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
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Plant Maintenance Resource Center
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