An　approach　to　stiffness　analysis　of　ball　screw　mechanism　is　proposed,　and　the　relationship　between　the　axial　force　applied　on　the　nut　and　the　pressure　on　the　raceway　of　nut　or　screw　is　obtained　by　force　analysis.　The　relationship　between　the　axial　displacement　of　nut　with　respect　to　screw　and　the　normal　elastic　approach　between　the　ball　and　raceway　at　the　contact　point　is　analyzed.　The　model　of　the　contact　angle　of　ball　screw　mechanism　is　established　considering　the　geometric　parameter　of　the　ball　and　the　raceway　of　nut　and　screw.　On　this　basis,　the　model　of　the　axial　stiffness　of　the　ball　screw　mechanism　is　established　according　to　the　theory　of　Hertzian　contact,　in　which　the　coupling　relationship　between　the　pressure　of　the　raceway　and　the　variation　of　the　contact　angle　is　considered.　According　to　the　model,　the　stiffness　characteristic　of　the　single　nut　ball　screw　mechanism　and　the　preloaded　double　nut　ball　screw　mechanism　is　studied.　The　result　shows　that　the　stiffness　of　the　preloaded　ball　screw　mechanism　is　larger　than　the　stiffness　of　the　single　nut　ball　screw　mechanism　when　the　applied　axial　load　is　small,　and　the　difference　of　stiffness　between　them　reduced　as　the　increasing　of　applied　axial　load.　And　the　stiffness　of　preloaded　ball　screw　mechanism　increase　with　the　preload　as　the　applied　axial　load　is　small.　©　2014　Journal　of　Mechanical　Engineering.