Ball, Roller and Needle Bearings

§1Rolling replaces sliding

A rolling bearing’s friction is a small fraction of even a good oil film’s — and, decisively, it is just as low at the first degree of rotation as at full speed.

The plain bearing’s whole drama — the wedge that needs speed, the boundary contact at every start — vanishes when the elements roll: friction coefficients drop to the thousandths, essentially independent of speed, so a rolling-borne shaft starts free, runs cool, positions precisely and holds its place without the plain bearing’s film to build or lose. That is why every machine axis, wheel, spindle and motor of the modern catalogue rides on them by default. The costs are the mirror of the gains. Load that a plain bearing spread over a broad film now passes through points and lines of near-glass-hard steel at contact stresses in the gigapascals — survivable only because the contact is momentary and moving, and terminal, eventually, by the sub-surface fatigue §4 describes: a rolling bearing has a life, computable and finite, where a full-film plain bearing in principle has none. Add the practical ledger — rolling bearings cost more, drum audibly, dislike shock and dirt, cannot be split over a crankshaft, and end at sizes plain bearings sail past — and the two families settle into the division the last page promised: rolling wins the general case, plain keeps the extremes.

§2The catalogue family

One idea, a dozen geometries — each member of the family is the answer to one question: what mix of radial load, axial load, speed, space and misalignment must this position carry?

§3The life equation, worked

The catalogue gives every bearing one number — the dynamic capacity C — and one line turns it into a life: with a cube law inside it that dominates every bearing decision ever made.

§4The statistics of fatigue

L10 is not a promise to your bearing — it is a percentile of a population: the life by which one bearing in ten has already failed, from a mechanism that is fatigue, not wear.

Each passage of a rolling element drives a pulse of shear stress a fraction of a millimetre below the raceway surface; after enough millions of pulses a crack nucleates there, grows to the surface and pops out a flake — spalling, the crater that announces the end, first as noise and vibration, then quickly as roughness breeding roughness. Because crack nucleation is statistical, identical bearings on identical duty scatter widely in life, and the standard deals in percentiles: L₁₀ is the 90%-survival life — one in ten gone sooner, the median comfortably longer — which is why critical positions are designed to generous computed lives and monitored by vibration, not run to the number. Two riders complete the picture. The equation assumes an adequate lubricant film: rolling contacts survive their gigapascal stresses partly because oil trapped in the contact pressure-thickens and keeps the steels apart (the elastohydrodynamic story the lubrication page tells), and a starved or contaminated film shortens life far below anything §3 computes — dirt denting a raceway pre-loads the fatigue clock at every dent. And a bearing damaged statically — brinelled by a press or a hammer blow through its elements — carries permanent dents that turn the same clock forward before the first revolution, the sin §5’s fitting rules exist to prevent.

§5Fits, location and float

Mounting has two laws — interference on the ring that rotates relative to the load, and one bearing located while the other floats — and both exist to stop quiet, slow failures.

The fit rule: the ring that rotates relative to the load direction gets the interference fit; the stationary-relative ring may be a close slide. On the common machine — rotating shaft, fixed radial load — that means inner ring pressed on the shaft, outer ring located but not jammed in the housing. A loose rotating ring creeps around its seat a fraction of a turn per revolution, fretting the shaft and itself into scrap; the interference fit is simply the cure for creep. The location rule: a shaft on two bearings is fixed axially at exactly one of them; the other must be free to float — via a sliding outer-ring fit or §2’s cylindrical roller with its built-in freedom — because a shaft warms and grows, and two hard-located bearings turn thermal growth into a fierce mutual axial preload that eats both. The handling rule follows from §4: press only on the ring being fitted, never so the force crosses the rolling elements — a press through the balls brinells the raceways in one stroke — and for stubborn interference fits, heat the inner ring (oil bath or induction) and let expansion do the pressing. Three rules, one theme: rolling bearings die less often from load than from the fits, floats and fingerprints around them.

§6Preload and pairs

The one-way bearings come as opposed pairs with an adjustment between them — and that adjustment, clearance through zero into preload, is a genuine design variable with stakes at both ends.

Angular-contact and taper pairs face each other so each carries the thrust the other cannot, and drawing them together sets the pair’s internal state. Left with clearance, the assembly is free and cool but the shaft can shuttle and tilt within the slack — fine for a conveyor, fatal for a spindle. Drawn into preload, every element is always engaged: stiffness multiplies, the shaft runs true to microns, no element ever skids unloaded — the setting of machine-tool spindles and pinion heads — but every revolution now works against the preload, so friction and heat rise, and heat in an over-preloaded pair expands parts into more preload: the thermal spiral that seizes spindles, and the reason preload is specified, not maximised. Ordinary single bearings carry the same variable factory-set as an internal clearance grade, chosen so the running fit and temperature land the bearing near zero in service. The craft’s classic exam is the taper-roller wheel hub, set by feel: tighten while turning to seat everything, back to the setting, lock — a few hundredths of adjustment deciding between a wheel that wanders and a hub that runs hot, on exactly the physics of this section. The whole family, in the end, is managed at its margins: fits, floats, films and this last handful of micrometres.

§7Quick reference

The working core of the page on one card rack.