Survey results - What's your biggest hydraulics problem?|
In the April Issue of Inside Hydraulics we asked you to tell us about your
biggest hydraulics problem. Thank you to all our readers who participated in the survey.
Many of the problems raised have been covered in previous issues of our Newsletter.
Not suprisingly, external leaks featured strongly in the survey results.
While the nuisance-value of leaks is well understood, their real cost is often overlooked.
Those leaks you have been meaning to fix could be costing you a small fortune,
this article explains why.
explains how to achieve leak-free port, tube-end and hose-end connections, and
presents a method for eliminating drips from 37-degree flared connections.
Overheating was another problem high on the list.
offers some guidance on understanding and solving heat issues.
Noise and vibration also featured prominently and is addressed in
as did water contamination of hydraulic fluid, covered in
and air contamination, covered in this article.
Proper flushing after major failures was also raised, its covered in this article.
Hose failures were a common concern, dealt with in this article,
as was effective troubleshooting. This article
covers the basics and this article offers some guidance on avoiding troubleshooting mistakes.
Internal leakage (wear) was another major concern. While all hydraulic components will
wear out eventually, an effective maintenance program is the key to maximizing component life
and minimizing operating costs.
This article examines the issue of proper equipment maintenance.
You can look forward to 'Inside Hydraulics' articles dealing with many more of the problems
submitted by respondents to our survey. In the meantime, a complete list of articles from past Newsletters is available at:
REMINDER: Hydraulics Workshop at the University of Western Australia, this Saturday June 10.
Brendan Casey will show you, step-by-step, how to keep hydraulic equipment running and
minimize its operating cost. Owners, mechanics, technicians and maintenance
professionals who attend this half-day workshop will be able to make a measurable contribution towards extending component life,
reducing downtime and cutting the operating cost of hydraulic equipment.
Be quick, time is running out. To enrol, visit the UWA web site, call
(+61) 08 6488 2433 or download this form.
Get a lifetime of hydraulics knowledge...
Get 'Industrial Hydraulic Control'.
Find out more
Putting the brake on water hammer|
One of our readers wrote to me recently about the following problem:
We have a hydraulic power unit that runs a calendar roll on a paper machine. The calendar
roll has multiple zones inside it to vary the pressure on the paper to maintain uniform
thickness. Moog servo-valves control the adjustments to the different zones. The problem we
have is high vibration in the pumps, lines and tank. Lines have broken and we had to
disconnect one bank of filters from the tank to keep it from breaking off. This has been
going on for some time. Do you have any ideas?"
As always, there are a number of possibilities and issues to consider.
Some background reading on vibration and noise in hydraulic systems
is available in this article. One explanation that jumps to the top of the list in this application is water hammer.
Coining a phrase
Water hammer is the term used to describe the effect that occurs when the velocity of the fluid moving through a pipe suddenly changes. Sudden change in fluid velocity causes a pressure wave to propagate within the pipe. Under certain conditions, this pressure wave can create a banging noise, similar to that you would expect to hear when beating a pipe with a hammer. Hence the phrase. Not surprisingly, common symptoms of this problem are high noise levels, vibration and broken pipes.
Hitting the wall
When a moving column of fluid hits a solid boundary - when a directional control valve closes suddenly for example, its velocity drops to zero and the fluid column deforms, within the rigid cross-sectional area of the pipe, to absorb the (kinetic) energy associated with its motion - similar to a car hitting a concrete wall. However unlike a car, the fluid is almost incompressible so the deformation is small and a store of energy accumulates in the fluid - similar to compression of a spring. The magnitude of the pressure rise that results from the subsequent release of this stored energy can be expressed mathematically as follows:
Pr = P + u p c
where P is initial pressure, u and p are initial fluid velocity and density respectively and c is the speed of sound through the fluid.
In our reader's application, uniform paper thickness is dependent on the constant adjustment of the calendar roll zones by the servo-valves. Under certain conditions, rapid switching of these valves could result in something that resembles peening a pipeline with a thousand hammers.
The significance of the pressure rise equation shown above is that fluid velocity is the only variable that can be altered by the troubleshooter when addressing this problem. Put simply, reducing the velocity of the fluid column that hits the solid boundary, reduces the magnitude of the subsequent pressure rise. Returning to the traffic crash analogy - the slower the car is travelling when it hits the wall, the less damage is caused. In hydraulics, the easiest way to do this - on paper at least, is to increase the diameter of the pipe. This reduces fluid velocity for a given flow rate. The other alternative is to control deceleration of the fluid column by choking valve switching time to the point where the pump's pressure compensator and/or system relief valve reacts fast enough to reduce flow rate through the pipe and therefore velocity of the fluid.
"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 is 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.
The benefits of load sensing|
To understand how load-sensing control makes hydraulic equipment more efficient,
read Brendan Casey's article in the March-April 2006 Issue of Machinery Lubrication magazine,
To receive a complimentary subscription to this informative magazine (US and Canada only) go to:
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About the Author: Brendan Casey has more than 16 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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