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With a motor whose bearing has collapsed, the decision depends on whether the damage stayed in the bearing. If the shaft, bearing housings and winding are sound, replacing the bearing is usually a sensible repair; if the rotor has rubbed on the stator, the housing has gone oval or the winding has overheated, a new motor is the safer route.
Finding out how far the damage goes
When a bearing collapses the rotor drops off centre. Because the air gap is very narrow, the rotor can touch the stator; shiny rub marks on the lamination stack and damage to the winding insulation may follow. Heat from the failed bearing passes into the shaft and the end shield seat, which can widen. The size and condition of these parts become clear once the motor is opened up and examined in a workshop.
Decision table
| Finding | Assessment |
|---|---|
| Only the bearing is damaged, shaft and housing within size | Bearing replacement is generally enough |
| End shield bearing seat has widened | Repair if the end shield can be renewed, new motor if not |
| Bearing journal on the shaft is worn | Shaft machining or a new shaft needed; cost and time grow |
| Rotor has rubbed the stator, marks on the winding | A new motor is the better choice in most cases |
Finding what destroyed the bearing
Bearings do not fail on their own. An over-tensioned belt, a misaligned coupling, lack of grease or the wrong grease, a pulley hammered onto the shaft, and dust or water ingress are among the commonest causes. Whether the motor is repaired or replaced, a new bearing will have the same short life unless that cause is removed. On belt-driven machines, fitting the pulley as close as possible to the shaft shoulder, that is to the motor body, reduces the load on the bearing.
Motor age and downtime
For a motor that has already been rewound several times and whose parts are hard to source, waiting for repair can take longer than the lead time of a new motor. On a machine that halts production, that difference alone often settles the decision.
If you decide to replace, you can choose a model matching the old motor's frame size from the general-purpose industrial motors and the power and speed list in the shop.