Dynamic Load Rating of Spherical Roller Bearings
The self-aligning spherical roller bearing is often referred to as "the workhorse of Canadian heavy industry". Because of its widespread use, a number of manufacturers, each with its own design philosophy, offer their spherical roller bearing in the marketplace.
When faced with the variety of inner ring and roller configurations and cage designs and materials, the designer may tend to rely upon the published catalog ratings as the basis for selection. However, the best bearing for the job is rarely the one with the highest dynamic load rating1.
This article is intended to familiarize the reader with the concepts of load ratings and life as well as other, important factors that should be considered when deciding on a particular spherical roller bearing for design or replacement.
All bearing manufactures' catalogs include, among other things, the dynamic load rating for each bearing listed in the bearing tables. This value is both an Engineering and a Marketing tool. On the technical side, the dynamic load rating is a measure of the bearing's ability to carry load while rotating and is used to determine a theoretical (L10) life. From a Marketing standpoint, the higher the dynamic load rating, the greater the commercial value.
What is the Dynamic Load Rating?
The Dynamic Load Rating is calculated according to an International Standard established by ISO2. In November of 1962, standard ISO281 was proposed. This standard "...specifies methods of calculating the basic dynamic load rating of rolling bearings, manufactured from hardened steel, as well as the basic rating life and the adjusted rating life..." The standard has been revised twice since then and a third revision is about to be adopted.
Basic Load Rating is defined within this standard as "that constant stationary radial load which a group of apparently identical roller bearings with stationary outer ring can endure for a rating life of one million revolutions of the inner ring".
Rating Life is further defined in the standard as "the number of [revolutions or hours] that 90% of a group of [apparently identical] bearings will complete or exceed before the first evidence of fatigue develops".
Adjusted Rating Life is the Rating Life modified for reliability, material and operating conditions.
How is the Dynamic Load Rating calculated?
The ISO formula for calculating the dynamic load rating of a roller bearing is:
| Where: |
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= |
Dynamic Load Rating |
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= |
Factor which depends on the units used, the exact geometry of the load-carrying surfaces, the material and the accuracy to which parts are made |
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= |
Number of rows of rollers |
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= |
Effective length of contact between one roller and that ring where the contact is the shortest (overall roller length minus roller corner radii or minus grinding undercuts) |
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= |
Nominal angle of contact |
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= |
Number of rollers per row |
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= |
Roller diameter |
From the definition of the Dynamic Load Rating, it follows that a radial bearing carrying a pure radial load equal to its load rating Theoretically has a 90% chance of surviving one million or more revolutions before metal flaking (> 6 mm2) occurs on any of the rolling surfaces. Alternatively, there is a 10% chance it will fail before attaining one million revolutions.
This does not mean a bearing should be loaded up to its dynamic capacity. Consider, for example, a bearing at the drive end of an electric motor rotating at 3,600 rpm and subjected to a load equal to its dynamic capacity. Based on the foregoing, this bearing theoretically has a 90% chance of surviving for just over four and a half hours!
This is obviously unacceptable. So in practice, a designer will typically select a bearing with a Dynamic Load Rating at least ten times greater than the applied load.
General guidelines established by the bearing industry classify load conditions (Pr) are as follows:
| Light |
Pr ≤ 06 × Cr |
| Normal |
.06 × Cr ≤ Pr ≤ .12 × Cr |
| Heavy |
Pr ≥ .12 × Cr |
How are dynamic load rating and bearing life related?
The simple relationship between dynamic capacity of a roller bearing and theoretical life, developed in the '40s is shown below. (As mentioned previously, this formula has been modified several times over the decades to account for reliability, materials and operating conditions.)
| Where: |
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= |
life in millions of revolutions attained or surpassed with 90% reliability |
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= |
Dynamic Load rating |
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= |
the applied radial (or radial equivalent) load |
Combining the Life and Capacity formulae, it can be seen that a small increase in capacity will have a significant effect on theoretical life. For example, if the length of the spherical roller is increased by 5%, the capacity increases by 3.87% and the theoretical fatigue life is increased by 13.5%. A 5% increase in roller diameter will produce a 5.38% increase in capacity and 19% increase in theoretical life.
Spherical Roller Bearing Internal Design and capacity
The first spherical roller bearing design, referred to as the B-type, utilized asymmetrical rollers and a three-ribbed inner ring. Asymmetrical meaning the largest diameter is not located midway between the roller ends, but closer to the inboard end. Thus the roller is slightly tapered as well as spherical.
