The efficiency of a transmission is limited by its power loss. The power loss of a geared transmission can be divided into no-load and load-dependent power losses of gearings, bearings, sealings and auxiliary units. Load-dependent power losses relate to dissipation from loaded tribocontacts, whereas no-load power losses originate from inner fluid friction due to interaction of rotating machine elements with transmission fluids. Water-containing lubricants like water-containing polyalkylene glycols (PAGs) show the potential for superlubricity in highly loaded rolling-sliding contacts, whereas superlubricity is related tribosystems with ultra-low friction with a coefficient of friction less than 0.01.  

This presentation discusses the superlubricity and elastohydrodynamic film thickness of water-containing PAGs depending on the water content [1] and evaporation [2]. It then shows a measured mean coefficient of friction of less than 0.01 of a spur gearing [3]. Based on this, the efficiency potential of geared transmissions is discussed, when machine elements with rolling-sliding contacts like gearings operate in superlubricity [4]. Primary superlubricity effects of reducing load-dependent power losses and secondary superlubricity effects of reducing no-load power losses are quantified in the context of different operating conditions. The term superefficiency is categorized [4].  

References
[1] S. Hofmann, T. Lohner, K. Stahl: Influence of water content on elastohydrodynamic friction and film thickness of water-containing polyalkylene glycols. Front. Mech. Eng 9:1128447. doi: 10.3389/fmech.2023.1128447 (2023)
[2] S. Hofmann, T. Lohner, K. Stahl: Influence of evaporation from water-containing polyalkylene glycols on elastohydrodynamic lubrication. 9^th International Tribology Conference, 25-30 September, Japan (2023)
[3] M. Yilmaz, T. Lohner, K. Michaelis, K. Stahl: Minimizing gear friction with water-containing gear fluids. Forschung im Ingenieurwesen. 83 (3), 327–337. doi:10.1007/s10010-019-00373-2 (2019)
[4] T. Lohner, C. Paschold, K. Stahl: Towards Superefficiency in Transmissions. 9th International Tribology Conference, 25-30 September, Japan (2023)