Belt creep in a friction-drive, power-transmission system comes when the drive moves slightly faster than the belt. As shown in the figure, two arcs divide the 180i of wrap between drive pulley and belt: power transmits at the effective arc (also called the angle of creep) while none transmits at the idle arc.
Within the idle arc, the belt and pulley surfaces are in static contact. The belt runs onto the pulley with tight-side tension T1 and speed V1, which matches the surface speed of the drive pulley. Both speed and tension remain constant as contact continues through the idle arc.
Within the effective arc, belt speed is slower than that of the pulley. This results in sliding contact between belt and pulley. This is caused by dimensional changes in the belt due to differential forces acting on it as it passes around the pulley.
Because the tensile member is the metal belt, with its high modulus of elasticity, creep in a metal belt is less than for belts made of most other materials. When not controlled, however, creep in a friction drive metal belt results in a loss of repeatability. Fortunately, creep in metal belts is easily controlled. Timing teeth are the most common way to combat creep. The number of timing locations should be the smallest number possible: four to six will usually suffice.