How to understand dynamic balancing?

We know balancing reduces vibration which is good, what exactly does it good for have ？

Assume you have a wheel and tire that is perfectly balanced -- the mass is evenly distributed radially and fore/aft about the axis of rotation. Now, imagine that you embed a 1-oz rock between the tread of the tire and then spin the wheel. If you are watching the wheel at the 12 o'clock position, you'll notice that there is now an imbalance force applied up when the rock is at 12 o'clock and applied down when the rock is at 6 o'clock. That's the radial component of the imbalance.

Also, there are two components of dynamic unbalance: static unbalance and couple unbalance. The first occurs when the rotation axis is offset from the centre of mass, picture a wheel with the axle off centre. The measure of this unbalance would be the mass of the wheel x the distance from the centre of mass to the rotation axis. If you spin the wheel and hold the axle it will try to move in small circles.

Our steel spools are perfectly balanced as same as wheels that can work for the copper foil and Aluminum foil on the fabricating line, every spool has to inspection dynamic balancing, and then shipping to customers.