Everyone's under their van's today!!
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Diamond Hell
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No need to dumb everything down, this is the Syncro forum - isn't it? Glad it made some sort of sense Euan, whirling might sound like a fairground ride or a neat move at a barndance, but there's also reams of engineering formulae for predicting what is essentially an unstable and therefore somewhat unpredictable critical mode.
A good analogy of a critcal static mode would be Euler buckling in compression...
Take a foot rule (or a metre one!) and holding it vertically so its weight doesn't distort its flatness, gently start compressing it between your hands until it suddenly whips into a curve (buckled state)...
now do same with simlar a 6" one...
If it's similar in all other respects than length, it should take about 4 times the force. Thickness or diameter has a very strong stabilising influence, just as it does with whirling... so short fat columns don't suffer this mode, just as simple look and feel tells us.
The exact force though will have a high degree of 'scatter', but if its prefctly pin-jointed at the top and bottom dead centrally (co-axially) that scatter will reduce a lot but still be there to an extent. So you stay well away from Euler buckling predictions of long beams in compression... ditto whirling which is a good degree more complex being dynamic, but similar in the approach engineers take in ensuring it doesn't happen in practice.
Another good reason not to drop, knock or dink that propshaft... but hopefully, that margin is so large by design, you'd only get vibration, not whirling, though its been known of course and extremely dangerous and frighteneing to anyone nearby large machinery when it happens (old dark satanic mills!)
A good analogy of a critcal static mode would be Euler buckling in compression...
Take a foot rule (or a metre one!) and holding it vertically so its weight doesn't distort its flatness, gently start compressing it between your hands until it suddenly whips into a curve (buckled state)...
now do same with simlar a 6" one...
If it's similar in all other respects than length, it should take about 4 times the force. Thickness or diameter has a very strong stabilising influence, just as it does with whirling... so short fat columns don't suffer this mode, just as simple look and feel tells us.
The exact force though will have a high degree of 'scatter', but if its prefctly pin-jointed at the top and bottom dead centrally (co-axially) that scatter will reduce a lot but still be there to an extent. So you stay well away from Euler buckling predictions of long beams in compression... ditto whirling which is a good degree more complex being dynamic, but similar in the approach engineers take in ensuring it doesn't happen in practice.
Another good reason not to drop, knock or dink that propshaft... but hopefully, that margin is so large by design, you'd only get vibration, not whirling, though its been known of course and extremely dangerous and frighteneing to anyone nearby large machinery when it happens (old dark satanic mills!)
Last edited by HarryMann on 28 Mar 2006, 22:05, edited 1 time in total.
The 80-90 Tech Wikipedia Your 1st port of call
1.9TD Syncro Doka / Syncro Kastenwagen / 16" Kombi Camper
Syncronaut No. 1
As I understand it Whirling would be Making Like a Skipping Rope.
That's the one, not sure if shape is a catenary or part of a sinewave - and I'm sure you don't want to know
The 80-90 Tech Wikipedia Your 1st port of call
1.9TD Syncro Doka / Syncro Kastenwagen / 16" Kombi Camper
Syncronaut No. 1
Your van would not just vibrate, but become a veritable Jumping Jack Flash, and be a right gas for a few seconds until you slowed down or something broke.. 
Lots of energy goes into whirling, forces would be high, it would obviously try to shorten the distance between the diff and transaxle if the prop didn't have a plunging joint in it, and it would likely hit the underrun bars... but then, it might just whirl a bit, shake your specs off, and quieten itself down again. If it keeps whirling, prop UJ damage in short order at a minimum, buckling of the prop or prop strike and instant destruction might eb on the cards...
The first point I was making back there was that as the prop is turning at pinion speed, the torque it transfers is lower by far than that going to each pair of wheels, when in low gears. So designing a shaft of that length running at that speed (potentially full engine speed) to resist whirling, likely easily covers the sizing required to simply transmit the torque.
Lots of energy goes into whirling, forces would be high, it would obviously try to shorten the distance between the diff and transaxle if the prop didn't have a plunging joint in it, and it would likely hit the underrun bars... but then, it might just whirl a bit, shake your specs off, and quieten itself down again. If it keeps whirling, prop UJ damage in short order at a minimum, buckling of the prop or prop strike and instant destruction might eb on the cards...
The first point I was making back there was that as the prop is turning at pinion speed, the torque it transfers is lower by far than that going to each pair of wheels, when in low gears. So designing a shaft of that length running at that speed (potentially full engine speed) to resist whirling, likely easily covers the sizing required to simply transmit the torque.
The 80-90 Tech Wikipedia Your 1st port of call
1.9TD Syncro Doka / Syncro Kastenwagen / 16" Kombi Camper
Syncronaut No. 1
