July 2007 Electrical Apparatus

July 2007 Electrical Apparatus

This is a summary of the Electrical Apparatus July 2007 featured technical article,  by Richard L. Nailen, P.E.     

In any electric motor, end play is the axial movement of the shaft that can occur under the influence of some external force. That force may be electromagnetic or aerodynamic within the motor itself, or imposed by the driven machine.

One axial force always present is that of thermal expansion in the motor shaft resulting from the motor temperature rise under load. To accommodate that, at least one of the bearings must be loose enough in its mounting to slide as the shaft expands.

In a standard horizontal ball bearing machine, total end play includes that looseness in bearing mounting plus whatever internal clearance necessarily exists within the bearings themselves. That clearance is standardized in the radial direction only, and difficult to quantify in the axial direction. Nominal end play in a standard motor is therefore quoted as a conservative figure (typically between 1 and 3 millimeters depending upon motor and bearing size).

Usually the bearing at the drive end of the motor is closely held in position while shaft expansion is permitted at the other end. That minimizes transmission of axial movement to the driven machine. When a cylindrical roller bearing is used on the drive end, to sustain high belt loads, that bearing allows for shaft expansion while the other one is restrained.

End play in a sleeve bearing machine is not governed by close fits between bearing components, and movement is much freer. Limitations required by the driven equipment are provided by limited end float couplings.

End play in vertical motors is much more complex. Many such machines must sustain high downthrust loads but may also be subjected to either momentary or continuous upward thrust. Several different types of thrust bearings are used to suit the application conditions, and both the end play limits themselves and the method of end play adjustment must follow specific manufacturer recommendations to avoid bearing damage.

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