Maximum fan speed
When the impeller runs, stresses are developed in the shaft, impeller blades, back plate and other rotating parts. These stresses limit the speed at which the fan can safely operate. The critical speed of the fan shaft depends on the nature of the material, its mass, its diameter and the distance between the bearings. The critical speed must be much greater than the maximum allowable running speed, otherwise the bending and torsional stresses set up in the shaft material will approach dangerous limits. The fan manufacturer sets the maximum allowable, safe speed but, in general terms, maximum running speeds should not exceed 66 per cent of the critical speed according to ASHRAE (1993) and a limit of 55 per cent is better, when the effects of operational wear and temperature are considered.
All fans used should be dynamically balanced by the manufacturer, as well as statically balanced.
Posted in Engineering Fifth Edition