Jewel Bearings, Jewels, and Pivots Information
Jewel bearings, jewels and pivots are miniature components used in instruments and controls that require low friction, long life, and non-magnetic properties. They are usually made of sapphire, ruby, or hard glass. Jewel bearings, jewels and pivots exhibit rare combinations of design characteristics. For over a century, jewel bearings, jewels and pivots have been used as reliable, low-friction devices to keep watches and timepieces working accurately. Cup bearings are jewel bearings that fit a ball bearing between two-cup jewels or semi-spherical pivots. Cup bearings are often used in vertical shaft applications, such as compasses and heat monitors. V-bearings are jewel bearings that are suitable for sensitive magnetic meter movements, compasses, encoders, turbine flow devices, and galvanometers. Unlike a miniature bearing that uses line contact, a V-bearing provides a low-friction coefficient because its moving shaft is near the point of contact. Glass V-bearings have slightly lower friction because their surfaces are fire polished. Synthetic jewel bearings, jewels and pivots are sometimes selected because of their superior hardness and ability to accept a high polish. Synthetic jewels are ideal for instruments, meters, control devices, and other precision mechanisms where low friction, long life, and dimensional accuracy are important. Ring jewels are journal or annular bearings that contain a contoured hole shaped like an hourglass or an olive, rather than a straight cylindrical bore. The advantage of ring jewels over conventional bearings is that a ring jewel provides a reduced contact area that causes lower friction between the shaft and the bearing. Ring jewels are commonly found in instruments such as watches, dial indicators, and gimbal suspensions. A V-jewel (Vee-jewel) is used in highly sensitive instruments requiring low torque bearings and where the moving element is light. V-jewels offer minimum friction when used with conical, small-radius pivots and permit slight lateral movement. V-jewels also provide for minimum friction when used with conical pivots. Jewel bearings, jewels and pivots are suitable for loads under 500 grams (g). Product specifications include friction coefficient, hardness, temperature stability, concentricity and size. For steel on sapphire, friction coefficients range from 0.1. to 0.15. In terms of hardness, sapphire has a compressive strength of 300,000 pounds per square inch (psi). High temperature stability results in a melting point about 2,050 degrees Celsius (C). Jewel bearings, jewels and pivots typically cost 80% less than a ball bearing and 90% less than a flexural pivot. Small sizes provide for compact designs.