US Patent 7445387 Ball bearing

A ball bearing includes a ball supporting member having a hemispherical concave surface, small balls freely disposed on the hemispherical concave surface, a large ball disposed on the small balls so as to be rotatable in all directions, and a casing that has an aperture through which an upper portion of the large ball projects, the casing accommodating the ball supporting member, the small balls, and the large ball; the small balls include primary small balls contacting the large ball so as to support the large ball, and auxiliary small balls, which are smaller than the primary small balls, for supporting the large ball and not contacting the large ball.