Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/680544
Oxidation can be kept to a minimum by normal fluid maintenance practices. The rate of all chemical reactions, including oxidation, will approximately double for every increase in temperature of 10 degrees C (18 degrees F). For most mineral-oil- based hydraulic systems, the maximum recommended temperature is 140 degrees F (60 degrees C). For every 15 degrees F (5 degrees C) above this temperature, the oil's life span will be cut in half. System pressure can make a difference, too. As pressure increases, so does fluid viscosity, which causes an increase in fric- tion and heat generation. Also, increased pressure results in an increase of entrained air (and therefore oxygen). The additional oxygen will accelerate the oxidation reac- tion of the oil. It is recommended that system pressure be kept as low as possible for maximum system efficiency and longevity of the oil and system components. Contaminants are another factor that can affect oxidation. A 1-percent sludge concentration in hydraulic fluid will double the rate of oxidation as compared to fluid with no sludge at all. Certain metals, most notably copper, act as catalysts for oxida- tion reactions, particularly in the presence of water. The presence of water and copper is a common occurrence when a heat exchanger ruptures. When you find that your hydraulic oil has darkened in color, don't assume that it needs to be replaced. It is quite likely that there are years of service remaining in your fluid. Get a good representative sample and have it analyzed. The most representa- tive sample will be taken immediately downstream of the pump. The second best location is from the exact center of the reservoir, obtained either while the system is running or immediately after shutdown. If you are just beginning a fluid sampling program, a good starting point is every 13 weeks. Adjust the frequency of sampling based on the analysis results. Keep at least a full year of analyses on file for compar- ison and spotting trends. Only then will you truly know the condition and service- ability of your hydraulic fluid. About the Author Jack Weeks is a hydraulic instructor and consultant for GPM Hydraulic Consulting. Since 1997 he has trained thousands of electricians and mechanics in hydraulic troubleshooting methods. Jack has also taught radio-wave propagation for the U.S. Air Force and telecommunications equipment operation and repair for the Central Intelligence Agency at American embassies overseas. www.machinerylubrication.com | May - June 2016 | 25 A change in oil color alone tells you nothing about the service- ability of the oil. ML