Why in a CFM56-3 is vibration measured in displacement for N1 and in velocity for N2?

N1 is measured in mils (thousandth of an inch) and N2 is measured in IPS (inches per second). Why are different methods used?

• Sep 18, 2021 at 12:44
• Can you provide a source for your statement?
– Afe
Sep 18, 2021 at 16:23
• Sorry I should've made myself clear. Yes, the vibration sensor is located in No. 1 bearing and turbine rear frame. From CFM56-3 Engine Shop Manual in Engine Operating Limits section, the vibration level limit for N1 is in mils and for N2 is in ips. Sep 19, 2021 at 2:26
• Both seem very odd units for vibration, as it is oscillation that by definition has frequency and amplitude. Neither alone tells enough about the nature and more importantly, the energy of the vibration. Sep 20, 2021 at 15:04
• @Jpe61 both are measured synchronously. i.e. at whatever frequency the corresponding rotor is spinning. Sep 20, 2021 at 16:00

So, what units are most convenient for analysis? It depends on the frequency range. If we have a sinusoidal vibration with displacement amplitude A and frequency $$\omega$$, then the corresponding velocity will be $$A \omega$$, and the corresponding acceleration will be $$A \omega^2$$. So for very low frequencies, i.e. $$\omega$$ very small, so that acceleration and velocity will both be small. Thus of the three, displacement will be the largest, and most convenient to work with. For very high frequencies, i.e. $$\omega$$ very large, then acceleration i.e. $$A \omega^2$$ will be the largest, thus more convenient to work with. For intermediate frequencies, velocity is the largest and thus most convenient.