Jewel bearing

From Academic Kids

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. Originally natural jewels were used, such as sapphire, ruby and garnet. In the early 1900s a process to make manmade sapphire and ruby was invented, making jeweled bearings practical at much less expense.

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 usually meant greater precision. Note, though, the quality of the rest of the watch is as important. Some watches would add either non-functional jewels or jewels in areas that were technically functional but entirely unnecessary. Taken to extremes, this lead to watches with up to 100 jewels, most of them of no use.

A typical "fully jeweled" time-only watch will have 2 cap jewels and 2 pivot jewels and an impulse jewel for the balance wheel, 2 pivot jewels and 2 pallet jewels for the pallet fork, and two pivot jewels each for the escape, fourth, third and center wheels.

Modern electronic watches achieve accuracy entirely separate from the friction of the mechanism. Early quartz watches continued to use jewels to increase battery life, and high-grade quartz watches will use jewels to reduce friction and wear. 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.

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