September 14, 2005 

'Inside Hydraulics' Newsletter

1. Accurate flow measurement using 'clamp-on' sensors
2. Hydrostatic balance 101
3. Troubleshooting hydraulics - from your lap top
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Accurate flow measurement using 'clamp-on' sensors

The accurate measurement of flow using clamp-on flowmeters can be a problem if the sensor does not take into account that the oil's flow condition can suddenly change from laminar to turbulent. Similarly, if the electronics are not fast enough to detect minute changes in flow, accuracy is further affected.

Consider this example: A one inch diameter carbon steel pipe conducting ISO VG 100 oil with a density of 0.825 g/cm3 at 100 degrees C. At a velocity of 1 m/s, this oil has a Reynolds number of 1,993 and flow is considered laminar. The flowmeter must correct for volume if this is so. If velocity increases to 2 m/s, the Reynolds number has increased to 4348, flow is now in a turbulent condition and a different volume correction must be applied. EESIFLO flowmeters predict flow condition transitions and report accurate volumes based on oil and pipe parameters entered by the user.

EESIFLO's clamp-on flowmeters

EESIFLO clamp on flowmeters
are available for purchase or hire.

EESIFLO's portable 6000 and stationary 7000 series flowmeters not only measure and display flow - but also Reynolds number. One application where knowing the fluid's Reynolds number is essential is hot oil flushing, where turbulent flow conditions are required for effective results. An option to measure in a fast transient mode where a static condition suddenly goes into a fast flow condition for a very short period of time, is also available.

EESIFLO is a manufacturer of high precision, easy to use ultrasonic flowmeters and water in oil sensors. To contact your nearest EESIFLO representative, or for more information, log on to

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2.   Hydrostatic balance 101

Hydraulic components are unique in that it is often possible to offset or balance hydrostatic forces to reduce loads on lubricated surfaces. By reducing surface loading, the maintenance of full-film lubrication is improved and therefore boundary lubrication conditions are less likely to occur.

Hydrostatic force is the product of pressure and area. Expressed mathematically: F = P x a. The balancing or offsetting of hydrostatic force is achieved by exposing opposing areas to the same pressure. The double-acting cylinder in Figure 1 illustrates this concept.

hydrostatic balance

Figure 1. Hydrostatically balanced cylinder loading two lubricated surfaces.

The rod-side area of the piston, area B, is 80% of area A. This means that the force exerted on the lubricated surfaces at the end of the cylinder rod is 20% of the force developed by the pressure acting on area A. This is due to the balancing or offsetting force developed by the same pressure acting on area B. Assuming the speed of the rotating surface (C) and fluid viscosity are adequate, full-film lubrication of the sliding surfaces is achieved.

hydrostatic balance

Figure 2. Typical cross-section of an axial design piston.

The same principle applied to a typical axial design piston is illustrated in Figure 2. Area A is exposed to system pressure during outlet (pump) or inlet (motor) and the force developed is transmitted to the lubricated surfaces of the slipper and swash plate. System pressure also acts on area B, the balancing area of the slipper, via the drilling through the center of the piston. Area C is the sliding (lubricated) area of the slipper. While the ratio of these three areas varies, in this particular piston, area B is 50% of area A and area C is 140% of area A. This means that the force transmitted to area C is half that developed by area A and is spread over 1.4 times the area, further reducing the load on the lubricated surfaces.

hydrostatic balance

Figure 3. Loss of hydrostatic balance increases load on the lubricated surfaces.

If the hydrostatic balancing force is lost, that is there is no pressuring acting on area B (Figure 3), the force exerted on the lubricated surfaces at the end of the cylinder rod will be 100% of the force developed by the pressure acting on area A. If full-film lubrication was dependent on the hydrostatic balance of the cylinder, boundary lubrication conditions will eventuate and two body abrasion is likely.

Editors note: To learn more about the construction of hydraulic components, their modes of failure and how to prevent them, read 'Preventing Hydraulic Failures'

"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.   Troubleshooting hydraulics - 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 your lap top or desk top computer! 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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