When it comes to balancing blades (this is generally called moment weighing and the instrument used is a moment weight scale) there is no commonly agreed measurement procedure. Depending on the measurement method used, accuracy can vary greatly. In this accuracy tips section we attempt to give insight to the best practices.
Absolute vs. Relative Measurements
“Should I take absolute measurements, or should I compare my blades to a master blade? What difference does it make?” Let our chief engineer, Kurt Wiener, answer:
“Both methods are used, but the accuracy will be greater with relative measurements. For small blades the difference is not significant because of the proximity to absolute zero. But when it comes to large blades such as fan blades, we strongly recommend using relative measurements. Using the same scale, the accuracy of an absolute measurement is in fact typically 4 to 5 times worse than that of a relative measurement.
“If a set of blades is being sorted to achieve balance on an engine, then it is only important that the measurement be relative to a master blade. In other words, you are looking for the difference between blades. Since all measurements are made using the same setup, this will result in greater balance.”
“However, if blades are being sorted for later use, and they may be combined with blades measured using other fixtures or moment weighing machines, then it is more important that the true moment value be measured. It is much more difficult to make an absolute measurement of blade moment than a relative measurement. A major source of absolute measurement uncertainty is in the fixturing used. The contact between the fixture and the blade will determine the absolute moment measurement accuracy.”
Sensitivity to Temperature and Drafts
Measuring blades in an open environment is a challenge for conventional moment weight scales. Temperature variations and drafts induce large errors and make it impossible to take repeatable measurements. Space Electronics moment eight scales were specially designed to address these issues. The transducers used in Space Electronics moment weight scales are temperature compensated.
Space Electronics moment weight scales come with optional shrouds that cover the blades during measurement. Alternatively you may want to have a temporary wall around your moment weight scale to protect it from drafts while measuring turbine blades.
Measuring at engine radius vs. optimal measurement radius
Most turbine drawings specify measuring blades at engine radius. However in some cases, in particular for most power generation turbine blades, this is not practical because the engine radii are large and the blades short. Measuring at these radii causes much more rocking inertia and sensitivity to windage than measuring at a short radius would generate.
Our moment weight scales allow measurements at any radius. The results can be translated to engine radius by the computer system. The only disadvantage to measuring at a different radius is the need to measure each blade weight (this is called pan weight) very accurately, in order to perform the conversion from measurement radius to engine radius. This means two measurements per blade, which takes more time.
Advantages of Precise Measurements
By following these easy balancing tips you ensure that your blades are measured more accurately. The blade distribution software is more likely to find a better-optimized balancing solution, with a lower residual unbalance. Your rotor will be better balanced, and therefore easier to integrate with other rotors when re-building the turbine.