Sustained torsional oscillation 1

[tokitalk]

Anyone has ideas on effective countermeasures?

 

[prex]

Oscillation of what? Sustained by which effect?

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads

 

[asimpson]

Damping

 

[ivymike]

an engine crankshaft undergoes sustained torsional vibration (oscillations), and there are many different damper designs available to address that. Look up "torsional vibration damper", "TV damper" or (ughhh) "harmonic damper."

(or even worse, "harmonic dampener")

 

[rb1957]

tacoma narrows bridge ?

 

[electricpete]

Another strategy may be to separate torsional resonant frequency from excitaitng frequency and it's harmonics.

=====================================
(2B)+(2B)' ?

 

[btrueblood]

Eliminate the periodic force/displacement function...

 

[drawoh]

tokitalk,

Stop twisting it back and forth.

JHG

 

[tokitalk]

Would a longer shaft intended to reduce shaft stiffness be a viable option?

 

[jmw]

I thought I'd already posted:
"Dog bones"
i.e. dog bone inertial dampers.
Or something on that principal.

JMW

http://www.ViscoAnalyser.com

 

[asimpson]

Changing the stiffness or mass of system will change its natural harmonic frequency ( stiffer is higher, heavier is lower) and prevent system resonating if exiting frequency and natural are similar. However that only reduces resonating. System may still vibrate, Just at a different frequency.

It is easier to damp higher frequencies so increasing stiffness and adding damper will help.

http://www.heavydutycomponents.com/construction.html

 

[Japo]

I agree with Electricpete, the first usual suspect you should look at is resonance. Check that the excitation causing this has a frequency component close to the natural frequency of the vibration mode that's responding. I'm assuming of course that you have no control over this excitation, but still you must know what it is.

You should know if it's more practical adding or removing mass or stiffness, specially since you have provided preciously little information on your problem.

The dumbest solution is adding mass. If this is a rotating shaft, make sure it remains equilibrated, that is: add something symmetric across the shaft, or equal weights on diametrically opposite sides; and either add that at the exact axial position of the center of mass of the rotor, or if you can't place it there, equilibrate two sets at either sides of the center.

 

[PNachtwey]

You can try active damping with a motor and controller but it would be best to remove the cause of the oscillations.

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[unclesyd]

Here is some information on torsion bars from Schroeder and Swayaway. Look at the hollow torsion bar, sometimes called a torque tube.

The third link is to C-flex spring dampener. We use this approach in a lot of machinery with relatively small size and low loads .

http://www.schroedersteering.com/HomePage.html

http://www.swayaway.com/TechRoom.php

http://www.c-flex.com/brochures.html

 

[unclesyd]

Forgot to add in the above post Swayaway has some online calculators that maybe of some help.

 

[electricpete]

Would a longer shaft intended to reduce shaft stiffness be a viable option?

That would generally decrease the resonant frequency. It certainly might be a good solution if you are experiencing resonantly-amplified vibration from a constant-frequency excitation (like an electric motor-driven load, no vfd). On the other hand if you have a varying-frequency source of excitation (like in a car), it may not be practical to separate resonance from all possible exciting frequencies in which case the other solutions like damping would be more effective.

=====================================
(2B)+(2B)' ?

 

[flash3780]

Usually the best bet is to change the stiffness of your part or the location of the supports to move the resonance out of the operating range. Increasing or decreasing the length would certainly change the frequency. A longer shaft would have a lower natural frequency, while a shorter shaft would have a higher natural frequency. You also may consider changing the diameter or wall thickness (?) of the shaft.
If it's impossible to get the natural frequency out of the operating range, dampers may be used. However damping is difficult to quantify, so it's preferred to avoid resonance.

 

[gruntguru]

tokitalk (Mechanical) 27 Jan 11 19:12
Would a longer shaft intended to reduce shaft stiffness be a viable option?

AKA "quill shaft". Commonly used to avoid TVs by lowering the resonant frequency.

Engineering is the art of creating things you need, from things you can get.

 

[unclesyd]

Have you looked a composite shafts?

1: Link to a producer of composite shafts.

2: A paper on composite shaft vibration. You may have to order the complete paper from Linda Hall Library.

http://www.bactechnologies.com/shafts.htm

 

[tokitalk]

Thanks for all the inputs....our current system utilized a relatively longer shaft, weight is a premium, so anything in the realm of adding mass/increasing diameter are not viable options. I must also add that the shaft is designed to see torsional loading only....no bending loads are intended, that is not to say parasitic bending from say misalignment may not arise.