The pulley ratio is defined as the ratio of what?

The pulley ratio is most accurately defined as the ratio of the pitch diameter of the driven pulley to the pitch diameter of the driver pulley. The pitch diameter is the effective diameter at which the belt engages with the pulley, meaning it directly affects the rotational speed and torque transfer between the pulleys.

When calculating the pulley ratio, this pitch diameter ratio provides a clear understanding of how the speed and force are transmitted from one pulley to another in a mechanical system. A larger pitch diameter on the driven pulley compared to the driver pulley results in a speed reduction and increases torque, which is essential in various applications such as conveyor systems and automotive engines.

Using the outside diameter may not provide an accurate representation of how the belt interacts with the pulleys, and shaft diameter is irrelevant for determining the pulley ratio since it doesn’t affect the belt's movement or speed relationship. Therefore, focusing on pitch diameters is crucial for effective pulley system design and analysis.