The week after last month's newsletter went out, I was talking to one of our members;
Mike Smith from Isco Industries in Huntsville Alabama. He told me he
showed the leaking cylinder simulation video to a few of his guys. At least one of them refused to believe it.
And I know that chap wasn't the only one who didn't 'get' it. In fact, some of the mail I got was a real eye-opener - even for me.
One member wrote: "Obviously your software does not take into effect the slippage of the oil past the piston". Say what? I'm not sure which video he watched.
Another member (with a college diploma) referred to pressure intensification on the rod side
of the cylinder. There is a big difference between pressure equalization and
pressure intensification. Do not confuse them.
While another tried to prove the simulation model wrong by machining up a "piston" and "rod" and
using it to displace water in an open test tube.
Once I pointed out the flaw in his experiment - that he needed to insert the piston rod,
completely fill the test tube with water, and insert the stopper to make a seal between the tube and rod - he was converted.
But I also received many considered replies. This one from Richard Hassebrock summed them up:
"This video is good for stimulating thought on the topic. But I have to tell you that when I worked as a heavy equipment mechanic on mobile equipment I saw numerous times cylinders drifting under a load due to leaking piston seals… Perhaps the pressure in the cylinder, as in your example, will increase to the set pressure of the port relief valve (missing in your simulated circuit)… allowing the piston to drift?"
Give the man a cigar.
Another of our members; Rob Cullen, correctly pointed out if there was a three-port
counterbalance valve on the cap end, the directional control valve center condition
(see page 14 of Industrial Hydraulic Control) could be fully open center; or float center if the pump was variable displacement - instead of tandem center used in the simulation, allowing the piston rod to drift.
The point is, and this may come as a shock; even a disappointment to some, I ain't always going
to connect all the dots for you. Reason being, in hydraulics, the number of variables is
unlimited. So to succeed in this game you need to be able to do that for yourself.
And if you 'get' the how and why, you can figure out all the possible exceptions - with or without the aid of computer simulation - on your own.
What you saw in the video was a double-acting cylinder turn into a 'displacement' cylinder -
also known as a ram. And as you may be aware, a ram does NOT have a piston seal.
Can a ram drift? Yes, of course. But ONLY if captive oil volume can escape from the cylinder somehow.
And if you really paid attention, you would have 'got' two other valuable lessons from the video.
One on how to troubleshoot cylinder drift - to quickly nail the root cause.
And another for machine designers when designing load-holding circuits.
There are a lot of clueless people out there, running around working on hydraulic equipment.
If you're a hydraulic equipment owner, be afraid. If you're an aspiring hydraulics specialist,
be emboldened - at how easy it is to stand head and shoulders above the crowd. And if you're a
regular reader of this newsletter, pat yourself on the back. You already have an unfair
advantage over the mediocre majority.
No one actually asked, but many of you were probably wondering if my sceptical reader from
Machinery Lubrication magazine actually ate his college diploma. I doubt it. But he did write
me to ask if I would allow him to add some sauce to make it more appetizing.
And to his credit, once he 'got' the concept involved, he made it his business to connect
the rest of dots for himself.
I couldn't ask any more of him than that.
Editors note: I will be based in Vancouver Canada, December thru January 2009 and currently
have a few openings in my schedule for training and/or consulting assignments in North America
during this time. I can be contacted on 440-499-6575 or at email@example.com
YOUR Services As a High-Paid Hydraulics Specialist...
Find out more
One type of hydraulic system you don't want|
One of our members (name withheld for reasons you'll understand in a moment) sent me this beauty:
"Our hydraulic power units are Kubota 86 hp and Deutz 76 hp 4 cylinder diesel systems.
We use a 40 gpm gear pump with a splitter valve to create two circuits of 20 gpm, each with a variable flow control. We have a diverter valve upstream to combine flows if needed. The gear pump can handle the trash in offshore oilfield hydraulics and is inexpensive compared to piston pumps. Relief is set at 3000 psi.
