7. How the calculator works, and what it does not do
Where the numbers come from
Everything here is ISO 281 as reproduced in NTN's published lifetime-calculation note: the basic rating life and its exponents, the life in hours, the equivalent dynamic load, the modified rating life with a₁, and the static safety factor. The a₁ table is quoted exactly. The equal-damage mean load is a consequence of the same cube law rather than a separate source.
How it was checked
node test-bearings.cjs runs 71 checks: the cube law (halving the load gives eight
times the life), the roller exponent, the hours conversion, every a₁ value, both branches of the
equivalent load, the required-C inversion round-tripping back to the life you asked for, the static
check with and without a governing axial component, and the equal-damage mean exceeding the
arithmetic one.
What is deliberately not modelled
The X / Y / e table. These are per bearing series and every accessible source publishes them as an image. They are inputs, not built-in data — see chapter 3.
- aISO, the life modification factor for lubrication, cleanliness and the fatigue load limit Cu. It defaults to 1, which is the optimistic assumption for poor lubrication and the pessimistic one for a clean, well-lubricated bearing. In practice it swings the answer by more than anything else on this page.
- Friction torque and heat. Not modelled at all, and it is what sets the real speed limit.
- Grease life, which frequently expires long before the fatigue life does.
- Preload and stiffness, which is why an angular contact pair exists.
- Fits and mounting. A bearing pressed onto an oversized shaft loses its internal clearance and its life with it — that belongs to a tolerances tool.