Basic Dynamic Load Rating Bearing Life vs. Stress-Based Bearing Life

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Article Tags:
Bearing
bearing-life
load-rating
Roller
Dynamic Load Bearing

When considering a bearing for their application, an end user engineer will generally select a bearing based on its basic dynamic load rating, speed limit, dynamic/static alignment, bearing carry thrust load, and of course, price.

One of the most predominant characteristics of the buying decision is the bearing’s basic dynamic load rating. The higher the rating, the more bearing life is expected for any shaft size; however, not all bearings are created equal.

When calculating roller bearing life, the following equation is typically used by both the ABMA and ISO:

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.
 
Key:
  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 contamination)
Pe = Equivalent load normal to the bearing’s raceways
  N =   Shaft Speed – rpm

When using this equation to calculate bearing life, it is relatively easy to compare competitive bearings. That said, this equation has a drawback. The standardized approach for calculating the basic dynamic rating of a bearing considers the size, length and number of rollers around a pitch diameter at a contact angle. The previously mentioned standardized L10 life equation assumes a condition of stress in the raceways of the bearing with a given C1 rating and an applied Pe load.

The following ABMA and ISO standardized equation is used when determining a roller bearing’s basic dynamic load rating:

C1 = fcm (i le Cos(α))7/9 Z3/4 D29/27 = lbs.
 
Key:
fcm =    Material x design & quality of manufacture factor
i =        Number of rows of rollers
le =      Effective length of the rollers = in
α =       Contact Angle of the bearing = °
Z =       Number of rollers
D =      Diameter of rollers = in

Bearing life is directly proportional to its stressed condition under load.  For any given load applied to a bearing, its projected lifespan is determined by the condition of stress exerted in the bearing’s raceways. That means it’s critical to consider a bearing’s design when determining its life potential in any application.

As a rule, manufacturers use the fcm factor given in the ratings standards to calculate the basic dynamic rating for any bearing. If a catalog rating is more than the standardized rating from a bearing’s geometry, the manufacturer is increasing the fcm factor over what is considered standard.

While this practice may prove to be consistent with laboratory testing, often their fcm factor is overstated relative to bearing raceway stress versus bearing life.

As shown in the following tables, the PT Select P2B207 bearing ranked third in cataloged ratings, however it ranked first in load handling, with the lowest overall stressed condition under load when compared to six other manufacturers.

With that information, the question then becomes, “Which bearing would you buy: a competitor’s brand or Rexnord PT Select.?

Bearing Description Table 1
 
Bearing Description Table 2 and 3

If you have questions or would like to talk to one of our application engineers about our PT Series for your high-temperature application, call (317)273-5781 or email bearing.tech.support@rexnord.com. For videos and other demos, visit our YouTube channel.

Marlon Casey

Marlon Casey is an Engineering Manager with the Rexnord Bearing Group. He has spent 11 years with Link-Belt and over 31 years with Rexnord managing the Research & Development, Testing, Engineering Analysis, Product Design & Development and Product Engineering activities. Marlon’s responsibilities include conceptualizing, developing, maintaining, and improving bearing products required to support MB, Link-Belt, REX and Filament Industrial Bearing products.  His skillsets and vast experience are regularly utilized by the manufacturing, commercial, and applications’ departments to provide an expert level of product knowledge to support customer needs.