Motivation Lubricating oils should form a stable liquid film in rolling contacts and thus protect the surfaces of the contact partners and reduce friction and wear. Friction losses are generated, for example, in the rolling contacts of steel gears and rolling or sliding contacts of bearings. Those losses reduce the efficiency and increase warming. The heat, resulting from those friction losses, is dissipated into the environment and contributes to global warming. For example, current efficiencies of gearboxes in wind turbines, are said to be between 97.5 and 98 %. Even with the greatest efforts, the efficiency of large gearboxes is not exactly measurable. Also, the calculation options, related to gearing losses and bearing losses, are based on old approaches and use research results from that time [1, 2, 3]. The smallest improvements in efficiency of mass-produced products, such as wind turbine gearboxes, gearboxes in general, and motors will have a significant impact on overall global power losses and thus on global warming. As example: a 3-stage planetary gear unit with 2 planetary stages and one helical gear stage has more than 13 tooth contacts and more than 20 contacts in bearings, where frictional losses are generated. If the efficiency of this wind turbine gearbox could be improved by 0.2 % from 97.5 % to 97.7 %, the power gain at 5 MW/ h would be 10 kW/ h, and with a population of 40,000 gearboxes, this would result in 400 MW/ h. With 7,000 production hours per year, this represents an enormous value of 2,800 GW per year, which could be saved. Selecting lubricants with low friction coefficients for machinery will therefore make a significant contribution in reducing global warming. Goal setting The most important challenge of today is, reduce global warming. There are various possibilities to support this goal. A new approach to support the reduction of global warming is, reduce friction losses in all lubricated contacts. Therefore, it is necessary to agree specifications for lubricants with respect to their property of friction loss, by specifying the maximum allowable friction coefficient or moment and / or wear value. The goal is, to propose general limits for the friction behavior of lubricants in contact surfaces of gears and bearings. State of the art knowledge about friction losses in lubricated contacts Methods for calculating the heat generation and heat dissipation in gears were presented in [1] as early as Science and Research 25 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0031 Can lubricants with lower frictional torque in rolling contacts of bearings, gears and machines significantly reduce global warming? Dirk-Olaf Leimann* Lubricants for machines with machine elements as steel bearings and gears, such as used in gearboxes or engines, are supposed to reduce friction and wear in the rolling surface contacts, but, dependent on the lubricant, as shown below, they themselves generate lower or higher levels of friction, and thus energy losses, which influence global warming. This article compares various lubricants, as mineral and synthetic oils, to determine the differences of friction coefficients, wear values and frictional torque of those lubricants. It will be shown, how the appropriate selection of lubricants with lower friction or wear values can reduce power losses and thus have a positive impact on the climate and reduce global warming. Friction coefficients, wear values, and