Jewel bearing
A jewel bearing is a bearing which allows motion by running a shaft slightly off-center so that the shaft rolls inside of the bearing rather than sliding. As the shaft rolls, the center precesses.
Jewel bearings were used widely for mechanical (escapement) watches because their low and predictable friction made the watch more accurate. Hence, a typical mark of quality on watches was a note such as "17 jewels". Using more jewel bearings meant greater precision. Note, though, the quality of the jewel is important -- cheap watches use cheap jewels with higher friction, hurting accuracy. Modern electronic watches achieve accuracy even with high-friction bearings, but today jewel bearings are used widely in sensitive measuring equipment.
Jewel bearing advantages include high accuracy, very small size and weight, low friction, predictable friction including good temperature stability, ability to operate without lubrication and in corrosive environments. Disadvantages include limited availability/applicability in medium and large bearing sizes and capacities, and friction variations if the load is not axial.
Jewel bearings are typically used for very small applications such as high-precision instruments. Bearing bores are typically less than 1mm and typically support loads of under 1 gram; large jewel bearings are as large as 10mm and support loads up to about 500g.
Historically, jewel pivots were made by grinding. Modern jewel pivots are often made using high-powered lasers, chemical etching, and ultrasonic milling.
Many bearings have higher starting friction than moving friction. Jewel bearings have very smooth surfaces and so have lower friction variability. Flexure bearings have even lower variability, but also have a more limited range of motion.
