Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1433576
6 | November - December 2021 | www . machinerylubrication.com is might occur if a filter bursts or is in bypass mode. You can use the online calculator linked to in Figure A to get a better idea of how increased cleanliness might affect machine reliability. It is based on Noria's well-known life extension table. For rolling element bearings, I recommend that you consult the charts in Annex A of the ISO 281:2007 stan- dard (Rolling Bearings — Dynamic Load Ratings and Rating Life). 10X Times Drier e relationship to water contam- ination is also rather easy to predict. Similar to the impact of particle contamination, the detrimental effects of water on bearing service life have been shown through ALT testing. is data is explicit and authenticates the rela - tionship between water concentration and life expectancy. An example of one such study is summarized in the graph in Figure 2. Just like particles, there are many different factors at play, including bearing type, operating temperature, lubricant type (base oil additives), the state of water (dissolved, emulsified, free), etc. e damage caused by water includes corrosion, impaired pressure-viscosity coefficient (poor film strength), hydrogen embrittlement, two-body abrasion and lubricant oxidation. Like particles, water contamination is disproportionate to service life. For example, by going from 1,000 ppm (0.1%) water to 100 ppm (0.01%), bearing life is roughly 4.5 times longer. Take a look at the article linked to in Figure B, which discusses nine ways in which water is responsiblefor damage and failure of rolling element bearings: 20 Degrees Cooler Determining the relationship between lubricant temperature and bearing life is more complex. e best place to start is to establish the change in viscosity resulting from the change in tempera - ture. Instead of referencing "10 times" with temperature, we'll say 20 degrees cooler instead. How would reducing an oil's temperature by 20 C impact bearing life? For bearings, too much viscosity (too cool) is destructive, as is too little viscosity (too hot). Ultimately, we're trying to optimize the viscosity relative to factors like energ y consumption, fatigue life, abrasion (and other forms of mechanical "contact" wear) and general reliability objectives. is can be done by tweaking the ISO viscosity grade, the viscosity index, additives (AW/EP) and operating temperature. 5X Life Extension Example: • Let's say our bearing at operating temperature (40 C) requires an ISO VG 68 R&O oil. This would achieve a Kappa (K) of one. Kappa is the viscosity at operating tempera - ture divided by t he required minimum viscosity. • Furthermore, let's say current condi- tions normally run at a temperature of 55 C. Using a standard ASTM temperature-viscosity chart for ISO VG 68, the actual viscosity is 34 cSt at that temperature, resulting in K=0.5. e service life factor (A 23) is 0.3. See Figure 3. • If we reduce the temperature of the lubricant by 20 C (from 55 C to 35 C), the viscosity rises to 135 cSt and K=2 and A 23=1.7. is 20 degree temperature reduction, using the chart in Figure 3, increases the bearing life estimate by five times (5X). So, in this example, a 20 C reduction in oil temperature trans - lates to five times longer bearing life. Figure 1: Relationship of particle contamination to pump life. Figure 2: Relationship of moisture contamination to bearing life. Ref. SKF Figure 3: How temperature and viscosity affect bearing life. Figure B AS I SEE IT https://www.machinerylubrication. com/Read/1367/water-bearing-failure