|01:07Ball Bearing tone ring supports were introduced by Gibson around 1919. Their construction consisted of a perforated tubular tone ring sitting on ball bearings that rested in cavities in the top of the wooden rim (source: "The New Gibson Banjos", Gibson Banjo Catalog, 1920). By around 1924 the cavities had been deepened and the ball bearings rested on springs inserted into holes drilled down into the rim from the top.
The theory usually advanced for this is that changes in humidity that cause the calf skin head to loosen or tighten would be counteracted by the constant spring pressure maintaining constant head tension. For this to work, however, the height of the head and thus, the action, would be variable. Also, the springs would absorb some of the sound that should be transmitted to the rim. This tone ring is sometimes rated as a decent compromise for those who play a wide variety of styles. Buell Kazee (clawhammer) played one. Most people who play a variety of styles use a variety of banjos.