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# Weight of Accelerometer pick up for natural frequency test

## Weight of Accelerometer pick up for natural frequency test

(OP)
During natural frequency test of LP rotor of ulility turbine, we have to mount accelerometer pick up on blade.

I want to know whether there is any empirical formulae for calculating what should be the weight of accelerometer pick up with respect to weight of LP blade.

or in other words, whether there is any defined ratio of weight of accelerometer pick up and weight of LP blade.

Kindly also tell where we should mount the accelerometer pick up on LP blade during natural frequency test.
(steamturbine)

### RE: Weight of Accelerometer pick up for natural frequency test

Ok, here's the neat trick. first measure the natural frequency with one accelerometer installed (I guess you are doing a hammer test?).

Then glue another one alongside it, and repeat the test. Look at the difference in the FRF. If the difference is too great then use a smaller accelerometer, or else you can do some maths to compensate for the extra mass. Cunning heh? If you are trying to correlate an FEA model then it can be worth getting the node pushers (sorry, FE analysts) to add the accelerometer's mass to their model.

Assuming you want the first bending mode of the blade it would be normal practice to mount the accelerometer at the end of the blade, but you'll learn far more about the system if you use a few different positions across the length and width of the blade.

Cheers

Greg Locock

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