Continued from page 22
To try to get better discrimination between greases, it is necessary to couple more bearings together to magnify the individual losses. Work is ongoing to using a four bearing test machine, to measure the efficiency of greases used in bearings.
Acknowledgements
The author wishes to acknowledge many co-workers and departments within The Lubrizol Corporation for their contribution to this work.
References (1) Wyrley-Birch, O., “Era of Efficiency: Trends & Implications” presented at The 10th ICIS Pan American Base Oils & Lubricants Conference, New Jersey, 5th December 2014.
(2) Fish, G. “Friction Modifiers for Lubricating Greases” NLGI Spokesman 2014, volume 78(1), pp18-29.
(3) Harinarain, A.K., Pappy, S., Singh, I., Jaiswal, A.K., Sayanna, E., Basu, B., and Malhotra, R.K. “Development of a Bench Test Methodology for Performance Evaluation of Energy Efficient Industrial Gear Oils” NLGI Spokesman 2014, volume 78(2), pp25-31.
Figure 7. Schematic of the 4 tapered roller bearing tester
Initial work focussed on setting up the test rig to measure the effect of pre-loading on performance. The rig was originally designed to run oil and therefore oils were initially used to investigate efficiency. The bearings were first run-in to remove the effects seen in the FE8 test. An SAE 75W90 was run under different loading conditions with a speed ramp. It was observed that the lower the pre-load, the lower the torque losses. Reducing oil viscosity was investigated by running the same oil at higher temperatures. At the lower temperature the film thickness was sufficient to separate the surfaces fully but at the higher temperature, the losses increased suggesting that the surfaces were running under mixed lubrication. Comparing oils of three different viscosities showed similar results. At lower speed the thinnest fluid obtained the highest torque losses but at higher speed gave the lowest losses. Cann (10) had reported that after running-in, greases typically see a “parched” lubricant flow between that of fully flooded and starved. Some tests were run at restricted oil flow to try to simulate this behaviour. The fully flooded bearings showed higher losses than those with restricted flow. Initially work on greases showed that consistently packing the bearings is a problem. A grease packer was machined to facilitate uniform grease distribution in test bearings. Work is ongoing to look at greases and the work will be reported when complete.
Summary
This work and that of others has shown that measuring the efficiency of greases is a particular challenge. The energy losses in small deep groove ball bearings are very small and difficult to quantify. Moving to bearings with more sliding has led to the development of a modified test machine for measuring the energy efficiency of tapered roller bearings.
In bearings and components with sliding it has also been shown that it is necessary to apply molybdenum complex containing formulations to achieve low friction coefficients. The complexes do not work on their own but have to be combined with other surface active additives.
Conclusions
By optimising the lubrication of the bearings, theoretical improvements in energy efficiency are possible, but as bearings are extremely efficient, measuring the losses is a significant challenge.
This work has shown that using molybdenum additives continues to be the best way to achieve low friction coefficients in grease, and that their combination with other molybdenum- free additives provides an effective way to formulate optimized low friction additive packages.
(4) Yamamoto, M. and Imai, J. “Development of Grease Focusing on Improved Energy Efficiency” NLGI Spokesman 2014, volume 78(4), pp18-29.
(5) Bowden, F.P. and Tabor, D., “The Friction and Lubrication of Solids” Oxford University Press (1950).
(6) Dowson, D., “History of Tribology (Second Edition)” Professional Engineering Publishing (1997), ISBN 1-86058- 070-X.
(7) Fein, I.-M., Perilstein, W.L., and Adams, M.R., “Solid Film Deposition and Non-Sacrificial Boundary Lubrication” ASLE Transactions 1963, Volume 6, pp 60-66.
(8) Fish, G. “Constant Velocity Joint Greases” NLGI Spokesman 1999, volume 63(9), pp14-29.
(9) DIN 51834 part 6 “Standard Test Method for Measuring Friction and Wear Properties of Lubricating Grease Using a High-Frequency, Linear-Oscillation (SRV) Test Machine” Deutsches Institut für Normung e. V (1991).
(10) Cann, P.M. and Hurley, S “Grease Composition and Film Thickness in Rolling Contacts” NLGI Spokesman 1999, Volume 63(1) pp12-22.
(11) Morales-Espejel, G.E., Lugt, P.M., Pasaribu, H.R. and Cen, H. “Film Thickness in Grease Lubricated Slow Rotating Rolling Bearings” Tribology International, June 2014, Volume 74, pp7–19.
(12) Saita, O. “Evaluation of Greases Contributing to Maintenance Interval Extension of Shinkansen’s Traction Motor” NLGI Spokesman 2009, Volume 73(5) pp38-45.
LINK
www.lubrizol.com
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LUBE MAGAZINE NO.129 OCTOBER 2015
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