frictional torque are measured in tests on FE8 (bearing), FZG (gear) or two disk test rigs. The used data base in this paper contains 227 data sets with test results on gears (FZG, 6 oils), bearings (FE8, 41 oils) and two disk tests (11 oils), with overall more than 12000 data. Even, if the effect of reduced friction due to the lubricant selection may be small per individual bearing or gear, the total impact on global warming is enormous, given by the use in billions of machines and machine elements worldwide. Keywords energy loss, mineral oils, synthetic oil, friction coefficients, wear values, frictional torque Abstract * Dipl.-Ing. Dirk-Olaf Leimann Düsseldorfer Straße 4 47441 Moers ons regarding the properties and load behavior for the lubricant selection of rolling bearings and a few regarding the load behavior of gears. Examples of these can be found in [4, 5]. Tables 1, 2, and 3 show these accustomed requirements [4,5]. As visible from these specifications, no requirements for friction properties are present. Friction moment tests on FE8 and FZG test rigs FE8 tests for bearings and FZG tests for gears are very common to determine lubricant behavior. The images 1 and 2 show those test rigs. The purpose of the FE8 bearing tests and FZG gear tests, as shown in table 1, 2 and 3 is, to give results for lubricants with respect to wear and fatigue behavior. As already mentioned, no tests are recommended for friction behavior. Science and Research 26 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0031 1982. Methods for reducing losses in gear designs measures were described in [2, 3] in 1993 / 1994. All of these and other design measures have led to better efficiencies, being achieved in current gear designs. Due to the fact, that gear box designs, as mentioned before in the example wind turbine gear box, have more than 33 different lubricated contacts with different designs, sizes, load and speeds, the choice of the best lubricant for each contact leads at least to a high number of required lubricants with a veriety of viscosities and properties. But, in a gear box, only one lubricant can be used. Means, the properties may not be the best choice for all contacts. Therefore, a compromise in the choice of the final lubricant is mandatory. A lot of information for the selection of a lubricant for gears and bearings are given in [1, 11,16], where also tables and equations to calculate friction losses are given. However, to date, there are generally no specifications for the friction behavior of lubricants, i.e., friction coefficients or friction torques or wear values, when selecting lubricants for machines. There are many specificati- Property Test method Test conditions Criteria pass Oxidation ASTM D2893 95 °C < 6 % Oxidation ASTM D2893 120 °C < 6 % Foaming ASTM D892 3 x max. 75/ 10 Cupper corrosion ISO 2160 120 °C max. damage degree 2 Wear DIN 51819-3 DIN 51819-3 < 15 / 30 mg Ripplings DIN 51819-3 DIN 51819-3 no Micropittings DIN 51819-3 DIN 51819-3 no Rust, distiled water ISO 11007 ISO 11007 max. damage degree 1 Rust 0,5 % NaCl ISO 11007 ISO 11007 max. damage degree 3 Fatigue > 800 Wear < 30 mg Fatigue with water > 600 hrs Sludge slight Filter blokking no Wear of rollers < 15 mg Cage wear < 40 mg FE8 FAG Step 2 IEC 61400-4 FE8 FAG Step 4 Table 1: Criteria for selecting lubricants for rolling bearings, fresh oil [4] Standardized test methods for bearings acc. to IEC 61400-4 [5] bearing type Load Speed