Characteristics
The advantages of jewel bearings include high accuracy, very small size and weight, low and predictable friction, good temperature stability, and the ability to operate without lubrication and in corrosive environments. They are known for their low static friction and highly consistent dynamic friction. The static coefficient of friction of brass-on-steel is 0.35, while that of sapphire-on-steel is 0.10–0.15. Sapphire surfaces are very hard and durable, with Mohs hardness of 9 and Knoop hardness of 2000, and can maintain smoothness over decades of use, thus reducing friction variability. Disadvantages include brittleness and fragility, limited availability/applicability in medium and large bearing sizes and capacities, and friction variations if the load is not axial.
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