Study on the Influence of Initial Tightening Times on the Loosening Behavior of Bolt Connections

In response to thread stripping and damage in the locomotive-bogie threaded holes， wire thread inserts were applied to repair the internal threads， and the post-repair loosening behaviour of the bolted structures was investigated.After the experiment， the feasibility of using wire thread inserts to repair damaged threaded holes was explored.Optical microscopy(OM) and scanning electron microscopy (SEM) were used to analyze surface damage in the internal threads after different tightening times.Results showed that damage to the internal threads intensified and anti-loosening performance deteriorated as the number of tightening times increased.Fatigue wear and abrasive wear were identified as the main wear mechanisms on the thread surface.Micro-sliding and fretting at the thread contact interface were identified as the primary causes of bolt-connection loosening.Damaged threaded holes were repaired with wire thread inserts， enabling their continued reuse.