Temperature Runtime Roller wear Fatigue damage Rippling Micropitting P n t L kN rpm ° C h mg FE 8 stage 1 81212 100 7,5 80 80 < 30 small small FE 8 stage 2 81212 100 75 70 800 < 30 - - - FE 8 stage 4 with added water 81212 60 750 100 > 600 < 30 No - - Test method - - - - Table 2: Criteria for selecting lubricants for rolling bearings, fresh oil [5] Standardized test methods for gears acc. to IEC 61400-4 [5] Procedure name Test method Test conditions Recommended minimum requirement Gear wear (adhesive) FZG scuffing test ISO 14635-1 A/ 8.3/ 90 Fails > Load step 12 Gear wear (fatigue) FZG micropitting test FVA 54 / I - IV CGF/ 8.3/ 60 Fails > Load step 10 Gear wear (fatigue) FZG micropitting test FVA 54 / I - IV CGF/ 8.3/ 90 Fails > Load step 10 Table 3: Criteria for selecting lubricants for gears, fresh oil [5] Calculation methods for power losses and friction behavior In [1], equations, methods and examples are given for the calculation of power losses of gears, i.e. spur and helical gears, bevel gears, worm gears, rolling bearings, plain bearings, oil bath and seals and heat dissipation. There are calculation methods available to calculate friction coefficients and friction moments. These methods can be found in [1, 10, 11,16] The power loss P VZ of gears is the sum of load loss P VZP and no-load loss P VZ0 : (1) The load loss P VZP , can be calculated acc. to equation (2), where a friction coefficient µ mZ and a factor H V can be calculated according to [1, 10, 11]. H V is an important factor, which can also take the gear micro geometry into account. The calculation of the no-load loss P VZ0 contains a lot of influence factors and is difficult to determine. (2) The calculation method for bearings is like the calculation method of gears. The friction torque “loss” of a bearing T VL is the sum of the “load loss” T VLP and “no-load loss” T VL0 . (3) The load dependent friction torque T VLP is: VZ VZ = VZP ZP + VZ0 Z0 VZP mZ v VL VLP VL0 VZ VZP VZ0 VZP ZP = × mZ mZ × v VL VLP VL0 VZ VZP VZ0 VZP mZ v VL VL = VLP LP + VL0 L0 (4) with the load P 1 (Nm) and the bearing pitch diameter d m (mm). The no-load dependent friction torque T VL0 is: if (5) The no load dependent T VL0 friction loss is: if (6) For the detailed calculation equations and units, please refer to the equations, mentioned in [1, 10, 11,16]. For gears, the friction coefficient must be calculated, for bearings, friction coefficients are given in combination with a load dependent factor f 1 for different bearing types. See table 4. Friction coefficients vary between 0,0005 and 0,0040. Friction coefficient and f 1 values are dependent on source [1,10, 11,16]. To calculate the no-load friction moment for bearings, a f 0 factor is given for different bearing types and lubrication methods. See table 5. (f 0 values are dependent on source [1, 10,11,16]) These calculations and factors are later used in the tables for the comparison of the measured friction moments to the calculated friction moments. VLP LP = 1 × 1 × m VL0 0 (t) m (t) VL0 0 m (t) VLP 1 1 m VL0 L0 = 0 ×( (t) t) × ) , × m (t) VL0 0 m (t) VLP 1 1 m VL0 0 (t) m (t) × ≥ VL0 0 m (t) VLP 1 1 m VL0 0 (t) m (t) VL0 L0 = × 0 × m (t) VLP 1 1 m VL0 0 (t) m (t) VL0 0 m (t) × < Science and Research 27 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0031 Image 1: FE8 test rig [8,14] VZ VZP VZ0 VZP mZ v Image 2: FZG test rig [4] VZ VZP VZ0 VZP mZ v stress relationship, c, exponent in the stress-life equation, e, Weibull exponent, η a , hoop and residual stress factor, η b , lubrication factor, η c , contamination factor. The factor for the lubrication influence η b is calculated as: (8) with: ψ, bearing characteristic number, M, viscosity ratio related factor, κ, viscosity ratio, m, viscosity ratio related factor. The factors for equation 7 and 8 can be used from table 6 [12]. 