An apple a day keeps the doctor away|
Just the other day I was reading a marketing guy's commentary on wants versus needs. One of the
examples he used to make his point was blood dialysis treatment. You don't need to sell dialysis
to someone with kidney failure because she needs it and will die without it. Back up a
few years and try to sell the same person a nutritional supplement proven to protect the
kidneys from infection and that sale is much harder to make.
Despite the fact we all know prevention IS better than cure, what he was saying is
prevention is a lot harder to sell than cure. Kind of ironic, don't you think?
This article resonated strongly with me, because I read it just after I presented my
Hydraulic Breakdown Prevention Workshop in Melbourne recently. The reason I say this is,
of the 10 guys who attended, only two were hydraulic equipment users - the others all make
their living fixing hydraulic equipment - after it breaks. And most of them drove long
distances to be there - 5 hours in one case.
To return to the medical analogy, this would be like presenting a seminar called
"An Apple A Day Keeps The Doctor Away" and 80 percent of the audience turns out to
be doctors. The irony certainly wasn't lost on me.
Its not that I believe hydraulic equipment users aren't interested in saving money -
they are. But it seems that an opportunity to save money is not as appealing as an
opportunity to make money. This explains why a doctor would attend a seminar on
preventative medicine and why a hydraulics professional would attend a seminar on
So if you really do believe prevention is better than cure - regardless of whether
you're interested in saving money or making more of it, why not join me at one of the
Hydraulic Breakdown Prevention Workshops I'm presenting between now and the end of this year:
Sydney Australia - October 12
Perth Australia - November 16
Cleveland, Ohio USA - December 3
The Nice Things People Say …
"For the last 13 years I was technical manager for Demag Komatsu mining shovels Southern Africa.
And I KNOW there is a SHOCKING amount of IGNORANCE in the field of hydraulics; oil sampling,
oil viscosity, filtration, component change-out schedules and cooling systems are often
blatantly ignored or misunderstood and the supplier is then put under pressure for warranties.
Your books Insider Secrets to Hydraulics
and Preventing Hydraulic Failures are right up my street."
Dosco Precision Hydraulics
Edenvale South Africa
Troubleshooting cylinder creep|
In tutorial #5 of my hydraulic maintenance mini-course, I debunk the myth that creep in a double-acting cylinder is caused by a leaking piston seal.
A popular misconception about hydraulic cylinders is that if the piston seal is leaking, the cylinder can creep down.
Fact is, if the piston seal is completely removed from a double-acting cylinder, the cylinder is completely filled with oil and the ports are plugged, the cylinder will hold
its load indefinitely - unless the rod-seal leaks.
What happens under these conditions - due to the unequal volume either side of the piston, is fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this occurs, the only way the cylinder can move is if fluid escapes from the cylinder via the rod seal or its ports.
If you grasp the theory at work here, you'll probably realize there are a couple of exceptions. The first is a double-rod cylinder - where volume is equal on both sides of the piston.
And the second is when a load is hanging on a double-acting cylinder. In this arrangement, the volume of pressurized fluid on the rod side can be easily accommodated on the piston side. But as the cylinder creeps a vacuum will develop on the piston side - once again due to the unequal volumes - and depending on the weight of the load, this vacuum may eventually result in equilibrium that arrests further creep.
This is not quite the end of the story though, but it's important to at least grasp this theory before we move on.
Notwithstanding the two exceptions mentioned above, if a double-acting cylinder's service ports are blocked - by a closed-to-actuator or cylinder spool and the piston seal does bypass, pressure will eventually equalize on both sides of the cylinder. As already stated, at this point a hydraulic lock is effected and no further creep can occur - unless fluid is allowed to escape from the cylinder or the cylinder circuit.
But because of the loss in effective area - due to pressure now acting on the rod-side annulus area, the static pressure in the cylinder must increase to support the same load.
For example, if the load-induced pressure on the piston side of the cylinder was 2,000 PSI and zero on the rod side when the DCV closed, assuming no leakage past the spool, the equalized pressure may be 3,000 PSI - depending on the ratio of the areas.
But what if this circuit has a service port relief valve set at 2,500 PSI? As pressure starts to equalize across the piston seal and the increasing static pressure on the piston side of the cylinder reaches the cracking pressure of the port relief, the cylinder WILL creep down.
While the root cause of the problem IS the leaking piston seal, the physics that applies is fundamentally different to what many people believe. And if you understand the theory, you can see how the humble pressure gauge can be extremely useful when troubleshooting cylinder creep.
"This book has the potential to save many
organizations lots of m0ney. It should be on the bookshelf of every engineer, supervisor, planner and
technician that deals with hydraulic equipment... it's worth its weight in gold." Find out more
Alexander (Sandy) Dunn
Plant Maintenance Resource Center
In search of the perfect hydraulic fluid|
For an entertaining discussion about the 'perfect' hydraulic fluid,
read Brendan Casey's article in the July-August 2007 Issue of Machinery Lubrication magazine,
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About the Author: Brendan Casey has more than 17 years
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