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September
4, 2001 Subscribers: 10,754 |
"How do I extend the life of my gear oils in hot applications?"
Heat is a problem for oil. For every 10°C increase above 60°C in temperature the rate of oxidation is increased by 100 percent. As the oil gets hotter the viscosity drops. As the viscosity drops the hydrodynamic film is compromised, which leads to more metallic contact, which leads to more heat. At some point, the oil is too thin to offer much protection and the life of the gearing and bearings diminishes dramatically.
The use of a heat exchanger (air or water) can help reduce internally generated heat. If the source of heat is from the process, or is radiant (from proximity to hot systems), then a more difficult challenge arises. Passive heat shields can be very helpful at limiting radiant heat. If the problem is a hot shaft running into a reducer, then perhaps a different coupling configuration and heat shields and/or heat exchangers can make a difference.
Some base stocks are better for high temperature applications than others. Synthetics have a reputation for resisting oxidation in hot applications. Severely hydroprocessed and hydrocracked based lubricants may provide a similar benefit. Using one of these alternatives should deliver the life extension that you wish to see.
Some antioxidants, such as BHT (butylhydroxy toluene), will turn oil bright red when oxidizing during normal service. While not always a cause for alarm, it is clear evidence that antioxidant is depleting.
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If your supplier is re-using your drums, you might ask them to provide you with documentation showing that the drums are being reconditioned using a standardized procedure including periodic inspection tests that are performed. If the supplier cannot provide this information you might consider either specifying that all products be sent in new drums, help them find a viable reconditioning resource, or suggest that liners be used or find a new supplier.
Each tip published will earn the sender $10. Send your tips to webmaster@noria.com or fax to 918-746-0925.
A slight discoloration of the oil is nearly always present during the initial stages of oxidation. This discoloration increases as the process continues and the decoloration is accompanied by a drop in interfacial surface tension and a rise in viscosity, specific gravity and the neutralization number. The rise in viscosity and acidity are not necessarily inter-dependent.
Primary oxidation will cause a rise in acidity which will ultimately level off, at which point the viscosity may rise at an extremely rapid rate. Such increases in viscosity rate usually occur due to the generation of oxidation products of high molecular weight, held in fluid suspension. Paraffinic base oils are less apt to produce sludge at an earlier stage as those of a polynapthenic base.
The oxidation rate and the degradation products increase not only with increasing temperature, but also with increases in agitation (turbulence) and contamination - air, water, metallic particles, dirt and dust. The oxidation stability varies not only with the viscosity grade of the oil but also with the quality of the base stock. In addition, the actual base stock from which the hydraulic fluid was derived and refined also affects the rate of oxidation.
For more information about the book "Proactive Maintenance for Mechanical Systems" go to:
http://www.noria.com/secure/product_detail.asp?catalogid=48
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Lube-Tips is published by Noria Corporation. Oil Analysis and Lubrication Experts