Machinery professionals intuitively know that by doing alignment and balancing jobs to tighter tolerances, and by reducing internal clearances in machinery, that vibration levels will be reduced with a corresponding increase in machinery reliability. However, it is often difficult to justify what needs to be done. Alignment and balancing jobs are often skipped or postponed because everyone is in a hurry to get the process back up and running. Entire vibration programs are scrapped because no one can document how much, if any, payback is occurring.
Reliability and replacement costs for rolling element bearings are major concerns in most plants. By examining the additional forces that mechanical problems exert on a bearing, we can estimate the bearing’s useful life reduction.
For those involved in predictive maintenance activities, especially vibration monitoring and analysis, two questions have always been present.
- What is the correlation between changes in vibration level and the corresponding impact on bearing life?
- What is the value in knowing this correlation if there is one?
There is a direct correlation between vibration level changes and bearing longevity! A simplified definition for vibration can be phrased as follows. Machine Vibration: A Dynamic Response to a Dynamic Force!
It is critical to note that typically vibration responds to a dynamic force in a linear fashion. Exceptions include machines where structural resonance, shaft criticals, component looseness, etc., occur.
Seven predominant factors impact rolling element bearing life:
RPM of the shaft
- Design load rating of the bearing (as defined by the manufacturer)
- Type of rolling element bearing (ball or other rolling element type-cylindrical roller, spherical roller, needle roller, tapered roller)
- Actual load (force) applied to the bearing
- Lubricant ability
- Contamination level
- Operating temperature.
BASIC BEARING LIFE EQUATION
Examining the basic bearing life equation we find that speed, load and the type of bearing are factors:
L10h = (16667 / rpm) x (C / P)r
L10h = 90th percentile of life in hours (the point at which only 10 percent of bearings in identical applications fail); Note: average life = 5 x L10h
Rpm = Rotational speed of the bearing
C = Published catalog load rating
P = Effective load (actual force applied to the bearing)
r = 3 for ball bearings
r = 3 1/3 for other types of rolling element bearings
First, let’s investigate the impact of rotational speed on bearing life. Reviewing the basic bearing life equation:
L10h = (16667 / rpm) x (C / P)r
The impact of increasing speed is obvious. Doubling the rotational speed (while maintaining a constant load) = L10h / 2 = 1/2 the original life.
RULE: Bearing life is inversely proportional to speed changes.(1 / speed change ratio)
2 x rpm = 1/2 life
3 x rpm = 1/3 life
1.25 x rpm = 0.8 life
Next, we need to investigate the impact of load on bearing life. Reviewing the basic bearing life equation again: L10h = (16667 / rpm) x (C / P)r. The impact of increasing load (force) is pronounced. Doubling load (while maintaining a constant speed) = L10h / 8 or 1/8 life (1/2)3 for ball bearings.