As a pulley rotates, centrifugal forces act on the pulley, and if its mass is not evenly distributed around the axis of... moreAs a pulley rotates, centrifugal forces act on the pulley, and if its mass is not evenly distributed around the axis of rotation – that is, if it is unbalanced – these centrifugal forces will also be unbalanced and cause the pulley to vibrate. (Uneven mass distribution can be due to imperfections in machining or inconsistencies in the material structure.)