Correlation between Tribofilm Formation and Friction Coefficient in Continuously Variable Transmission at the Initial Stage of Rubbing

Abstract

To improve the transmission efficiency of a continuously variable transmission (CVT) system, it is effective to form a tribofilm with higher friction coefficient. Therefore, understanding of the correlation between the tribofilm formation and friction coefficient in a CVT fluid has been a great deal. In this study, the tribofilm formation on a bearing steel ball surface under the simulated additive oil was investigated. We used ball-on-disk tribological test and optical microscopy imaging to observe the temporal changes in the tribofilm growth. Atomic force microscopy and energy-dispersive X-ray spectrometry analyses revealed that the tribofilm consisted of pad-like structures derived by tricresyl phosphate and calcium sulfonate additives. A binary imaging analysis showed that the total area of the pad-like structure was a key parameter determining the friction coefficient. The pads were repeatedly desorbed and reformed during the sliding test. These results improve the design of future CVT systems and lubricants.

Relationship between Nano/Macrofriction Behaviors and Mechanical Properties of Tribofilms: An Investigation Using Multidimensional Nanoindentation Equipment

“…Table 1 shows the sliding conditions. The sliding conditions were determined with reference to previous research reports [1,10]. In these reports, the sliding conditions of the belt element and the pulley were clarified based on the driving situation investigation in CVT systems of the actual vehicles.…”

“…We prepared three lubricants containing TCP, OBCS, and DBDS; the amounts of PAO4 and additives were measured using an electronic balance (AUW220D, Shimadzu, Japan). Table 2 shows the amount of additives based on previous studies [1,3,11]. To compare the frictional properties of these lubricants with those of a commercial CVTF and base oil, friction tests were also conducted in PAO4 alone and a commercial CVTF (NS-3, Idemitsu Kosan, Japan) under the same conditions.…”

“…The adhesion friction is also the product of the real contact area A r and the shear strength τ on the sliding surface, as expressed in Eq. (1).…”

“…Therefore, almost all lubricants are designed by combining various lubricant additives to adapt them to each application. In continuously variable transmissions (CVTs), the frictional properties of the tribofilm have a significant effect on the performance and transmission efficiency [1,2] because the contact region is under boundary lubrication [3,4] and CVT fluids (CVTF) contain various lubricant additives, including phosphorus-, calcium-, and sulfur-based additives. In addition, the CVTF has great demand for generating high friction [5] because a CVT transmits torque power through the frictional force between the elements and the pulley.…”