Hidden Errors in Turbine Blade Moment Weight Measurement
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.
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)
Moment weighing is standard for turbines, both for power generation turbines and aircraft engines.
The following steps are necessary for balancing one rotor stage:
- Moment weighing all the blades
- Determining the unbalance of the hub and entering it into the optimization software of the moment weighing scale
- 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 on our moment weighing scale, please refer to our main Moment Weighing Scale page.
The cradle (also called tooling or fixturing) which holds the blade being measured is an integral part of the AWS 800 Series TURBINE BLADE MOMENT BALANCE system.
If a blade is not held for measurement exactly as it would be held in the actual engine assembly; any measurements achieved are meaningless.
Custom cradles are unique for each blade. While the root of one blade may be the same as another, the factors of weight, length and nominal moment must all be taken into account when designing and manufacturing the cradle for a particular blade.
Additionally, a custom designed weight which exactly duplicates the root of the blade and is correct to 2 decimal places of optimum moment is critically necessary for proper set-up and calibration of the system.
By first measuring the static moment of the individual blades and then sorting them into the ideal order, jet engine manufacturers have found that they can greatly reduce the time and effort required to balance the rotor of an engine. More recently a new concept has emerged: if a computer record is kept of the moment of every blade in every engine manufactured, then a damaged blade can be replaced with one of identical moment without the need to disassemble the engine and rebalance the rotor. This saves both money and time, but it places new demands on the accuracy of the moment measurement. If blade moments are in error, then the engine will be unbalanced, resulting in premature wear, or possibly a fatal accident. The concept of blade replacement by matching blade moment requires that the blade be measured with a high degree of accuracy. For example, a 35 pound fan blade might have nominal moment of 17,000 oz-inch and need to be balanced to within 0.5 oz-inch. This represents a required measurement accuracy of 0.003% of value!
Space Electronics manufactures instruments to measure turbine blade moment (these instruments are often called «moment weight scales»). Our instruments [see them here Single Axis Moment Weight Scales; Multi-Axis Moment Weight Scales] use a new technology which is as much as 40 times more accurate than the conventional knife-edge and load-cell technology that has been employed for the last 30 years. As a result, the moment measurement error of our instruments can be considered insignificant. This has led us to more clearly identify other sources of measurement error which appear to be widespread throughout the industry. The problems show up in two ways: (1) a blade is replaced in the field with one of supposedly identical moment, and the engine is then found to be unbalanced; (2) a set of blades is measured at Plant A and then sent to Plant B for installation in the engine. If the blades are measured at Plant B before they are installed, the data differs from the original set of measurements. However, it often isn’ t just a simple change in scale factor (i.e the blades aren’ t just 0.5% higher in moment at Plant B). There are several factors involved, resulting in what appears to be random differences. I believe I have identified the sources of these errors. This paper identifies each type of error, and gives recommendations for their elimination.
This is a paper delivered to the 56th Annual Conference of Society of Allied Weight Engineers which won the best paper award. Click here to access the complete paper in PDF format after a login – please provide your email/contact info address after the link.