Fundamental Frequency of a Powertrain

[mert1]

I am trying to determine the first couple of frequencies of a drive shaft. It consists of three seperate pieces connected with elastic couplings. It is attached to the vehicle, so that the boundary conditions as in real life. How can I do the test?
for example, could I do a hammer test and get data from three accelerometers attached to each of the pieces? Or hit all three pieces and find each parts fundamental frequency?? Yet, I am assuming there is one fundamental frequency for the whole system.
Thanks.
Mert

 

[Rob45]

Are you looking for bending modes?
Unless your couplings are VERY elastic, you're correct that there will be a fundamental frequency for the whole system as well as individual modes for the separate pieces.
Why don't you try the roving accelerometer method, where you hit one place and move the accelerometers around?

BTW, this would be known as driveline vibration - "powertrain" vibration is generally considered to involve the vibrational modes of the engine/transmission unit, and little else.

You should also be concerned about torsional modes; I am currently involved in a severe driveline torsional vibration that is excited upon deceleration by the partial engine braking, so it is excited at a lower frequency than would be expected based on engine firing.

 

[GregLocock]

To get the bending frequencies and rigid body frequencies, move the accelerometer to the centre of each shaft and hit it there.

However that tells you very little of interest, and will probably be difficult to interpret.

If you want the mode shapes (and you probably do) then use Rob's roving accelerometer technique, once you have established a good driving point.

You may have to pull the driveline out of the truck, and hang it on elastic, to get a good understanding of the basics, although that won't have the right boundary conditions.

Incidentally in cars we often manage to get the bending stiffness of the powertrain (ie engine and gearbox) into the same frequency range as that of the driveline. That causes some fun. You will find it very difficult to excite a complete powertrain+driveline with a hammer from one location.








Cheers

Greg Locock

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