Insane Hydraulics

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What Should You Read to Learn Hydraulics?

I get this question a lot in emails - "What should I read to learn hydraulics?"

I once stated in an old post that I consider component catalogs to be, by far, the best resource for getting usable industrial hydraulics knowledge, and I stand by it. So you already know my answer to "what". But I also want to tell you "how". And so - this article is a practical guide on how one would use a catalog to learn hydraulics.

Before we go any further, please note that we are at the intermediate level here. The three-step exercise I am about to describe is not for beginners. And if you are one - the internet is full of resources on basic hydraulics, and the only two things you need to become a stellar "hydraulic rookie" are: a) to be in a place where you can practically apply your recently acquired knowledge (i.e. service or work with hydraulic equipment) and b) an internet connection. So let's make sure we're on the same page here - some hydraulic knowledge and experience are a must for this post to be of any use to you.

It's like one of my back-engineering exercises, only instead of physical parts, we will be looking at catalog pages.

Step One - "Pick a Name"

Pick a well-established name. It can be any name, as long as it has something to do with hydraulics. I call Oil Control for this post. One of the first names I got to know when I began working in this field. And they have pretty decent catalogs. Believe it or not - I still have paperback copies from the 90s back in the office! But you can still find decent-quality PDF files online as well.

A brief history note: Oil Control is an Italian company that was founded in 1973, and grew into a full-blown industrial group that held world-famous hydraulic brands EDI System and Oleodimanica LC, and then got acquired by Bosch Rexroth in 2004/2005. These guys know what they're doing when it comes to screw-in-type cartridge valves.

Step Two - "Pick a Component Type"

Now you pick a random component type. Let us pick the relief valve. "Oh, but I know everything there is to know about relief valves!" - to which I reply - "No, you don't. And neither do I." As a matter of fact - nobody does, which is why such an exercise is actually useful even for techs "of the highest levels" (IMHO).

So, let us take a concrete Oil Control's relief valve lineup as an example - the Special Cavity Relief Valves. Special cavity valves are the valves that you get when you buy their complete valves with manifolds, but once again - I could have picked their standard cavity line of valves, or anything else, for that matter - it's just an example, and a very small one - the file is only 16 pages long!

Step Three - "Follow the "Magic Pattern" One Page at a Time"

This is the fun part where you go through the complete lineup of a single component type, and follow this "magic pattern" for every listing:

Symbol and Name -> Design -> Application

Let me explain what every step of the "magic pattern" means:

Symbol and Name

Both the symbol and the name of a component must make 100% sense to you. Every word in the name and every single line in its symbol. If something does not - you don't go further until you figure it out. This teaches you the vocabulary and the many ways a component can be represented as a symbol in a hydraulic diagram. This also teaches you another very important thing - and you will see this as soon as you look at more than one brand - there are many ways to represent the same component in a schematic, and it largely depends on the personal preference of the engineer that created the catalog (I'll give you an example at the end of this article if you don't believe me). A symbol may be intuitive, and it may not - but it is important that you follow the logic an engineer applied when he was drawing the lines. These are all relief valves:

Every single dashed line, a rectangle, or an arrow in the symbols must make sense to you. If you are an "intermediate level tech", and you do this exercise for the first time - you may be amazed by how many valves can be called a "relief valve".

The same goes for the name. After going through this tiny 16-page file, for example, word combinations like "direct-acting", "poppet-type", "guided-popped-type", "spool-type", "differential area", "pressure-compensated", "pilot-operated", "external-drain", "unloading relief valve" should have an immediate functional association in your head.

Once again - when you come across a line in a symbol or a word combination that is new/strange to you - you use the internet (and what other resources you may have at your disposal) to find out what exactly it means, and do not turn the page until you have zero doubts.


At the same time as you are looking at the symbol and the name of a hydraulic component, you also study the cutaway view. In detail! Every design detail matters! Oil Control catalogs are pretty good in that regard because they present detailed cutaways for all of their valves. Understanding how a component is built and operates is very important. Just like every term in the name and every line in the symbol, every part in the cutaway view should make sense to you. These three relief valves are built for the same cavity, but can you explain the differences in their execution and, therefore, operation?:


And finally - the application. With every page that you turn, you must think about (or discover) the real-life purpose of the component you are looking at. The 16-page example file starts with the tiny valves that are used in thermal relief applications - and if you have never heard about it - this would be a good incentive to find out. Then you will come across a relief valve with a hardened seat and a comment that it provides increased durability in high-pressure high-cycle applications - and this will make you realize that the cycle-endurance of a relief valve is yet another factor to consider when you are choosing this seemingly simple component for an application. Then you come across a pressure-compensated relief valve that applies the so-called balanced piston design, which makes the valve's setting independent of the back pressure therefore making its operation especially consistent, and then you may even realize that you can use the very same cartridge as a sequence valve! Then you will see that spool-type relief valves are capable of accommodating large flows...

And then, when you think you know everything there is to know about relief valves - you will come to the page containing the so-called "unloading" relief valve - something that may be entirely new to you, and this will make you go online and look for the potential use cases for such valves, and you will discover that what the folks form Oil Control named "an unloading relief valve" is often referred to as a "kick-down" relief valve, and then you'll probably run into one of the darkest use-case examples for such a valve provided by (our beloved) SUN Hydraulics:

"... When the farmer gets caught in the auger, the kick-down relief trips and allows him to escape..."

Wow! I especially appreciate the use of the present tense in this sentence. To my ears, this sounds almost like an everyday ordeal for that poor farmer (like - wake up, have a breakfast, go to the grain silo, get caught in the auger, escape from the auger, fuel the tractor, feed the animals...) But I also see how the use of a kick-down relief valve in this highly graphical situation can be a literal life-saver! And by the way - I prefer the name "kick down". The term "unloading" would be for valves with external pilots one would use for accumulator charging or maybe two-stage systems. But that's just me.

Remember that I promised a concrete example of how different engineers draw "highly personalized symbols" for the same components? Well, the kick-down relief is a good example. These three symbols represent the same valve - the first one is from an old Oil control catalog (from 99), the second one is the same valve but from the example file that I just gave you, and the third one is the SUN's RQEB, which employs the same design - a side hole in the relief piston, that connects it to the return gallery when it lifts, and thus switches it to the "always open" mode:

Both the old and the new Oil-Conttrol/Rexroth symbols kind of show you that "there's more to the vale than just a simple relief function", while the SUN's symbol clearly shows you how shifting the relief piston connects the pilot chamber to the return line. SUN is a clear winner here, in my opinion! But it is still very interesting to see how two competing engineering schools addressed the creation of what is supposed to be an explicit and intuitive symbol for the same component. Props to you, Mr. SUN! You keep on doing what you are doing!

And now - the final secret - consistency. You already saw that a half-an-hour exercise with a dozen catalog pages can teach you a lot of new things. Now, all you have to do is turn it into a habit - i.e. do this once a week. Just find free 15 minutes, sit down, pull out a catalog, pick a random component type, and go through a listing from top to bottom. Such "brain teaser" sessions with OEM catalogs are a great way to learn hydraulics!