Last week I spoke about post compensated flow controls and how they ensure reliable flow sharing among multiple functions even when the pump is saturated, and so it's only logical that in today's post I am addressing the flow sharing in its "pre-compensated flavor".
Let us briefly revise what we already know:
The main idea behind flow sharing is making sure that all the metering elements (variable orifices) of our multiple-consumer flow divider are subjected to the same delta-P, which guarantees that the relation of speeds between the functions is constant, and only defined by their opening.
In a post compensated flow divider, we "take" the pressure drop of the metering element with the highest load pressure, and impose it on all other functions with a pressure-inducing compensator, placed downstream the variable orifices.
The fact that the compensator has to be placed downstream of the flow metering element (hence the term "post-compensation") makes creating valves with directional control quite a challenge. This is how a typical post-compensated DCV cross-section looks like:
As you can see, the flow from the pressure gallery (1) enters the spool's metering section in the center (a.k.a. the variable orifice), then goes out of the metering section (2), through the compensator (4), returns to the spool in the "forked" pressure gallery, which then gets "distributed" to the works ports in a "normal" fashion. Designing and manufacturing both the valve bodies with all these internal galleries and the spools with all these notches is a serious challenge.
However - there's another solution, which allows you to achieve flow sharing without using a compensator placed downstream the metering element!
Yes - you read this right, you can achieve flow sharing in a pre-compensated DCV, and use the classic T-A-P-B-T valve gallery design (if you find this combination of letters "odd" - I suggest checking out "Which Way Will the Oil Go?").
So, let us try to recreate this marvelous flow sharing solution, starting with the classic pre-compensated arrangement:
This is the good old design we all know and love. A simple normally open compensator with a fixed-setting bias spring, which will keep the function flow "in check" so long as the available pump flow is enough to keep the delta-P across the variable orifice higher than the bias pressure. And when there's not enough oil to "go around" our multiple simultaneously activated functions, the compensators stay open, and the oil takes the path of least resistance, naturally prioritizing the function with the lowest pressure.
But let us think about it for a minute. The compensator in itself is perfectly capable of controlling any flow, its "only problem" is the bias spring - which "locks" its switching pressure to a fixed value, forcing this flow controlling system to be working with a constant pressure differential.
We already know that to secure flow sharing, we need a way of imposing an equal delta-P on all of our compensators, and we need to do this dynamically - i.e. work with the pump compensator's delta-P setting when the flow demand is met, and work with whatever the delta-P our highest pressure function happens to be experiencing when the pump flow-supplying capability is saturated.
And this is our answer right here! What we need to do is basically throw away the fixed-setting bias spring of our compensator, and replace the force it was "providing mechanically" with the dynamic force created by the pressure differential between the pump outlet and the highest pressure load signal. Like so:
Genius, don't you agree? Of course, the compensator now needs to be "seriously re-designed", and I will definitely look into how such compensators are built in a future post, but still - the idea that you can take a pressure differential, and "toss it across a spool", turning it essentially into a dynamically set bias spring is very neat! Whoever came up with it was light years ahead of his time!
So now, if you hear a tech say that flow sharing and post compensation is the same thing (alternatively - you need post-compensatoin to ensure flow-sharing), point him to this post. Hopefully he or she will be pleasantly surprised!