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Derek Sweetman - n/a
8-Jan-08, 06:40 PM
Have a look at this link

http://www.prime-mover.org/Engines/Torsional/contact1/contac (http://www.prime-mover.org/Engines/Torsional/contact1/contact1.html) t1.html

The BD-5 Drive system in this page looks very similar to an integrated craft engine fan layout!

What do you think?

Is torsional vibration a hovercraft problem too?

I have certainly seen it on long lift drive shafts and may be on other failures too.



Regards

Derek Sweetman



S.E Branch Chairman

chairman.southeast@hovercraft.org.uk

Anne Scrimshaw - n/a
8-Jan-08, 07:43 PM
Hi Derek,



We used to regularly calculate the whirling speed of ballscrews at the linear bearing company I worked for. Any shaft will resonate at a certain speed, which is dependant on the length, and diameter, of the shaft and how the bearings are mounted. From memory I would think that you could have a problem with a shaft drive craft, and certainly needs checking. However I doubt if a standard integrated drive shaft layout is run anywhere near the critical speed.



I can dig out the formulas if you want.



Anne

tonybroad - n/a
9-Jan-08, 08:21 AM
I agree with Anne (who wouldn't)in fact i feel a techie magazine article coming on - typical integrated shafts go nowhere near critical speed



Shaft Drives are a completley different issue - I've done a load of calcs on different shafts/bearing spacings and calculated the Turnbull-style single long aluminium shaft actually goes through critical speed or 'whipping speed' as it's sometimes known at around 1200 - 1600 rpm depending on distance between bearings/shaft diameter/wall thickness



Nick Long told me once his driveshaft sometimes whips and the only thing to do is stop the engine and start it again, it won't pull through the whip



there's an online calculator (below) - it's for racecar driveshafts but the principle is the same



http://www.wallaceracing.com/driveshaftspeed.htm



where theory meets practice - welcome to my world

Nick Long - n/a
9-Jan-08, 08:37 AM
This is so, some of the long, lightweight drive shafts are operating above their first resonance. Its not usually a problem as they are not operated at the critical speed, unless you get a combination of circumstances, such as a bump while taxying at the critical low revs. Then it is better to stop and restart. It has never happened to me in a race or on the start grid.



Note though, that here we are talking about lateral vibration, not torsional vibration as in the original article.



Nick

tonybroad - n/a
9-Jan-08, 04:58 PM
the only torsional issues i can think of is when a flexible coupling is soft or worn or doesn't match and goes into a resonance the engine can't pull through but i presume that's mainly down to the coupling not the shaft