Machinery Lubrication

Machinery Lubrication Jan Feb 2016

Machinery Lubrication magazine published by Noria Corporation

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TYPES OF WORM DRIVE EFFECTS ADVANTAGE OF A SYNTHETIC GEAR OIL OVER A MINERAL OIL Reduction of total losses 30% or more Improved efficiency 15% or more Reduction of operating temperature 20˚C or more 46 | January - February 2016 | LESSONS IN LUBRICATION Also, because of the sliding nature of the worm drive, metals with a low coefficient of friction are generally used. The worm wheel typically contains yellow metals, while the worm is usually made of steel. This results in more favorable wear characteristics, better loading ability and less heat genera- tion not found in other metal combinations. Yellow metals like bronze that are used on the worm wheel can present unique lubrica- tion challenges when selecting a compatible additive package. With this metallurgical combination, it is also expected that the worm wheel act sacrificially in comparison to the worm due to the relative effort and costs in worm drive rebuilds. Lubrication Solutions Gearing designs and materials have been modernized through the years to achieve better load-carrying capability, higher torque conversions and improved longevity. Sophisticated testing platforms and computerized methods have provided a better understanding of common worm drive failure modes and offered clues for optimizing the solutions. Lubricants are no exception to these enhancements for worm drives. Generally speaking, a high-quality worm drive lubricant will have low friction, high oxidation resistance, good anti-wear protection and high viscosity index. The Right Base Oil While using lubricants formulated with mineral oil is quite common within worm drives, employing synthetic base oils gener- ally results in improved gear efficiency and lower operating temperatures. Figure 5 illus- trates lubricant life and oil change interval expectations for polyalphaolefins (PAOs), polyalkylene glycols (PAGs) and mineral oils over a range of oil sump temperatures. This is supported by the Arrhenius Rate Rule, which states that for every increase of 10 degrees C in the average oil temperature, the chemical reactions double. The energy transmission efficiency of the gear system's input and output can be significantly influenced by the lubricant selected. Figure 6 specifies the improved efficiency when choosing a synthetic over a mineral oil, particularly PAGs, which have an inherently low coefficient of friction. PAGs are also known to reduce operating temperatures and total losses. Additional comparisons between mineral and PAG base oils are seen in Figure 7. PAGs do have some drawbacks, most notably their higher costs. They also are not compatible with some seal materials, plas- tics and paint coatings, so always confirm compatibility when switching to PAGs. The Right Additives One of the most important jobs of a gear oil additive is to form a protective or sacrificial barrier between contacting surfaces when the conditions exceed that of the bulk oil's film strength. An additive package for a lubricant in a worm drive must be selected with care, since the yellow metals often contained within worm wheels can be adversely affected by corrosion from the activated sulfur within the extreme-pres- sure (EP) additive, particularly in the presence of heat. Nevertheless, advance- ments in additive formation with Figure 6. Advantages of synthetic gear oil over mineral oil Figure 7. Polyalkylene glycol vs. mineral oil Polyglycol Mineral Oil 1500 1000 500 1.6 1.4 1.2 1.0 0.8 0.6 110 100 90 80 70 60 50 400 600 800 1000 1200 1400 40 60 ISO 220 ISO 150 ISO 220 80 100 120 140 160 50 100 150 200 250 300 350 400 450 Torque Limit Nm Power Loss P. Temperature at Meshing Height °C Mineral Oil Polyglycol Viscosity @40°C Torque T, (daNm) Torque T, (daNm) Mineral Oil Polyglycol

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