Bearing rating life

Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

For estimating the expected bearing life, you can either use basic rating life, L10, or SKF rating life, L10m.
If you have experience with the operating conditions related to lubrication and contamination, and know that the conditions you are working with do not have a dramatic effect on the life of your bearings, use the basic rating life calculation, otherwise SKF recommends using the SKF rating life.

Bearing life definition
Bearing life is defined as the number of revolutions (or the number of operating hours) at a given speed that the bearing is capable of enduring before the first sign of metal fatigue (spalling) occurs on a rolling element or the raceway of the inner or outer ring.

Tests on seemingly identical bearings, under identical operating conditions, result in a large variation in the number of cycles, or time, needed to cause metal fatigue. Therefore, bearing life estimates based on rolling contact fatigue (RCF) are insufficiently accurate and so a statistical approach is needed to determine bearing size.

The basic rating life, L10, is the fatigue life that 90% of a sufficiently large group of apparently identical bearings, operating under identical operating conditions, can be expected to attain or exceed.

To determine a relevant bearing size using the definition given here, compare the calculated rating life against the service life expectations of the bearing application, using experience from previous dimensioning where available. Otherwise, use the guidelines regarding specification life of various bearing applications provided in table 1 and table 2.

Due to the statistical spread of bearing fatigue life, an observed time to failure for an individual bearing can be evaluated in relation to its rated life only if the failure probability of that particular bearing is determined in relation to the general population of bearings running under similar conditions.

Numerous investigations on bearing failure, in a variety of applications, have confirmed that design guidelines based on 90% reliability, and use of dynamic safety factors, lead to robust bearing solutions in which typical fatigue failures are avoided.

Basic rating life
If you consider only the load and speed, you can use the basic rating life, L10.

The basic rating life of a bearing according to ISO 281 is

Basic rating life
Perform calculation

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

Basic rating life in hours

where
L10basic rating life (at 90% reliability) [millions of revolutions]
L10hbasic rating life (at 90% reliability) [millions of hours]
Cbasic dynamic load rating [kN]
Pequivalent dynamic bearing load [kN]
nrotational speed [r/min]
pexponent of the life equation
= 3 for ball bearings
= 10/3 for roller bearings


SKF rating life
For modern high-quality bearings, the calculated basic rating life can deviate significantly from the actual service life in a given application. Service life in a particular application depends not only on load and bearing size, but also on a variety of influencing factors including lubrication, degree of contamination, proper mounting and other environmental conditions.

ISO 281 uses a modified life factor to supplement the basic rating life. The SKF life modification factor aSKF applies the same concept of a fatigue load limit Pu (→ Fatigue load limit, Pu) as used in ISO 281. Values of Pu are listed in the product tables. Just as in ISO 281, to reflect three of the important operating conditions, the SKF life modification factor aSKF takes the lubrication conditions (viscosity ratio κ → Lubrication condition – the viscosity ratio, κ), the load level in relation to the bearing fatigue load limit, and a factor ηc for the contamination level (→ Contamination factor, ηc) into consideration using

SKF rating life
Perform calculation

If the speed is constant, the life can be expressed in operating hours, using the equation

SKF rating life in hours

where
LnmSKF rating life (at 100 – n1) % reliability) [millions of revolutions]
LnmhSKF rating life (at 100 – n1) % reliability) [operating hours]
L10basic rating life (at 90% reliability) [millions of revolutions]
a1life adjustment factor for reliability (table 3, values in accordance with ISO 281)
aSKFSKF life modification factor
Cbasic dynamic load rating [kN]
Pequivalent dynamic bearing load [kN]
nrotational speed [r/min]
pexponent of the life equation
= 3 for ball bearings 
= 10/3 for roller bearings

1) The factor n represents the failure probability, which is the difference between the requisite reliability and 100%.

For 90% reliability:

Lnm = SKF rating life (at 100 - n1)% reliability) [million revolutions]

Becomes:

L10m = SKF rating life [million revolutions]

Since the life adjustment factor a1 is related to fatigue, it is less relevant for load levels, P, below the fatigue load limit Pu. Dimensioning with life adjustment factors reflecting very high reliability (such as 99%) will result in large bearings for given loads. In such cases, the bearing load must be checked against the minimum load requirement for the bearing. Calculating minimum load is described in Requisite minimum loads.

Table 4 provides commonly used conversion factors for bearing life in units other than million revolutions.

Calculating bearing life with variable operating conditions, fluctuating load

In some applications – for example, industrial gearboxes, vehicle transmissions or windmills – 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 (diagram 1).

For continuously changing loads, each different load level can be accumulated and the load spectrum reduced to a histogram plotting constant-load blocks. Each block should characterize a given percentage or time-fraction during operation. Heavy and normal loads consume bearing life at a faster rate than light loads. Therefore, it is important to have peak loads well represented in the load diagram, even if the occurrence of these loads is relatively rare and of relatively short duration. 


Within each duty interval, the bearing load and operating conditions can be averaged to a representative, 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 L10m1. Under variable operating conditions, bearing life can be rated using

Bearing life

where

L10mSKF rating life (at 90% reliability) [million revolutions]
L10m1, L10m2, ...fraction SKF rating lives (at 90% reliability) under constant conditions 1, 2, ... [million revolutions]
U1, U2, ...life cycle fraction under the conditions 1, 2, ...
U1 + U2 + ... Un = 1


The use of this calculation method is well suited for application conditions of varying load level and varying speed with known time fractions.

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