A ball bearing is the most precision-demanding part in this wiki: hardened steel balls matched to within +-0.01 mm rolling in ground, hardened races, which is why a shop should not attempt one until it can routinely hold +-0.02 mm. Before then, salvage, clean and repack old bearings, and reserve them for fast, clean shafts where a bushing would run too hot.
Part of Bearings and Bushings
A rolling-element bearing of hardened balls between inner and outer races, held apart by a cage, trading manufacturing difficulty for very low friction at speed.
The Balls Must Be Better Matched Than The Shop That Makes Them
A ball bearing has four parts: an inner race that sits on the shaft, an outer race that sits in the housing, a ring of balls between them, and a cage that keeps the balls spaced. Everything in it is hard, small and exact. The carrier article states the requirement plainly: hardened steel, grinding to within 0.01 mm, polished surfaces, and a machine shop that can reliably hold +-0.02 mm before the attempt.
Notice the ratio. The ball set must agree to +-0.01 mm, twice as tight as the +-0.02 mm that the carrier treats as the shop’s general capability. That is why balls are not turned on a lathe. They are ground between two grooved plates with progressively finer abrasive (see Making Steel Balls). Every ball is rolled through the same random path against the same plates, so the process averages out errors instead of copying a master, much as the three-plate method generates flatness. That is the only way a set comes out tighter than the lathe that started it.
A tiny contact patch carries each ball’s load (Roller Bearings), so stress is high; if one ball is bigger than its neighbours it takes more than its share. Even sizing is what spreads the load.
What You Need
- High-carbon steel for balls and races, and a way to harden and temper it (Tool Steel, Hardening)
- Lapping plates and graded abrasive for the balls
- A lathe that holds +-0.02 mm routinely, and a grinder for the race grooves
- A gauge to measure ball diameter to 0.01 mm (Micrometers)
- Grease, and clean conditions for assembly
Method: The Build Order
Step 1: Make the balls (Making Steel Balls).
Step 2: Machine the races (Race Machining).
Step 3: Harden and temper both, then smooth the grooves.
Step 4: Assemble with a cage (Cage and Assembly). It should spin freely, smooth and quiet; roughness means dimensional errors or surface defects.
Step 5: Fit it with the inner race pressed on the shaft and the outer race sliding into the housing (Interference Fits).
If you cannot reach Step 1 honestly, skip to salvage: press the bearing off with a tube against the inner race only, soak it in kerosene, diesel or denatured alcohol for 30 minutes, spin it by hand and feel for roughness, clicking or looseness, and repack only if it runs smooth (steps from Roller Bearings).
Worked Example
Why choose rolling at speed? Take the 50 mm shaft under 2,000 N from Friction Fundamentals and run it at 3,000 rpm. Surface speed = 3.1416 x 0.05 x 3,000 / 60 = 7.85 m/s.
- Lubricated bronze bushing, friction force 40-120 N: heat 40 x 7.85 = 314 W up to 120 x 7.85 = 942 W
- Ball bearing at 1/10th to 1/100th of the sliding friction force: 0.4-12 N, heat 3.1 W up to 94 W
That assumes the bushing’s coefficient stays at its low-speed value, which is pessimistic if a full oil film forms. It also assumes the ball bearing is packed to 30-50% with grease; over-packing makes churning heat. The sums show the direction, not a promise.
Common Mistakes
- Making a ball bearing before the lathe is ready. Out-of-round balls and rough races fail early and noisily.
- Putting a salvaged bearing on a shock-loaded machine. A hard race with a tiny contact patch dents under a blow (see Sliding vs Rolling Friction).
- Striking the outer race to remove a bearing. It dents the race.
- Overpacking with grease.
Safety
Hardened steel fragments fly
Hardened balls and races are brittle and a failing bearing at speed throws fragments. Wear eye protection when hardening, grinding, pressing and first running, and test-run a new bearing behind a guard with the machine’s stop within reach. Quench oil and a forge are fire hazards.
What’s Next
- Learn next: Making Steel Balls - the part that is hardest to get right
- Combine with: Precision Tool Making - the measuring tools that tell you whether the balls are good enough