Optimization of Gearbox Efficiency

Although mechanical gearboxes used as torque and speed converters have already very high efficiency it is not only a task in automotive applications to further decrease gearbox power losses but also in many industrial applications. Different methods are discussed for power loss reduction in a gearbox. No load losses can be reduced, especially at low temperatures and part load conditions when using low viscosity oils with a high viscosity index and low oil immersion depth of the components. This in turn influences the cooling properties in the gear and bearing meshes. Bearing systems can be optimized when using more efficient systems than cross loading arrangements with high preload. Low loss gears can contribute substantially to load dependent power loss reduction in the gear mesh. Low friction oils are available for further reduction of gear and bearing mesh losses. All in all a reduction of the gearbox losses in average of 50 % is technically feasible. The challenge is substantial power loss reduction with only minor impact on load carrying capacity, component size and weight and noise generation. Adequate compromises have to be proposed. INTRODUCTION Future energy shortages have not only to be fought with exploitation of new renewable energy resources but also with reduction of energy consumption in all technical fields. For automotive applications optimisation attempts are made in all operating areas and for all components of vehicles to achieve minimum fuel consumption. Weight reduction and thermal management are possible approaches as well as hybrid systems and mechanical and software features for high efficient engines. Power loss reduction at the end of the power train has a large impact on overall optimization although absolute efficiency in gearboxes and rear axles is already high, (Xu, 2007). However, 1 kilowatt savings in the gearbox mean 4 kilowatts savings in fuel energy. Looking at wind turbines as a growing market for alternative energy production a modern equipment of the 5 megawatt class consist of 8 or more gear meshes and more than 12 bearing meshes. A reduction of the overall losses by 50 % would save some 200 kW power losses per wind turbine unit. B.-.R Höhn, K. Michaelis, M. Hinterstoißer Optimiranje... goriva i maziva, 48, 4 : 441-480, 2009. 463 The challenges are therefore a substantial power loss reduction with only minor impact on load carrying capacity, component size and weight and noise generation. BASIC CONSIDERATIONS Power loss in a gearbox consists of gear, bearing, seal and auxiliary losses (Fig. 1). Gear and bearing losses can be separated in no load losses which occur even without power transmission, and load dependent losses in the contact of the power transmitting components. Besides operating conditions and internal housing design no load losses are mainly related to lubricant viscosity and density as well as immersion depth of the components of a sump lubricated gearbox (Changenet, 2007). Load losses depend on transmitted load, coefficient of friction and sliding velocity in the contact areas of the components. Fig. 1: Composition of transmission power loss For nominal power transmission the load losses of the gear mesh are typically dominant. For part load and high speed high no load losses dominate total losses. For an optimisation of the whole operating range of a gearbox load losses and no load losses have to be addressed. In the following the major contributors to gearbox power losses namely bearings and gears are considered. BEARING POWER LOSS No load bearing losses depend on bearing type and size, bearing arrangement, lubricant viscosity and supply. Lowest no load losses of radial bearings are expected for cylindrical roller bearings. The also low values of taper roller bearings are valid for unloaded bearing arrangements, however, for the typical cross loading bearing arrangement axial preloading is required. This requirement of preload in a cross locating bearing arrangement with taper roller bearings increases the no load losses substantially. Optimiranje... B.-.R Höhn, K. Michaelis, M. Hinterstoißer 464 goriva i maziva, 48, 4 : 441-480, 2009. Fig. 2: Influence of bearing type on load losses (according Wimmer, 2003) Load dependent bearing losses depend also on bearing type and size, load and sliding conditions in the bearing and on lubricant type (Wimmer, 2003). Fig. 2 shows load dependent losses of bearings with same load capacity C = 20 kN and same utilisation ratio P0/C = 0,1. Again cylindrical roller bearings show the lowest power loss of radial bearings. Taper roller bearings for same load capacity have also low load power loss due to small diameters for same load capacity. A comparison of the bearings losses for the 6 gear in a manual transmission of a middle class car for original design with preloaded cross locating taper roller bearing arrangement and alternative design with locating four-point contact ball bearings and non-locating cylindrical roller bearings on the gearbox shafts and cross locating angular contact ball bearings of the final drive wheel (Fig. 3) was calculated according SKF, 2004. For medium load and medium speed conditions at low gear oil temperatures of 40°C, relevant for the new European drive cycle NED C, a reduction of the bearing losses of more than 50% was found for the alternative design, because of the preload on the cross locating taper roller bearings. At high gear oil temperatures of 90°C, where the preload is reduced to almost zero, the bearing loss reduction is still around 20% for the alternative design (Fig. 4). B.