Pre-loading,　both　in　terms　of　application　method　and　actual　applied　force,　significantly　affects　the　stiffness　and　natural　frequency　of　a　high　speed　spindle　system.　The　gyroscopic　moment　at　high　speed　leads　to　whirling　of　the　spindle,　and　the　whirl　frequency　is　not　equal　to　the　system’s　natural　frequency.　To　discover　the　relationship　between　pre-load　and　whirl　frequency,　theoretical　and　experimental　research　was　undertaken.　Two　numerical　models　of　the　angular　contact　ball　bearings,　based　on　rigid　and　constant　pre-load　mechanisms,　were　established.　The　shaft　is　considered　as　a　set　of　Timoshenko　beam　elements,　and　gyroscopic　moment　and　centrifugal　force　are　both　considered.　Adding　bearing　stiffness　in　the　form　of　springs　to　this　finite　element　system　produced　a　spindle-bearing　coupled　model.　Iteration　was　used　to　deduce　the　interactions　among　bearing　groups.　The　exact　whirl　frequency　of　a　spindle　subjected　to　different　pre-load　mechanisms　has　been　calculated.　To　validate　the　proposed　theory,　frequency　analysis　was　carried　out　on　a　Siemens　CAT40　spindle.　Experimental　results　agreed　with　theoretical　calculations.　The　result　shows　that　speed　had　a　great　influence　on　bearing　stiffness　and　spindle　whirl　frequency.　Adopting　a　reasonable　pre-load　method　and　pre-load　force　improved　the　spindle　critical　frequency.　©　JVE　INTERNATIONAL　LTD.　JOURNAL　OF　VIBROENGINEERING　2014.