A flat belt pulley is used to operate machinery commonly driven off of a single power source. While appearing flat, the flat belt pulley actually has a slight pitch machined into it and works in concert with the opposing pulley on the drive shaft; the pulley on the machinery or tool aids in keeping the belt on the equipment and operating without throwing the belt off. The typical design of the flat belt pulley consists of a very heavy cast iron pulley with a keyed center hole. This keyed hole allows the pulley to be slid onto a solid steel drive shaft and held from slipping with a key way and a fastening bolt. In order to remove some unneeded weight as well as to aid the unit in remaining cool, most flat pulley designs use a spoked center instead of a solid steel center in the pulley.
In early manufacturing plants, a single engine was used to power all of the machinery and tools in the plant. This was accomplished through a series of drive shafts and pulleys running through the ceiling of the factory. Each work station had its own drop-down flat-belt and flat belt pulley that was driven off the overhead system of drive shafts and powered the particular equipment at the individual work station. The worker simply engaged a tensioner that brought the belt tight, and the workstation became powered. This was much like engaging a type of clutch which brought the tools and machinery at the work station to life.
The centrally-located engine and the use of a flat belt pulley drive system allowed the plant to operate without the added heat that individual engines would produce at the work stations. This also saved on expenses, since only one engine operator needed to be employed for each work shift. A flat belt pulley was also able to be slid down the drive shaft to any location a new work station required. By changing the diameter of a flat belt pulley, the speed of a machine could be altered. Typically, a larger-diameter flat belt pulley would cause the machine to run slower, but with added torque, a smaller-diameter flat pulley would result in a faster machine with less torque.
Some outside farm equipment was equipped with a flat belt pulley to power stationary machinery. Threshers and saw mill equipment both used the flat belts driven off a tractor flywheel to operate. Belt tension was adjusted in these setups by changing the position of the tractor.