When isn't vibration bad?

In standard electric motors, techniques of design, manufacturing, and installation are intended to reduce mechanical vibration that can damage windings and bearings. Some industrial machinery, however, makes use of high levels of vibration intentionally created by a driving motor.

Such "vibrator motors" agitate granulated material in bins or chutes so that it remains free-flowing. The vibration may also be used to move material along a conveyor path. Motor-generated vibratory motion will also separate materials of different densities and particle sizes, or agitate grinding material in a mix of small parts to create a smooth finish.

The vibration is produced by eccentric weights mounted on shaft extensions at each end of the motor. If the two sets of weights are aligned with each other, the vibrating motor moves parallel to itself; weight misalignment produces a simultaneous tilting or wobbling motion. High vibratory forces are either applied directly by the motor to some attached structure, or used to excite resonant vibration in a spring-mounted assembly.

For adequate strength and stiffness, the motor shaft is greatly oversized for the rating. That, plus limited ventilation, dictates electrical parts larger than normal. So are the bearings. Heavy radial loads require roller bearings in most motors, as well as special lubricants. Because the load rotates with respect to the outer race, the bearing must be a tight fit in its housing rather than the sliding fit in a standard motor. Other component fits throughout the machine may also be tight, calling for unusual assembly/disassembly practices.

Windings need particular attention to end turn lacing and void-free impregnation. Electrical supply connections, and lead wires into the winding, must be secured against flexing and abrasion.

Vibrator motors are often repaired improperly because service shops aren't aware of some of those special concerns. Not providing the correct replacement bearings; using the wrong grease; pressing tightly fitted parts together (or driving them apart) without heat--those are all common causes of early failure.

From  "When Isn't Vibration Bad" ...by Richard L. Nailen, EA Engineering Editor  - 
published in Electrical Apparatus February 2004  © 2004 Barks Publications, Inc.  All Rights Reserved.