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Maintenance of Equipment

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.

Particles per milliliter
ISO Scale Number More than Less than
22 20000 40000
21 10000 20000
20 5000 10000
19 2500 5000
18 1300 2500
17 640 1300
16 320 640
15 160 320
14 80 160
13 40 80
12 20 40
11 10 20
10 5 10
9 2.5 5
8 1.25 2.5

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. 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.

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