Abstract: A vacuum pump has a back-up bearing assembly for allowing its rotor to spin down safely without damaging the pump if the primary bearing system becomes disabled. The drive shaft of the rotor is tubular and contains the back-up bearing assembly in its hollow interior. Back-up bearings are mounted onto a tubular center shaft which is coaxial with and inside the drive shaft. An axial position sensor is nested inside the center shaft and light emitted from its light emitter is reflected by a reflective target inside the drive shaft to detect the relative axial position of the drive shaft with respect to the center shaft. A polymer sleeve is attached to the end of the drive shaft as a thermal compensator, which restricts the axial movement of the drive shaft.