Jewel bearing
From Wikipedia, the free encyclopedia
A jewel bearing is a bearing which an unlubricated metal shaft spins in a jewel-lined pivot hole. The hole is typically shaped like a donut hole and is slightly larger than the shaft diameter. In operation, the shaft tilts slightly so as to contact the jewel pivot hole at two opposite points. The shaft rolls inside of the bearing rather than sliding. As the shaft rolls, the center precesses. It was invented by Joseph Johnson in the early 1800s. Originally natural jewels were used, such as sapphire, ruby and garnet. In the early 1900s a process to make synthetic sapphire and ruby (crystalline aluminum oxide also known as corundum) was invented, making jeweled bearings practical at much less expense.
Jewel bearings were used widely for mechanical (escapement) watches, where their low and predictable friction improved watch accuracy. A typical mark of watch quality was a note such as "17 jewels". More jewel bearings often meant better precision. Some makers added non-functional or unnecessary jewels to give the impression of accuracy. Some watches had 100 jewels, most of them of no use. A typical "fully jeweled" time-only watch has 2 cap jewels, 2 pivot jewels, an impulse jewel for the balance wheel, 2 pivot jewels, 2 pallet jewels for the pallet fork, and 2 pivot jewels each for the escape, fourth, third and center wheels. Modern electronic watches achieve accuracy entirely separate from the friction of the mechanism, but early quartz watches used jewels to increase battery life, and high-grade quartz watches use jewels to reduce friction and wear.
Today, jewel bearings are used widely in sensitive measuring equipment.
The advantages of jewel bearings 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 1 mm and typically support loads of under 1 gram; large jewel bearings are as large as 10 mm and support loads up to about 500 g.
Historically, jewel pivots were made by grinding. Modern jewel pivots are often made using high-powered lasers, chemical etching, and ultrasonic milling.
Jewel bearings are known for their low breakaway friction and highly consistent moving friction. The jewel surfaces are very hard and durable and can maintain smoothness over decades of use, thus reducing friction variability. Flexure bearings have even lower variability, but also have a more limited range of motion.
