The fan was balanced, but a heavy spot appeared on the static counter. What's the problem?

It is completely normal to find a heavy spot on a static balancing stand even after successfully balancing a fan on a dynamic balancing machine. This doesn't mean there's a problem; rather, it highlights the fundamental difference between dynamic and static balancing. Static balancing stands only detect and measure mass unbalance in a single plane (i.e., static unbalance). These stands check if the part's center of gravity coincides with its geometric center. If the center of gravity isn't perfectly centered, the part will show a heavy spot on a static stand.

However, dynamic balancing machines operate differently. Dynamic balancing corrects both static unbalance and the dynamic unbalance (couple forces) that occur during rotation. These machines measure vibrations and forces while the rotor spins under conditions similar to its operational environment, which allows them to compensate for all of the part's unbalances. Therefore, a fan that has been dynamically balanced may still show a heavy spot on a static stand because the stand cannot detect that the dynamic unbalance has been corrected. The fan has been properly balanced for operation on its own bearings and its performance will not be affected. For this reason, a dynamically balanced part appearing heavy on a static stand means the process was successful, not that it failed.

The point that further clarifies this situation is that the two methods measure different quantities. The static stand sees the resultant (static) component, perpendicular to the axis, of the residual unbalances in the two planes and gives a heavy point as direction; whereas dynamic balancing, even if it brings this static component within tolerance, manages the couple (moment) component separately. In addition, the threshold of the static stand is often independent of the dynamic tolerance; even a very small static residue within tolerance can reveal the heavy point on a frictionless stand.

In the evaluation, the criterion is not the static stand's subjective sense of the heavy point, but the numerical result the dynamic machine gives in gram-millimeters and compares against the ISO 21940-11 tolerance. If the fan is dynamically balanced while running on its own bearings, a slight tilt on the static stand does not affect operational performance. MBS Balance takes as reference the dynamic measurement based on the part's real operating condition and, when necessary, reports and interprets the difference between the static and dynamic results.