accelerometer pick up for natural frequency test of rotor blades
accelerometer pick up for natural frequency test of rotor blades
(OP)
During natural frequency test of rotor blades 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 rotor blade.
or in other words, whether there is any defined ratio of weight of accelerometer pick up to the weight of rotor blade.
Kindly also tell where we should mount the accelerometer pick up on rotor blade during natural frequency test, for reliable results (distance from the blade tip to accelerometer pick up)
What are the best methods for fixing accelerometer pick up on blade surface.
(steamturbine)
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 rotor blade.
or in other words, whether there is any defined ratio of weight of accelerometer pick up to the weight of rotor blade.
Kindly also tell where we should mount the accelerometer pick up on rotor blade during natural frequency test, for reliable results (distance from the blade tip to accelerometer pick up)
What are the best methods for fixing accelerometer pick up on blade surface.
(steamturbine)





RE: accelerometer pick up for natural frequency test of rotor blades
One test I would do in your case is to mount the accelerometer on the blade and do the test. Then repeat the test with the acclerometer mounted at the root of the blade (space permitting). While the signal at the root will be much weaker, you can still compare the frequencies and if they change too much the accelerometer is probably too heavy.
I would mount the accelerometer at 3/4 blade length from the root. Then you will not be at a node in the first mode and at an anti-node in the second mode.
Since you can't drill into a blade, use "crazy glue" or expoxy. They can be cleaned off afterwards. I prefer "crazy glue" since it is very fast acting and very stiff.
C. Hugh (www.Hatch.ca)
RE: accelerometer pick up for natural frequency test of rotor blades
More practically, you can run reciprocity test (impact at blade tim, measurement at blade root and then reverse and check the modifications of the FRF by plotting them on a single plot) which will give you the linear behavior of the structure and show possible peak drifts due to added mass (in the case of a linear structure)...Or you can measure natural frequencies with a non contact system (laser, proximity tranducer) with and without the accelerometer.
Check out ENDEVCO 2222C models (1.2gr)
Thomas.
RE: accelerometer pick up for natural frequency test of rotor blades
For mounting accelerometers, there are waxes available that are great at room temperature (beeswax and others).
As far as transducer mass effects go I remember seeing a tech sheet from PCB Piezotronics some years ago that rigorously covered this issue. Try contacting them for a copy. (they have the wax too)
As a last thought. Have you considered using a sound level meter as your vibration transducer? I have used this on axial compressors -it's fast and repeatable.
Regards,
Gunnar
RE: accelerometer pick up for natural frequency test of rotor blades
RE: accelerometer pick up for natural frequency test of rotor blades
Measure the FRF with one accelerometer.
Stick another accelerometer alongside.
Repeat the measurement.
Assuming that your accelerometer is only a small proportion of the modal mass then you now have a graphic illustration (offtopic : what the hell is a non-graphic illustration?) of the effect of the extra mass. If it is enought o invalidate your results then get the FE model you are correlating with to add the accelerometer mass for the correlation runs.
Cheers
Greg Locock
RE: accelerometer pick up for natural frequency test of rotor blades
RE: accelerometer pick up for natural frequency test of rotor blades
RE: accelerometer pick up for natural frequency test of rotor blades