1 0 a b c u b = × � , × � − �� Science and Research 28 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0031 New approaches for bearing life calculation In 1999, a new approach for the life time equation for bearings was published by E. Ioanides, G. Bergling and A. Gabelli [12]. The new equation included a new lubrication factor η b . The general life time equation is: (7) with : L 10 ,Basic rating life [Mrevs], A, scaling factor, C, Basic dynamic load rating [N], P, equivalent dynamic bearing load [N], P u , fatigue load limit ( see catalogue) [N], p, exponent in life equation, w, exponent in the load- 10 = � 〈 −� a × b × c × u � 〉 � × � � b Friction coefficients and determination of friction factor f 1 P / C = 0,1 - Min Max Deep groove ball bearing 0,0015 0,0030 0,0020 to be calculated 0,0009 (P 0 / C 0 )^0,55 Self-aligning ball bearing 0,0010 0,0030 0,0010 to be calculated 0,0003 (P 0 / C 0 )^0,4 Angular contact ball bearing single row 0,0015 0,0020 0,0015 (F/ C)^0,33 (40°) 0,0013 (P 0 / C 0 )^0,33 Angular contact ball bearing double row 0,0024 0,0030 0,0020 (F/ C)^0,33 0,0010 (P 0 / C 0 )^0,33 Cylindrical roller bearing 0,0010 0,0030 0,0005 1 0,0003 1 Needle roller bearing 0,0020 - 0,0005 1 - - Spherical Roller Bearing 0,0020 0,0030 0,0010 to be calculated 0,0035 1 Tapered roller bearing 0,0020 0,0050 0,0010 to be calculated 0,0005 1 Axial deep groove ball bearing 0,0012 - 0,0015 (F/ C)^0,33 0,0012 (P 0 / C 0 )^0,33 Axial spherical roller bearing 0,0030 - 0,0015 1 0,0006 1 Axial cylindrical roller bearing 0,0040 - 0,0035 1 0,0018 1 Bearing type Friction coefficient load dependent [1] f 1 Basic μ Factor load Friction coefficient general [exact determination see 10,16] μ Friction coefficient load dependent [exact determination see 10,16] f 1 Basic μ Factor load Table 4: friction values µ and load dependent friction factor f 1 Factor f 0 for no-load loss calculation for different bearing types and lubrication methods Min Max Min Max Min Max Min Max Min Max Deep groove ball bearing 1,5 2,0 1,5 2,0 3,0 4,0 0,7 1,0 0,7 1,0 Angular contact ball bearing single row 1,5 2,0 0,7 1,0 Angular contact ball bearing double row 3,0 4,0 1,6 2,0 Cylindrical roller bearing 2,0 3,0 2,0 3,0 4,0 6,0 1,0 1,5 1,5 2,0 Needle roller bearing 6,0 12,0 6,0 12,0 12,0 24,0 3,0 6,0 3,0 6,0 Spherical Roller Bearing 4,0 6,0 4,0 6,0 8,0 12,0 2,0 3,0 2,0 3,0 Tapered roller bearing 3,0 4,0 3,0 3,5 6,0 8,0 1,5 2,0 1,5 2,0 Axial deep groove ball bearing 1,5 2,0 1,5 2,0 3,0 4,0 0,7 1,0 0,7 1,0 Axial spherical roller bearing 3,0 4,0 3,0 4,0 6,0 8,0 Axial cylindrical roller bearing 2,0 3,0 - - 2,5 5,0 - f 0 2,0 4,0 1,0 4,0 8,0 2,0 Bearing type f 0 f 0 f 0 f 0 Oil bath / grease horizontal shafts [1] Oil bath / circulating [10] Oil bath / grease vertical shafts [1] Oil mist lubrication [1] Lubrication oil mist, drop, grease [10] Table 5: no-load dependent friction factor f 0 10 a b c u b Factor 0,1051 < κ < 0,41 use κ = 0,1051 if κ < 0,1051 0,41 < κ < 1 1 < κ < 4 use κ = 4 if κ > 4 Radial ball bearing Radial roler bearing Thrust ball bearing Thrust roller bearing M 0,87830 