August 17, 2004 

'Inside Hydraulics' Newsletter

1. The next generation of hydraulic test stands
2. Which oil would I use?
3. More hydraulic maintenance tips
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


The next generation of hydraulic test stands

As every reputable repair shop owner knows, thoroughly testing a rebuilt hydraulic component prior to dispatch ensures that the component will work the way it should and will perform within its design parameters.

This is particularly important with pumps. It is a common misconception that all that is required to return these components to 'as new' condition is to fit a new rotating group. Not only must a new rotating group be fitted, it must be correctly toleranced during assembly so that volumetric efficiency equals or exceeds that of a new unit. The only way to confirm this is to dynamically test the unit under load, on a test stand designed for this purpose.

Many repair shops build their own test stands, which are almost always a work in progress. Valuable time is often spent modifying and adapting the test bench instead of focusing on the core business of repairing pumps.

The Next Generation of Test Stands designed and developed by Flow Technology Systems, offer the following benefits:

  • Improved turn-around time.
  • Reduced warranty claims.
  • Reduced test time.
  • Automated test report - proof of performance.
  • Increased customer satisfaction.
  • Historical data on tested pumps.
  • Comparison of performance with OEM specifications.
  • Measured variables in metric or imperial units.
  • Presentation of data in tabular or graphical format.

Flaretite seal in cross-section.

Flow Technology Systems' product range includes pump/motor and cylinder test stands, and portable test equipment, including a pocket PC that can be connected to eight sensors. They also offer upgrades to existing test benches with their purpose-built electronics and software packages. For more information, visit or send email to

2.   Which oil would I use?

The May 2004 issue of Inside Hydraulics carried an article that examined the use of multigrade, detergent engine oil as a hydraulic fluid in mobile equipment. If you missed this article, it's available here.

Following the publication of this article, I received a lot of feedback. Some folk thought my recommendations were not definitive enough. But when it comes to hydraulic fluid, it is not possible to make one, definitive recommendation that covers all types of hydraulic equipment in all applications.

In an effort to clarify this matter, I decided to pose (and answer) the following question:

What type of oil would I use in my excavator hydraulics and why?

Assuming that my hydraulic excavator is a late model machine fitted with variable displacement piston pump(s) which operate at pressures up to 5,000 PSI or higher, my selection would be based on the following factors.

Multigrade versus monograde

The operating temperature range of the excavator is the factor that will determine whether I use a multigrade or monograde oil. If my excavator is required to operate in freezing temperatures in winter and tropical conditions in summer, then multigrade oil will be required to maintain oil viscosity within optimum limits across this wide operating temperature range.

If my excavator has a narrow operating temperature range and it is possible to maintain optimum fluid viscosity using monograde oil, I wouldn't chose a multigrade. This is because the viscosity index (VI) improvers used to make multigrade oils can have a negative effect on the air separation properties of the oil. This is not ideal in a mobile hydraulic machine like an excavator, which typically has a relatively small reservoir with correspondingly poor de-aeration characteristics. In addition, by not using a multigrade oil when it's not required, I don't need to concern myself with loss of viscosity as a result of VI improver sheardown.

Detergent versus non-detergent

Detergent oils have the ability to emulsify water, and disperse and suspend other contaminants such as varnish and sludge. This keeps components free from deposits but means that contaminants are not precipitated out - they must be filtered out. These are desirable properties in a mobile hydraulic machine like an excavator which, unlike an industrial hydraulic system, has little opportunity for the settling and precipitation of contaminants at the reservoir, due to its relatively small volume.

I'll be using an oil with detersive/dispersive additives in my excavator and closely monitoring water content on the fluid condition reports. Water accelerates aging of the oil, reduces lubricity and filterability, reduces seal life and leads to corrosion and cavitation. Emulsified water can be turned into steam at highly loaded parts of the system. I'll be avoiding these problems by maintaining water content below 100 ppm.

Anti-wear versus no anti-wear

The purpose of anti-wear additives is to maintain lubrication under boundary conditions. The most common anti-wear additive used in engine and hydraulic oil is Zinc dialkyl dithiophosphate (ZnDTP).

The presence of ZnDTP is not always seen as a positive, due to the fact that it can chemically break down and attack some metals. However, it is an essential additive in oil used in any high-pressure, high-performance hydraulic system, such as a modern excavator fitted with piston pumps and motors. I'll be selecting an oil containing ZnDTP at a concentration of at least 900 ppm for use in my excavator hydraulics.

Fill 'er up

In summary, if optimum fluid viscosity can be maintained using monograde oil, then I would select and use one that contains detersive/dispersive additives and at least 900 ppm of ZnDTP. If multigrade oil were required, I would use either multigrade hydraulic fluid (with detergent additives) or heavy-duty diesel engine oil (API classification CD, CF-4, CG-4 or CH-4) with a minimum ZnDTP content of 900 ppm.

"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

3.   More hydraulic maintenance tips

For more hydraulic maintenance articles like the ones above, read Brendan Casey's regular column in Machinery Lubrication. This magazine contains practical information on a wide range of maintenance issues including lubricant selection and application, contamination control, filtration and condition-monitoring of hydraulic and other rotating machinery. To receive a complimentary subscription to this informative magazine (US and Canada only) go to:

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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