August 17, 2016
Reduce Force to Maximize L10 Bearing Life
By Brad Case
Simply put, L10 bearing life is the number of service hours that 90% of bearings will survive or conversely 10% of bearings will have failed. L10 life: is the preferred term in specifying bearing life and is defined by The American Bearing Manufacturers Association (ABMA).
A bearing life equation is:
This calculation assumes proper installation and lubrication, no shock or vibration, perfect alignment, debris free environment and temperatures that are not extreme. Obviously, all of these factors are important to maintenance personnel, however one of the critical factors of this calculation we can control with precision shaft alignment is FORCE.
If we simplify this calculation to:
it is apparent that the amount of force created by misalignment is very detrimental to bearing life.
For twice the misalignment;
For three times the misalignment;
“Perfect Alignment” is not practical as rotating machinery does not operate in a “perfect” world. IF it did maintenance personnel possibly wouldn’t be needed. Fortunately flexible couplings allow for machines to operate in less than “perfect” environments.
That being said, rotating machinery does need to be aligned to close shaft alignment tolerances, not coupling tolerances. See a previous blog on this subject. Misalignment creates forces at the coupling which are exerted on the shafts. This results in reduced bearing and seal life.
The force effects of misalignment can be simplified by considering the misalignment as a simple lever. Misalignment at the coupling creates a moment of force acting on an effort arm. This will create a first class or second class lever with either the inboard or outboard bearing acting as a fulcrum. The length of the motor shaft between its bearings is the resistance arm. The objective of shaft alignment is to minimize coupling forces by making two rotational axes form a single line while the machine operates.
As the above simplified calculation illustrates, when the force is increased the reduction in bearing life is exponential. Doubling the force reduces the L10 bearing life to 1/8. Tripling the force reduces the L10 life to 1/27.
So minimize “The Force” with precision shaft alignment for maximum L10 life so the bearings, seals, & etc. with “Live Long and Prosper”!
Save The Machine!