Pan Weighing vs Moment Weighing

Balancing is one of the most critical steps of any rotor assembly or repair. An unbalanced rotor occurs due to uneven weight distribution of the individual blades and any connected hardware when mounted to the rotor. Unbalanced blades can create vibrational forces that significantly affect a turbine’s operating performance, service life and capacity to function safely.

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 means measuring the actual mass of the blades. This would simply be called “weighing the blade” in layman’s terms. We call it pan weighing to avoid confusion with moment weighing.

Pan weighing the blades gives a first idea of how to roughly balance the rotor. 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 average mass of the blades centered on the axis of rotation, the overall rotor should be balanced, right?

Wrong. This is only true if the average center of mass of all the blades is located at the center rotational axis of the shaft. To balance a rotor, one needs to locate the average mass of all components on the designed axis of rotation by changing blade positions, adding counterweights and/or removing material from sacrificial locations.

The moment weight of a single blade is equal to:

Radial Moment Weight = (Mass of the blade) x (Distance from the assembly axis of rotation to the center of mass of the blade)

Moment balancing of large blades, such as jet engine fan blades, involves weighing three moments of the blade — the radial, axial and tangential — which can be accomplished with the proper machine.

Real-World Examples of Moment

One of the most practical ways to describe moment is imagining a see-saw on a playground. Putting equal weights on both sides of the see-saw creates a balanced moment. Applying more or less weight to one side makes an unbalanced moment. To rebalance, you can either shift the weight positions or add or remove weight on one side.

Trying to loosen a tight bolt with your fingers is often impossible. However, a wrench makes turning much easier because the distance between the force and the pivot grows, increasing the turning moment. The further away from the pivot you apply a force, the easier the task becomes.

Static balancing — also called moment weighing — is one of the most critical initial steps for balancing a turbine rotor. High-accuracy static balancing begins with measuring the moment that each turbine blade will add to the assembly. The resulting measurements enable you to complete preliminary static balancing of your rotor quickly and precisely, showing good results for lower-precision applications and reducing the need for corrections if you are proceeding to dynamic balancing.

What Is Moment Weighing?

Moment weighing is a standard operation for turbines, being viable for power generation turbines, aircraft engines, and turbo pumps.

The following steps are necessary for balancing a rotor stage:

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

Typical Applications

Moment weight scales from Raptor Scientific are ideal for both original equipment manufacturers (OEM) and maintenance and repair operations (MRO) providers in various turbo machinery applications, including:

• Jet engines: Turbine and fan blade manufacturing and turbine assembly.
• Helicopters: Blade balancing for assembly and manufacturing.
• Industrial fans: Fan blade balancing for assembly and manufacturing.
• Power generation: Steam, gas and wind turbine blade assembly and manufacturing.

We offer an extensive range of measuring solutions, from small single-axis devices for compressor blades down to 100 grams and 2 inches long to multi-axis moment weight scales for larger blades up to 200 pounds and 20 feet in length.

Discover More With Raptor Scientific

Raptor Scientific is a leading global provider of testing, measurement and engineering services, including a comprehensive range of precision measuring instruments. Our moment weight scales are just one example of our many superior products. For more information about our blade balancing products and services, visit our Blade Balancing  page.

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