-.R Höhn, K. Michaelis, M. Hinterstoißer Optimiranje... goriva i maziva, 48, 4 : 441-480, 2009. 465 Fig. 3: Alternative bearing design in a manual transmission with final drive Fig. 4: Influence of design and operating temperature on bearing losses GEAR POWER LOSS No Load Gear Losses Besides operating conditions no load gear losses mainly depend on immersion depth in sump lubricated gearboxes as well as on lubricant viscosity. Otto, 2009 investigated systematically the influence of oil immersion depth in a sump lubricated test gearbox. Compared to the reference oil level at shaft centre line, three times module at pinion (3*m pinion) with pinion and gear immersed in oil, three times module at gear (3*m gear) as well as one times module at gear (1*m gear) with only the gear immersed in oil were investigated. The situation in the test gearbox for the different oil levels is shown in Fig. 5. The test gearbox was equipped with transparent front and top covers to visualize the oil churning in the test gearbox at different conditions of oil level, pitch line velocity and sense of rotation. Fig. 6 shows the distribution of an ATF ISO VG 32 at room temperature in the test gearbox at medium speed of v = 8,3 m/s and outward rotation. The reduction of churning losses with reduced immersion depth is clearly visible. Optimiranje... B.-.R Höhn, K. Michaelis, M. Hinterstoißer 466 goriva i maziva, 48, 4 : 441-480, 2009. Fig. 5: Immersion depth in test gearbox (according Otto, 2009) Fig. 6: Gear churning as a function of immersion depth (according Otto, 2009) B.-.R Höhn, K. Michaelis, M. Hinterstoißer Optimiranje... goriva i maziva, 48, 4 : 441-480, 2009. 467 No load loss measurements at pitch line velocities v = 8,3 and v = 20 m/s with a mineral oil ISO VG 100 at oil temperatures of 90 and 120 °C showed a substantial reduction of the gear no load losses with decreased immersion depth (Fig. 7). As expected the effect is higher at high speed conditions compared to lower speeds. However, in both speed conditions the churning losses can be reduced by more than 50 % when the immersion depth is reduced from centre line to 3 times module of the gear. In contrary to the beneficial effect of churning loss reduction with reduced immersion depth at the same time the detrimental effect of reduced cooling of the gear mesh has to be considered. Fig. 8 shows measured pinion bulk temperatures at different immersion depth. For high loads and high speeds the bulk temperature may even exceed the tempering temperature of the case carburised material. A substantial reduction of the load carrying capacity has then to be expected. Fig. 7: Influence of immersion depth on gear churning loss (accord. Otto, 2009) There are different opinions of the influence of lubricant viscosity on no load gear losses. Terekhov, 1975 reports increasing gear churning losses for increasing gear oil viscosities when using relatively high viscosity oils (Fig. 9). Michaelis, 1994 confirms increasing gear churning losses with increasing lubricant viscosity independent of the oil type (Fig. 10) also for low operating viscosities. Depending on the operating conditions a change from e.g. ISO VG 150 to VG 100 can reduce the no load power losses by some 10%. Investigations of Mauz, 1987 showed with increasing viscosity increasing churning losses for low speeds and decreasing churning losses for high speeds (Fig. 11). He explains this phenomenon that less oil volume is in motion at higher viscosities and thus less losses are generated. Optimiranje... B.-.R Höhn, K. Michaelis, M. Hinterstoißer 468 goriva i maziva, 48, 4 : 441-480, 2009. Fig. 8: Influence of immersion depth on pinion bulk temperature (accord. Otto, 2009) Fig. 9: Influence of oil viscosity on gear churning losses (accord. Terekhov, 1975) B.-.R Höhn, K. Michaelis, M. Hinterstoißer Optimiranje... goriva i maziva, 48, 4 : 441-480, 2009. 469 Fig. 10: Influence of oil viscosity on gear churning losses (accord. Michaelis, 1994) Fig. 11: Influence of oil viscosity on gear churning losses (accord. Mauz, 1987) Optimiranje... B.-.R Höhn, K. Michaelis, M. Hinterstoißer 470 goriva i maziva, 48, 4 : 441-480, 2009. Load Gear Losses The load gear losses PVZP in the mesh while power is transmitted follow the basic Coulomb law