May 10, 2005 

'Inside Hydraulics' Newsletter

1. Silt-size filtration at the pump inlet
2. Pressure intensification in hydraulic cylinders
3. Advanced hydraulics troubleshooting
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Silt-size filtration at the pump inlet

From a filtration perspective, the pump intake is an ideal location for filtering media. Filtering efficiency is increased by the absence of high fluid velocity and pressure-drop. These advantages are outweighed by the restriction barrier filters create in the intake line.

The restriction caused by a conventional suction filter increases the chances of a vacuum developing at the pump inlet. This results in the formation of gas and/or vapor bubbles within the fluid. When these bubbles collapse in proximity to a metal surface, erosion occurs. Cavitation erosion damages critical surfaces and contaminates the hydraulic fluid.

The mechanical forces induced by the vacuum itself can cause catastrophic failure. Vacuum in the pumping chambers of an axial pump puts the piston-ball and slipper-pad socket in tension. This joint is not designed to withstand excessive tensile force and as a consequence, the slipper becomes detached from the piston. This can occur either instantaneously or over many hours of service as the joint is put in tension repetitively during inlet.

In vane pump designs, the vanes must extend from their retracted position in the rotor during inlet. As this happens, fluid from the pump inlet fills the void in the rotor created by the extending vane. If excessive vacuum exists, the vanes lose contact with the cam ring during inlet and are then hammered back onto the cam ring as pressurized fluid acts on the base of the vane during outlet. The impact damages the vane tips and cam ring, leading rapidly to catastrophic failure.

Gear pumps are mechanically the least susceptible to vacuum-induced forces. Despite this fact, research has shown that vacuum at the inlet can reduce the service life of an external gear pump by at least 50 percent.

For the reasons described above, conventional barrier filtration at the pump inlet - even at the relatively large blocking size of 125 microns, is not recommended by most pump manufacturers. And the capture of silt-size particles before the pump has not been possible - until now!

The Magnom Hydraulic Pump Mate incorporates new, patented filter technology that allows filtration of ferrous particles down to less than one micron - with no pressure drop and therefore no risk of vacuum-induced pump damage.

Magnom Hydraulic Pump Mate

The Magnom Hydraulic Pump Mate - designed
to replace conventional suction strainers.

Designed to replace conventional suction strainers, the Magnom Hydraulic Pump Mate has been tested and endorsed by a major, mobile hydraulic equipment manufacturer. In one field trial, the Hydraulic Pump Mate was removed for inspection after 1,250 hours. Over four grams of contamination was harvested from the core of the Magnom unit (see below).

Magnom in action

Particles captured by the Magnom. Note the heavy loading of the
top (upstream) core, which illustrates the efficiency of the unit.

Subsequent analysis of the debris revealed that 85% of the particles captured were smaller than 20 microns - debris that would have passed straight through a conventional suction strainer, causing pump wear.

The inconvenience of servicing a filter located inside the reservoir is a common reason why conventional suction strainers go unserviced - until after the pump fails. The Magnom Hydraulic Pump Mate shown is designed to hold 200 grams of wear metal - with no pressure drop across the filter. This means that all the components in the system would have to be replaced several times before the unit needs to be cleaned, making the Magnom unit virtually service free.

For more information, visit or in the U.S. contact Keith Day on 312-738-1147 or email

2.   Pressure intensification in hydraulic cylinders

A question that I'm asked regularly is "What is the best way to test the integrity of the piston seal in a double-acting hydraulic cylinder?"

There is a simple bench-test for doing this but it involves the intensification of pressure in the cylinder. While the test procedure is safe if you understand the concept of intensification in a hydraulic cylinder - it is inherently dangerous if you don't.

In this article I will explain the dangers of intensification in a double acting cylinder and in next month's newsletter I will explain the test procedure.

Cylinder force

Force produced by a hydraulic cylinder is a product of pressure and area (F = p x A). In a conventional double-acting cylinder the effective area and therefore force produced by the piston and rod sides of the cylinder are unequal. It follows that if the rod side of the cylinder has half the effective area of the piston side, it will produce half the force of the piston side for the same amount of pressure.

Pressure intensification

The equation F = p x A can be transposed as p = F/A that is, pressure equals force divided by area. If the rod side of the cylinder has to resist the force developed by the piston side, with only half the area, then it needs double the pressure. This means that if the piston side is pressurized to 3,000 PSI a pressure of 6,000 PSI will be required on the rod side to produce an equal force. This is why pressure intensification can occur in a double-acting cylinder. Note that pressurizing a cylinder rated at 3,000 PSI, to 6,000 PSI can have devastating consequences.

If, for any reason, the piston side of a double-acting cylinder is pressurized and at the same time fluid is prevented from escaping from the rod side, pressure will increase (intensify) in the rod side of the cylinder until the forces become balanced or the cylinder fails catastrophically. Consider the following scenario one of our newsletter readers described to me recently:

"It was minus 36 degrees here the other day and we had a cylinder at about minus 10 degrees. The boss was attempting to press out a pin. He turned on the pump and moved the lever, and the gland end of the cylinder blew out. It was a 7.5" cylinder with a 2,500 PSI operating pressure."

The gland on this cylinder blew out as a result of pressure intensification due to a blockage between the rod side of the cylinder and tank. Possibly due the cold conditions, that is the ambient temperature had fallen below the pour point of the hydraulic fluid.

Safety is paramount

As you can see, pressure intensification in a double-acting cylinder is a dangerous phenomenon and the concept must be thoroughly understood when testing hydraulic cylinders.

"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.

3.   Advanced hydraulics troubleshooting

'Advanced Hydraulics Troubleshooting' covers four case studies representing the most common types of hydraulic service calls technicians encounter on hydraulically powered equipment. The viewer is challenged to develop their own diagnosis before the cause of each problem is explained. Common mistakes technicians make when diagnosing hydraulic problems are illustrated along with procedures for avoiding these costly errors. Find out more

4. Content for your web site or e-zine

Need some fresh content for your web site or e-zine? You now have permission to reprint these 'Inside Hydraulics' articles on your web site or in your e-zine, provided:

1. Each article is printed in its full form with no changes.

2. You send an e-mail to to advise us where you'll be publishing them.

3. You include the following acknowledgement at the end of each article:
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:

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