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Dynamic bearing loads and life

The general information about bearing life calculation and basic load ratings provided under Selecting bearing size is also valid for super-precision bearings. It should be noted that all life calculations based on ISO 281:2007 are valid for normal speeds. For applications where the speed factor A ≥ 500 000 mm/min, contact the SKF application engineering service.

A = n dm

A=speed factor [mm/min]
dm=bearing mean diameter [mm]
= 0,5 (d + D)
n=rotational speed [r/min]
Rated bearing life can be calculated for fatigue conditions based on statistical assumptions. For detailed information, refer to Basic rating life.
Basic dynamic load rating

The basic dynamic load rating C is used for life calculations involving dynamically stressed bearings, i.e. bearings that rotate under load. It expresses the bearing load that will result in an ISO 281:2007 basic rating life L10 of 1 000 000 revolutions. It is assumed that the load is constant in magnitude and direction and is radial for radial bearings and axial, acting centrically, for thrust bearings.

Values for the basic dynamic load rating C are listed in the product tables.

Equivalent dynamic bearing load

To calculate the basic rating life for a bearing using basic dynamic load ratings, it is necessary to convert the actual dynamic loads into an equivalent dynamic bearing load. The equivalent dynamic bearing load P is defined as a hypothetical load, constant in magnitude and direction, that acts radially on radial bearings and axially and centrically on thrust bearings. This hypothetical load, when applied, would have the same influence on bearing life as the actual loads to which the bearing is subjected.

Information and data required for calculating the equivalent dynamic bearing load is provided in each product section.

Basic rating life
The basic rating life of a bearing in accordance with ISO 281:2007 is

Perform calculation

If the speed is constant, it is often preferable to calculate the life expressed in operating hours using

L10=basic rating life (at 90% reliability) [million revolutions]
L10h=basic rating life (at 90% reliability) [operating hours]
C=basic dynamic load rating [kN]
P=equivalent dynamic bearing load [kN]
n=rotational speed [r/min]
p=exponent of the life equation
= 3 for ball bearings
= 10/3 for roller bearings
Rating life for hybrid bearings

When calculating the rating life for hybrid bearings, the same life values can be used as for bearings with steel rolling elements. The ceramic rolling elements in hybrid bearings are much harder and stiffer than steel rolling elements. Although this increased level of hardness and stiffness creates a higher degree of contact stress between the ceramic rolling elements and the steel raceway, field experience and laboratory tests show that the same rating lives can be used for both bearing types.

Extensive experience and testing show that in typical machine tool applications, the service life of a hybrid bearing is significantly longer than the service life of a bearing with steel rolling elements. The extended service life of hybrid bearings is due to the hardness, low density and surface finish of the rolling elements. Low density minimizes internal loading from centrifugal and inertial forces while increased hardness makes the rolling elements less susceptible to wear. Their surface finish enables the bearing to optimize the effects of the lubricant.

Requisite minimum load
In bearings that operate at high speeds or are subjected to rapid accelerations or rapid changes in the direction of load, the inertial forces of the rolling elements and the friction in the lubricant can have a detrimental effect on the rolling conditions in the bearing arrangement and may cause damaging sliding movements to occur between the rolling elements and raceways. To provide satisfactory operation, rolling bearings must always be subjected to a given minimum load. A general “rule of thumb” indicates that minimum loads of 0,01 C should be imposed on ball bearings and 0,02 C on roller bearings.
Calculating life with variable operating conditions

In some applications, the operating conditions, such as the magnitude and direction of loads, speeds, temperatures and lubrication conditions are continually changing. In these types of applications, bearing life cannot be calculated without first reducing the load spectrum or duty cycle of the application to a limited number of simplified load cases.

In case of continuously changing loads, each different load level can be accumulated and the load spectrum reduced to a histogram of constant load blocks (diagram 1). Each block should characterize a given percentage or time-fraction during operation. Note that heavy and normal loads consume bearing life at a faster rate than light loads. Therefore, it is important to have shock and peak loads well represented in the load diagram, even if the occurrence of these loads is relatively rare and limited to a few revolutions.

Within each duty interval, the bearing load and operating conditions can be averaged to some constant value. The number of operating hours or revolutions expected from each duty interval showing the life fraction required by that particular load condition should also be included. Therefore, if N1 equals the number of revolutions required under the load condition P1, and N is the expected number of revolutions for the completion of all variable loading cycles, then the cycle fraction U1 = N1/N is used by the load condition P1, which has a calculated life of L10 1. Under variable operating conditions, bearing life can be rated using


L10=basic rating life (at 90% reliability) [million revolutions]
L10 1, L10 2, ...
 =basic rating lives (at 90% reliability) under constant conditions 1, 2, ... [million revolutions]
U1, U2, ...=life cycle fraction under the conditions 1, 2, ...
Note: U1 + U2 + ... + Un = 1

The use of this calculation method depends very much on the availability of representative load diagrams for the application. Note that this type of load history can also be derived from a similar type of application.
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