Industrial hydraulic control|
An excellent book that effectively tackles the
complex subject of hydraulics, by explaining the underlying
operating principles and function of components and systems - for all levels
of understanding, is now available in our online store.
It's called Industrial Hydraulic Control by Peter Rohner.
This is not just another book covering basic hydraulic theory and concepts.
Peter Rohner is a University Professor with over
30 years of teaching experience. He is one of the most
successful fluid power lecturers and authors in the world today.
And even though the book's title is Industrial Hydraulic
Control, the information it contains is just as relevant to
mobile hydraulics. These days, topics such as slip-in cartridge
(logic) valves, variable pump controls, proportional and servo
control, and PLC programming are just as relevant to mobile
hydraulics as they are to industrial hydraulics.
Industrial Hydraulic Control is the most comprehensive guide to
understanding the complexities of hydraulic circuits and
components you could ever hope to get your hands on. For
details of what's included in each chapter, and the powerful
knowledge you'll gain, go here now: http://www.IndustrialHydraulicControl.com
Testing hydraulic cylinders|
In last month's Newsletter, I described the danger associated with the intensification of pressure in double-acting hydraulic cylinders. In this article, I will explain how to use the intensification effect to test the integrity of the piston seal in a double-acting cylinder. Before attempting this test procedure, it is absolutely essential that the danger associated with pressure intensification in a cylinder is fully understood. Therefore, read this article first!
The conventional way of testing the integrity of the piston seal in a double-acting cylinder is to pressurize the cylinder at the end of stroke and measure any leakage past the seal. This is commonly referred to as "end-of-stroke bypass test" (demonstrated here).
The major limitation of the end-of-stroke bypass test, is that it generally doesn't reveal ballooning of the cylinder tube caused by hoop stress as a result of under designed cylinder wall thickness or reduction of wall thickness through excessive honing. The ideal way to test for ballooning of the cylinder tube is to conduct a piston-seal bypass test mid-stroke. The major difficulty with doing this is that the force developed by the cylinder has to be mechanically resisted, which in the case of large diameter, high-pressure cylinders is impractical.
However a mid-stroke bypass test can be conducted hydrostatically using the intensification effect. The necessary circuit is shown in Figure 1 below.
Figure 1. Hydraulic cylinder test circuit.
The procedure for conducting the test is as follows:
- Secure the cylinder with its service ports up.
- Fill both sides of the cylinder with clean hydraulic fluid through its service ports.
- Connect ball valves (1) and (2), gauges (3) and (4), relief valve (5) and directional control valve (6) as shown in Figure 1.
- With ball valves (1) and (2) open, stroke the cylinder using the directional control valve (6) multiple times to remove all remaining air from both sides of the cylinder - take care not to 'diesel' the cylinder.
- Position the piston rod mid-stroke and close ball valve (2).
- With the adjustment on the relief valve (5) backed out, direct flow to the rod side of the cylinder.
- Increase the setting of relief valve (5) until the cylinder's rated pressure is seen on gauge (3).
- Close ball valve (1) and center directional control valve (6). Note: it is assumed that the hydraulic power unit used to conduct the test has its own over-pressure protection - not shown in Figure 1.
- Record the respective pressure readings on gauges (3) and (4) and monitor any change over time.
If the ratio of effective area between the piston and rod side of the cylinder is 2:1, then if the rod side of the cylinder has been pressurized to 3,000 PSI, gauge (2) on the piston side should read 1,500 PSI. If the differential pressure across the piston is not maintained, this indicates a problem with the piston seal or tube.
Safety is paramount
Under no circumstances should flow be directed to the piston side of the cylinder with ball valve (1) closed. Failure of the cylinder and personal injury could result. When conducting this or any other hydrostatic (pressure) test, always wear appropriate personal-protection equipment.
"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 true value of hydraulic circuit diagrams
For an insight to the real value of hydraulic circuit diagrams,
read Brendan Casey's article in the March-April 2005 Issue of Machinery Lubrication magazine,
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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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