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Old 28 September 2008, 12:09   #1
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Fracture in A Frame

Hi All

Has anyone on this fourm had problems with the A Frame fracturing. It has happened to me twice both times at a 45 degree nuckle near the transom. The first time I thought is was due to the weight of the Aux engine so I had it removed but now it has fractured again. I dont know if it is as a result of vibration (resonance) or just excessive bending moments. The frame is 1.5 inch tublar s.s. marine grade

You thoughts
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Old 28 September 2008, 12:23   #2
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Any photos?
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Old 28 September 2008, 15:27   #3
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Flexing fatigue..same as Codders' U-bolts. You need to brace it somehow. Also, imho, it is better not to have too small and neat a weld because the area around the weld cools rapidly and the structure within the steel may not be at its best. (Tried to word that carefully so's I don't get a row from Jono...) Have a look closely at the broken ends, I bet you can see the grain structure of the steel.
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Old 28 September 2008, 17:15   #4
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Need phots of the fracture. Did the weld itself break? did the weld separate from the tubing? did the tubing tear out?
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Old 29 September 2008, 03:34   #5
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Brought the A frame back to the man who made it this morning. He is of the opinion that the failure is as a result of vibration. he is going to weld in some gussets to make the unit more rigid
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Old 29 September 2008, 04:48   #6
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I think any A'frame that is higher than 3 inches will fail at some point, it's just a matter of time.


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Originally Posted by two stroke mick View Post
Brought the A frame back to the man who made it this morning. He is of the opinion that the failure is as a result of vibration. he is going to weld in some gussets to make the unit more rigid
The problem with this is you'll just pass the vibration onto somewhere else which will fail in the future ( apprecciate you don't have a choice to repair. )

Mark
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Old 29 September 2008, 07:49   #7
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I think any A'frame that is higher than 3 inches will fail at some point, it's just a matter of time.
3 inches?
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Old 29 September 2008, 14:24   #8
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Originally Posted by MarkWildey View Post
I think any A'frame that is higher than 3 inches will fail at some point, it's just a matter of time.




The problem with this is you'll just pass the vibration onto somewhere else which will fail in the future ( apprecciate you don't have a choice to repair. )

Mark
No it's not-if the flex in the frame is the source of the vibration then bracing it will remove the problem.
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Old 29 September 2008, 14:37   #9
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bracing or thicker tube should stop it happening again, I have come across this sort of failure many times with home made bracketry on landrover engine conversions etc, the problem happens when resonance is set up in a component, bracing the bracket or using thicher material changes the resonant frequency(raises it) and also makes the item stronger.
can you sleeve the tube internally? that should fix it. the frame probably resonates at a low frequency so sleeving at the base will reduce whip.
Lighting columns are like this for just that reason, double skinned tapered out to the base.
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