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Low-loss gears are an interesting design approach for increasing the efficiency and thermal load limits of gearboxes. The loss-optimized gear geometry concentrates sliding around the pitch point, which results in low load-dependent gear power losses. In this study, a method for modeling transient EHL (elastohydrodynamically lubricated) contacts in gear mesh considering mixed lubrication and thermal effects is introduced and applied to analyze the tribological behavior of a low-loss gear geometry. Special focus is placed on local frictional losses to analyze the role of the thermal effects of the gear mesh. Although a thermal reduction in fluid friction is observed, the overall effect on total frictional losses of the low-loss gear geometry is evaluated to be very small. The edge geometry strongly influences the lubricant film thickness and frictional power losses. [ABSTRACT FROM AUTHOR]

Low-loss gears are an interesting design approach for increasing the efficiency and thermal load limits of gearboxes. The loss-optimized gear geometry concentrates sliding around the pitch point, which results in low load-dependent gear power losses. In this study, a method for modeling transient EHL (elastohydrodynamically lubricated) contacts in gear mesh considering mixed lubrication and thermal effects is introduced and applied to analyze the tribological behavior of a low-loss gear geometry. Special focus is placed on local frictional losses to analyze the role of the thermal effects of the gear mesh. Although a thermal reduction in fluid friction is observed, the overall effect on total frictional losses of the low-loss gear geometry is evaluated to be very small. The edge geometry strongly influences the lubricant film thickness and frictional power losses.

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This article evaluates the frictional performance of different bearing grease formulations in full rolling bearings and a ball-on-disc rig and subsequently assesses whether the ball-on-disc test results can be used to predict the grease performance in actual bearings. A selection of custom-made greases with systematically varied formulations as well as their base oils were tested. Bearing torque was measured in two different cylindrical roller thrust bearings and a thrust ball bearing. The same lubricants were tested with ball-on-disc tribometers, a mini traction machine (MTM) to measure friction and an optical elastohydrodynamic (EHD) rig to measure film thickness. Both lithium complex and diurea greases were observed to produce lower friction than their base oils within the low speed, low nominal lambda ratio region, whereas the greases and oils had the same friction at high nominal lambda ratio values. These relative trends were the same in full bearing and single-contact MTM tests. The reduction in friction was seen to be related to the level of film thickness enhancement provided by greases at lower speeds, which leads to an increase in the effective lambda ratio and hence reduced friction. By extracting the sliding torque component from the overall measured bearing torque, a plot of the friction coefficient against the effective lambda ratio was produced encompassing all bearing and single-contact tests and all lubricants and test conditions. This plot was seen to follow a general shape of a master Stribeck curve, indicating that the numerical values of the friction coefficient from ball-on-disc and full bearing tests overlap and can be related to each other using this approach over the range of conditions employed here. Thus, single-contact ball-on-disc tests can provide a fast and economical way of establishing the frictional performance of bearing greases in full bearings in terms of both relative performance rankings and quantitative values of bearing frictional power losses. [ABSTRACT FROM AUTHOR]

This study aims at finding an analytical model of ball continuously variable transmission (B-CVT) behavior and performance, based on the existing model of the half-toroidal traction drive. The geometrical and kinematical aspects of this model have been found. Then, force-moment equilibrium equations have been applied for a B-CVT. The contact behavior has been modeled using the isothermal elastohydrodynamic lubrication (EHL) theory, and spin and relative slip losses have been estimated for this CVT. Finally, the overall efficiencies of the B-CVT have been analyzed and compared with the halftoroidal traction drive. This study results have shown that B-CVT has higher torque efficiency and lower speed efficiency in comparison with half-toroidal CVT. Also, it has shown that the adjustment of axial forces in B-CVT leads to higher efficiency in lower input torques. [ABSTRACT FROM AUTHOR]

