1. INTRODUCTION The development of fuel efficient vehicles is being actively promoted as a result of growing awareness in environmental and energy-related issues. In addition to improving transmission efficiency, key measures for transmissions include increasing the number of shift speeds, introducing continuously variable transmissions (CVTs), and widening shift speed ranges. Tribology technology plays a major role in the development of traction-drive type CVTs since such transmissions transfer power through a solid film (hyaloid). The maximum surface contact pressure of the transmission contact area that forms the oil film exceeds 4 GPa and requires a normal load of more than 1 ton. This large load is one factor that makes it difficult to reduce the size of theCVT. Furthermore, relative rotation (generally referred to as “spin”) is generated at the contact surface, which is one cause of efficiency reduction. The amount of spin differs in accordance with the geometrical structure of the variator. One such structure is referred to as a half-toroidal configuration, which can be designed with low spin at the transmission contact area. According to a recent report, a double-cavity half-toroidal variator has been developed with a transmission efficiency of 97% or more in the overdrive ratio and an increased ratio range of 7.5 [ 2]. However, such a high transmission efficiency and wide ratio range requires excess normal load at the transmission contact area to be minimized. Accurately estimating the maximum traction coefficient ( μmax) is important to accomplish this goal. 2013-01-0366 Published 04/08/2013 Copyright © 2013 SAE International doi:10.4271/2013-01-0366 saepcmech.saejournals.org Study of the Prediction Method for Maximum Traction Coefficient Toshinari Sano and Mitsuaki Tomita Toyota Motor Corporation Masashi Inoue, Yasuhiro Takeuchi and Muneo Yorinaga Nippon Soken Inc ABSTRACT This report proposes a rheological model and a thermal analysis model for oil films, which transmit power through a variator, as a prediction method for the maximum traction coefficient, and then describes the application and verification of this method. The rheological model expresses the conditions inside the contact ellipse using a combination of viscosity and plasticity. The thermal analysis model for oil films was confirmed by comparison of previously obtained temperatures directly measured from the traction contact area of the four-roller experimental apparatus [ 1]. The measurement used a thin-film temperature sensor and the consistency between the calculated and measured values was verified in the estimation model by reflecting the precise thermal properties of the thin film. Most values were consistent with the calculated values for the middle plane local shear heating model inside the oil film. However, under some conditions, the values were closer to those calculated for homogeneous shear. Based on these results, this paper proposes a calculation method that combines both heating models within the contact ellipse. In addition, the variator in a continuously variable transmission (CVT) was verified for the first time by using a test box to take direct temperature measurements from the transmission contact area in the CVT. This test verified the precision of the thermal analysis model, including the spin motion. Finally, the maximum traction coefficient was estimated using these calculation models and compared with measured values. CITATION: Sano, T., Tomita, M., Inoue, M., Takeuchi, Y. et al., "Study of the Prediction Method for Maximum Traction Coefficient," SAE Int. J. Passeng. Cars - Mech. Syst. 6(2):2013, doi:10.4271/2013-01-0366. ____________________________________ 568Downloaded from SAE International by University of Auckland, Saturday, August 04, 20182. CALCULATION MODEL PROPOSAL 2.1. Rheological Model and Calculation Method Film shearing under high traction oil pressures is generally categorized as viscous, visco-elastic,