PT Select Series spherical roller bearings offer rugged cast iron housings, total misalignment up to two degrees, are available in expansion and non-expansion units and offer a 90-degree mounting collar for superior holding strength.
We’ve conducted studies that show the PT Series has a longer lifecycle than competitive products.
When calculating the life of roller bearings, the following industry standard equation is typically used by both the ABMA and ISO organizations:
L10 = a1 a2 a3 (C1 / Pe) 10/3 * 1,000,000 / N 60 = hrs.
L10 = Statistical life where 90% of the time, 10% of the bearings in any given population of bearings running at the same application conditions will fail due to raceway spalling
a1 = Reliability Factor (typically normalized to equal 1)
a2 = Material Factor (typically normalized to equal 1)
a3 = Environmental Factor (typically calculated to be within .2 to 3, considering lubrication and
Pe = Equivalent load normal to the bearing’s raceways
C1 = Basic dynamic load rating = lbs.
N = Shaft Speed – rpm
Bearing life is directly (inversely) proportional to its stressed condition under load. For any given load applied to a bearing, it’s the condition of stress developed in the bearing’s raceways that determine its life. It is important to consider a bearing’s design when determining its life potential in any application.
Let’s look at a recent comparison study performed between a PT Select 2 7/16” bore unit and those of several other manufacturers.
Here are the fatigue life estimates calculated from their load ratings at a 25% C1 rating:
Competitor E - L10 = a1 a2 a3 (46,400 / 9600)10/3 * 1,000,000 / (1200) 60 = 2651 hrs.
Competitor F - L10 = a1 a2 a3 (38,300 / 9600)10/3 * 1,000,000 / (1200) 60 = 1399 hrs.
PT Select - L10 = a1 a2 a3 (38,400 / 9600)10/3 * 1,000,000 / (1200) 60 = 1411 hrs.
Fatigue life estimate calculated from their stressed conditions would be:
Competitor E - L10 = 1247 hrs.
Competitor F - L10 = 455 hrs.
PT Select - L10 = 1411 hrs.
The PT Select fatigue life was considered valid at a maximum inner ring compressive stress of 326,552 psi, because its Basic Load Rating (BLR) of 38,400 pounds was validated by Weibull testing a population of 16 bearings.
As you can see, when you increase the inner ring stress to 367,478 psi for Competitor F, it will be as if the bearing is carrying 13,450 pounds instead of 9,600 pounds. In addition, the inner and outer ring compressive stresses for Competitor E are slightly more than for the PT Select bearing, which causes that bearing to have lower fatigue life potential.
When considering the geometry of a bearing’s components with an A2 and A3 factor of 1.0, the actual resulting bearing life can be less than estimated when using the standardized L10 calculation, due to the deleterious effects of higher-than-assumed stress conditions within the bearing.
If you have questions about this topic or any other bearing-related issues, call (317) 273-5781 or email [email protected]. For additional videos and demos, visit our YouTube channel.