←
BearingLab rolling bearing life
Open the interactive version →

This is a static copy of the chapter for search engines. The interactive version has animated figures, check questions, and buttons that load the example into the calculator.

6. Sizing one, in the order that works

  1. Get the loads. From the gear, the belt, the payload and the arm's own weight. If you have used GearLab, Ft and Fr are already there; ShaftLab turns them into bearing reactions.
  2. Split them between the bearings. Radial by the beam reactions; the whole axial load onto the located one.
  3. Pick a type from the load directions, not from a catalogue price.
  4. Decide the target life and the reliability you actually need.
  5. Compute the C you need — invert the life equation:
C = P · ( L10h · 60 · n106 · a1 · aISO )1/p
  1. Find the smallest bearing with that C that fits the shaft.
  2. Check s₀ against the worst static case, which is usually not the running one.
  3. Check the speed against the catalogue limit, and the load is not too light.
A bearing can be too lightly loaded. Below about 1 % of C the rolling elements skid instead of rolling, scuffing the races. Minimum-load requirements are real, and are why a lightly loaded bearing is often deliberately preloaded.
Try it

A gearbox output bearing: heavy radial load, moderate speed, 30 000 h target.

Formulas in this chapter

C — the rating a target life demands
C = P (L₁₀ₕ · 60 n / (10⁶ a₁ a_ISO))^(1/p) [N] ISO 281 inverted
R_A, R_B — bearing reactions from one radial load
R_A = F(L−a)/L, R_B = F a/L [N] statics