Oil—How 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 range.

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.


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 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. Fig. 1 shows a shaft in a journal bearing lubricated by oil.