Belt and pulley systems can be used for speed reduction. Most high power motors operate at speeds between 1000 to 3600 RPM (revolutions per minute). The machines that these motors power usually operate at half these speeds or less. The solution to this situation is to place a small pulley on the motor and place a larger pulley on the machine being driven by the motor. When a small pulley drives a large pulley, a speed reduction takes place.
Determine the speed of the driving motor and the needed speed of the driven equipment. For example: If the motor's normal speed is 1800 RPM then that is the motor speed. If the equipment that the motor will be driving operates at a normal speed of 900 RPM, then that is the needed driven speed.
Divide the driven speed by the motor speed to get the pulley ratio. For example, 900 RPM divided by 1800 RPM equals 0.5. The pulley ratio is also equal to the diameter of the motor pulley divided by the diameter of the driven pulley. This means that to get a speed reduction, the driven pulley must be twice the size of the motor pulley.
Place a 2 inch pulley on the motor shaft and place a 4 inch pulley on the driven machine shaft. Connect the two pulleys with a new belt and tighten the belt to the needed tension.