Calculation of frequencies for vibration analysis

[will792]

In order to match output of SK Data Viewier Vibration Analyzer to a specific part I am trying to create a list of frequencies of different rotating parts in my helicopter for vibration analysis. Each spike in a graph points to a specific component in the heli vibrating at specific frequency and correlated with rotational speed. Can someone confirm my calculations?

My heli is Gaui X5 with 15T motor pinion, 120 main gear, 61 autorotation gear, 15T tail gear. I target 2100RPM for the main blades which I can measure with optical tachometer. Therefore I start my calculations with main blades rotational speed.

Ratios:
Main blades to motor: 120 / 15 = 8
Main blades to TT and tail blades: 61 / 15 = 4.07

Rotation per minute and per second:
Main blades: 2100RPM 35RPS
Motor: 17600RPM 293RPS
TT & blades: 8540RPM 142RPS

Expected vibration frequency (1x of RPS)
Main blades: 35Hz
Motor: 293Hz
TT and tail blades: 142Hz

I am not sure if the expected vibration frequency would be single, double or multiple of rotational frequency. Common logic says that something like a bent shaft should produce vibration equal in frequency to the rotation. Calculation of frequency for TT is a mystery to me. I cannot figure out if there is a constant multiple between TT rotational speed and frequency of TT harmonics.

[Larry227]

Edit/erase: I thought I was in a different forum

[will792]

I added a couple of sentences explaining how this is related to SK. Vibration analysis log is one of distinguishing features for SK but most of us do not have the knowledge how this extra information can be used to point to the source of the mechanical problem.

[georgi UK]

=] post a log, see if we can help you better understand it?

[Mostly]

I think the calculation on the tail is wrong.
The first reduction on the main gear for the tail is 61/120.

If all the other gears have equal teeth this would give 135 hz

I may be wrong as I have never seen a TT heli, all those gears!

Mostly got to go