Bearing re-greasing interval time is an issue which tends to be as much of an art as it is a science.
From the scientific end of the spectrum, greasing intervals are dependent on bearing type, bearing size, grease type and rotational speed. If ideal conditions were to exist in all motor/ bearing applications, re-greasing intervals would be easy to calculate.
The challenge comes into play when we apply the motor/ bearing combination into the real world and then try to make a responsible re-greasing interval recommendation that will give rated bearing life in 99% of the cases. Required re-greasing intervals might be shortened by a multitude of external influences such as vibration, moisture, dust and other contaminants, the type of grease used, high ambient temperatures and high radial load applied to the shaft of the motor. Every motor in an industrial application tends to be subject to a unique combination of varying degrees of the previously listed factors. This makes it virtually impossible to recommend a blanket re-greasing interval that satisfies the bearing’s needs and satisfies the customer’s wishes of having the longest greasing interval possible and still allow the bearings to attain rated life expectancy in 99% of all applications.
Most motor manufacturers are conservative in their recommendations for greasing intervals. They mimic the recommendations that are published by such bearing manufacturers as SFK, NTN and NSK. The lubrication manufacturers themselves publish almost identical recommended greasing intervals as the bearing manufacturers do. (Most manufacturers follow the guidelines set forth by such organizations as the National Lubricating and Greasing Institute.) The one factor that the recommended greasing intervals have in common besides being very similar in time frames is that they are published on the conservative side.
This is where the “art” aspect of recommending bearing greasing intervals comes into play. Bearing manufacturers point out that greasing intervals can be significantly stretched out if ideal conditions exist.
This increase in re-greasing intervals can be as much as 2.5 times for roller bearings and as much as 3 times for ball bearings. Somewhere in between the “worst case” scenario and the “ideal” situation lies a re-greasing interval which is appropriate for each specific motor/ bearing application. We feel that it would be inappropriate to make a blanket statement that extends the recommended greasing interval beyond the time frame recommended by the bearing and grease manufacturers unless very specific external parameters are known. Basic guidelines and types of lubricant used from the factory are usually included in the manufacturer’s instruction or maintenance material shipped with each motor.
The single most significant thing that increases greasing intervals, that is under the control of the manufacturer, is to supply ball bearings in lieu of roller bearings. The greasing interval for a ball bearing is almost exactly double that of a roller bearing if both bearings are the same physical size. Ball bearings have the capacity of handling a limited amount of radial thrust as compared to roller bearings, but are still suitable for many applications and therefore should be seriously considered when making motor/bearing selections. Please note that higher HP motors, will require roller bearings for belt drive applications but the ultimate decision in this instance rests with the user.
Common Bearing Greasing Intervals
(in all cases deference to manufacturers specific recommendations is preferred)
|3600 RPM||6202- 6208 6305 - 6309||8 months||4 months||1 month|
|1800 - 900 RPM||6202- 6208 6305 - 6309||30 months||12 months||4 months|
|3600 RPM||6211 6309 - 6314||8 months||4 months||1 month|
|1800 - 900 RPM||6211 6309 - 6314||24 months||12 months||4 months|
|3600 RPM||6216, 6313 - 6324 NU317-NU328||8 months||4 months||1 month|
|1800 - 900 RPM||6216, 6313 - 6324 NU317-NU328||18 months||8 months||3 months|
SERVICE CONDITIONS DEFINED
For the purposes of this article, Service Conditions are outlined so as to tie into Intervals chart shown above.
Eight hours per day, Light to normal loading, Clean condition, free from dust
24 hours per day, Light to normal shock loading, vibration, Exposure to dirt or dusty conditions
VERY SEVERE DUTY
24 hours per day, High ambient, Normal to high shock loading, vibration, Dusty conditions, Confined mounting conditions
Article Provided by:
Lafert North America, MOTR: Motor Operation & Technical Review