Assemble a ball bearing by offsetting the races to open a gap, loading the balls into it, centring the races and spacing the balls evenly, then fitting a brass or steel cage with a pocket for each ball and packing grease into 30-50% of the free space. A good bearing then spins freely, smooth and quiet; any roughness means dimensional errors or surface defects in the parts.
Part of Bearings and Bushings
Getting the balls between the races, holding them apart with a cage, and finishing the bearing with grease and shields.
The Cage Stops Balls Rubbing Each Other
Two neighbouring balls in a loaded bearing roll in the same direction around the bearing, but at the point where they would touch their surfaces move in opposite directions. Left alone, they rub. The cage holds each ball in its own pocket so they never meet, keeps the spacing even, and lets the ring of balls travel round together. The carrier article calls for a machined brass cage or a stamped steel one with a pocket for each ball.
Balls cannot simply be dropped in: with the races concentric, there is room for a ball only where the gap is, and the gap is the same size all the way round. The trick is to offset the inner race against one side of the outer race so that a crescent-shaped gap opens on the other side, drop the balls into the crescent, then bring the races back to centre and spread the balls evenly.
What You Need
- Finished inner and outer races and a matched set of balls
- A cage with one pocket per ball, in machined brass or stamped steel
- Clean grease (see Lubricant Types) and shields or seals if the bearing will see dirt
- A clean bench: grit between ball and race scores both
Method: Assemble A Ball Bearing
Step 1: Offset the races to open the crescent gap, and load the balls into it.
Step 2: Centre the races and distribute the balls evenly around the track.
Step 3: Insert the cage so each ball sits in its own pocket.
Step 4: Pack with grease. Fill about 30-50% of the internal space. More than that causes churning heat (Roller Bearings).
Step 5: Fit shields or seals if needed.
Step 6: Spin it by hand. It should run freely, smoothly and quietly. Roughness points to dimensional errors or surface defects.
Worked Example
How many balls fit? The balls sit on a pitch circle that runs through their centres. For 10 mm balls on a pitch circle of 40 mm diameter, adjacent ball centres subtend an angle of 2 x arcsin(10 / 40) = 2 x 14.48 degrees = 28.96 degrees. Dividing a full circle by that angle gives 360 / 28.96 = 12.4, so at most 12 balls can touch in a ring.
A ring with 12 balls touching has no room for cage webs between them, so a working bearing carries fewer; the wiki gives no standard count, so choose the number so that the web between pockets is thick enough to be machined and strong, and keep the balls evenly spaced. For example, 9 balls leave a gap between neighbours of 360 / 9 = 40 degrees of arc minus the 28.96 degrees a ball occupies, about 11 degrees. This arithmetic is the geometry of the example, not a sourced rule for how many balls to use.
Common Mistakes
- Forcing balls in with the races concentric. It dents the race grooves; offset the races first.
- Overpacking grease. It heats rather than protects.
- Assembling in a dusty place. A single grit grain scores a groove.
- Mixing balls from different size classes. One big ball carries the load.
Safety
Solvent and grease are fire hazards
Cleaning parts in kerosene, diesel or denatured alcohol and drying them near a forge or open lamp starts fires, and the vapours collect in poorly ventilated rooms. Work outside or with cross-ventilation, keep ignition sources out of the area, and do not test for vapour with a flame. Finished bearings should be test-spun behind a guard with the machine’s stop in reach.
What’s Next
- Learn next: Fitting and Alignment - mounting the bearing so it lasts
- Combine with: Ball Bearings - the full build order and the precision threshold