» Sampling Cleanliness
The method and cleanliness by which an
oil sample is taken has a critical effect on the accuracy of the laboratory
results. If the sample is falsely contaminated by taking it from the wrong
point or in the wrong way, or if the sample-taking equipment or method
introduces contaminants, then false contamination levels will be reported.
Agood sample is one that is cleanly taken from the circulating oil flow.
The proper sample-taking method and procedure should be agreed with the
laboratory and if necessary the laboratory should be asked to provide
training for the sample takers.
How clean should oil be?
Many original equipment manufacturers
have accepted the indisputable evidence from numerous field and laboratory
trials that oil cleanliness has a major effect on wear within their
equipment. Some of them are now specifying how clean the oil used in their
equipment must be if warranty claims are to be honored.
For example, Caterpillar Inc. specifies new oil to have a particle count of
ISO 16/13. If new oil is above this level of contamination it will not
warranty the equipment. When new oil from a leading international oil
manufacturer was tested before putting it into new Caterpillar equipment,
the solid particle contamination was found to be 17/14. This was new oil
from a never previously opened container. In this case the new oil had to be
further filtered to bring it to below the required specification
Table 2 is a list of the recommended target oil contamination levels for
close tolerance equipment from a survey of hydraulic oil equipment and oil
|RECOMMENDED TARGET OIL
||< 3000 psi
||> 3000 psi
|Fixed displacement pumps
|Variable displacement pumps
For extremely low wear rates and long
equipment life, the evidence indicates that oil needs to be filtered down
below 5 micron size and preferably down to 1 micron size. Care needs to be
taken that the filter does not remove any solid additives, such as graphite,
in the oil. Additives dissolved in the oil will not be removed unless the
additive is attached to a solid particle.
Oil filtration can be done under full oil flow or with bypass flow or
offline. There are several filter types such as pleated paper and wrapped
fibre cord. In all cases the filter must capture a large proportion of
greater than 5 micron particles if it is to clean the oil.
The correct way to measure filter
performance is by use of the Beta Rating. which compares the number of
particles entering a filter to the number leaving. It is an accurate way to
measure true in-service performance. Nominal filter micron size ratings from
manufacturers are meaningless. And absolute filter micron size ratings are
unreliable since the softer particles in the oil can be squeezed through the
filter and reappear as contaminants.
Numerous tests on a range of hydraulic (e.g., piston pump) and
oillubricated equipment (e.g., truck engine) have been conducted that
confirmed filtering oil and removing particles deliver exceptionally long
equipment life. The cost of suitable filtration systems is not expensive.
For expensive hydraulic and oil-lubricated equipment the cost of filtration
is easily and quickly returned by the large gain in equipment working life
This article is an excerpt from Rotating Machinery Essentials
by Mike Sondalini (1999). Detail at www.feedforward.
Mike Sondalini is an equipment longevity engineer and author. He edits the
subscription-based newsletter Process & Plant Equipment UPTIME
(www.feedforward.com.au). He can be reached at Lifetime Reliability, P. O.
Box 578, Bentley, WA, 6102 Australia; (+61 8) 9457 0742;
www.lifetime-reliability. com. Don Irvine is with Donamar Filters that
supplied the data for this article.