This month marks the fifth anniversary of our Inside Hydraulics newsletter.
From humble beginnings back in February 2002, membership has grown steadily to over
Due to the significant growth in membership over this time, it's fair to say that
only a small number of our current members ever received the early issues. For those of
you who missed out, you can now get copies of these early issues - in fact you can get all
five years worth, 60 in total. But more on that in a moment. First, here's the main article
from our very first issue:
Understanding the diesel effect
I was recently engaged to conduct failure
analysis on a large (and expensive) double-acting cylinder
off a hydraulic excavator. This cylinder had been changed-out
due to leaking rod seals after achieving only half of its
expected service life.
Inspection revealed that apart from the rod seals, which had
failed as a result of the 'diesel effect', the other parts
of the cylinder were in serviceable condition.
What is the 'diesel effect'?
The diesel effect occurs in a hydraulic cylinder when air is
drawn past the rod seals, mixes with the hydraulic fluid and
explodes when pressurized.
How does this happen?
When a double acting cylinder retracts under the weight of its
load, the volume of fluid being demanded by the rod side of the
cylinder can exceed the volume of fluid being supplied by the
When this happens, a negative pressure develops in the rod
side of the cylinder, which usually results in air being drawn
into the cylinder past the rod seals. This occurs because most
rod seals are designed keep high-pressure fluid in the cylinder
and are not designed to keep air out. The result of this is
aeration - the mixing of air with the hydraulic fluid.
Aeration causes damage through loss of lubrication and
overheating, and when a mixture of air and oil is compressed in
a cylinder it can explode, damaging the cylinder and burning its
seals. As you have probably gathered, the term 'diesel effect'
is a reference to the combustion process in a diesel engine.
In the example described above, the cause of the aeration was a
faulty 'float' valve. The function of a float valve on a
hydraulic excavator is to allow the boom or arm to be lowered
rapidly under its own weight.
When activated, this valve connects the ports of the cylinder
together allowing the cylinder to retract under the weight of
the boom or arm. The fluid displaced from the piston side of
the cylinder is directed with priority to the rod side of the
cylinder, before any excess volume is returned to the reservoir.
An orifice controls the speed with which the cylinder retracts.
If this valve malfunctions or is set incorrectly, a negative
pressure can develop on the rod side of the cylinder, causing air
to be drawn past the rod seals, leading to failure of the
How can this type of failure be prevented?
This example highlights the importance of checking the operation
and adjustment of circuit protection devices at regular intervals.
As in this case, if the faulty float valve had been identified
early enough, the failure of this cylinder and the significant
expense of its repair could have been prevented.
Now you can get your own personal copy of this and all 59 other
Inside Hydraulics articles, all nicely formatted and indexed in a single 120-page
Adobe Acrobat document. It's called 'How to Solve and Prevent Hydraulic Problems'.
You can save this ebook to your desktop and/or laptop computer, search it by
keyword and print all 120 pages - or individual articles to pin up on your notice board or
hand out to your guys.
You get all this knowledge at your fingertips for a measly 50 cents per article. That's a
paltry six bucks a year for each of the five years worth information. And at under 30 bucks,
it's less than a pale of hydraulic oil. Plus, just one timely tip from this ebook could easily
return you 10 times you modest investment.
So don't wait, get your copy today:
"As a mechanic with more than 30 years experience, I think Industrial Hydraulic Control is
excellent. I use it as my hydraulics reference."
Find out more ...
Equipment Maintenance Supervisor
Oilfield Service Company
How well do YOU know YOUR hydraulic equipment?|
My wife often asks me why I still take on consulting work. She wonders why I happily leave the comfort of my office to crawl all over hot, dirty, smelly hydraulic equipment.
Well for one, I actually enjoy it. Two, it keeps my sharp. But perhaps most importantly, it keeps me in touch with the issues that hydraulic equipment users grapple with.
One thing I have learnt over the years is that in the initial stages of a consulting assignment,
it's better to ask intelligent questions rather than dispense good advice.
A recent client had a set of pumps worth fifty grand fail after achieving only half of their expected service life. And they wanted some answers. At the initial meeting the client opened proceedings with a brief history on the machine, an account of the events leading up to the failures and then pushed a stack of oil analysis reports across the table.
After I finished taking notes on what I'd just been told, I fired off my first question:
"What is the system's normal operating temperature?"
Stunned silence. Client shrugs his shoulders.
"O-K … what's the system's usual operating pressure range?"
Blank look from client. "Err dunno … we don't monitor either of those things."
At the end of the meeting we took a walk through to the control room. Turns out, both operating pressure and temperature were displayed on the default PLC screen - albeit along with a lot of obviously more important production information. Say no more.
But could YOU answer these two simple questions about the "vital statistics" of your hydraulic equipment? If not, I strongly recommend you make the effort to get to know your equipment better.
This information is easy to collect, can give valuable insight to the health of your equipment and is essential data if failure analysis is required. Here's how I recommend you do it:
First you need an infrared thermometer, also called a heat gun. If you don't have, you'll need to invest around 100 bucks to get one. Next, using a permanent marker, draw a small target on the hydraulic tank below minimum oil level and away from the cooler return. Label it "1". This marks the spot where you'll take your tank temperature readings. The idea behind these targets is that regardless of who takes the temperature readings they'll be taken from the same place each time.
If the system is a closed circuit hydrostatic transmission, mark a convenient location on each side of the transmission loop and number them "2" and "3". Skip this step for open circuit systems.
Next, mark a target on the cooler inlet and outlet and number them "4" and "5".
This records the temperature drop across the cooler. The benefit of doing this is
explained in this article.
With that done, now draw up a table like the one below to record these temperatures and a few other essential parameters. Note that there is little point in recording the temperature across the cooler if the fan isn't running. And charge pressure is only relevant to closed circuit hydrostatic transmissions.
|(1) Tank Temperature||30||68|
|(2) Transmission A||41||60|
|(3) Transmission B||44||65|
|(4) Cooler In|| ||64|
|(5) Cooler Out|| ||53|
|Fan On Y/N||N||Y|
In terms of compiling the data, it's a good idea to take readings on the hottest and coldest days of the year and on a couple of average temperature days in between. This provides a baseline of data. Beyond that, taking readings at regular intervals - daily, weekly or monthly, can provide early warning of system problems. And if the system starts to give trouble, taking a set of readings will reveal if it's operating outside of its normal parameters.
"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
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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