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source: Popular Mechanics 1920 Volume 34
The roller or keg float is a familiar way farmers used to keep the bull wheel on mowers,reapers, binders and harvesters from sinking and then sliding in the mud. It is made by passing a piece of 1/2 in. pipe, about 3 ft. long, through holes bored in the center of the heads; this axle is then attached to the binder frame with iron brackets, so as to be carried behind the bull wheel, and about 3 in. above the surface, when the binder is set. By using the tilting lever, a part of the machine's weight may be thrown on the keg at any time.
See Bull Wheel Improvements