Aqueous Gear Lubrication – Influence of Water Evaporation

Image source: FZG, Technical University of Munich

FZG gear efficiency test rig

The use of aqueous polyalkylene glycols (PAGs) in gears has demonstrated significant reduction in friction, power loss and temperature. Challenges in its application include material incompatibilities and the risk of corrosion. Furthermore, water tends to evaporate from aqueous PAGs during operation. To ensure a stable and robust operation, the impact of water evaporation must be considered. Based on the research work conducted, the influence of water evaporation on gear power losses has been quantified. The results can be used to derive monitoring and control strategies for gearboxes lubricated by aqueous PAGs.

The following article is an excerpt version of a full paper that is currently accepted for publication in the special issue Best of Gears 2025 in Engineering Research [1].

Balkendiagramm zeigt Einfluss des Wassergehalts auf den Leistungsverlustfaktor bei PAGW22-Schmierstoffen.

Figure 1: Load-dependent power loss factor over the evaporated water content ∆ξ for an operating point at a contact pressure of 1343 N/mm² (pitch point) and circumferential speed of 8.3 m/s (pitch point) [1]. (PAO as reference from Yilmaz et al. [2])

Energy saving potential and lubrication mechanisms

The use of aqueous polyalkylene glycols (PAGs) for gear lubrication has attracted significant attention, as these types of lubricants have demonstrated superlubricity with coefficients of friction of less than 0.01, even for gears [2]. In elastohydrodynamically lubricated (EHL) contacts such as those found in gears or roller bearings, aqueous PAGs have shown to reduce friction by up to 95% compared to polyalphaolefin (PAO) [3], while providing adequate film formation [3,4]. Using aqueous PAGs in electrical drive units can achieve power loss savings of 74% [5]. Furthermore, the favorable calorimetric properties of aqueous PAGs make them suitable for a holistic thermal management, including the function of lubrication and cooling [6,7]. The predominant EHL mechanism resulting in superlubricity is related to the low pressure-viscosity coefficient and hence low contact viscosity [3,8]. Consequently, increasing the functional water content continuously decreases friction [3,8,9]
 

Challenge and Aim of the Study

Apart from the possible issues related to corrosion and material incompatibilities, water evaporation poses a challenge in the application of aqueous PAGs to gears. Water evaporation changes the formulation of aqueous PAGs, particularly their viscosity [9, 10], and influences both the load-independent and load-dependent power losses. Therefore, this study investigates the influence of water evaporation on the power loss behavior in cylindrical spur gears. Based on the experimental research work, strategies for monitoring, controlling, and operating gearboxes lubricated by aqueous PAGs are introduced.
 

Experimental Setup

A FZG gear efficiency test rig was used to study the influence of water evaporation on the power loss behavior. Cylindrical test gears of type-Cmod with a superfinished surface were considered. The aqueous lubricants considered had an initial water content of 20 and 40 wt%, are of ISO VG 22, and are referred to as PAGW2220wt% and PAGW2240wt% respectively. Targeted amounts of evaporated water content were achieved using a heat plate and a magnetic stirrer. The water content within the test and transmission gearbox of the gear efficiency test rig was monitored using the linear relationship between the refractive index and water content of aqueous PAGs [9]. Unwanted water evaporation during testing was found to be below 1 wt%. The operating cycle and evaluation procedure at the FZG efficiency test rig were similar to the method according to FVA 345 [11,12]. 
 

Results and Discussion

The load-dependent power loss factor XLP(vt,c) based on the measured load-dependent loss torque indicates for both PAGWs a steady increase with increasing evaporated water content . Figure 1 exemplarily shows the load-dependent power loss factor XLP(vt,c) for PAGW2220wt% and PAGW2240wt% at an operating point with a moderate load and speed. For classification, the results of a polyalphaolefin (PAO) of ISO VG 22 are included in Figure 1. 

At the initial state without evaporated water content, the power loss factors XLP(vt,c) are reduced by 58% (PAGW2220wt%) and 63% (PAGW2240wt%) compared to PAO. This clearly demonstrates the drastic energy saving potential by using PAGWs for gear lubrication. However, the load-dependent power losses with aqueous PAGs remain also low at high evaporated water contents, especially for PAGW2220wt%. Hence, even aqueous PAGs with a low functional water content can offer significant energy saving potential for gear lubrication. For PAGW2220wt%, the load-dependent power loss factor is approximately 27% lower at the nearly water-free state compared to PAO. For PAGW2240wt% it is comparable to PAO at the highest investigated evaporated water content. It should be noted, that water evaporation results in drastically increasing kinematic viscosity, which increases at 40 °C lubricant temperature to approximately 64 mm²/s for PAGW2220wt% at ∆ξ  = 20.0 wt% and to 164 mm²/s for PAGW2240% at ∆ξ  = 32.7 wt%.

