As 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.)
Pulley vibrations can transfer to the support bearings and other components of the machine, causing premature or even catastrophic failure. This is why pulleys used in belt drive systems almost always undergo some form of balancing.
Centripetal vs centrifugal: There is much debate in science, physics, and engineering circles about which is the correct term to use when discussing forces on a rotating body. For a great explanation of the difference (or lack thereof) between centripetal and centrifugal force, check out this article by Phil Plait of Discover Magazine.
Virtually all pulleys undergo static balancing – also referred to as “one-plane balancing” – after manufacture. This method ensures that the pulley’s weight is equally distributed around its center of rotation. As its name implies, static balancing can be done while the object is at rest and is relatively easy to demonstrate through a simple experiment.
Rotate the pulley by hand and let it come to rest on its own. Mark the point at the very bottom center of the pulley. Rotate it again and let it come to rest. If it stops with the same point at the bottom center, then its weight is not balanced – the pulley is heavier at that point.
Correcting this is typically done by one of two methods: by removing mass from the “heavy” point (which is commonly achieved by drilling a small hole in the pulley) or by adding mass to a point 180 degrees from the “heavy” point.
Static balancing is typically sufficient for pulleys that travel at 6500 ft/min (33 m/s) or less. For speeds above this, or when the pulley diameter is less than 7 to 10 times the face width, dynamic balancing is recommended.
Dynamic balancing – also referred to as “two-plane balancing” – goes one step beyond static balancing and ensures that the pulley’s center of mass is on the same axis as its center of rotation. It is possible for a pulley to be statically balanced but dynamically unbalanced (although the reverse is not true), so dynamic balance must be measured while the pulley is turning.
Because it involves forces in two planes, dynamic balancing requires that masses be added in two planes to counter the imbalances and prevent pulley vibration.
The measure of unbalance is given in units of g-mm (oz-in), based on the mass of the pulley and the eccentricity (the distance between the center of mass and the center of rotation). The Mechanical Power Transmission Association provides guidelines for both static (one-plane) and dynamic (two-plane) balancing in their standard, MPTA-B2c-2011: Standard Practice for Sheave/Pulley Balancing.