Electrical Apparatus October 2014

Electrical Apparatus October 2014

This is a summary of the Electrical Apparatus October 2014 featured technical article,  by Richard L. Nailen, P.E.

In most industrial a-c motors up to about 400 kilowatts, grease-lubricated ball bearings are standard components. Easier to handle than oil, grease is less subject to leakage. However, age, heat, and contamination cause grease to deteriorate. New grease must be added periodically. How much to add, when to add it, and how to add it are questions that have been answered in many conflicting ways.

The answers will depend upon the severity of the motor application, including bearing size, speed, and radial load; operating temperature; and environmental cleanliness. Some bearings contain single or double shields, affecting grease flow into the bearing. Others contain no shields.

The bearing housing assembly affects the path grease takes into and out of the bearing. Some of these conditions are known only to the motor manufacturer, whose instructions should be consulted first when creating a lubrication schedule. The methods selected must then be modified, based on experience.

Grease is best added with the motor at normal operating temperature. When relubrication is impractical or unsafe while the motor is running, add grease at standstill, then restart the motor to run for 30 minutes with drain plugs removed to allow discharge of old grease.

The amount of grease added must be measured by volume, by weight, or by calibrated operation of a grease gun. Volume measures, often quoted in milliliters or cubic centimeters, are impractical in the usual motor environment. Weight in grams is preferable.

Relubrication intervals are given in operating hours, months, or years, as a function of either motor power rating, motor frame size, bearing bore, or shaft diameter. The smaller the motor, and the lower its speed, the less frequently regreasing is required. However, little correlation exists among the many published recommendations; the same is true for grease quantities.

Automatic lubricators can provide regular addition of small amounts of grease, with pressure-sensitive drains for escape of old grease. But the proper lubricator setting must be derived from experience, to avoid either over- or under-lubrication. No simple formula can substitute for careful monitoring and good judgment.

The arc flash phenomenon first became widely publicized primarily as a burn hazard. The radiant heat—“plasma cloud”—constituted the serious danger to the safety of personnel. That was because most arcs occurred while someone was working on the equipment, and in a position to suffer injury or death even if not the inadvertent cause of the arc itself.

To order a back issue with the full article, “Relubricating with Grease: Is there a right way?” call 312-321-9440 or visit our online webstore.

Share Button