Hydraulic regulation - of a different kind|
Last month I was invited to speak at Enzed's bi-annual conference in Sydney.
Enzed is the brand Parker Hannifin's Hose Doctors operate under throughout
Australia and New Zealand.
In addition to speaking at the conference, I had the opportunity to talk to many
of the 200 delegates. One of the many interesting things I learnt was, before a new
recruit can begin working as a Parker/Enzed Hose Doctor, he or she must attend
a two-week training program. And apparently it's intensive - 80 contact hours plus homework.
This commitment to rigorous training is reassuring to those of us who from time to time
find ourselves having to work in close proximity to an operating hydraulic system. I know
any time I find myself working near a large diameter, multi-spiral hose pressurized to
6,000 PSI I hope the guy who fabricated it knew what the heck he was doing.
And for good reason too. The consequences of being in the vicinity of a hose failure can
be devastating. For a better appreciation of what I mean,
read this article.
The week following the Enzed event I had a meeting with a lecturer at a local vocational college.
He was in the process of developing a hydraulics training module for a group of apprentices and
had chosen Industrial Hydraulic Control as the class text.
During our conversation it was revealed that the basic hydraulics program he was preparing involved 36 hours of contact time. As you know, I'm a big advocate of continuous learning and the pursuit of knowledge, but I found myself thinking this is less than half the amount of training an Enzed Hose Doctor gets in one specialized area.
I guess my point is, without any further education in hydraulics, when these apprentices become tradesman, there's nothing preventing them from working on any type of hydraulic equipment. But should it be this way?
In most developed countries the Electrician is a licensed trade. This means if you're a 'sparky' you must be appropriately trained and licensed for the type of electrical work you're doing. And for those of us who aren't - it's illegal to fiddle.
Why is this so? Simple really. If you don't know what you're doing your chances of getting fried (or frying someone else) increase significantly. There's similar parallels in hydraulics of course and those of us who've been around a while have seen the hydraulic equivalent of electrocution occur all too often: trying to remove a bladder from an accumulator with the bladder still gassed; using compressed air to remove the piston rod from a cylinder, and so on.
Should hydraulics be regulated in a similar way to electrics? I think it's inevitable. But I'd like to hear your views on this issue. And if you have any good "hydraulic electrocution" (or near miss) stories I'd like to hear about those as well.
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An important fact about spool valves|
In response to my article on troubleshooting cylinder drift in
a couple of our members thought the influence of spool configuration
on cylinder drift warranted discussion.
Charlie Field from Perry Slingsby Systems in the UK sent me this message:
"By far the most common reason for cylinder drift is the DCV controlling it. Closed to actuator spools almost always leak pressure to both service ports. If you effectively plug the actuator lines with gauges you will see something like 30% to 50% of the "P" line pressure in the actuator lines."
This CAN be a problem with closed-center cylinder spools
(all ports blocked in the center position) when the pump is not unloaded.
Bud Trinkel posed the question this way:
"How about a horizontally mounted cylinder powered by a pressure compensated pump at 3,000 PSI and using an all ports blocked center condition valve? Will that cause the cylinder to drift?"
As Bud knows and Charlie has pointed out, you can almost bet it will.
As you've probably gathered this has nothing to with the integrity of the
piston seal - which we were talking about in last month's article.
It's due to the radial clearance of the spool.
Because radial clearance is required for the spool to slide in its bore,
this valve design in not leakless. To say this another way, even when a port
in a spool valve is closed off - a small amount of leakage should be expected.
Whereas the other main valve design used in hydraulics - the poppet type,
where the valve 'poppet' closes against a seat IS generally considered leakless.
That is, if the valve is closed and the poppet and its seat are in good condition - there
is no leakage across the valve's ports.
BUT there's an important exception to this rule. Slip-in cartridge valves, also called
logic elements are a type of poppet valve commonly found in today's hydraulic systems.
Even though a logic element can be configured for flow in two directions, it is only leakless
in one direction. To understand why,
read this article
Getting back to cylinder drift caused by the radial clearance of the spool,
if the pump can't be unloaded then a float center spool (A and B open to T) with
load holding check or counterbalance valves is the typical solution.
This is an application issue and hopefully one which the technician in the
field won't have to solve!
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Plant Maintenance Resource Center
Hydraulic troubleshooting - from your lap top|
'Fundamentals of Hydraulics and Troubleshooting' explains when and how to use diagnostic
tests, including the direct pump test, system T test, spool valve leakage test and
cylinder piston seal leakage test - all from the convenience of your lap top or desk top computer.
Find out more
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About the Author: Brendan Casey has more than 17 years
experience in the maintenance, repair and overhaul of
mobile and industrial hydraulic equipment. For more
information on reducing the operating cost and increasing
the uptime of your hydraulic equipment, visit his
web site: http://www.InsiderSecretsToHydraulics.com
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