Study On Reliability Allocation For Planetary Reducer In High Torque Hub Drive System

The Made in China 2025 Initiative emphasizes that the reliability of domestic key products shall reach the advanced international level,and reliability allocation is one of the key contents of system reliability design which is an important technology to ensure and enhance product reliability during developing.The 2-stage NGW planetary reducer is at the core of the domestic high-torque hub drive system,so studying its reliability allocation would not only advance its system reliability design but also elevate the reliability level for the high-torque hub drive system.To address existing issues in the modern system reliability allocation of the planetary reducer,such as incomprehensive failure analysis,allocation models lacking integrity,and poor allocation results,FTA and FMECA are employed,and an allocation of system reliability design objectives and an optimal allocation of system reliability improvement targets are proposed.The details are as follows:(1)Considering failure correlation,the FTA is implemented for the planetary reducer system.Key components and their primary failure modes are analyzed based on the system fault tree under independent failure,clarifying the basic failure logic of the reducer.Then,it is deduced that there is mainly positive failure correlat ion in the planetary reducer,and taking into account the action effect of positive failure correlation,the fault tree of the planetary reducer is established.The analysis concludes that both single and coupling failures of key components,and both single and coupling occurrence of their primary failure modes are sufficient conditions for the planetary reducer’s failure under correlation failure,which lays a foundation for studying system reliability allocation theory.(2)Considering failure correlation,a FMECA model of the planetary reducer is established.After analyzing the planetary reducer’s basic characteristics,subsystem complexities are evaluated from the perspective of failure-sensitive links,fully characterizing complexity.A series of FMECA formulas with a modifying operator of positive failure correlation are proposed,reflecting that subsystem complexity and occurrence change in the same direction as the failure correlation degree,respectively.Comparing the subsystem reliability allocation weights obtained through different FMECA methods,results show that the weights from the proposed method are more credible.Hence,it would provide more accurate weight data for the allocation of system reliability design objectives.(3)Targeting components of the planetary reducer,a deeper allocation method of system reliability design objectives is presented.The system reliability of the planetary reducer is proposed to be the product of torsion reliability and function reliability based on the system failure mechanism.Then,reliability allocation models for “systemsubsystem” and “subsystem-component” are formulated using the Gumbel Copula function and order statistic theory,respectively.Furthermore,the high-dimensional failure correlation coefficient formula is constructed by using the Kendall harmony coefficient.Based on the proposed “system-subsystem-component” reliability allocation model,the system reliability design objectives of the planetary reducer are allocated to components,deepening the system reliability allocation level.(4)Taking into account multiple correlation failures and risk uncertainty,an optimal allocation method for the system reliability improvement targets is proposed.Two new concepts are put forward: independent failure resulting in basic risk and correlation failure leading to disturbance risk,and a risk assessment machinery of “actual risk =basic risk + disturbance risk” is proposed.The action mechanisms of three correlation failures are studied using cooperation game theory,and generalized risk models are provided under probability measure.Considering improvement costs,the expectation and the variance of system risk reduction,a multi-objective optimal allocation model is developed,which is solved by using the PSO algorithm.Finally,the proposed optimal allocation is implemented at the 2-stage NGW planetary reducer,with results indicating greater efficiency and feasibility for engineering practice.