Recently, an automatic transmission is one of the most popular systems for passenger cars. But it has more power losses than a manual transmission. Various studies have been conducted to improve transmission efficiency in automatic transmissions due to the regulation of greenhouse gases due to climate change. For the development of bearings for automatic transmissions that are becoming smaller and smaller with various driving environments, designing bearings considering durability and power loss is an important technology. In this study, the technology of the bearing preload and lubrication condition analysis, the technology of the vehicle running condition and bearing load condition analysis, the technology of the bearing power loss analysis, the technology of the bearing design to reduce friction torque, and the bearing test evaluation method are presented. Therefore, this study contributes to the development of the analysis technology and the durability evaluation technology for bearings for automatic transmissions.

First, it is important to define the load conditions of bearings for automatic transmissions. The operating temperature and lubrication conditions of the bearing, which have the highest correlation with the performance and durability of the bearing, are measured, and the operating preload condition of the bearing is calculated. The bearing temperature is measured according to the transmission oil temperature. In addition, by using the specifications of the automatic transmission and the vehicle specifications, a method of calculating the load case for each transmission speed from NEDC(new european driving cycle) conditions is presented.

Second, numerical analysis of bearings for automatic transmission is performed. In order to calculate the durability, power loss and static strength of the bearing, a numerical analysis model of the transmission system is presented. The dynamic equivalent load, equivalent rotational speed, rating life, maximum contact stress, and power loss of each automatic transmission bearing are calculated. The numerical analysis results show that the dynamic equivalent load of all bearings is 20% or less of the dynamic rated capacity, the equivalent rotation speed is 2,000 rpm or less, the maximum contact stress is 4,000 MPa or less, the theoretical life is 80 × 10⁴ km or more, and the sum of the power losses is 108.7 W. Based on the results of numerical analysis, bearings capable of designing to reduce power loss are selected.

Third, the main parameters for reducing the power loss of bearings by bearing type are determined. As tapered roller bearings have a rotational speed of 1,000 rpm or less, it is effective to improve the roughness of the rolling surface of the rolling element, and for ball bearings and thrust needle bearings, power loss can be reduced by changing the size of the rolling element. The bearing design equation is proposed to reduce power loss by combining the bearing rating capacity and rating life theory with the design constraints of the number of rolling elements and diameter. Based on this, the optimum bearing is designed by comparing the NEDC rating life and power loss of the bearings. The power loss reduction of the newly designed bearing is 23.9 W, which means an improvement of 29.1%.

Fourth, a comparison test of the friction torque between the existing bearing and the newly designed bearing is performed. The friction torque measurement value of the newly designed bearing is low in the entire rotational speed range measured by the test. The numerically analyzed friction torque results for the test conditions are similar to those of both the existing bearings and the newly designed bearings, which means that the numerical analysis results of NEDC power loss reduction are valid. Fatigue life tests are performed on the new design bearings. The test results show that the test life of all bearings is more than the theoretical rating life. Therefore, it is verified that the life of the newly designed bearing is more than 30 x 10⁴ km under NEDC operating conditions. In addition, static strength tests are performed on the newly designed bearings to verify that the bearings are not damaged under the maximum transmission load condition.

This study propose a new design and test evaluation process to reduce the power loss of bearings for automatic transmissions. The presented analysis method and test results can be used as the basis for the design technology of bearings for automatic transmissions.