Insane Hydraulics

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Pre-Compensated Flow Control With Flow Sharing - Compensator Design

In my article on pre-compensated flow control with flow-sharing I mentioned that designing a compensator for such a valve is a serious engineering feat, and today I want to study a concrete example using Parker's L90LS DCV, and also talk a little bit about hydraulic component catalogs and how it is important to not only study them, but also keep track of all the existing editions, because most manufacturers tend to issue a new catalog every year, sometimes even more often, and aside from registering the natural development of this or that series of a component line, they often bring along "other improvements", which actually make them worse in certain aspects in comparison with earlier versions. You'll see why I decided to put these two, seemingly unrelated topics, in a single post in a minute. Let us start with the compensator design.

A quick recap: our pre-compensated flow-sharing system needs a compensator that takes two pressures - one from the pump gallery and one from the function with the highest load, and uses the differential between them as a "hydraulic bias spring" of sorts, creating a flow-limiter that works with an arbitrary and dynamically set pressure differential as opposed to the standard compensator that works with a mechanical bias spring and therefore has a fixed delta-P:

To be honest, I wouldn't even know where to begin if I had to "invent" such a compensator, but engineers from Parker (an old Voac team perhaps?) did an excelling job designing the following engineering marvel:

What a beauty! This cutaway view shows it all!

You can clearly see how the pressure from the pump gallery acts on the left piston and pushes the spool to the right (open) position, and how the pressure from the "highest load gallery, or LS", using another piston with an equal area, pushes it to the left, creating a perfect hydraulic bias force on the compensator, without affecting its "normal "acting areas - i.e. the upstream area (closing), that works with the pressure of the spool's metering inlet, and the downstream area (opening), that works with the pressure of the metering outlet, which in this case, since the spool is directional, is read from the work-ports, and is provided to the compensator through the spool's own LS channel. Some cool engineering going on here.

Ok, so you studied the cutaway, the schematic, wrapped your head around all this pre-compensated flow-sharing business, and now you ask - Sergiy, but what did you mean by the "catalog talk" above? Ah! I am so glad you asked!

I've said before and I will say again and again that catalogs are the best literature to learn industrial oil hydraulics. If you know how components work, how they are designed, and what manufacturers recommend to do with them - you're golden!

But there's one part that I always forget to mention. Catalogs are great, they really are. But they also change all the time. This is normal - series evolve, engineers come and go, management teams get swapped as brands are bought by brands, that are later bought by other brands, and so on. And so - each year or so a new shiny catalog is launched.

However, new does not always mean better. If you ever did any type of documentation for a hydraulic project you'll know what I mean. If you visit a brand's official website looking for technical docs, you will most likely be able to download only the latest versions of the catalogs, and if don't compare them to the previous versions - you may be missing a lot of important stuff.

Here's a concrete example for you - the beautiful cutaway view from the above comes from the L90LS Parker catalog from 2004! Here's the cutaway from the 2021 catalog:

Now tell me (and be honest!) which of the two does the best job explaining the valve's function? I am all in favor of 3-d modeling, but a nicer-looking picture does not make a technical drawing better!

Here's another example, showing, among other things, the LS copy spool I talked about a couple of weeks ago:

I think the cutaway view is much more educational than the fancy-looking 3d model. But it's best to have both in your catalog collection!

So, the "catalog tip of the day" is:

If you're "advanced enough" to have a catalog folder in your computer, always look for all the files you can find for this or that component series, and not just the lastes version. Place them all in the same folder and give the files consistent names (like L90LS_2004.pdf, L90LS_2007.pdf, L90LS_2021.pdf). Then do the same for the respective parts lists as well.

This gives you an excellent opportunity to study a component's evolution, and you'll be surprised how often an older catalog can be better in explaining or showing certain things in comparison with the latest one. My L90LS folder has files from 1998 (yes - freaking 98, when it was still more often than not referred to as VOAC) all the way up to 2021. Older files are harder to find, so grab them and store them whenever you can.