0,77860 0,77890 - - - m 0,05760 0,19090 0,07174 - - - ψ - - - 0,50 0,15 0,16 0,06 w - - - 1 / 3 1 / 2,5 1 / 3 1 / 2,5 p - - - 3 10 / 3 3 10 / 3 c / e - - - 9,3 9,2 9,3 9,2 A - - - 0,10 0,10 0,10 0,10 Table 6: Constants for the factors in the new approach for bearing life time calculation [12] Equation 8 was applied to the data for the axial cylinder roller bearing 81212 from table 8 for 6 different lubrication oils and different speeds, tested on a FE8 test rig. The diagram 1 shows the calculated factor η b and the measured friction moment with respect to the viscosity ratio κ. It is an interesting observation, that there is a relation between the friction moment and η b , unfortunately, there is no equation available to convert η b values to friction moments. The research continued in [17] with the “SKF Generalized Bearing Life Model”, where an equation for the surface risk function R S , as part of the general model, L 10GM , contains the influence of surface stress and conditions as lubrication, contamination, wear, and others. (9) S = ( , , c , 1 , 2 ) Data analysis friction behavior of lubricants from available research results In some research studies (references [6, 7, 8, 9]) with the goal to determine the load carrying capacity of gears and rolling bearings, friction coefficients, wear, and frictional torques values were measured and documented. In the cited papers, information about these lubricants is available on mechanical properties such as viscosity grade and, in some studies, also on chemical components, which results in total > 227 data cases. In [8], 4 different bearing types in combination with 10 lubrication oils and several load and speed combinations were examined and the frictional torque values were measured. The measured friction moment was compared with calculated friction moments acc. to the equations 4 to 6. The data and the results are shown in table 7. Table 7 shows, using data from [8] with the axial cylindrical roller bearing 81212 and gear information as an example, that with ten different lubricants, significant differences in the friction torques of the rolling bearing and the gear scuffing test results can be seen. From table 7 it can be assumed, that a direct influence on the friction moment by different chemical components and their amount is not visible. In [8], all lubricant tests were carried out with 4 bearing types: axial cylindrical roller bearing 81212, axial spherical roller bearing 29412, axial deep groove ball bearing 51212, and angular contact ball bearing 7312. A comparison of the Science and Research 29 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0031 S c 1 2 Friction moment Friction moment load calculated [1] Load Value Speed Temperature Hertzian Stress T T (Load Cal.) P C / P n t p H ν 40 ν 100 ϱ 15 S P Zn Ca Nm Nm kN rpm ° C N/ mm² mm²/ s mm²/ s kg/ dm³ ppm ppm ppm ppm 1 81212 [ 8 ] I1.M.A99.184 Mineral 1 5,30 0,87 12,2 5 30,0 80 1282 184 16,9 0,909 2680 996 6 57 12 2 81212 [ 8 ] I1.M.A99.184 Mineral 1 5,70 0,87 12,2 5 7,5 80 1282 184 16,9 0,909 2680 996 6 57 12 3 81212 [ 8 ] I2.P.SP.100 PAO 2 5,40 0,87 12,2 5 30,0 80 1282 100 14,0 0,860 4500 332 12 19 12 4 81212 [ 8 ] I2.P.SP.100 PAO 2 5,90 0,87 12,2 5 7,5 80 1282 100 14,0 0,860 4500 332 12 19 12 5 81212 [ 8 ] I3.M.SP.100 Mineral 3 8,60 0,87 12,2 5 30,0 80 1282 100 11,0 0,890 1900 115 0 24 12 6 81212 [ 8 ] I3.M.SP.100 Mineral 3 8,70 0,87 12,2 5 7,5 80 1282 100 11,0 0,890 1900 115 0 24 12 7 81212 [ 