DIN 732 — Thermally Safe Operating Speed of Rolling Bearings, Explained

To determine the thermally safe operating speed of a rolling bearing under your actual operating conditions. It takes the bearing's thermal speed rating from DIN ISO 15312 — the speed at which friction heat and dissipation balance under standardised reference conditions — and corrects it with a lubricant parameter and a load parameter to give n_θ = n_θr · f_n. It is used in high-speed machine-tool spindle design, where the catalogue reference speed often does not reflect the real load, lubricant or preload.

No, and the Thermal Analysis Calculator no longer claims to. DIN 732 solves for a SPEED using coefficient tables, a DIN ISO 15312 thermal speed rating, a reference heat-flow density and a defined reference surface. The MechanixCalc tool solves a spindle-assembly heat balance for a TEMPERATURE, using a bearing maker's published simplified constant-coefficient friction estimate plus a motor-loss term and a cooling-circuit capacity that DIN 732 has no concept of. It is an engineering estimate with no governing standard, and it is labelled as one on every panel. Earlier versions of this page and of the calculator described it as a DIN 732 implementation; that claim was wrong and has been removed.

Through the Palmgren M = M₀ + M₁ friction-moment split. M₀ is load-independent and depends on the lubricant viscosity and the speed, roughly as (ν·n)^(2/3); M₁ is load-dependent. The frictional power is Q = M·ω, so the heat rises faster than linearly with speed. A simplified constant-coefficient estimate — M = µ·P·d_m/2 with a single µ, the form published in bearing-maker catalogues — has no M₀ term at all and is therefore optimistic at high speed or high viscosity. The two are often confused; only the M₀ + M₁ form is Palmgren's, and only that form is what DIN 732 builds on.

The reference speed (formerly "thermal speed rating") is the heat-balance point under the standardised conditions of DIN ISO 15312. The limiting speed is a separate MECHANICAL limit from cage stability, lubricant film and centrifugal loading. DIN 732 is the method that corrects the reference speed to your actual application — actual loads, actual lubricant, actual cooling — and the result frequently differs substantially from the catalogue figure in either direction. Both limits must be respected; neither supersedes the other.

Thrust roller bearings are excluded, because heat balance is not what limits their speed. Bearings with rubbing contact seals are covered only if the seal friction is accounted for separately, since the seals can dominate the friction moment. The method also assumes the heat leaves through the defined seating surfaces and lubricant flow, so an arrangement with a substantially different heat path needs its own analysis.