n+1 and n-1 excitation of crankshaft torsional vibrations 1

[GregLocock]

Please could someone (Tom?) point me in the direction of a good discussion of the source and the 'correct' way of thinking about this. I know that it is concerned with the difference between rotating and stationary frames of reference, and that it affects orders, but not frequencies.

I'd even hazard a guess that it has a great deal to do with intermodulation.

I'm pretty sure it is covered in Kerr-Wilson, but the only copy I've ever read of that is two decades and 10000 miles away.



Cheers

Greg Locock

 

[noisun]

Sorry, I forgot signature...my name is Noisun.

Cheers
Noisun

 

[lyngengr]

Greg:

Without going into a lot of detail, a reciprocating engine can produce both integer and half-integer frequency components of running speed. The exact orders and strength of the components depends on the number of cylinders, cycle type (2 or 4), and firing order. These components are primarily torsional in nature but can couple to produce lateral vibrations. We have tested 12 cylinder diesels and those engines produce harmonics starting at 1/2 of running speed and go up to 10/rev, with predominate components at 2, 2 1/2, 3, 3 1/2, 4, 4 1/2, and 5 per rev. If you are interested, I can send you a sample spectrum.

 

[GregLocock]

Well, thanks for the offer but I have engine spectra coming out of my ears. I am specifically interested in n+1 and n-1 torsional vibrations. noisun's explanation sounds close. Bengisu wrote a paper on that very subject in Sound and Vibration, 2001, I'll get the library to chase it up for me.

Cheers

Greg Locock

 

[lyngengr]

Greg:

We have copies of S&V going back to the mid-90's, if I can find the article I can scan it and send it to you.

 

[GregLocock]

Thank you very much. It is in

S&V 2001 Volume 35 fascicolo 9

22 Computation of Vibration Order Shift M. Turgay Bengisu

Could you send it to the very spammed up address :

greglocock at yahoo dot com dot au

thanks



Cheers

Greg Locock

 

[GregLocock]

OK,I've got the paper, it's in S&V Sept 2001

Here's the mickey mouse explanation, he goes into maths later:

The simplest form is an unbalanced shaft rotating at 600 rpm. In an idealised experiment a strain gauge on the shaft will measure a static bending only, ie f=0Hz. An accelerometer on the bearing will measure a vibration at 10 Hz. So in this case the true physical situation has been shifted by +1 order.

He also does a worked example with some real data, and shows where n-1 comes from as well. If I sound a bit unenthusiastic, well, there ya go.



Cheers

Greg Locock

 

[autonoise]

Greg,

FWIW, I worked briefly on this. The order transformation can help explain how resonances in mount vibration can be related to crankshaft torsion & bending. It's extremely difficult to measure unless one "demodulates" the entire waterfall, as Turgay did in his paper. His references give good additional background, especially Ochiai