8 ] I4.M.PD.100 Mineral 4 8,60 0,87 12,2 5 30,0 80 1282 100 11,0 0,880 12200 1831 1178 546 12 8 81212 [ 8 ] I4.M.PD.100 Mineral 4 8,70 0,87 12,2 5 7,5 80 1282 100 11,0 0,880 12200 1831 1178 546 12 9 81212 [ 8 ] SG1.M.A20.146 Mineral 5 3,40 0,87 12,2 5 30,0 80 1282 146,5 14,5 0,895 26900 1322 16 32 12 10 81212 [ 8 ] SG1.M.A20.146 Mineral 5 5,10 0,87 12,2 5 7,5 80 1282 146,5 14,5 0,895 26900 1322 16 32 12 11 81212 [ 8 ] SG2.M.SP.79 Mineral 6 8,90 1,67 12,2 5 30,0 80 1282 79 9,8 k.A. 10400 489 0 275 no data 12 81212 [ 8 ] SG2.M.SP.79 Mineral 6 10,50 0,87 12,2 5 7,5 80 1282 79 9,8 k.A. 10400 489 0 275 no data 13 81212 [ 8 ] A1.M.SP.32 Mineral 7 8,50 0,87 12,2 5 30,0 80 1282 32 7,1 0,876 2600 54 0 819 10 14 81212 [ 8 ] A1.M.SP.32 Mineral 7 8,40 0,87 12,2 5 7,5 80 1282 32 7,1 0,876 2600 54 0 819 10 15 81212 [ 8 ] A2.M.SP.35 Mineral 8 9,40 0,87 12,2 5 30,0 80 1282 35 7,0 0,856 2300 188 0 52 8 16 81212 [ 8 ] A2.M.SP.35 Mineral 8 10,00 0,87 12,2 5 7,5 80 1282 35 7,0 0,856 2300 188 0 52 8 17 81212 [ 8 ] C2.M.SP.34 Mineral 9 7,70 0,87 12,2 5 30,0 80 1282 34 7,1 0,867 1100 466 697 678 11 18 81212 [ 8 ] C2.M.SP.34 Mineral 9 18,40 1,30 18,4 3,33 7,5 80 1282 34 7,1 0,867 1100 466 697 678 11 19 81212 [ 8 ] M2.M.ZP.106 Mineral 10 9,20 0,87 12,2 5 30,0 80 1282 106 11,8 0,896 10800 517 1166 4262 12 20 81212 [ 8 ] M2.M.ZP.106 Mineral 10 9,80 0,87 12,2 5 7,5 80 1282 106 11,8 0,896 10800 517 1166 4262 12 8,31 0,93 18,40 1,67 3,40 0,87 Maximum value friction moment Minimum value friction moment Count oils Oil mechanical data Oil chemical data Gear type: FZG- A/ 8.3/ 90 Load step fail > Average value friction moment Count cases Bearing type and size Source Test oil designation Oil type Table 7: Data for the frictional moment of 10 lubricants, measured on FE8 and FZG test and more detailed information about the lubricants 10 a b c u b 0,00000000E+00 5,00000000E-03 1,00000000E-02 1,50000000E-02 2,00000000E-02 2,50000000E-02 0,1051 0,1051 0,1051 0,1051 0,1051 0,1051 0,2841 0,2042 0,1732 0,1716 0,2360 0,1499 0,6078 0,4370 0,3706 0,3671 0,5049 0,3207 1,0805 0,7768 0,6587 0,6526 0,8976 0,5701 1,9208 1,3808 1,1710 1,1601 1,5956 1,0135 3,4146 2,4547 2,0817 2,0623 2,8364 1,8017 Lubrication factor η b [12] Friction moment x 1000 Nm [8] Viscosity ratio κ eta b Measured friction moment Nm x 1000 Diagram 1: Comparison of measured friction moments with calculated factor η b [12] Science and Research 30 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0031 Table 8: Measured friction moment and comparison with calculation results with data from [8] measured friction moment with the calculated friction moment acc. to the equations 4 to 6 for the bearings with the bearing load in table 7 and 8 clearly illustrates differences between calculation and measurement. The difference between measurement and calculation is big, near to 1000 %. In [9], six different lubricants with different viscosities and base oils were used. Table 9 shows results for gears and 2 different test procedures. Table 10 shows results from [9] for the bearings and gears, tested with the same or quite similar oils. Table 11 shows measured friction coefficients on a twodisk test rig with data from [7] and lubricant details. The data contain two chemical components and the viscosity data. Here it can be observed, that the measured friction values do not differ very much with respect to the speed. The question is, is this test suitable to gain friction informations. Could a FE8 bearing test also be represent for Gears? This question should be more examined, at least, both contacts have similar conditions regarding the frictional behavior. Diagram 2 gives some information to it. Probably the answer is no. Science and Research 31 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0031 Oil type short name Phosphorus Zinc ν 40 ν 100 ? 