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Find bearing loads and interpret rating life

Two bearings do not necessarily split load equally. Start with the load position and bearing centres to find each reaction.

Simply supported shaft with one downward load, upward support reactions, bearing span and load position dimensions.FyAyByABxL
Side view of a simply supported shaft at A and B. Blue Fy is a downward point load; red Ay/By are upward reactions. L is bearing-centre span, x the load position from A. Shaft self-weight must be included separately.

What you will understand

  • How support/load positions change reactions
  • Radial, axial and equivalent load
  • Basic rating life versus actual service life

Blue: defined conditions · Red: calculated quantities. Diagrams are schematic.

  • L, x: bearing centres and load position
  • Ay, By: signed reactions
  • P: equivalent dynamic load, C: basic dynamic load rating

The examples below are illustrative. Each example button loads its fixed values, units and mode. Ordinary calculator links do not load an example.

Resolve each bearing reaction

Only a central load in the symmetric model gives equal reactions. Overhung loads can reverse a reaction; a negative result is a direction change, not an error to discard.

Ay=Fy(1−x/L), By=Fy x/L

Separate radial and axial loading

Combine radial reaction components vectorially. Axial load sharing depends on locating/floating arrangements and bearing pairs. X and Y must match the actual bearing and equation regime.

For practical review

Fa/Fr, threshold e, rotating ring, contact angle and induced axial loads in pairs can alter equations and sharing. X=Y=1 is not universal, nor is the general equation automatically valid for pure axial loading.

General form P=XFr+YFa (verify product applicability)

Read basic rating life in revolutions and hours

For constant load, the basic-life exponent is 3 for balls and 10/3 for rollers. L10 is a fatigue-life statistic at 90% reliability for a bearing population under stated conditions, not a guarantee for an individual bearing.

L10=(C/P)^p ×10⁶ revolutions, L10h=L10/(60n) (n in rpm)

Example: reactions first, then life

Inputs

  • L=300 mm, x=120 mm, Fy=1000 N, Fz=Fa=0
  • Illustrative radial-ball-bearing regime X=1, Y=0
  • At A: C=6000 N, n=1000 rpm

Results

  • A: 600 N, B: 400 N; A has P=600 N
  • A has L10=1 billion revolutions; L10h≈16,667 h

C=6000 N is illustrative and not a product rating. The result does not promise failure-free operation for that duration. Lubrication, contamination, fits, static loads and speed limits remain separate.

Load the fixed values, units and mode for this illustration. Then edit them for your conditions.

Review operating conditions beyond basic life

Even with adequate calculated fatigue life, static indentation, displacement, heat or poor lubrication can govern. A changing duty cycle needs more than one arbitrary constant load.

  • Static rating C0, shock, minimum load and limiting speed
  • Lubrication, sealing, contamination, temperature, fits and clearance
  • Shaft/housing stiffness, misalignment and locating/floating arrangement

Keep a review checklist

This checklist is a personal reminder, not a safety score or an automatic approval. It is not sent or saved.

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Reference information and illustrative calculations. This content does not guarantee suitability or safety of a design. Confirm the actual materials, component specifications and operating conditions.