applied sciences Article Patent Analysis and Structural Synthesis of Epicyclic Gear Trains Used in Automatic Transmissions Huafeng Ding1,2,3,* and Changwang Cai1,2 1Hebei Provincial Key Laboratory of Parallel Robot and Mechatronic System, Yanshan University, Qinhuangdao 066004, China; cai1990@stumail.ysu.edu.cn 2School of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China 3School of Mechanical Engineering and Electronic Information, China University of Geosciences (Wuhan), Wuhan 430074, China *Correspondence: dhf@ysu.edu.cn Received: 10 December 2019; Accepted: 18 December 2019; Published: 20 December 2019 /gid00030/gid00035/gid00032/gid00030/gid00038/gid00001/gid00033/gid00042/gid00045 /gid00001 /gid00048/gid00043/gid00031/gid00028/gid00047/gid00032/gid00046 Abstract: A patent survey is conducted to investigate the application status of epicyclic gear trains (EGTs) used in automatic transmissions (ATs). In this regard, 673 AT patents are investigated firstly, and 274 di erent EGTs are obtained after the patent processing. EGTs are presented by corresponding double bicolor graphs (DBGs) and they are sorted into 13 groups in accordance with the degree of freedom (DOF) of DBGs and the degree of hollow vertices. Then, structural characteristics of obtained EGTs, which are used to detect whether the EGTs obtained in the future can be used in ATs, are analyzed and summarized. Finally, a new approach is proposed to obtain new configurations of the EGTs with one main shaft based on the transformation relation between DBG and the basic graph. Several examples are presented to illustrate the validity of the proposed approach. Keywords: epicyclic gear trains; automatic transmission; characteristic analysis; synthesis approach 1. Introduction In the automotive industry, automatic transmissions (ATs) with high e ciency and low consumption are desired to overcome the problems originating from the energy crisis and environmental concerns. Moreover, studies show that vehicles equipped with ATs have remarkable advantages, including simple operation, smooth shift and long service life, which can provide a good driving experience. Since the advent of the first cars equipped with ATs, these vehicles have continuously developed so that they attracted a vast number of consumers, and quickly occupied the dominant place in the field of automotive, especially in U.S., Japan, and European countries [1]. An AT mechanism typically consists of two parts: the hydraulic transmission and mechanical transmission. The hydraulic transmission mainly includes a torque converter, while the mechanical transmission mainly consists of a set of clutches and brakes (hereafter called shifting elements) and a gear train. Operation of the gear train working together with several shifting elements provides various speed ratios, where these speed ratios contribute to the vehicle performance and the fuel economy. Reviewing the literature indicates that gear trains in ATs are mainly divided into two types, namely the ordinary gear train (OGT) and the epicyclic gear train (EGT). It should be indicated that EGTs are more widely used in the AT industry, because they have remarkable superiorities, including the compact structure, larger speed ratios, higher e ciency, and longer working life, over OGTs. Therefore, studying the structural characteristics of EGTs is of great significance to obtain ATs with better performance. Johnson and Towfigh [ 2] proposed a synthesis method for EGTs based on the synthesis approach of linkage-type kinematic chains. Moreover, they synthesized gear mechanisms with one degree of Appl. Sci. 2020 ,10, 82; doi:10.3390 /app10010082 www.mdpi.com /journal /applsciAppl. Sci. 2020 ,10, 82 2 of 25 freedom (DOF) and up to eight links. L évai [ 3] classified planetary mechanisms in detail and defined the simple planetary gear train (PGT) consisting of two central gears, one or more planet gears and one arm. Buchsbaum and Freudenstein [ 4], an