Insane Hydraulics

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His name was Hysteresis...

"His name was Hysteresis" was written for those who still think that hysteresis is a foreign guy's name. Please note that one part of this article should be taken seriously, while the other should NOT!

Through my hydraulic troubleshooting and technical assistance practice I've come to learn, appreciate and apply certain "psychological" approaches to problem solving. Those of you, who already have assistance practice, know that every technical assistance call or troubleshooting situation has two sides - technical and psychological. Technical side is nothing more than "bolts and parts" behind the problem, and normally doesn't constitute that much of a problem for a skilled and properly equipped technician. The psychological or human-related part, however, can be most problematic, as behind every hydraulic problem there is a always an individual (the owner, the operator, service mechanic, company engineer, the-guy-who-is-in-charge, etc...) or a group of individuals, whom you will have to "sell your opinion to", convincing him/them that a certain course of action must be taken in order to address the problem. No matter how good a technician you are, if you are a poor "convincer" - there is a good chance the client will ask for a second opinion somewhere else.

You might ask me here - what the hell does all this have to do with the above mentioned HYSTERESIS matter?! Bear with me for one more minute, please, - I am getting there...

Now, this very interesting matter of human relation skills that every hydraulic troubleshooter must master is a topic that can be extended into a separate article, or even a book, and surely is not going to be discussed here and now. But one this is for sure - in order to gain the client's unconditional trust, and convince all that his technical diagnosis is undoubtfully correct, and that his solution of the problem is the best, what every technician must do is to look confidently competent no matter what!

That's right - confidence is the key to looking like a skilled professional whichever might be your area of expertise. Make a doubtful face - and you are as good as dead! That is why an experienced troubleshooter develops a series of tactics in order to gain the client's trust as early as possible, preferrably at first sight.

Many techniques, particularly suitable for hydraulic technicians, can be named, among them the classics, like presenting sophisticated testing gear with superior look on your face (digital pressure-gauges and data-loggers work best, especially in remote and lightly populated rural areas...), or throwing a tool in the air and catching it after it has flipped a couple of times (requires training and works wonders if you can catch it behind your back, but has a drawback of producing exactly the opposite impact when the tool is a hammer and you catch it with your unprotected head), and, my personal favorite and the classiest of them all - the confident use of expensive words, by which I mean, of course, calling things by their names. After all - there are correct scientific names for certain technical phenomenae and, when you know what you are talking about, using these terms is no shame at all. Words like "hydrostatic", "volumetric","delta P", "resonance", "regenerative circuits", "load sensing", "reservoir", "carry-over" and what not may sound dull and everyday for you, but there's a good chance that they will be new to your client, and will skyrocket your credibility and status. With one of the sneakiest words to throw at people, instantly making them surrender to your technical opinion, being the aforementioned HYSTERESIS.

Please understand, that although I defend the use of correct technical vocabulary without simplifications, I am still against the use of special terms without understanding or out of the context. The only positive thing that can come out of this is a solid basis for popular anecdotes and cartoons (like the Simpsons). Why "hysteresis" you say? Well, it's because I "confronted" many mechanics with this term in the past, and more often than not got stumbled faces as a silent reply, you know, like - what did you just call me? And yet understanding the phenomenon of hysteresis is very important in hydraulics, especially when working with pump and motor controls. This is the main reason why I, perhaps in a slightly satiric manner, allowed myself to make the following attempt to enhance the technical vocabulary of those, who picture a bearded face whenever they hear the word "hysteresis".

A little bit of history behind the term - the word HYSTERISIS is derived form an Acient Greek word that means "lagging behind", and was introduced by a Scottish physicist Sir James Alfred Ewing in the late nineteenth century. He applied the term to describe certain magnetic properties of metals, but later on this term began to be used to describe "lagging behind" of any system's response to an input. The "book" definition of the term would be "the lagging of the effect behind its cause".

Let me illustrate with a hydraulics related example. Say you have a 100cc pump equipped with an electro-hydraulic proportional control. By increasing the control electric signal you will get to the stable half displacement - 50cc - at, say 400ma, however if you increse the input current to the max. displacement, and then dial it back to the 400ma, you will see that the displacement of the pump is now 53cc, and not 50cc. The same input - 500ma - resulted in two different outputs - 50 and 53 cc! That's hysteresis for you! And in this particular case you could say that this displacement control has hysteresis of (53-50)/100=3% of the full displacement.

ANY hydraulic control - whether it is pressure, flow, position, torque, displacement, or anything else related - has hysteresis, and will result in two different outcomes from the same input, depending on whether the input level was increasing or decreasing. Even the simplest relief valve will crack at one pressure (pressure increasing) and re-seat at a lower one (pressure decreasing). However, it is important not only to understand the notion of the hysteresis, but also to know when it really becomes important to take it under consideration.

With hydraulic controls things are simple - the best hysteresis is no hysteresis. Why? Because what we want from them controls is predictable results. In general, hysteresis presented by standard hydraulic controls is tolerable for most industrial applications. Still, normal wear of control mechanisms can increase the hysteresis to unacceptable levels, that is why it is a good practice to evaluate hysteresis when testing overhauled pumps and motors equipped with "aged" control modules.

There are applications when especially precise control is required. Take, for example, hydraulic generator drive systems, which must produce fixed flow to maintain the current frequency stable. One possible solution for such a system is a simple load sensing controlled pump and a fixed orifice. There are brands which manufacture special pumps for this purpose, and these babies are equipped with LS controls that have extremely low hysteresis. So, whenever overhauling such a pump, an extra care should be taken to evaluate hysteresis of the control during testing phase, to check whether it is still suitable for the application.

Also, detecting high hysteresis of a hydraulic control during tests gives you an idea of what to expect when servicing or adjusting it on the actual machine in the future. In some cases, knowing that a control has confirmed high hysteresis, can be a key factor to explain certain phenomenae in a system and help to find a "plan B" solution.

It should be noted that in the case of hydraulic controls the hysteresis level is rate-dependent, which means that it will change (increase usually) with the rate of the input signal change.

Main causes for hysteresis in mechanical control systems are freeplay/backlash (feedback links), friction, and wear of spool/sleeve assemblies. There are other causes, but from practical point of view the first two are the only ones that you can to something about.

Let us recapitulate:

- In control systems a certain result is expected from a certain input, and the same input produces two different results, depending on whether the input level was increasing or decreasing earlier. This difference is the hysteresis. A smart Scottish scientist invented the word. Scotland rules!

- In hydraulics hysteresis is everywhere, because hydraulics is all about controls. Hydraulics rules!

- It is important to evaluate hysteresis of hydraulic controls during tests of overhauled components. For certain applications high levels of hysteresis are unacceptable. Knowing the hysteresis rate of a component you just repaired or tested will give you an idea of what to expect when you re-commission it in a hydraulic system.

- Hysteresis level is rate-dependent.

- Best hysteresis is no hysteresis (at least in hydraulic controls). Normal wear of parts causes the hysterysis to increase.

- Hysteresis has nothing to do with Hysterical

- Next time you hear the phrase "Her cheeks flushed when she saw his massive hysteresis..." you will know that what is meant here is actually a polite conversation between two hydraulic company coworkers, and the female coworker got very upset with the hydrauic pump test results, her male colleague had just showed her, because the test uncovered unacceptably high hysteresis of the pump's displacement control...