# Optimization Design of RV Reducer Crankshaft Bearing
## 1 Introduction  
The rotate vector (RV) reducer is widely used in industrial robots because of its compact structure, small size, high torsional rigidity, large transmission ratio, and great load capacity [1, 2]. Service life is one of the most important performance parameters of a reducer. However, the harsh working environment makes the reducer prone to fail easily [3]. Thus, it is important to investigate the fatigue life of the RV reducer.  
For an RV reducer, the service life mainly depends on the rated life of crankshaft bearings [4, 5]. Both the RV reducer sample manuals of Nabtseco and standard GB/I37718 take the rated life of the crankshaft bearing as the service life of the reducer. However, the load working condition of the crankshaft bearings are very complex, and the radial size is very small. Therefore, the crankshaft bearings often suffer fatigue failure, which limits the service life of the RV reducer [4]. Therefore, crankshaft bearings use non-standard cylindrical roller bearings [4, 5]. For nonstandard bearing, it takes a lot of time to design the bearing with the traditional trial and error method [6]. In addition, the design of the bearing must satisfy various constraints, e.g., geometrical, kinematical, and mechanical constraints. Therefore, it is necessary to use optimization techniques for crankshaft bearings to improve the service life of the reducer.  
In recent years, there has been abundant research focused on RV reducers. [PERSON] [7] proposed an analytical model for the cycloidal gear considering tooth profile modification. [PERSON] [8] analyzed the sensitivity of transmission accuracy of RV reducers. [PERSON] [9] researched the measurement of thebacklash of RV reducers. [PERSON] [10] established an analytical model of system dynamics in cycloidal pin gear reducers with small tooth differences. [PERSON] [11] analyzed the transmission error of the RV reducer by using virtual machine technology. [PERSON] [12] proposed a new contact model of cycloidal meshing pair and analyzed the transmission accuracy of RV reducer. The influence of tooth profile modification was analyzed. [PERSON] [13] analyzed the meshing characteristics of RV reducers considering the manufacturing error. [PERSON] [14] analyzed the vibration characteristics of the RV reducer. [PERSON] [4] established a dynamic model of the RV reducers. The transmission mechanisms of cycloidal gear were analyzed. [PERSON] [15] analyzed the lubrication characteristics of RV reducers. [PERSON] [5] analyzed the mixed lubrication of crankshaft bearings of an RV reducer. [PERSON] [16] used genetic algorithm to optimize the design of cycloid reducer. [PERSON] [17] proposed a selective assembly method for RV reducers using genetic algorithms. [PERSON] [18] proposed a multi-objective optimization method for cycloidal speed reducer. The objective functions are the reducer volume, loads on the crankshaft bearings, and stress of cycloidal gear.  
The above review shows that most of the published works focus on the meshing characteristics of the cycloidal pair, tooth profile modification, and performance characteristics of the RV reducer. However, few scholars have focused on optimizing the fatigue life of the reducers. Although studies [16; 18] have optimized the fatigue life of cycloidal reducers by using a multi-objective optimization method, the geometric parameters and roller profile of the bearing have not been comprehensively considered during the optimization design.  
Therefore, an optimization method for RV reducer crankshaft bearings considering the bearing geometry and crowned roller profile is presented. First, the performance of the bearing considering the load working condition and roller profile is analyzed. Second, a mathematical model for the optimization design is developed. Third, two types of crankshaft bearings are optimized by the proposed optimization method. A comparison of the parameters of the bearing before and after optimization is presented. Finally, a research summary of the study is provided.