Basic relationships and physical concepts used for the development of a bearing dynamic tool (called CAGEDYN) are explained. Emphasis is put on some key physical concepts affecting the calculated cage impact forces: the appropriate roller-cage contact stiffness (derived herein using a novel equivalent stiffness), the tangential hydrodynamic rolling and pressure forces, the hydrodynamic effect on the roller normal load (for which a new relationship is given for calculating a load, even when there is no interference between the roller surface and counter race or cage bridge surface), and the appropriate traction force calculated using a nonlinear viscous model and contact temperature increases.