Dynamics of Nature - Rolling Ball Bearings

Leonardo recognised the mechanical principle behind the use of ball bearings to reduce friction. In this design, the three spherical balls can move freely, nestled in a hemispherical base. The balls spread the friction created by the pressure of the vertical shaft. Leonardo noted that three spheres, not four, are needed. Four spheres would move unevenly and produce greater resistance, making the device less efficient.
Like many of Leonardo's inventions, this was forgotten for several centuries: it was not until 1791 that the idea was patented by a carriage-maker in Wales.