###### Abstract
[PERSON] et al. [13] proposed a thermal network model to predict power loss of a manual transmission. The model consists of power loss factors including gearing, bearing, and oil churning affected by temperatures, and power efficiency of the manual gearbox was predicted by considering thermal influences on power loss factors. However, these studies performed in the system level did not cover transmission structures consisting of multiple decks of planetary gear sets, thus it was not presented that the correlations of bearing power losses and thrust loads resulted due to helix angles of a planetary gear train. There are several studies on transmission power efficiency including planetary gear sets [14; 15], however efficiency analysis and methodologies have been proposed for conceptual structures not considering bearings. In order to analyze bearing power losses, modeling an automatic transmission including planetary gear sets is one of main tasks in this study, and the related literature was reviewed accordingly. For investigating basic performances of a concept design, [PERSON] et al. [16] and [PERSON] [17] proposed systematic methodologies for analyzing kinematics and power flows of a schematic structure. [PERSON] [18] proposed a graph-based method to construct a basic structure of a planetary gear train by enumerating connections for central members of planetary gear sets. Related to a given basic structure, [PERSON] and [PERSON] [19] presented a graph-based method to investigate if a given schematic structure can be assembled in rotation plane. For designing planetary gear sets, several earlier publications [20; 21; 22; 23] suggested design methods defining gear geometry parameters and avoiding interference conditions. [PERSON] et al. [24; 25] proposed a design search for planetary gear sets, which considers geometry and performance parameters of external and internal gear meshes simultaneously. While there are diverse studies cover ing concept structure design, kinematics and gear designs, it is difficult to find literature regarding bearings investigated in the system levels, which are installed in an automatic transmission consisting of several planetary gear sets. Although there is an extensive quantity of literature related to power losses, limited studies analyzing them in a system level are found amongst them. Furthermore, it is quite difficult to find any research considering the directions of gear helix angles systematically affecting power losses in planetary gear trains. Some analysis and design studies concerning helix angles of gears are found in gear research areas, however their objects are mostly focused on dynamic effects or design optimizations [26; 27]. In this manuscript, in order to fill a void in the research areas of bearing power losses affected by the designs of helix angles in a complete transmission system, the influences of thrust loads resulting from helix angle directions of planetary gear sets on bearing power losses are investigated in a system level of a planetary gear train. As an automatic transmission structure changes speed drives, various amounts of loads are applied to bearings accordingly. Therefore, bearing power loss must be analyzed in an entire transmission system for diverse drive situations. For the system level analysis, first a planetary gear train model is constructed referring to an exemplary automatic transmission [28], and the structural analysis for diverse combinations of helix angle directions of planetary gear sets is executed for predicting thrust loads on bearings. Bearing power losses occurring in the transmission are calculated by utilizing the bearing power loss model, and the results of total bearing power losses of the entire combinations are discussed to show the influences of helix angle directions on bearing power losses in a planetary gear train. Figure 1 displays a flowchart of distinct processes for the bearing power loss analysis in this study.