Machinery Lubrication

Machinery Lubrication May June 2016

Machinery Lubrication magazine published by Noria Corporation

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Page 26 of 75

By Jack Weeks, GPM Hydraulic consultinG When hydraulic oil turns from that golden honey color of new oil to a dark brown, does that mean it must be changed immediately? Is the system suffering from lost lubricating properties or gross contamination when this occurs, or is this a normal aging characteristic to be dismissed so long as the oil analysis results are within accept - able parameters? These types of questions are often asked whenever hydraulic fluid mainte- nance is discussed. Many people compare the oil in their industrial hydraulic systems to that of their automobile, assuming if the oil has turned to a dark brown that it must be changed as soon as possible regardless of how long it has been in service. It's easy to forget that the oil in an industrial hydraulic system is kept in a much different environment than the oil in an internal combustion engine. A color change in hydraulic oil is a good reason to be alert but not a good reason to go running for the oil skid to replace it right away. You first need to determine why the oil has changed color. The two most common causes of oil darkening are thermal stress and oxida- tion, neither of which will necessarily require that the oil be replaced. The first step is to take a representative sample of the oil and have it analyzed. I have seen hydraulic oil that has darkened consider - ably but was still perfectly good to remain in service. I have also seen hydraulic oil that has retained its original color but could not meet the parameters necessary to provide adequate system protection. In short, a change in oil color alone tells you nothing about the serviceability of the oil. However, darkening of the oil can direct you to potential problems that may need to be addressed. Perhaps a system has one or more "hot spots" where the oil is heated up significantly in a localized area, but the temperature is brought down again once it reaches the relatively cool reservoir. I once found a valve that had failed, forcing oil through a small orifice with a significant pressure drop. This generated a relatively large amount of heat, but it was localized in only a very small amount HYDRAULICS Changes Why Hydraulic Oil Color 22 | May - June 2016 |

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