Page 12 - Vibrations 3

The cepstrum of the bearing with a fault in its outer ring

will show up in the spectrum.

The cepstrum of the bearing with a fault in its inner ring

will also show up.

Cepstrum analysis is particularly useful for identifying

faults in rotating machines.

Measurement of Vibrations.

The principal sensor for measuring vibrations us the

accelerometer.

This can be a piezo-electric crystal which has a non- cento-

symmetric design.

Calcium titanate CaTiO 3

The titanium ion does not

reside in the centre of the

structure.

Off-set from the centre.

As the crystal is stressed

this causes the Ti ion to shift and the structure becomes

polarised – separation of + and – charges.

Other materials are lead zirconate titanate (PZT), lithium

niobate and quartz.

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