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calibrated automatic counting devices are used
to measure contamination; three scale-numbers
are used to describe solids contamination. These
are the 4 micron and larger, 6 micron and larger
and 14 micron and larger. When the count is done
by optical microscope two size ranges are used
– the 5 micron and larger and 15 micron and
larger.
For example oil
solid particle contamination can be described as
ISO 20/18/16. This would mean there are between
5000 and 10000 particles larger than 4 micron
per milliliter sample. Also there are between
1300 and 2500 particles larger than 6 micron in
per milliliter of sample and between 320 to 640
particles larger than 14 micron. If a two scale
number is used the contamination result could be
18/16. In this case there are between 1300 and
2500 particles larger than 5 micron in per
milliliter of sample and between 320 to 640
particles larger than 15 micron.
HOW
CONTAMINATED OIL DESTROYS EQUIPMENT
Dirty oil spells
rapid death for hydraulic machinery and
lubricated equipment. Fine tolerance equipment
can have clearances between parts of 5 to 10
microns (0.005 – 0.01 mm, 0.0002” - 0
0.0004”). Solid particles larger than the
clearance gap will jam into the space. The solid
particles will further be broken-up and mangled
while ripping out more material from the
surfaces. In equipment with larger tolerances
the oil film between parts can get as thin as 3
– 5 micron. Solid particles larger than the
oil film will be broken up into smaller pieces
and produce more solids contamination. Figure
No. 1 shows a shaft in a journal bearing
lubricated by oil. In the drawing the solid
particles are larger than the oil film thickness
and when they enter the bearing pressure zone at
the bottom of the shaft they will tear into the
metal, be broken up and make more particles that
cause further wear.
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