The present invention relates to an alarm system suitable for use in warning of overheating in rotatable bearings (4) having first and second bearing portions. The alarm system comprises an alarm signal generator means having a thermally activated engagement element (8) on a first bearing portion or body (6) connected thereto. The element (8) is in thermal connection with the bearing (4) and is formed and arranged so as to be driven generally axially of the rotatable bearing from a first primed retracted position into a second deployed extended alarm position upon reaching a predetermined temperature. In the alarm position the element (8) contacts an engaging portion (12) on the second bearing portion so as to produce a distinctive alarm signal indicating that said bearing (4) has exceeded the predetermined temperature.
