» Maintenance of Equipment
Maintenance of Equipment
OilHow Clean Does It Have To Be?
Oil cleanliness has a major effect on
wear within equipment.
Clean, dry oil can extend equipment life between failures up to 8-10 times
the normal operating life. The Timken Co., a bearing manufacturer in Canton,
OH, reports that reducing water levels from 100 ppm to 25 ppm increases
bearing life two times. British hydraulics research indicates that if solids
contamination with particles larger than 5 micron is reduced from a range of
5000-10,000 particles/ ml of oil to 160-320 particles, machine life is
increased five times.
It is clear there is great benefit to be gained in having clean oil and
that it may well be worth spending a lot of money to achieve it. This would
be the case where expensive equipment was used and the cost of maintenance
was high or where the equipment was costly but not highly profitable to
operate. Increasing the equipment life and the period between maintenance up
to 10 times normal would be highly profitable in both cases. On the other
hand, if the cost of replacement equipment is inexpensive, it is unlikely to
be justifiable to spend money on oil filtration.
Grading Oil Cleanliness
Solid particle counts in oil can be done
with optical equipment (microscope, light extinction), with an
electron-scanning microscope (ESM), or by sifting through screens. Each
procedure produces slightly different particle counts due to the varying
sensitivity in detecting particles of different sizes. The ESM detects many
more smaller particles than the optical methods.
Counting standard ISO 4406-1999 is used internationally to rate solids
contamination of oils. This standard classifies the cleanliness of oil and
provides a basis to define acceptable solids contamination. It also means
oil filters can be tested to prove their performance meets acceptable
standards. Table 1 is part of the ISO 4406 method of coding the level of
solid particles in an oil sample. The solid particle content of oil gets a
classification that represents the number of particles of a particular size
Where calibrated automatic counting devices are used to measure
contamination, three scale numbers are used to describe solids
contamination: 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: 5 micron and larger and 15 micron and larger.
For example, oil solid particle contamination can be described as ISO
20/18/16. This means there are between 5000 and 10,000 particles larger than
4 micron/ml sample. Also there are between 1300 and 2500 particles larger
than 6 micron/ml of sample and between 320 and 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/ml of sample and between 320 to 640 particles larger than 15 micron.
|ALLOCATION OF PARTICLE COUNT
|Particles per milliliter
|ISO Scale Number
Contaminated Oil Destroys Equipment
Dirty oil spells rapid death for
hydraulic machinery and lubricated equipment. Fine tolerance equipment can
have clearances between parts of 5-10 microns. Solid particles larger than
the clearance gap will jam into the space. The solid particles will be
further broken up and mangled while ripping out more material from the
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. Fig. 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.
Solids suspended in oil are like grinding paste. They scour and gouge
surfaces, block oil passages, and make the oil more viscous. The longer the
oil is left dirty, the faster the rate of failure. Even expensive synthetic
oil is of no use if it is contaminated by solid particles. Though synthetic
oil has better high temperature and surface tension characteristics than
mineral oil, all advantages are lost if the synthetic oil is so contaminated
that it is destroying the machine. The only solution is to keep the oil
clean by filtration.