Hidden Errors in Turbine Blade Moment Measurement and How to Avoid them

by

Richard Boynton, President
Space Electronics, LLC, Berlin, CT 06037

Abstract

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 sometimes called "moment weight scales"). Our instruments 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 re-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. We believe we have identified the sources of these errors. This paper identifies each type of error, and gives recommendations for their elimination.

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