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"What
Determines Motor End Play"
With
trilingual summary
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.
From “What
determines motor end play,"
published in the
Electrical Apparatus July 2007 issue
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order
copies. © 2007
Barks Publications, Inc. All Rights Reserved.
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