Every rotating shaft needs a bearing. Choosing the wrong one means premature failure, excessive heat, or wasted money. The core job of a bearing is to constrain relative motion while minimizing friction between moving parts. Here are the types you'll encounter most often.

Ball Bearings (Deep Groove) — The workhorse. A single row of steel balls sits between an inner and outer race. They handle radial loads (perpendicular to the shaft) well and moderate axial loads (along the shaft). You'll find them in electric motors, skateboard wheels, and hard drives. They're cheap, widely available, and work at high speeds.

Roller Bearings (Cylindrical) — Replace the balls with cylinders. The larger contact area means they carry significantly higher radial loads than ball bearings of the same size, but they handle almost no axial load. Used in conveyor systems, gearboxes, and heavy machinery.

Tapered Roller Bearings — Conical rollers arranged at an angle. These carry both high radial and high axial loads simultaneously. Car wheel hubs are the classic application. They're always installed in opposing pairs so the axial forces balance.

Thrust Bearings — Designed exclusively for axial loads. The ball or roller variant sits in a flat plane. Found in lazy Susans, rotary tables, and the steering columns of vehicles.

Plain (Sleeve) Bearings — No rolling elements at all, just a lubricated bushing. Extremely simple, quiet, and compact. Used where loads are light and speeds are low to moderate: door hinges, small linkages, and oscillating joints.

Selection criteria — When choosing a bearing, evaluate these factors in order:

• Load direction and magnitude — Radial, axial, or combined? This eliminates whole categories immediately.
• Speed — Every bearing has a rated max RPM. Ball bearings win at high speed; roller bearings win at high load.
• Life expectancy — Bearing life is calculated using the L10 formula: L10 = (C / P)^n × 10⁶ revolutions, where C is the dynamic load rating (from the datasheet), P is the equivalent applied load, and n = 3 for ball bearings or 10/3 for roller bearings.
• Environment — Temperature, moisture, contamination. Sealed bearings (2RS suffix) keep grease in and dirt out. Shielded bearings (ZZ suffix) offer less protection but less friction.
• Space and cost — Sleeve bearings are smallest and cheapest. Tapered rollers need the most axial space.

Quick calculation: A 6205 deep-groove ball bearing has a dynamic load rating C = 14.8 kN. Under a radial load P = 2 kN: L10 = (14.8 / 2)³ × 10⁶ = ~1.6 billion revolutions. At 1,750 RPM (a common motor speed), that's roughly 15,500 hours — nearly two years of continuous operation. If you need more life, go up one bearing size.