In this proposal, the effect of the friction coefficient on the efficiency of spur gears with tip reliefs was analysed. For this purpose, the efficiency values using an average friction coefficient along the mesh cycle were compared with those obtained implementing an enhanced friction coefficient formulation, which is based on elastohydrodynamic lubrication fundamentals. In this manner, it can be established the differences between both formulations in the efficiency and friction coefficient values, as well as the advantages of using this enhanced friction coefficient with respect to formulations implemented in traditional approaches of efficiency calculation. In addition to studying the impact of the friction coefficient choice on efficiency, the profile modifications influence on the friction coefficient and efficiency was also assessed. In this regard, three tip relief case studies were set out; pinion tip reliefs, driven wheel tip reliefs and profile modifications in both gears. From the results, it was inferred that the choice of friction coefficient formulation clearly influences the efficiency in gear transmissions with tip reliefs, obtaining discrepancies between both formulations with regard to which tip relief case study provides the lowest efficiency values. [ABSTRACT FROM AUTHOR]

It has been suggested in the literature that the ability of a lubricant to produce low “end of test temperature” (EOTT) in the achsialrillenkugellager (ARKL)—that is, axial groove ball bearing—thrust bearing test is an indicator of its likely efficiency in vehicle transmissions. Based on this, the current study has used regression analysis to correlate the ARKL EOTT with a range of physical and friction properties of a set of 26 lubricants. The latter have been selected to span a wide variety of base oil and additives types. It has been found that the most important lubricant property in determining ARKL EOTT is the elastohydrodynamic (EHD) friction or traction coefficient at low slide–roll ratio. Lubricants with low EHD friction were found to give low EOTT. Two lubricant properties of secondary importance have been identified. One is the lubricant viscosity index (VI), where a high VI contributes to high EOTT. The second is the heat transfer capability of the fluid, where a high capability was found to correlate with lower EOTT. The findings help clarify how low EOTT in the ARKL test can be achieved via lubricant composition. If the correlation between ARKL and transmissions that has been suggested in the literature is valid, then these findings are transferable to the design of high efficiency transmission lubricants. [ABSTRACT FROM AUTHOR]

• A novel traction drive with large speed ratio and cam loading is proposed. • The performance evaluation model is developed under the new loading principle. • The model is established based on non-smooth surface under non-ideal contact. • Performances are investigated in terms of traction capability, and efficiency. • The spin loss is negligible, resulting in higher transmission efficiency. The development of high-speed motors has stimulated the demand for high-speed reducers. Aiming at the lack of high-speed reducer and the NVH problem of high-speed gear transmission, an asymmetric loading multi-roller planetary traction drive (AL-MPTD) scheme is proposed to fill the gap of traction drive with large speed ratio, zero spin and self-adaptively loading. By comprehensively considering the effects of axis tilt, elastic deformation, rough surface contact and elastohydrodynamic lubrication (EHL), this paper establishes a more refined theoretical analysis model and provides the analysis method under the condition of the non-smooth surface and non-ideal contact. This paper also studies the transmission performance of AL-MPTD from the aspects of traction ability, transmission efficiency and so on. The results obtained by theoretical calculations show that AL-MPTD can be considered to achieve zero-spin transmission in the entire operation range. In addition, the loading mechanism achieves a good self-adaptively loading effect, effectively reducing the sliding of the AL-MPTD and improving its efficiency, with the peak of the global sliding coefficient being only 0.402% and the maximum transmission efficiency of 98.5%. [ABSTRACT FROM AUTHOR]

A novel continuously variable transmission (CVT) mechanism (a shaft-drive CVT) was developed by the present authors. In the mechanism, the input and output shafts with conical disks are parallel and an idler shaft with conical rollers at both ends is placed perpendicular to the input-output shafts. This idler shaft transmits torque from the input shaft to the output shaft and its linear motion causes the speed to vary by changing the contact point between the input-output disks and the idler roller. In this paper, the use of a disk with a concave shape is proposed in order to decrease the amount of spin caused by the change in the contact point, because the spin has an adverse effect on traction contact performance and is responsible for an increase in power loss. The design procedure of the novel concave disk and the roller is discussed and their effect is evaluated using a prototype CVT. The speed efficiency was improved to 1.6 points in the case of a transmitted power of 0.6 kW. [ABSTRACT FROM AUTHOR]

