Fluid friction in elastohydrodynamically lubricated (EHL) contacts depends strongly on the lubricant considered. Synthetic oils can have significantly lower fluid friction than mineral oils. Water-containing fluids have the potential to significantly reduce fluid friction further. The aim of this study is to investigate the film formation and frictional behavior of highly-loaded EHL contacts with water-containing fluids. Comparisons are made with mineral and polyalphaolefin oils. Measurements at an optical EHL tribometer show good lubricant film formation of the considered water-containing gear fluids. Measurements at a twin-disk test rig show coefficients of friction smaller than 0.01, which is referred to as superlubricity, for all considered operating conditions.

Lubricants have a large influence on gear friction and therefore on gearbox losses and heat balance. Synthetic oils can exhibit significantly lower gear friction than mineral oils. Recent basic research on water-containing gear fluids has demonstrated superlubricity in highly-loaded elastohydrodynamically lubricated (EHL) contacts. Following up these results, the potential of water-containing gear fluids to improve the efficiency and heat balance of gearboxes was investigated. Experimental results from an efficiency gear test rig demonstrate significant lower load-dependent gear losses compared to mineral and polyalphaolefine oils. The mean coefficient of friction for a wide range of operating conditions is clearly smaller than 0.01, which is referred to as superlubricity.

Lubricants have a large influence on gearbox power losses. Recent investigations at a gear efficiency test rig have shown the high potential of water-containing gear fluids in drastically reducing load-dependent gear losses and temperatures. In this study, the bearing power losses with water-containing gear fluids were evaluated at a specific bearing power loss test rig explicitly and compared with mineral and polyalphaolefine oils. For all investigated lubricants, a Stribeck curve behavior of the load-dependent losses is observed. The water-containing gear fluids demonstrate lower no-load bearing losses and higher load-dependent bearing losses at higher rotational speeds. The comparison of measured bearing losses with typical calculation procedures showe partially large differences. The results underline the importance of having detailed knowledge of bearing losses when evaluating gear losses in gearboxes.