In later designs, the asymmetrical roller was traded for a symmetrical or truly barrel shaped roller. This became know as the C-type.
Why is there a difference in manufacturers' load ratings?
All manufacturers strive to maximize capacity within the envelope dimensions (bore, O.D. and width) established by ISO. The distinctive design philosophies involving internal geometry, inner ring configuration, the number of rollers, their diameter and length, result in different published load ratings for essentially the same bearing.
For any manufacturer wanting to increase the marketability of its spherical roller bearing by increasing the capacity, the simplest solution is to increase the length of the roller. This means altering the internal design to fit in a longer roller. This is essentially why the "C" style spherical roller bearing was developed. The apparent advantage of this style is the increase in dynamic capacity.
The next development was the "E" type, or Extra Capacity spherical roller bearing. The design features larger diameter, symmetrical rollers, thus offering an even higher load rating than its predecessor.
The "Extra Capacity" Spherical Roller Bearing
While a significant capacity gain can be realized by increasing the roller diameter, there is a limit because of the need for sufficient ring cross section. Also, as the roller diameter increases, there is less room at the pitch circle for cage bars. A strong cage bar is needed in the C type to prevent skewing of the barrel shaped roller. Manufacturers of the "E" type spherical roller bearing have addressed this problem in one of two ways.
The first compromise involved changing the cage material to molded polyamide. The molding process permitted a cage bar cross section that yielded sufficient strength and made assembly easier. A reduction in manufacturing cost of the cage was an added benefit.
Although the polyamide cage has some specific benefits, it is not suitable for all applications. Many industrial users have never fully accepted polyamide as a cage material and continue to specify metallic cages.
The second compromise was a redesigned steel cage that can be used with larger diameter rollers. The cage bar was dropped below the pitch diameter of the rollers in order to maintain sufficient cage bar thickness.
Theoretical Life versus Service Life
These differences in load ratings affect the theoretical fatigue life of the bearing. The term theoretical is emphasized because it considers only one mode of failure, subsurface metal fatigue (spalling). It is generally accepted that roughly 10% of the bearings put into service in North America fail from fatigue. The remaining 90% failed prematurely from other factors affecting bearing performance. The most common of these, in the experience of the writer, are:
- Contamination
- Lubricant and lubrication issues
- Improper installation
- Improper shaft and housing fits
- Skidding3 (lack of load or too much capacity)
Overall, how important is a high dynamic load rating?
NTN provides root cause failure analysis as a service to our customers. We have inspected literally thousands of spherical roller bearings of every make and from all industries. In the initial phase of the inspection, we examine the length of the contact between the rollers and the raceways. This is a good indicator of the magnitude of the load relative to the capacity of the bearing.
We typically find that the contact between rollers and raceways is limited to a narrow band, roughly centered on the roller outside diameter. In other words, no matter how long the roller is, only a small portion of its length is actually carrying the load.
Furthermore, we have found that for a given load condition, the contact band is pretty much the same width for all makes and bearing designs. Only in the most severely loaded applications (vibrating screens, continuous casters, and some press roll applications) is there a large percentage of the roller length in contact.
Our experience has shown that lack of capacity is rarely a problem in terms of premature failure.
In fact, in our collective experience, failures caused by too much capacity are far more common than those due to insufficient capacity.
The factors listed above affect bearing performance by placing a high demand on individual bearing components. This is especially true during the initial stages of a developing problem. Cage design, roller configuration, roller guidance and ease of lubricant flow are just as important as the load rating, if not more so, in attaining satisfactory service life.
Dynamic capacity does not dictate bearing performance and should not be used as the sole guide to bearing selection. A technical evaluation of the required dynamic load rating is necessary. However, a prudent bearing choice will place even greater emphasis on internal design and cage design and materials.
| 1 |
Dynamic Load Rating is shown as Cr in catalog bearing tables. The terms Basic Load Rating, Dynamic Load Rating, Load Rating, Dynamic Capacity and Basic Dynamic Capacity are generally used interchangeably. |
| 2 |
The International Organization for Standardization (ISO) has set the standards governing all aspects of antifriction bearings. There are approximately 50 standards related to anti-friction bearings. |
| 3 |
Skidding refers to the non-rolling contact between a rolling element and a raceway. It is most commonly found in roller bearings that are lightly loaded relative to the dynamic capacity of the bearing. Cylindrical roller bearings and spherical roller bearings are particularly susceptible to this form of damage.
Skidding damage frequently occurs at the point of entry into the load zone where the rolling elements (which have slowed down while out of the load zone) are required to return to full rotational speed instantly.
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