Torsional natural frequency

[Y123Y]

hi all,

I am doing some calculation with regards to torsional natural frequency
given one set of equipment with motor and pump along with respected moment of inertia for both.
and it also gives you "the first torsional natural frequency"

and here I really don't understand what it mean by " the first"

can anyone please explain what it mean by 'the first'? is it different to torsional natural frequency?

 

[drawoh]

Y123Y,

Most structures have more than one resonant frequency. If you are concerned about resonance, you usually (but not always) worry about the lowest resonant frequency. This is pretty basic vibrations stuff.

--
JHG

 

[GregLocock]

Although to be fair a pair of rotors connected by a single shaft you are actually interested in the second, and last, natural frequency.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[RolMec]

The basic stuff would be that mechanical systems have a number x of natural frequencies equivalent to the number x of degrees of freedom within this system.
By solving the system of equations of motion you get x natural frequencies, where the lowest (smallest number) of oscillations per second is named "first" natural frequency.
So if your system would have one degree of freedom (one rotating mass on a generalized shaft) you have one natural frequency. As this degree of freedom is rotation, your system is a purely torsional oscillation system and you have a "first torsional natural frequency".
However, for rotating systems with multiple masses the first natural frequency might well be a bending one, so for rotating systems bending and torsional modes of natural frequencies are common.
This is most importantly to be checked against the excitation frequencies occuring in your system, because if an excitation frequency gets quite near to one of the natural frequency then resonance occur. You might want to look this up.
Finally, I'd like to kindly propose you to invest some time into a machine dynamics handbook or else get professional support on this in order to ensure correctness of the dynamical aspects of the job you're doing.
Regards

RSVP

 

[Tmoose]

What type/make/model of the coupling?

Horizontal pumps often use flexible grid or elastomeric couplings.
Those couplings types are typically pretty soft torsionally.
I think that the coupling torsional stiffness is often so low that they control the first torsional to a large degree.

Pictures of the various torsional modes of a flat beam here -

http://pubs.sciepub.com/jmdv/1/1/4/image/fig5.png

A useful animation of first mode starts at ~ 2:00 here -

https://www.youtube.com/watch?v=JvuaNll9IZs

 

[GregLocock]

RoIMec, you are omitting the zero Hz mode, which is easy to do and what I was getting at. The two mode shapes for a two rotor single shaft system are [1,1} and {1,-something). The first is at zero Hz, the value of something in the second is easy to work out.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?