24th International Colloquium Tribology - January 2024 31 Towards Superefficiency Tribology Solutions Shaping Tomorrow’s Gearboxes Thomas Lohner 1* , Constantin Paschold 1 , Karsten Stahl 1 1 Technical University of Munich, School of Engineering and Design, Department of Mechanical Engineering, Gear Research Center (FZG), Boltzmannstraße 15, 85748 Garching near Munich, Germany * Corresponding author: thomas.lohner@tum.de 1. Introduction The main design criteria of gearboxes are (i) efficiency and heat balance, (ii) power density and load capacity, and (iii) noise, vibration and harshness. These classical design criteria are nowadays superimposed by sustainable aspects of product lifecycles and circular economy, in which the use phase is often dominant. Implementing low-loss technologies like superlubricity, low-loss gearings, and on-demand lubrication methods can push the gearbox efficiency to next levels. This study discusses the potential of low-loss technologies and introduces the term superefficiency. The results are based on the authors’ presentation at ITC 2023 [1]. 2. Methods and Materials This calculation study uses the program WTplus [2] for power loss and heat balance analysis. No-load (index: - 0) and load-dependent (index: - P) gearing (index: G) and bearing (index: B) power losses as well as sealing power losses (index: S) are considered: P L = P LG0 + P LGP + P LB0 + P LBP + P LS (1) The load-dependent gear power loss P LGP is calculated based on the input power P In , tooth loss factor H V and mean coefficient of gear friction µ mz : P LGP = P Ih × H V × µ mz (2) The object of investigation is the gearbox of the FZG efficiency test rig in Figure 1. A complete operating cycle is defined by a fully parametric combination of the wheel rotational speed n 2 -=-{87, 174, 348, 870, 1444, 1739, 2609, 3479}- min -1 , pinion torque T 1 -= {0, 35.3, 94.1, 183.4, 302.0}-Nm and oil temperature ϑ Oil -=-{40, 60, 90, 120}-°C with t-=-5-min per operating point. Figure 1: FZG test gearbox with high toothing gearing A high toothing (HT) gearing with H V -=-0.226 is used as a reference. Moreover, a moderate low-loss (mLL) and an extreme low-loss (eLL) gearing with the same load-carrying capacity as the HT gearing but strongly reduced tooth loss factors of H V -=-0.094 and H V -=-0.049 are considered. Besides different gearings, different gear oils with a viscosity of ≈10-cSt at 100-°C are considered: (i) mineral oil MIN10, (ii) polyalphaolefine PAO10, (iii) polyglycole PG10, (iv) polyether PE10 and (v) water-containing polyglycole PAGW09. The mean coefficient of gear friction µ mz is known from power loss measurements at the FZG efficiency test rig acc. to method FVA345 [3][4] and described by an oil-specific parametrized calculation equation. An oil-specific evaluation of µ mz|FVA345 and its comparison to the value of MIN10 provides values of 0.70 for PAO10, 0.61 for PG10, 0.48 for PE10, and 0.10 for PAGW09. PAGW09 shows superlubricity with µ mz -<-0.01 for a wide range of operating conditions [4]. Furthermore, different lubrication methods are considered: (i) dip lubrication (DL) with an immersion depth of half of the pinion and wheel (e = d a / 2), (ii) DL with a reduced immersion depth of three times the module of the pinion (e 1 = 3∙m n ) and (iii) minimum quantity lubrication (MQL) with an oil volume rate of 28-ml/ h supplied by a continuous air stream. To evaluate the results, the cumulative energy dissipation W L of a complete operating cycle is determined acc. to W L = S i P L,i × t, (3) and divided by a reference W L.ref , which refers to HT, MIN10 and DL with e 1 = 3∙m n . Such energy efficiency index EEI is similar to [5] described by: (4) 3. Results and Discussion Figure 2 shows the calculated EEI for the considered gearing geometries and gear oils for (a) DL with e 1 -=-3∙m n , (b) DL with e = d a / 2, and (c) MQL. Based on Figure- 2a with the reference EEI of 100- %, the variation of the gear oil for HT shows EEI values of 78-% for PAO10, 70-% for PG10, 61-% for PE10, and 37-% for PAGW09. This is mainly