Electrical Apparatus October 2013

Electrical Apparatus October 2013

This is a summary of the Electrical Apparatus October 2013 featured technical article,  by Richard L. Nailen, P.E.   

Despite the traditional emphasis on overheating and insulation breakdown, it’s generally agreed that most motor failures—as many as 75% to 90%–are bearing-related. Severity of operating environments is often responsible.

That is why most new motors are totally enclosed. To protect bearings and lubricant from damaging solid or liquid contaminants to which such motors are exposed in industrial surroundings, a variety of shaft seals have been developed over many years. Simple, inexpensive, and easy to replace, elastomeric disc “flingers” have been widely used. Many large and small sleeve bearing motors have also used rubbing felt seals, which absorb oil to reduce friction and maintain a seal despite variation in shaft centering.

More effective for anti-friction bearings, and more recently introduced, bearing isolators involve some combination of these conditions:

1. A close-fitting labyrinth path between stationary and rotating components, usually containing changes in path direction.

2. A provision for contaminant particles to collect in one area within the seal from which they can readily escape back to the environment.

3. A means to keep the labyrinth fully closed when the motor is at standstill, opening up at full speed to minimize rubbing and the associated wear and friction loss.

The aim is to provide protection at least equivalent to the IP55 dust and water exclusion of IEC Standard 60034-5 (also now included in National Electrical Manufacturers Association Standards MG 1 Part 5). Variations of such seals are available for several different motor assembly configurations, including sleeve bearing designs that include shaft end play, or “float,” of 6 to 12 mm.

Labyrinth seals are readily available on new motors. They can also often be installed in the repair shop. They are normally placed only on the exterior side of the bearing housing. Hence, they offer no protection against bearing or lubricant contamination by moisture or chemicals within the air inside the motor.

Totally enclosed machines are often (as in the IEEE 841 standard) fitted with automatic drains to allow escape of internal condensation, and even without such openings no motor can be considered vapor-tight and therefore proof against humidity. All seal designs involve some compromises and must necessarily allow a rotating shaft to move freely through a stationary enclosure.

To order a back issue with the full article, “Shaft seals: one problem, many solutions,” call 312-321-9440 or visit our online webstore.

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