Pan Weighing vs Moment Weighing

The terms used in turbine blade balancing can be confusing. The two main terms used are pan weighing and moment weighing.

What is Pan Weighing?

Pan weighing simply means measuring the mass of the blades. This could be called weighing the blade. We call it pan weighing to avoid the confusion with moment weighing. Pan weighing the blades gives a first idea on how to roughly balance the blades. By placing blades of equal mass opposite each other on the rotor, or by optimizing the distribution of mass around the rotor to have the overall mass of the blades centered on average, the overall rotor should be balanced, right?

Wrong. This is only true if the center of gravity of all the blade is located at an equal distance from the engine radius. In order to balance one rotor, one needs to balance the forces generated by the blades relative to the engine rotation axis. The force generated by a single blade is called the moment weight of the blade. The moment weight of one blade is equal to:

Moment Weight = (Mass of the blade) x (Distance from the engine centerline to the center of gravity of the blade)

What is Moment Weighing?

Moment weighing is standard for turbines, both for power generation turbines and aircraft engines.

The following steps are necessary for balancing one rotor stage:

1. Moment weighing all the blades
2. Determining the unbalance of the hub and entering it into the optimization software of the moment weighing scale
3. Running the blade optimization software to determine the optimal arrangement of blade that will minimize the total unbalance of the rotor. Note that the blade optimization software is generally integrated in the computer system that controls the moment weighing scale.

For more information about our blade balancing products and services visit our Mass Properties Lab Services.