A gear-type flow splitter would be better but is much more expensive. The flow controls we use are inexpensive - but the down side is they create unwanted heat.
The flow thru relief valve is another problem source of heat - it dumps past the cartridge port back to tank during normal operation.
The biggest problem we experience is not enough cooling - trying to find a 12 VDC cooler that is not too expensive that can handle salt water (Gulf of Mexico oilfield operations). We looked at hydraulic fan motor but it was too expensive. This problem could make an interesting article for your newsletter."
An interesting article for this newsletter? As you can see, he did offer.
There are four problems here:
1. Being cheap to the penny;
2. Technical ignorance;
And as a result of 1 & 2:
3. Asking the wrong questions;
4. And looking for 'silver bullet'.
Let's take a look at each in more detail.
Being cheap to the penny
With hydraulics, like most other things, you get what you pay for. Design on price alone (especially if you don't know what you're doing) and you'll end up with a load of rubbish. Which is exactly what we've got here.
Oh sure, there's lots of hydraulic equipment users out there who don't know any better. And plenty of equipment builders who are short-sighted enough to accommodate them.
But if this guy can't build a properly engineered system, fit for the purpose and get his oilfield customers to pay that price - whatever it may be, he should go find something else to do. I mean, someone please remind me what the price of oil is at the moment? Sheesh.
If you are going to go 'cheap' you need to know what you're doing. By that I mean: what you can and can NOT get away with. I don't think it would be possible to come up with a more inefficient hydraulic system than the one we've got here. Let's take a quick look at what it comprises:
Gear pump - a.k.a. heat generating device
Spool-type flow divider - a.k.a. heat generating device
Variable flow control(s) - a.k.a. heat generating device
Pilot-operated relief valve - a.k.a. heat generating device
Have I upset anyone yet? Write this in blood: EVERY component in a hydraulic system is a heat generating device. But some are much worse, read: more inefficient, than others.
So if you grab a whole bunch of the most inefficient ones and throw them together, you better make sure you've got a BIG cooler. But wait, the 'budget' won't allow for a cooler will it? Darn it then, let's just cross our fingers and hope it will work OK without one. It's false hope.
Asking the wrong questions and looking for a 'silver bullet'
He knows he's got an overheating problem. And he's also at least vaguely aware that his choice of components is a contributing factor. But he's completely paralyzed by his wallet.
Asking: "How can I get this hydraulic system to run cool without sending any more money?"
Because it presupposes there is a magic solution. One which will defy the physical laws at the root of the problem. There is no such solution.
So what is the answer? Well despite this member's protestations to the contrary, I reckon I could design a power unit around a variable displacement piston pump and still be cost-competitive enough to consign his cheap 'n nasty version to the scrap heap. It would definitely be a lot simpler. And it would certainly work out cheaper in the long run.
But that's a story - and possibly a simulation, for another time. Next year in fact.
Because in 2009 I'm launching my "Hydraulics Mastery Newsletter". It will comprise a print newsletter and CD delivered to your door each month by snail mail.
The print format will permit more detailed discussion of the issues covered, and the inclusion of drawings and other exhibits. While the companion CD will enable delivery of circuit simulation videos with analysis.
It's not for everyone, though. Because it will be available through paid subscription only. Stay tuned for more details.
"Thanks for the great work on the two publications, Insider Secrets to Hydraulics and Preventing Hydraulic Failures. I have been in the hydraulics business for the past 20 years and it's very difficult to find any decent material on hydraulic maintenance, troubleshooting and failure analysis. These two books cover it all in easy to understand language... I conduct hydraulic training courses and plan to purchase copies to distribute to my students to share your practical approach to understanding a not so understandable subject."
Paul W. Craven, Certified Fluid Power Specialist
Motion Industries, Inc.
Six hydraulic mistakes to avoid|
If you haven't read Brendan Casey's latest report:
'Six Costly Mistakes Most Hydraulic Equipment Users
Make -- And How YOU Can Avoid Them',
complimentary copy today
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