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    "His name was Hysteresis" is a short illiteracy training, provided by IH for those, who still think that hysteresis is a foreign guy's name. 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, is the one that can give the most headache, as behind every hydraulic problem there is a always a person (the owner of the equipment, the operator, mechanic, engineer, the-one-who-is-in-charge, etc...) or a group of persons, whom you will have to "sell your opinion to", convincing him/them that a certain course of action must be taken in order to fix the problem. No matter how good a technician you are, if you are not a good "convincer" - there is a chance that the client will ask for a second opinion somewhere else.

    You might ask me - what the hell does all this have to do with the above mentioned HYSTERESIS matter - be patient - I am getting there...

    OK, the so very interesting matter of human relation skills every troubleshooter must master (in any area, not just in oil hydraulics) 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 thing is for sure - one of the most important things a technician must do in order to convince a client that his technical diagnosis is undoubtfully correct, and that his solution of the problem is the best, is to gain client's faith in his qualification - in other words - to look confidently competent no matter what.

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

     Many techniques, particularly suitable for hydraulic technicians, can be named, among them classics, like presenting sophisticated testing gear with a superior look on your face (digital pressure-gauges work the best, especially in remote lightly populated agricultural areas...), throwing a tool  in the air and catching it after it has flipped a couple of times (requires training and works best if you catch it behind your back, but has a drawback of producing exactly the opposite impact when you catch a hammer 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", "carryover" and what not may sound dull and everyday for you, but there's a good chance that they will be news for your client, and will contribute to your credibility 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 absolutely 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, more often than not, see stumbled faces of many mechanics out there, when confronted with this term, and yet the phenomenon of hysteresis plays an important role in hydraulics, especially in the function of 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 imagine a bearded foreign man's 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 at the time to describe certain magnetic properties of metals, but later on this term began to be used to describe "lagging behind" of any system responding to a certain input. The "book" definition of the term would be "the lagging of the effect behind its cause".

   Let me clarify this with a hydraulics related example. Say you have a pump equipped with an electro-hydraulic proportional  1000ma control. Increasing the input electric signal form 0 to 500ma will result in a displacement, for example 50 cu cm, however if you increase the current to the maximum value of 1000ma, and then decrease it to the same 500ma, you will see that the displacement is now 52 cu cm. The same input signal resulted in two different outputs - 50 and 52 cu cm. If the pump's full displacement were 100 cu cm, you would say that the displacement control has hysteresis of (52-50)/100=2% 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 used control modules.

    There are applications, when especially precise control levels are 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. I
n 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 can be improved.

    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. 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 because for some applications certain levels of hysteresis can not be tolerated, and even in case of standard applications you will know what to expect.

    -  Hysteresis level is rate-dependent.

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

    - Hysteresis has nothing to do with Hysterical

    - Next time you hear the phrase like "Her cheeks flushed as he pulled out his huge hysteresis..." you will know that what is meant here is a conversation between two colleagues, who work in a hydraulics related company, and the female colleague got very upset with the pump testing results, her male colleague just showed to her, because the results proved high hysteresis levels of the pump's control unit....