Round belt pulleys or grooved pulleys are used in drives and mechanical systems with o-ring belts, round belts, cables, ropes and wires. The belt rides at the bottom of the pulley groove and requires elongation or stretch to maintain tension and friction against the pulley. Devices with an idler contain a center bearing which allows the pulley to rotate freely. Typically, idler pulleys are used as tensioners or free pulleys in larger mechanical systems. Round belt pulleys are designed to accept belts of a specific diameter and have one or more pulley grooves. Pitch diameter measures the line of action and is taken at the center of the circular pulley groove. Outside diameter is the full diameter to the outside of the pulley’s flanges. Bore diameter measures the size of the center hole for shaft mounting. Some round belt pulleys are measured in English design units such as inches (in) and fractions of an inch. Others use metric design units such as millimeters (mm) or centimeters (cm).