My first “up close and personal” acquaintance with a steam traction engine was several(?) decades ago at an Antique Tractor show in Portland Indiana. More recently, I watched a 1913 Case engine drive a threshing machine at the Dakota County (Minnesota) fair late this summer. When these engines drive threshing machines or saw mills, power transfer is by use of a very wide and tremendously long fabric belts. I once asked an engine operator about how taut to make the belt. He advised, “only taut enough to keep it from slipping. Any tighter stresses axles and bearings.” The following video (not mine) shows an engine driving a threshing machine.
Wander around and look at shopmade wooden treadle lathes. You’ll find all manner of drive “belts.” Many use rope or leather cords. In doing my research, I saw a lot of lathes where the builder used extra idlers to add tension, usually to cords or ropes. Those look like very fussy contraptions.
Instead of using a narrow cord or rope, I decided to model my drive after the traction engines, with a wide leather belt. My intent was to eliminate the need for a fussy tension idler and to keep tautness to something less than a gnat’s ass stretched over a rain barrel. My drive belt is 2″ wide material from Tandy. The longest belting strips they offer are 72″ long. It takes the better part of two lengths to make this belt.
I made it long enough to be a snug, but not particularly taut, fit over the flywheel and temporary spindle pulley. It doesn’t slip, and the rounded shape of the spindle pulley sure keeps it on track.