8/12/2019 Definition of Bearing Life

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mining Bearing Life http://www.epi-eng.com/BAS-Bearing

12/05/2005

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The prediction of the life of a rolling-element bearing (ball, roller, needle) is a statisticalcalculation of the fatigueproperties of the bearing components, in which life is stated as the

number of hours that a specified percentage of a large population ofapparently-identicalbearingswill survive under a specified loadwith a specified set of operating conditions.

From that definition, it should be clear that the prediction of the expected life of a rollingelement bearing in a specific application involves a bit more analysis than simply plucking aload rating from a bearing catalog. The dynamic load rating listed in bearing catalogs is theload at which 90%of a large population of apparently-identical bearingswill survive one millioncycles. A shaft turning at 2250 RPM will produce one million cycles in 7.4 hours. That'sprobably OK for a race car, but not exactly an aircraft-quality life expectancy.

The life of a given bearing is a nonlinear function of the applied load. For a ball bearing, it isrelated to load to the 3.00power (i.e. load x load x load); For roller and needle bearings, it isthe 3.33 power. That means that a relatively small increase in bearing load can cause adramatic reduction in bearing life.

The usual life rating for industrial applications is called "L-10" life. The L-10 life is the numberof hoursin service that 90%of a large population of apparently-identical bearingswill survivewhen subjected to the boundary conditions (load, speed, lubrication, material andcleanliness)that are specific to the application. Stated another way, 10% of that populationwill have failed in the L-10 number of service hours.

We think that the L-5 life (5% failure) criterion is more appropriate than L-10 for aircraft design.To achieve the 5% failure rate with a given set of boundary conditions requires 1.64 timesgreater bearing capacity (dynamic load rating); to achieve a 2% failure rate requires 3.0 timesgreater dynamic load rating. Put another way, for a given L-10 life, the L-5 life of the same

bearing is 61% of the L-10 value, and the L-2 life is 33% of the L-10 value.

Most rolling element bearing manufacturers publish detailed life-load analysis procedures. Thebearing life calculations which the manufacturers publish take into account factors including (a)the combined effect of applied radial and thrust loads, (b) RPM, (c) pitchline velocity, (d)lubricant viscosity, (e)contamination, (f) bearing load ratings, and (g) desired probability ofsurvival (failure rate). All of them are important, but the effects of lubricant viscosity andcleanlinessare huge.

These calculations have been implemented in a computer program which EPI wrote severalyears ago for doing bearing life analysis. This program uses a user-defined load modelof theexpected service to evaluate bearing life in an actual application. The load model is a set ofdifferent operating loads and speeds which the actual machine is designed to handle, and anestimate of the % of service that each load and speed represents.

For example, the load model of an an aircraft propeller gearbox on a normally-aspiratedengine might be described as 5% of the time at max power (takeoff), 10% at climb power, 70%at cruise power, and 15% in aerobatic maneuvers. Each one of those conditions imposesignificantly different loads and speeds on the gearbox bearings. The combination of thosedifferent conditions in a realistic load model allows for a more reasonable selection of bearingsfor an application.

Certain bearing manufacturers (SKF, for example, in late 2003) have made available, on theirwebsites interactive programs for predicting "L-10" bearing life.

8/12/2019 Definition of Bearing Life

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mining Bearing Life http://www.epi-eng.com/BAS-Bearing

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The SKF website provides a life-calculation program which produces THREE different valuesfor an L-10 life rating:

(1)the original (old)Arvid Palmgrenmethod, which predicts a relatively short life,(2)the seasoned, proven "A23" method (the one implemented in the EPI bearing

program), and(3)the new, SKF-"marketing-friendly" rating.

The one you pick depends on how realistic you want to be.

Our bearing life calculation program matches the SKF-website values calculated for an L-10life under the "A23" method for the same input values. However, our program also includes

the ability to calculate life ratings for complete load models (described above) as well as forsurvival probabilities more appropriate to aircraft applications (95, 96, 97, 98 and 99%).

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