15 Test temperature Speed Hertian stress p c Water content fresh oil Test Wear Gear type mg/ kg mg/ kg mm²/ s mm²/ s kg/ dm³ °C 1/ min N / mm² ppm mg - Polyglycol PG1 2231 1689 224,1 39,5 1,059 60 2250 1723 1500 DGMK 575 7 / 5 C-GF Polyglycol PG2 2267 4 209,5 38,1 1,061 60 2250 1723 3000 DGMK 575 13 / 12 C-GF Polyalphaolifin PAO 470 2 224,5 27,9 0,859 60 2250 1723 120 DGMK 576 5 / 8 C-GF Ester E 136 5 199,9 24,8 0,950 60 2250 1723 300 DGMK 577 6 / 4 C-GF Mineral oil M 358 73 214,6 18,9 0,898 60 2250 1723 80 DGMK 578 9 / 9 C-GF Manual Transmission Fluid MTF 243 0 63,1 12,5 0,837 90 2250 1723 240 DGMK 575 16 / 13 C-GF Oil type Short name Phosphorus Zinc ν 40 ν 100 ? 15 Test temperatu re Speed Hertian stress p c Water content fresh oil Test Wear Gear type mg/ kg mg/ kg mm²/ s mm²/ s kg/ dm³ °C 1/ min N / mm² ppm mg - Polyglycol PG1 2231 1689 224,1 39,5 1,059 60 13 1853 1500 DGMK 377-1 38 C-PT Mineral oil M 358 73 214,6 18,9 0,898 60 13 1853 80 DGMK 377-1 58 C-PT Manual Transmission Fluid MTF 243 0 63,1 12,5 0,837 90 13 1853 240 DGMK 377-1 19 C-PT Results micro pitting test on FZG test bench according to DGMK 575 Results wear test on FZG test bench according to DGMK 377-1 FVA 488 Part Gears [ 9 ] Table 9: Detailed information to FZG tests [9] ν 40 VI ν 100 S P Zn Ca Mg mm²/ s mm²/ s ppm ppm ppm ppm ppm mg Nm [ 9 ] SKL PG 220 235 42 3254 2267 4 0 No Data FE 8 - 5,90 [ 9 ] SKL PG 220 235 42 3254 2267 4 0 No Data FE 8 - 5,90 [ 9 ] AZRL PG 220 235 42 3254 2267 4 0 No Data FE 8 - 9,75 [ 9 ] Gear PG1 224,01 231 39,48 No Data 2231 1689 0 0 DGMK 575 7 / 5 - [ 9 ] Gear PG1 224,01 231 39,48 No Data 2231 1689 0 0 DGMK 377-1 38 - [ 9 ] Gear PG2 213,84 236 39,9 No Data 2267 4 0 0 DGMK 575 13 / 12 - [ 9 ] SKL Ester 220 164 28 338 170 0 0 No Data FE 8 - 8,80 [ 9 ] Gear Ester 209,8 158 26,8 No Data 136 5 7 0 DGMK 575 6 / 4 - [ 9 ] SKL Mineral 220 101 19,5 10406 326 12 25 No Data FE 8 - 12,80 [ 9 ] AZRL Mineral 220 101 19,5 10406 326 12 25 No Data FE 8 - 16,00 [ 9 ] Gear Mineral 223,24 95 18,9 No Data 358 73 29 0 DGMK 575 9 / 9 - [ 9 ] Gear Mineral 223,24 95 18,9 No Data 358 73 29 0 DGMK 377-1 58 - [ 9 ] SKL PAO 220 163 28 2870 495 0 1 No Data FE 8 - 9,50 [ 9 ] Gear PAO 227,2 166 28,73 No Data 470 2 0 0 DGMK 575 5 / 8 - [ 9 ] SKL MTF 64 183 9,5 700 257 4 19 No Data FE 8 - 6,70 [ 9 ] Gear MTF 63,35 175 11,8 No Data 243 0 25 0 DGMK 575 16 / 13 - [ 9 ] Gear MTF 63,35 175 11,8 No Data 243 0 25 0 DGMK 377-1 19 - Source Oil data Test results Oil Type Oil mechanical data Oil chemical data Test name Wear Friction moment Bearing Type / Gear Table 10: Results from [9] on wear and friction moment for bearings and gears Nevertheless, there are ways to reduce friction coefficient values and friction torques and thus contribute to reducing losses and thus heat. Even the smallest improvements with the help of a suitable lubricant have an enormous effect on most of all applications. There might be also the opportunity to use coatings to additionally reduce the friction in rolling steel contacts. Due to lack of data, it was not possible to evaluate the influence of coatings. For both, bearings