Aqueous Lubrication in Geared Transmissions: A Review on Mechanisms, Superlubricity, Efficiency Potential, and Remaining Barriers

The application of aqueous lubricants in geared transmissions offers enormous energy saving potential and can contribute towards climate neutrality. This potential is based primarily on the ultra-low friction within typical machine elements in geared transmissions. In elastohydrodynamically lubricated contacts, coefficients of friction lower than 0.01 have been measured in gears, which is referred to as superlubricity. Despite extensive research, this potential has not been transferred to industrial applications yet. This review summarizes the current state of knowledge on aqueous lubrication for geared transmissions. The primary focus is on aqueous polyalkylene glycols and aqueous glycerol as these base stocks have shown to be promising for the lubrication of machine elements such as gears, bearings and seals. The lubrication mechanisms as well as the friction reduction and energy saving potential, and the challenges arising by the application of aqueous lubricants with the specific machine elements of geared transmission are described. This includes gears, rolling element bearings, plain bearings, seals, and clutches. Challenges such as (tribo-)corrosion, material incompatibility, load-carrying capacity, and water evaporation are analyzed. The review concludes with a derivation of research gaps that need to be addressed to enable aqueous lubrication of geared transmissions.