the figure that there is a spring provided which is holding a check valve with a fixed orifice against the pump oil flow in one direction. If the oil pressure exceeds the spring pressure, the valve will open and let the full flow of oil to pass through. But in opposite direction, the oil pressure can only pass through the restricted orifice.

A meter-out valve is used when an actuator has the potential to run away or over speed which can cause cavitation to occur in the actuator. The meter-out flow control valve is placed between the control valve return and the actuator. This will control flow away from the actuator allowing the pump to displace sufficient oil preventing the cavitation to occur.

 

Pressure compensated or bleed off flow control

Fig. Pressure compensated or bleed off flow control valve

One method of bleed off is by placing the valve between the pump outlet and the control valve with a drain line to the reservoir. It meters the flow which is diverted to the tank rather than the working flow. As we can see in the figure above, there is a piston with an orifice in the center allowing the oil to pass through. That piston is being supported by a spring against the hydraulic oil pressure. The oil is pushing the piston against the spring and at the same time, the oil is also passing through the orifice. When the flow is increased, the piston will be pushed more to the right compressing the piston and opening the bled port. This will maintain a constant flow on the metered side. Here, the pressure compensated feature becomes a factor when the system pressure begins to rise. When it happens, the piston will try to bleed off more oil. However, the pressure on the metered side also increases. Then with the help of spring, it will push the control piston to the left closing the bleed off port maintaining a constant flow rate in the metered side. It’s not a precise means to control the flow but it is one of the methods it is being done in some equipment.

 

Flow dividers

To supply multiple hydraulic circuits from a single source, the flow dividers are used. They are normally available in spool type or gear type. A gear type flow divider has a common inlet branching into multiple outlets with gear sections connected to a common shaft rotating at same speed as shown in the figure.

Fig. Gear type flow divider splitting flow into three circuits

The sizes of the gear and the amount of oil it can trap within the meshed teeth dictates the flow it can pass through it. So, the gears can be selected to get required divided flow in different sections.

 

Another type of flow divider is called spool type. As we can see in the figure below, there are different passages for primary and secondary flow. Primary circuit is without any restrictions resulting in a direct supply of oil but secondary circuit is only supplied with oil if the pressure is high enough to compress the spring and open the port to the secondary circuit. A specialty of this type of flow divider is its capability for proportional or priority flow control. 

Fig. Pressure compensated spool type flow divider

As can be seen in the figure, the primary side is always provided the supply of oil but secondary side will be supplied only when the pressure is high enough to push the spring pressure and open the port to secondary circuit. A port releasing the oil to the reservoir tank is also seen which opens to drop the pressure on the secondary side if the valve moves to the right side enough to open the port due to the spring pressure. It happens when the oil pressure is less than the spring pressure. When the oil pressure increases high enough, the spring will compress again opening the port to secondary circuit and closing the port to the reservoir tank. So, it can be concluded that the secondary side will be supplied only when the pressure is higher than a set value but primary side will always be supplied with oil.