• Ball CVT performance evaluation model is established based on EHL traction theory. • Performances were investigated in terms of traction capability, and effectiveness. • Most design and performance factors are expressed as dimensionless values. • Conformal ball CVT has better traction capability than Non-conformal CVT. In this study, we established a performance analysis model of two types of ball continuously variable transmissions (CVT), conformal type and non-conformal type, by modifying the rheological model based on the elastohydrodynamic contact behavior for the existing toroidal traction drive to suit the working mechanism of ball CVTs. The main purpose of this study is, based on our analysis model, to identify the tendency of performance change according to design factors of ball CVTs and to compare the performance between two ball CVTs. Performance was analyzed in terms of torque capability, torque density and efficiency; factors affecting performance were selected through analysis of individual shapes and operating conditions. In this process, all design and performance factors were expressed as dimensionless quantities. It was confirmed that the conformal ball CVT has superior traction capability compared to the non-conformal ball CVT. In addition, by solving an example problem for ball CVT design and describing the process of designing optimal ball CVTs, it was proved that this study is applicable to real on-site problems. [ABSTRACT FROM AUTHOR]

Roller bearings are critical components in electrified powertrains. At high speeds, the critical role of the elastohydrodynamic lubrication (EHL) cannot be ignored, necessitating a tribo-dynamic analysis. This paper investigates the behaviour of roller bearings under operating conditions present in high-speed electrified powertrains. The bearing motion is obtained experimentally using a novel experimental rig. The bearing centre's orbital motion is acquired from the experiment and used as the boundary condition for tribological analysis. Results reveal that the EHL film thickness increases from 0.2 to 1.9 μm across a speed range of 0–15 000 rpm. A comparison with conventional dry contact analysis shows that roller load can be underestimated if the lubricant film is neglected in the roller-race contact deformation. • Tribo-dynamic study of rolling element bearings under high-speed operation. • Novel experimental and numerical methodology obtains bearing tribological behaviour. • Lubricant film is implicitly included at the roller-race contact within the model. • Dry Hertzian contact analysis underestimates roller load by 149% at high speeds. • Lubricant film thickness increases from 0.2 to 1.9 μm between 0 and 15 000 rpm. [ABSTRACT FROM AUTHOR]

Extensive wear appears in the case of dry contacts, or when the lubrication of the contacting surfaces is not appropriate. The aim of this research is to improve the mixed elastohydrodynamic lubrication in hypoid gears by the optimization of manufacture parameters for tooth surface processing. A full numerical analysis of the thermal mixed EHL in hypoid gears is applied. The equation system and the numerical procedure are unified for a full coverage of all the lubrication regions including the full film, mixed, and boundary lubrication. In the hydrodynamically lubricated areas the calculation method employed is based on the simultaneous solution of the Reynolds, elasticity, energy, and Laplace's equations. In the asperity contact areas the Reynolds equation is reduced to an expression equivalent to the mathematical description of dry contact problem. The real geometry and kinematics of the gear pair based on the manufacturing procedure is applied, thus the exact geometrical separation of the mating tooth surfaces is included in the oil film shape, and the real velocities of these surfaces are used in the Reynolds and energy equations. The transient nature of gear tooth mesh is included. The oil viscosity variation with respect to pressure and temperature and the density variation with respect to pressure are included. The non-Newtonian behaviour of the lubricant is considered. Using this model, the pressures, film thickness, temperatures, and power losses in the mixed lubrication regime are predicted. By using the developed method, the influence of the manufacturing parameters on the conditions of mixed elastohydrodynamic lubrication is investigated. On the basis of the obtained results recommendations are formulated to improve the mixed EHL and the efficiency of face-milled hypoid gears. • A full numerical analysis of the mixed EHL in hypoid gears is performed. • The influence of the manufacture parameters on mixed EHL in hypoid gears is investigated. • Recommendations are formulated to improve the EHL characteristics of hypoid gears by optimizing the manufacture procedure. [ABSTRACT FROM AUTHOR]