Based on the power loss factors, a sensitivity analysis was performed to quantify the influence of water evaporation on the power loss behavior. Assuming a linear relationship yields maximum sensitivities of the load-dependent power loss factors XLP(vt,c) of 4.9 %/wt% (PAGW2220wt%) and 6.0 %/wt% (PAGW2240wt%). The same approach was also performed for the no-load losses in terms of a no-load power loss factor XL0(vt,c). As the no-load losses are hardly affected by water evaporation when considering a constant oil filling level, the derived sensitivities are close to zero.

Finally, it should be noted that the effect of water evaporation is a reversible phenomenon. Adding the evaporated water content restores the initial power loss behavior. Thereby, the measurement of the refractive index may present an easy method to monitor the water content within a gearbox.
 

Conclusion

The findings of the study support the design of gearboxes lubricated by aqueous PAGs. The effect of water evaporation on power loss behavior was evaluated and a sensitivity analysis was performed to quantify its influence on the power losses. Although the power losses increase continuously with increasing evaporated water content, and higher initial water content shows greater sensitivity, power losses remain low compared to those of conventional lubricants. Thus, the use of aqueous PAGs with a low functional water content for gears may ensure robust operation with a low risk of water evaporation and high efficiency.

1Stefan Hofmann, M. Sc. (Teamleader Tribology and Sustainability)

1Dr.-Ing. Thomas Lohner 
(Division Head EHL-Tribological-Contact and Efficiency)

1Prof. Dr.-Ing. Karsten Stahl (Full Professor Institute of Machine Elements and Director of Gear Research Center (FZG))

1Technical University of Munich, School of Engineering and Design, Department of Mechanical Engineering, Gear Research Center (FZG), Boltzmannstraße 15, 85748 Garching near Munich, Germany

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References

[1] Hofmann, S.; Lohner, T.; Stahl, K.: Influence of water evaporation on the power loss behavior of cylindrical gears lubricated by aqueous polyalkylene glycols. Engineering Research. (2025) (accepted)
[2] Yilmaz, M.; Lohner, T.; Michaelis, K.; Stahl, K.: Minimizing Gear Friction with Water-Containing Gear Fluids. Forschung im Ingenieurwesen. (2019) doi:10.1007/s10010-019-00373-2
[3] Hofmann, S.; Lohner, T.; Stahl, K.: Influence of water content on elastohydrodynamic friction and film thickness of water-containing polyalkylene glycols. Frontiers in mechanical Engineering. (2023) doi:10.3389/fmech.2023.1128447
[4] Yilmaz, M.; Mirza, M.; Lohner, T.; Stahl, K.: Superlubricity in EHL Contacts with Water-Containing Gear Fluids. Lubricants. (2019) doi:10.3390/lubricants7050046
[5] Sedlmair, M.; Lohner, T.; Stahl, K.: Increasing gearbox efficiency of battery electric vehicles with water-containing fluids. 61th German Tribology Conference, Göttingen, Germany. (2020)
[6] Luther, R.: Mehr Kühlen beim Schmieren. Wasserhaltige Getriebefluide für den elektrischen Antriebsstrang. Reibung in Antrieb und Fahrzeug. (2020). doi.10.1007/978-3-662-63608-2_12
[7] Morhard, B.; Schweigert, D.; Mileti, M.; Sedlmair, M.; Lohner, T.; Karsten, Stahl.: Efficient lubrication of a high-speed electromechanical powertrain with holistic thermal management. Engineering Research. (2021). doi:10.1007/s10010-020-00423-0
[8] Hofmann, S.; Jingyu, H.; Lohner, T.; Stahl, K.: Elastohydrodynamic Lubrication Mechanisms of Aqueous Polyethylene Glycols. Tribology Letters. (2025). doi: 10.1007/s11249-025-01962-9
[9] Hofmann, S.; Lohner, T.; Stahl, K.: Influence of water evaporation on elastohydrodynamic lubrication with water-containing polyalkylene glycols. Friction. (2024). doi:10.1007/s40544-024-0916-1
[10] Hasan, M.; Björling, M.; Matta, C.; Meeuwenoord, R.; Jantel, U.; Larsson, R.: An Investigation of Film Formation and Pressure-Viscosity Relationship of Water-Based Lubricants in Elastohydrodynamic Contacts. Tribology International. (2025). doi:10.1016/j.triboint.2025.110654
[11] Doleschel, A.: FVA 345 I – Wirkungsgratest – Vergleichende Beurteilung des Einflusses von Schmierstoffen auf den Wirkungsgrad bei Zahnradgetrieben, Abschlussbericht, Forschungsvereinigung Antriebstechnik e.V. (FVA) (2002)
[12] Doleschel, A.; Michaelis, K.; Höhn, B-R.: Method to Determine the Frictional Behavior of Lubricants Using a FZG Gear Test Rig – Research Project No. 345, Forschungsvereinigung Antriebstechnik, e.V. (FVA) (2002)

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