due to the reduction of µ mz and, therefore, P LGP . The variation of the gearing geometry for MIN10 shows EEI values of 60-% for mLL and 48-% for eLL. This is mainly due to the reduction of H V and, therefore, P LGP . Hence, choosing an optimized gear oil and gearing geometry offers great potential for reducing the EEI. Switching from MIN10 to PAGW09 with superlubricity provides a more significant potential for reducing the EEI. The maximum potential with 32 24th International Colloquium Tribology - January 2024 Towards Superefficiency an EEI of 32-% is found when combining eLL and PAGW09. However, when using a low-loss gear oil, the energy dissipation is already very small, so the additional reduction of the EEI using a low-loss gearing geometry is therefore slight, c.f. HT+PAGW09 and eLL+PAGW09. Figure 2: Energy Efficiency Index EEI for the complete operating cycle of the FZG test gearbox The influence of the lubrication method can be seen by comparing the results in Figure 2a with DL with e = d a / 2 (Figure 2b) and MQL (Figure 2c). A higher immersion depth increases the no-load power loss and the EEI for all gear oils and gearing geometries. MQL can potentially reduce the EEI to a minimum of 32-% in this study. Note that the limited heat transfer with MQL results in critical gear bulk temperatures of ϑ M ->-160-°C for technologies with an EEI->-50-%. The minimum EEI with the considered low-loss technologies depends on the operating cycle. If operating conditions with T 1 -= {183.4, 302.0}-Nm are removed from the complete operating cycle, the minimum EEI increases to 45-%. This is due to the lower proportion of operating points with high load-dependent power loss in the operating cycle. In terms of gearbox efficiency, the calculated mean efficiency for the complete operating cycle increases from 98.5-% for the reference technology with HT, MIN10, and DL with e 1 = 3∙m n to 99.8-% for eLL, PAGW09, and MQL. For the considered FZG test gearbox complete operating cycle, superefficiency might be defined for operating points with an efficiency of h-> 99.8-%. It should be emphasized that the EEI and superefficiency are specific to a gearbox and the underlying operating cycle. The exemplary application of the presented low-loss technologies to a single-stage industrial gearbox with a gear ratio of 2.23 using an ISO VG 320 gear oil shows a potential to reduce the EEI to 44-% and to lower the oil sump temperature by 33-K for an operation condition of input torque T 1 -=-12-732-Nm, input rotational speed n 1 -=-230-min -1 (P In -= 307-kW) and an operating time of 6000 h/ a. This relates to an increase of h from 99.0-% to 99.6-% and the reduction of W L from 67 GJ/ a to 30 GJ/ a. 4. Conclusion The following conclusions can be drawn from this study: • The energy efficiency index of gearboxes can be reduced to below 50 % with low-loss technologies. • Low-loss gear oils, particularly when enabling superlubricity, and low-loss gearings strongly reduce the load-dependent power loss. • Minimum quantity lubrication reduces the no-load power loss and saves resources. It is enabled by low load-dependent power losses resulting in uncritical component temperatures. • The maximum low-loss potential is found for eLL, PAGW09 and MQL with the EEI-=-32-%. • For the considered FZG test gearbox and complete operating cycle, superefficiency might be classified for operating points with an efficiency >-99.8-%. References [1] Lohner T. et al.: Towards superefficiency in transmissions, 9 th ITC, 25 th -30 th September, Fukuoka, Japan (2023) [2] Paschold C. et al.: Calculating component temperatures in gearboxes for transient operation conditions, doi: 10.1007/ s10010-021-00532-4 (2021) [3] Hinterstoißer M. et al.: Minimizing load-dependent gear losses, doi: 10.30419/ TuS-2019-0014 (2019) [4] Yilmaz M. et al.: Minimizing gear friction with water-containing gear fluids. doi: 10.1007/ s10010-019- 00373-2 (2019) [5] EU Regulation 2017/ 1369 on setting a framework for energy labelling and repealing Directive 2010/ 30/ EU, Official Journal L198/ 1 (2017)