Metering In and Out

If you are throttling the oil into an end user (such as a cylinder or motor), you are metering IN. If you are throttling the oil OUT of the end user, you are metering OUT.

Metering in does not give as good a control as meter out BUT some devices such as end users with LOAD HOLD VALVES (like Counterbalance Valves) do not like meter out due to increased back pressure. This will have an effect on the pilot ratio!

The back pressure generated by a meter out device will become additive to the setting of any load hold device!

If it is vital to maintain 'tight control' of an end user and you MUST use meter out, any load hold valve must be one that is 'insensitive to back pressure'!

If you are still not clear, I will draw you a typical circuit or look on Google! Good luck with it!

C1 shows METER IN.

C2 shows METER OUT.

C3 shows METER IN with Counterbalance Valve and the centre spool position of the Direction Control Valve changed accordingly. I have ignored bits like Pressure Relief Valve and filters for this discussion!

Hope you can see enough of the drawing!

Hope this clarifys your issues!

My experience has been that metering in on pneumatic devices never works because air is compressable. There may be applications where it is called for but I don't know of them.

Another tip. I am assuming your flow controls will be free flow in the return path. You should use ball type checks if you can. I have seen the cheaper disk type hang up intermitently and cause puzzleing shutdowns that resulted in spending of big bucks unneccessarily (and often). Tell your maintenance people to suspect the flow controls if pressure and flow is mysteriously lost, especially intermitently.

Hi Skelley

I suppose that I should qualify my contributions as only relevant to oil hydraulics but then, I am sure you knew that! I hope that Satish does.

Oh and thanks for the useful tip! I'm not getting that much exposure to pneumatics of late!