ball bearing

A ball bearing is a rolling-element bearing in which hardened steel spheres sit between two concentric rings, or races. The inner race mounts on a shaft; the outer race sits in a housing. When the shaft rotates, the balls roll along grooved paths in both races, converting the high friction of sliding contact into the much lower friction of rolling. A cage, usually made of brass or steel, spaces the balls evenly around the circumference so they do not collide.

The geometry of a ball bearing is deceptively simple but critical. The balls are typically made from through-hardened bearing steel and ground to tolerances within a few micrometers. The races are also hardened and precisely formed with circular grooves, called raceways, that have a slightly larger radius than the balls themselves. This geometry creates a point contact between each ball and the raceway, concentrating the load into a small area and allowing higher speeds than flat-race designs. Single-row deep-groove ball bearings, the most common variant, can handle radial loads perpendicular to the shaft and some axial (thrust) load along it.

Size and load rating follow dimensional standards set by ISO 15. A typical bearing in an electric motor might be 6205: an outer diameter of 52 millimeters, an inner bore of 25 millimeters, and a width of 15 millimeters. A bearing's basic dynamic load rating, expressed in kilonewtons, tells you the constant radial load it can sustain for one million revolutions before spalling (flaking) begins on the raceway surfaces. Failure usually starts as subsurface fatigue in the steel, where stress cycles eventually exceed the material's endurance limit.

Wear, preload, and installation

Bearings wear from repeated stress cycles, from particles of dirt or metal in the lubricant, and from inadequate or degraded grease. Seals and shields protect against contamination, though no barrier is perfect. In applications where axial play must be eliminated, such as spindle bearings in machine tools, two ball bearings are often installed face-to-face with an adjustable preload that compresses the balls slightly against the races. This increases stiffness and reduces vibration at the cost of higher friction and heat generation.

The name reflects the form and function: spheres roll, friction drops, load moves smoothly. Ball bearings sit at the foundation of rotating machinery: electric motors, pumps, compressors, gearboxes, and machine tools all depend on them. They operate at speeds from near-static to over 10,000 rpm in small bearings and carry loads from tens of newtons to hundreds of kilonewtons. Once installed and lubricated, they require little maintenance, which explains their dominance over earlier sleeve and journal bearing designs.