and gears, test methods for test benches such as FE-8 (bearings) or FZG (gears) are suitable to compare different lubricants about their friction behavior. It is recommended that these tests are included in the lubricant specifications for machines. For bearings, the recommendation for a lubricant specification would be to conduct an FE8 test with the conditions i.e. bearing type 81212, C/ P = 5, speed 30 rpm at 80° C and determine the frictional torque. The frictional torque for a lubricant should be less than 5 Nm. Diagram 3, which is the result of 168 measured friction moments, shows, that a limit of 5,4 Nm is feasible. For gears, a FZG test according to DGMK 575 is seen as suitable, in which the maximum wear for a lubricant should be less than 5-7 mg. Science and Research 32 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0031 Conclusion Measuring overall or individual efficiencies and power losses on machines, such as gearboxes and engines (combustion or electric motors), and drawing conclusions about friction coefficients or friction values for machine elements such as lubricants, gears or bearings is very difficult, almost impossible. The inaccuracies in the measurements are usually greater than the small individual differences in the values, to be measured. Table 11: Results for the frictional coefficient measured on a two-disk test Spur gear Path of contact Axial Bearings Roller width 0 D d m d > 0 0 > 0 >> 0 >> 0 A C E Friction Value μ Relative loss friction Moment Δ T with corrections Diagram 2: Frictional behavior of gears and axial cylindrical bearings Literature [1] D. Leimann, Wärmeentstehung und Wärmeabfuhr bei Getrieben, Firmenschrift PEKRUN, Iserlohn, 1982 [2] D. Leimann, Wärmearm konstruieren, Teil 1: Einfluss des Zahnflankenspiels auf die Erwärmung bzw. Verlustleistung von Zahnradgetrieben, antriebstechnik 32, 1993, Nr.3 [3] D. Leimann, Wärmearm konstruieren, Teil 4: Einfluss von Zahnbreite, Motorauswahl und Schmierstoff auf Erwärmung und Geräuschverhalten, antriebstechnik 33, 1994, Nr.4 [4] D. 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Witzig et all, Zulässiger Wassergehalt in Getriebeschmierölen, insbesondere Polyglykol-Ölen und der Einfluss auf die Wälzlagerlebensdauer und die Zahnflankentragfähigkeit einsatzgehärteter Stirnräder, Vorhaben FVA 488, FVA Frankfurt, 2009 [10] Eschmann, Hasbergen, Weigand, Brändlein, Die Wälzlagerpraxis, zweite Auflage, Oldenburg Verlag München Wien, 1978 [11] NN, GfT Arbeitsblatt 5, Zahnradschmierung, Gesellschaft für Tribologie, Jülich [12] E. Ioanides et all, An analytical formulation for life of rolling bearings, Acta Polytechnica Scandinavica, The finnish Academy of Technology, Espoo, 1999 [13] NN, DIN/ ISO 281, Beiblatt 1, Wälzlager - Dynamische Tragzahlen und nominelle Lebensdauer - Lebensdauerbeiwert aDIN und Berechnung der erweiterten modifizierten Lebensdauer, DINMEDIA, Berlin, 2003 [14] N 060 mod 1, GfT Arbeitsgruppe „Datenbank Tribologische Prüfstände“ - Datenblatt FE 8 Prüfgerät, 2012 [15] NN, GfT Arbeitsblatt 7, Tribologie Definitionen, Begriffe, Prüfung, Gesellschaft für Tribologie, Jülich, 2002 [16] NN, GfT Arbeitsblatt 3, Wälzlagerschmierung, Gesellschaft für Tribologie, Jülich, 2006 [17] NN, SKF Group, PUB BU/ P9 15513 EN · March 2015 Science and Research 33 Tribologie + Schmierungstechnik · volume 72 · issue 6/ 2025 DOI 10.24053/ TuS-2025-0031 Diagram 3: Test results and limit value for frictional moment of FE8 bearing test