Machinery Lubrication magazine published by Noria Corporation
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deactivated sulfur have helped to reduce or eliminate these corrosive attacks. Worm drives can present a unique boundary lubrication challenge, with the focus more on friction reduction than on the effects of wear. In these applications, a specific type of mineral-based lubricant known as a compounded oil can be used. This lubricant is formulated with up to 10 percent fatty acid (natural oil) or acidless tallow as the compounding agent along with rust and oxidation inhibitors and other additives. This results in improved lubricity, reduced friction and decreased sliding wear. EP oils are still commonly used in worm drive applications where they are formu- lated with yellow metal compatibility. However, both compounded oils and EP gear oils have a working temperature limita- tion of approximately 80 degrees C before oxidation rates rapidly increase, resulting in acidic products that can attack cupric worm wheel materials. The Right Viscosity Aside from the ambient and operating temperature, the correct viscosity will depend on several variables of the final worm wheel, including the pitchline velocity, center distance and revolutions per minute. Figures 8 and 9 provide recommendations for the ISO viscosity grade selection on cylindrical and double-enveloped worm drives according to the American Gear Manufacturers Association (AGMA) 9005- E02 standard. As these recommendations and the oil change interval chart show, temperature has a significant impact on effective lubri- cation. Not only are the lubricant and machine longevity negatively affected by higher temperatures, but worm drives in particular have trouble with temperature spikes. As a result, if higher temperatures are expected, more effective alternatives for base oils and additives should be selected. Synthetic oils such as PAOs and PAGs perform better than mineral oils due to their naturally higher resistance to thermal degradation. Nevertheless, an increase of 32 degrees C above the ambient tempera- ture in single-throated worm drives (37 degrees C for double-throated worm drives) is not considered excessive for the oper- ating conditions. The Right Oil Level As with most splash-lubricated gear systems, the oil level in a worm drive is essen- tial to maintain accuracy. Depending on the position of the worm relative to the worm wheel, a small drop in oil level could be the difference between ideal lubrication and no lubrication. When monitoring the oil level in the three most common worm drive positions (Figure 10), adhere to the manufacturer's recommendations, which will often be in line with the standards for depth of oil immersion. When the pitchline velocity of the worm elements exceeds 10 meters per second, particularly with double-enveloping worm drives, a force-feed lubrication system is recommended to spray the entire face of the worm. PITCHLINE VELOCITY OF FINAL REDUCTION STAGE ISO VISCOSITY GRADES A Ambient temperature (°C) -40 to -10 -10 to 10 10 to 55 Less than 2.25 m/s 220 460 680 More than 2.25 m/s 220 460 460 NOTES: Worm gear applications involving temperatures outside the limits shown above or speeds exceeding 2,400 rpm or 10 m/s sliding velocity should be addressed by the manufacturer. In general, for higher speeds, a pressurized lubrication system is required along with adjustments in the recommended viscosity grade. This table applies to lubricants with a viscosity index of 100 or less. For lubricants with a viscosity index greater than 100, wider temperature ranges may apply. Consult the lubricant supplier. CENTER DIS- TANCE OF FINAL REDUCTION STAGE WORM SPEED OF FINAL REDUCTION STAGE (RPM) ISO VISCOSITY GRADES A Ambient temperature (°C) -40 to -10 -10 to 10 10 to 35 35 to 55 Up to 305 mm <300 460 680 1000 1500 300-700 320 460 680 1000 >700 220 320 460 680 More than 305 mm to 610 mm <300 460 680 1000 1500 300-500 320 460 680 1000 >500 220 320 460 680 More than 610 mm <300 460 680 1000 1500 300-600 320 460 680 1000 >600 220 320 460 680 NOTES: Worm gear applications involving temperatures outside the limits shown above or speeds exceeding 2,400 rpm or 10 m/s sliding velocity should be addressed by the manufacturer. In general, for higher speeds, a pressurized lubrication system is required along with adjustments in the recommended viscosity grade. This table applies to lubricants with a viscosity index of 100 or less. For lubricants with a viscosity index greater than 100, wider temperature ranges may apply. Consult the lubricant supplier. www.machinerylubrication.com | January - February 2016 | 47 Figure 8. ISO viscosity grade guidelines for enclosed cylindrical worm gear drives Figure 9. ISO viscosity grade guidelines for enclosed globoidal worm gear drives