Continued from page 7
Figure 5 shows some of the energy savings achievable when switching from a mineral gear oil to an equivalent viscosity fully synthetic gear oil made with high viscosity mPAO.
The two most common types of gear failures – pitting (rolling contact fatigue) and scuffing (adhesion) can also be reduced by choosing the correct lubricant. Pitting of the gear teeth (crack formation, growth and material removal) is a surface failure caused by the combination of rolling and sliding stresses experienced by the teeth. There is a compressive, pulsating stress at the contact point where the surface continually flattens and unflattens, while the sliding action creates further shear stresses before and after the contact (compressive and tensile respectively). Increasing the EHD film (i.e. higher effective viscosity) has a direct effect on reducing pitting, as does reducing sliding forces (i.e. lower traction co-efficient)[4].
Figure 6 shows the results from a roller disk machine consisting of two polished steel test rings which can be pressed together under load and independently speed controlled to create different sliding or rolling conditions. The specific film thickness in these tests averaged 1.2, which promoted higher surface interactions and boundary lubrication conditions. For equivalent viscosity gear oils, we can see the PAO-based oil has approximately 25% lower traction coefficient than the mineral gear oil and shows no scuffing even at higher load conditions [5].
In addition to reducing energy losses and scuffing, the lower traction coefficient of synthetic base oils helps reduce temperatures in the gearbox. Lower temperatures at the gear mesh inlet leads to higher EHD films, providing better wear protection, while the overall reduction in oil temperature helps extend seal and lubricant life (reduced oxidation). Replacing an ISO 460 mineral oil with an ISO VG 460 fully synthetic gear oil in a typical industrial enclosed gear box could cause the bulk oil temperature to drop by up to 10°C [6]. Bearing in mind the rule of thumb that oxidation rates double for every 10°C rise in temperature above 60°C (Arrhenius law), a reduction of 10°C could double the oil life. Extended oil life, a benefit that synthetics typically offer over conventional mineral oil grades, could be as high as 5 times longer depending on application and formulation [7].
Table I. Properties and benefits of synthetic gear oils versus the equivalent mineral oil viscosity grade.
13 trial gearboxes - 3 old and 10 new (smaller sumps) All switched from mineral to PAO-based gear oils
3 Average oil temperature reduced by 6°C 3 Energy consumption dropped by at least 2.2% 3 288 gearboxes on site = Potential 11.5 MWhrs per annum saving
Source: Charles Du Bois, “Reduce friction to save energy”, The South African Mechanical Engineer, Vol. 61, September 2011.
Table II. Benefits of changing mineral gear oil to fully synthetic gear oil.
Benefits of synthetic gear oils have not only been recognized by end users (see table II), but have also been used by gearbox manufacturers to increase the allowable power rating of gearboxes [6]. Alternatively, by using synthetic lubricants a smaller gearbox can be designed for the same power rating.
The predictions made in 1983 are still valid today. The global marketplace’s demand for improved energy efficiency, lower environmental emissions, higher equipment durability, and more sustainable energy solutions helps to drive the need for energy efficient lubricants based on synthetic base oils.
References [1] P.S Korosec and S. Norman, Ethyl Petroleum Additives Division, Gear Lubrication: Fifty Years of Progress”, NLGI Spokesman, November 1984.
[2] Kline & Company, Global Synthetic Lubricants: Market Analysis & Opportunities, June 2014
[3] Mobil EHL Guidebook, fourth edition, Mobil Oil Corporation Technical Publications
[4] Wilfried J. Bartz, Lubrication of Gearing, p44-45, Mechanical Engineering Publications Ltd London 1993
[5] Jackson,A.; Webster, M.N.; Enthoven, J.C. “The effect of lubricant traction of scuffing”, STLE Tribology Transactions, vol 37, No.2, p387-395
[6] ed. Leslie R. Rudnick and Ronald L. Shubkin Synthetic lubricants and high performance functional fluids, 2nd edition, p511/514. Marcel Dekker Inc. New York. (From Neale, M.J.,ed. Tribology Handbook, Butterworth, London 1973.)
Figure 6. Variation of traction coefficient with load for polished rings (source : ref [5]).
Gear applications are thus well suited to the use of synthetic lubricants, and the following table highlights the benefits they offer in comparison to equivalent mineral oil grades.
LINK
www.exxonmobilchemical.com/synthetics
[7] ed. Leslie R. Rudnick and Ronald L. Shubkin Synthetic lubricants and high performance functional fluids, 2nd edition, Figure 6, p429 - Service life depending on temperature for different synthetic oils containing inhibitors. Marcel Dekker Inc. New York. (From Neale, M.J.,ed. Tribology Handbook, Butterworth, London 1973.)
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