Insane Hydraulics

Site theme image

How to Transform Closed-Center Double-Acting Spools of Monoblock Directional Control Valves

Can you transform a "normal" double-acting closed-center spool of a mono-block directional valve into a single-acting one?

You may ask - why the doubt? Because while it is intuitive to think that one can easily transform a closed-center spool into an open-center by removing the excess material that is "closing", for the single-acting spool the transformation may seem "tricky", especially if you look at an "original" single-acting spool and see that it appears to have more material on it than its closed-center double-acting sibling:

Walvoil SD11 double-acting closed-center spool next to a single-acting spool

It is easy to remove parts of the spool with a grinding wheel and a steady hand, but you can't possibly add material to it now, can you?

This is a valid point, and I agree that "spool appearances" are misleading, however, I assure you that it is absolutely possible (and very easy) to transform a common double-acting closed-center spool of a mono-block DCV into single-acting through the same process of grinding off "excess spool". And this short post is aimed at explaining and demonstrating (with an interactive drawing!) how easy it is.

Before all - it is important to understand how common mono-block valves function. And the best way, in my opinion, is to study the cutaway view (and the hydraulic diagram) of a typical mono-block valve. Please note, that this example shows a typical mono-block valve with a side by-pass gallery - i.e. placed to the side of the work ports. There is another common design, in which the by-pass runs between the work ports (for example the Salami VDM06), and in that case, the spool design and appearance are different, but the main idea that this post (hopefully) conveys is still the same.

Typical monoblock DCV cutaway view
Typical monoblock DCV hydraulic diagram

Virtually all mono-block valve manufacturers follow the design pattern where a spool can be divided into two parts - one for directing oil to ports, and the other for closing the by-pass gallery when the spool is shifted. If you consider this - you will see that the only thing a single-acting spool needs to do to be called a "proper" single-acting spool, is stop closing the by-pass gallery when it is connecting its "business port" to the tank (thus not forcing the poor pump to work against a plugged port and giving the other spools the opportunity to use the pump as they please).

An "original" single-acting spool will have more material on the unused port side because it makes no sense machining grooves that are not needed, and while its double-acting brother does have an extra port groove, the part that needs to be modified (for the single port operation) - the by-pass gallery closing part - is still there, complete, and therefore, grind-able.

I am not good at explaining stuff with words, so let me show you! The image below is animated - you can use the radio buttons to toggle between the spool types and move the spool:

Animated Drawing (use the controls below the image)

A B P T T C C A B C P T C Plug Plug

Select Spool Type

Select Spool Position

Now, the last question is - how much of the spool do you need to grind off? In most cases, it would be about half of the spool's full travel (from A to B). You can actually get away with less when you transform a closed-center spool into an open-center one if your objective is just to vent the lines to tank (for example if you are to apply an over-center or a piloted check).


This drawing, by the way, is a good illustration of a "move-line" spool that I describe in "Which Way Will the Oil Go?"