January 21, 2004 

'Inside Hydraulics' Newsletter

1. Hydraulic cylinder rods turn black
2. Anatomy of a hydraulic pump failure
3. Hydraulic equipment slowdown - nailing internal leakage
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Hydraulic cylinder rods turn black

Black nitride is a relatively recent alternative to the hard chrome plated cylinder rod. With reports of achieved service life three times that of conventional chrome, longer seal life and comparable cost, black nitride cylinder rods are an option that all hydraulic equipment users should be aware of.

Available in standard diameters up to 5" (127 mm), black nitride cylinder rods offer the following benefits over conventional hard chrome:

  • Superior corrosion and wear resistance
  • Better oil retention (longer seal life)
  • Dimensional uniformity
  • Dent resistant - without the need for induction hardening
  • No pitting, flaking or micro-cracking
  • Environmentally friendly process

For a detailed technical description, go to http://www.hydraulicsupermarket.com/black-nitride.html

2.   Anatomy of a hydraulic pump failure

I was asked recently to give a second opinion on the cause of failure of an axial piston pump. The pump had failed after a short period in service and my client had pursued a warranty claim with the manufacturer. The manufacturer rejected the warranty claim on the basis that the failure had been caused by contamination of the hydraulic fluid. The foundation for this assessment was scoring damage to the pump's valve plate.

How does contamination cause this type of damage?

When hydraulic fluid is contaminated with hard particles that are the same size as the clearance between two lubricated surfaces, a process known as three-body abrasion occurs (Figure 1). Three-body abrasion results in scoring and heavy wear of sliding surfaces.

What other explanations are there for this type of damage?

In axial piston designs, the cylinder barrel is hydrostatically loaded against the valve plate. The higher the operating pressure, the higher the hydrostatic force holding the cylinder barrel and valve plate in contact. However, if operating pressure exceeds design limits or if the valve plate is not in proper contact with the cylinder barrel, the cylinder barrel separates from the valve plate. Once separation occurs, the lubricating film is lost, the two surfaces come into contact and a process known as two-body abrasion occurs.

A major clue that the damage to the valve plate (Figure 2) was not caused by contamination in this case, is the pattern of wear. Notice that the scoring (bright areas) is confined to the inner and outer edges of the sliding surface of the valve plate. If the scoring had been caused by three-body abrasion, the damage would be more evenly distributed across the entire surface, with the areas between the pressure kidneys, likely to exhibit the heaviest damage.

The pattern of wear on the valve plate is consistent with two-body abrasion resulting from uneven contact between the valve plate and cylinder barrel, caused by distortion of the valve plate and/or separation. Examination of the sliding surface of the cylinder barrel supports this assessment. The scoring of the cylinder barrel (Figure 3) is heaviest top right of the picture and lightest bottom left. Examination of the head of the pump also revealed uneven contact between the valve plate and head.

Root cause of failure

Although the valve plate was flat, its locating dowel was holding it off the head on one side. This in turn was causing the valve plate to be tilted against the cylinder barrel, resulting in uneven loading, separation and two-body abrasion of the two surfaces. The root cause of this hydraulic pump failure was not contamination; but rather improper assembly at the factory.

Editors note - For more case studies like this one, join Brendan Casey for a practical, half-day workshop on troubleshooting hydraulic failures, to be held in Nashville TN, March 21, 2004 and hosted by Lubrication Excellence 2004. For more information, visit the LE2004 web site or call 800-597-5460 or 918-749-1400

"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.   Hydraulic equipment slowdown - nailing internal leakage

For some practical advice on locating internal leakage in hydraulic systems, read Brendan Casey's article in the November-December Issue of Machinery Lubrication magazine, available here. To receive a complimentary subscription to this informative magazine (US and Canada only) go to: http://www.machinerylubrication.com/hydraulic1.asp

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 editor@hydraulicsupermarket.com to let us know where you'll be publishing them.

3. You include the following credit 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: http://www.InsiderSecretsToHydraulics.com

5. Help us spread the word

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6. Tell us what you think

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