Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/756816
30 | November - December 2016 | www.machinerylubrication.com PERSPECTIVE favoring high-temperature lubricants, along with simply desiring to enhance performance for improved reliability. Lubricant Selection The first step in the lubricant selection process for a high-tem- perature application is to estimate the average operating temperature or temperature range. At this point, a temperature-control initiative may be implemented, such as installing a heat exchanger, insulant or additional ventilation. Also, be sure the OEM requirements and current machine conditions match the lubricant's specifications. The expected performance improvement should be specific, such as to achieve increased varnish or deposit control, extended relubri- cation intervals or enhanced lubrication properties. Assess the pros and cons of switching lubricants in terms of cost, compatibility, change-out procedures, etc. To obtain the desired results, you may need to be more specific in terms of the lubricant's technical properties. The suggested parame- ters or attributes to consider include the viscosity required by the machine or components at the operating temperature, the film strength properties, the oxidation resistance, the compatibility of the new lubricant with the current lubricant as well as with the synthetic materials in the machine, the potential impact of typical contaminants around the machine, and any OEM or industry standards. Please note that synthetics are not the only suitable lubricants for high temperatures. Mineral oils with strong refining and additive packages may work as well. Synthetic lubricants must also be changed. Their advantage over mineral oils is that the change intervals are extended. Typically, the higher the operating temperature, the greater the benefit of using a synthetic lubricant. Base Stocks Used at High Temperatures The charts on page 29 compare the operating temperature ranges of different base stocks for mineral and synthetic lubricants. Note that these ranges represent normal values according to the base stock type. They do not reflect the comparative life of the lubricant. At high temperatures, the lifespan of synthetics is much longer than similar mineral oil products. There are also more synthetic base stocks that can work at high temperatures, such as diesters, poly- glycols, polyolesters, etc. 3 Simple Rules for Maximum Operating Grease Temperature 1. If the dropping point is less than 300 degrees F, subtract 75 degrees F from the dropping point. 2. If the dropping point is higher than 300 degrees F but less than 400 degrees F, subtract 100 degrees F from the dropping point. 3. If the dropping point is higher than 400 degrees F, subtract 150 degrees F from the dropping point. The values subtracted may be even larger if extended relubri- cation intervals are desired. Also, keep in mind that regreasing intervals must be more frequent at higher temperatures due to the Arrhenius rate rule. FLUID STRENGTHS WEAKNESSES APPLICATIONS Polyalphaolefins (PAOs) High VI, high thermal oxidative stabil- ity, low volatility, good flow properties at low temperatures, nontoxic and compatible with mineral oils Limited biodegradability, limited additive solubility, seal shrinkage risk Engine oils, gear oils, bearing oils, compressor oils, high-temperature grease, lube-for-life applications Diesters and Polyolesters Nontoxic, biodegradable, high VI, good low-temperature properties, miscible with mineral oils Low viscosities only, bad hydrolytic stability, limited seal and paint compatibility Compressor oils, high-temperature grease, co-base stock with PAOs, bearing oils, gear oils, oil mist, jet engine oils Phosphate Esters Fire-resistant, biodegrades quickly, excellent wear resistance, scuffing protection Low VI, limited seal compatibility, not miscible with mineral oils, moderate hydrolytic stability Fire-resistant hydraulic fluids used in power plants, factories, marine vessels, mining, aircraft and mobile equipment Polyalkylene Glycols (PAGs) Excellent lubricity, nontoxic, good thermal and oxidative stability, high VI Additives marginally miscible, not miscible with mineral oils, limited seal/paint compatibility Refrigeration compressors, brake fluids (water soluble), fire-resistant fluids (water soluble), gas compressors (low gas solu- bility), worm and high-temperature gears, chain lube (clean burn off), metal-working and quenchants, H1 food-grade lubricants Silicones and Per- fluoropolyethers (PFPEs) Highest VI, high chemical stability, excellent seal compatibility, very good thermal and oxidative stability Worst mixed and boundary film lubrication properties, not miscible with mineral oils or additives High-temperature fluids, specialty greases, lubricant-contacting chemicals, some brake fluids