Friday, 26 February 2016

METER IN METER OUT AND BLEED OFF HYDRAULIC CIRCUIT

METER IN METER OUT AND BLEED OFF HYDRAULIC CIRCUIT

We were discussing various types of hydraulics valves used in hydraulic system in our previous post e.g. Servo valve and proportional valve. Today we will be concentrated here with various types of flow control hydraulic circuits usually used in various hydraulic units.

 
In order to control the speed of an actuator such as hydraulic cylinder, hydraulic design engineer consider the flow control circuits and these are basically three types. Meter-in circuit, meter-out circuit and bleed of circuit.

Flow control circuit: Meter-in

Meter-in flow control circuit represents the controlling of fluid flow just before fluid enters to the actuator with the help of flow control valve. In meter-in flow control circuit, flow control valve will be installed in such a way that fluid flow to the actuator could be controlled or we can also say that there could be a restriction in fluid flow to the actuator.

Let us consider meter-in circuit for controlling the flow of fluid and hence speed of actuator e.g. hydraulic cylinder in following circuit. We can see here the bypass check valve that will force the fluid to flow through the adjustable orifice before fluid enters to the actuator i.e. hydraulic cylinder here.
Figure showing the meter-in flow control hydraulic circuit, first circuit represent the rest position of cylinder while pump will be in running condition. Second circuit represents the extension of hydraulic cylinder.

Therefore, speed of an actuator will be controlled here by controlling the flow of fluid before it enters to the actuators with the help of flow control valve which will be basically an adjustable orifice with bypass check valve as shown in above hydraulic circuit.

Let us consider, flow control valve is not provided then in that case flow of fluid and thereby extension as well as retraction of actuator i.e. cylinder will be carried out with unrestricted rate. 

Movement of actuator during extension and retraction will be controlled by providing flow control valve as we have seen above. When fluid flow direction will be reversed in that situation, fluid will flow through the check valve and hence will bypass the flow control valve.

Meter-in flow control circuit will have quite precise control if load is resistive load but if there will be overrunning load then in that situation meter-in circuit will not be able to control the actuator speed. 

In case of overrunning load, actuator will move faster and hydraulic circuit will not be able to fill it with hydraulic fluid and hence cavitation phenomena will be possible over there.

Meter-in flow control circuit applications could be seen in feeding welding machine, grinder table and also in milling machines.

Flow control circuit: Meter-out

As we have discussed above, if load on actuator is overrunning load in that situation meter-in flow control circuit will not be able to control the speed of actuator because overrunning load on actuator will make actuator rod i.e. cylinder rod to extend due to weight of vertical load i.e. overrunning load.
 
Flow control valve will be installed on discharge end or return side of actuator i.e. cylinder in order to control the discharge of fluid flow. Meter-out flow control circuit will control the flow of fluid leaving the actuator.

Figure showing below the meter-out flow control circuit, first circuit represent the rest position while pump is in running situation. When DC valve will be operated, we can see the second circuit to determine the fluid flow to the cylinder through the check valve. Fluid will flow via check valve by bypassing the flow control valve. When fluid will leave the cylinder, fluid will not be able to flow via check valve and will be restricted by an adjustable orifice.

Therefore actuator speed will be controlled in meter-out circuit by restricting the flow of fluid leaving the actuator. Meter-out circuit will work successfully with resistive load and also with overrunning load or running away load because actuator will not be able to move faster than fluid discharge it permits.

Bleed-off flow control circuit

Bleed-off flow control circuit is also work successfully only with resistive load because we are also controlling here the fluid flowing to the inlet of the actuator.


 
Figure showing below the Bleed-off flow control circuit, where we may see one needle valve installed at the inlet of the actuator. Inlet of needle valve is connected with a line which is going to the inlet of actuator i.e. cylinder. Outlet of needle valve will be connected here with tank.

Second circuit in above figure indicates the extension of hydraulic cylinder. Hydraulic fluid will flow to the inlet of actuator but during the flow to actuator, excess flow of fluid will be bled off to the hydraulic tank.

Image Courtesy: Google
We will start our discussion with another topic in our next post.

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