A　reducer　is　a　typical　hysteresis　system,　and　the　hysteresis　affects　its　transmission　performance.　The　existing　hysteresis　models　of　reducers　usually　ignore　the　hysteresis　characteristics　of　geometric　errors　and　treat　them　as　constants,　which　results　in　the　inability　of　these　models　to　fully　reflect　the　hysteresis　characteristics　of　reducers.　In　order　to　better　understand　the　mechanism　behind　gear　reducer　hysteresis,　this　paper　theoretically　analyzes　the　effects　of　friction,　elastic　deformation,　and　geometric　errors　on　the　phenomenon.　Additionally,　it　develops　a　new　model　for　gear　reducer　hysteresis　that　takes　geometric　error　hysteresis　characteristics　into　account.　The　model＇s　practical　application　reveals　the　dynamic　characteristics　of　the　lost　motion　in　the　reducer,　and　a　dynamic　lost　motion　formula　is　deduced.　Experimental　research　verifies　the　influence　of　geometric　errors　on　the　reducer　hysteresis　and　confirms　the　effectiveness　of　the　hysteresis　model.　By　changing　the　loading　rate　during　the　lost　motion　test,　the　dynamic　characteristics　of　the　lost　motion　are　verified.　It　was　found　that　different　materials　have　different　effects　on　the　loading　rate.　For　every　0.05　(N·m)/s　increase　in　the　loading　rate,　the　lost　motion　test　results　of　the　small　metal　gear　reducer　and　the　small　plastic　gear　reducer　with　a　modulus　of　0.22　mm　decrease　by　about　3′　and　10′,　respectively.　©　2025　Beijing　University　of　Aeronautics　and　Astronautics　(BUAA).　All　rights　reserved.