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I Can't Believe This Happend AGAIN!

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#26
Since it has happened twice with two different sets of screws I would like to see pics of the bridge plate, top view and bottom. It might be the shear force culprit. Question - did the other set of screws break at different points as well?
 
#27
Just reread you post about the last set of screws - all broke at the same spot. That throws out my hypothesis - something about the bridge plate was causing an outer screw to shear and put all of the tension on the remaining two screws which sequentially pulled out and sheared due to the tension, in a kind of twisting fashion.
 
#28
The fact that they broke below the surface makes me wonder if the holes are loose enough at the top that the screws are only firmly seated deep in the wood, so as the bridge pulls on them, it's actually bending them slightly. The threads closest to the head aren't biting into wood (much or at all) otherwise they wouldn't just pop out with the force that's pulling perpendicular to their orientation. Although I scoffed at the thought at first, I think I would revisit it as a problem with the holes -- I'd carefully glue and fill them with hardwood slivers -- something more substantial than toothpicks. I can't think of any other way the screws would be subjected to any force that far down.
 
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#30
lethargytartare said:
The fact that they broke below the surface makes me wonder if the holes are loose enough at the top that the screws are only firmly seated deep in the wood, so as the bridge pulls on them, it's actually bending them slightly. The threads closest to the head aren't biting into wood (much or at all) otherwise they wouldn't just pop out with the force that's pulling perpendicular to their orientation. Although I scoffed at the thought at first, I think I would revisit it as a problem with the holes -- I'd carefully glue and fill them with hardwood slivers -- something more substantial than toothpicks. I can't think of any other way the screws would be subjected to any force that far down.
Click to expand...

I think you may be on to something. The holes are very slightly rounded out at the top so the screws may not be grabbing hold at the very top. Add to the equation the very heavy bridge and non-hardened screws and that may be what's causing the failure. It's a combination of multiple factors. Thanks!
 
#36
lethargytartare said:
The fact that they broke below the surface makes me wonder if the holes are loose enough at the top that the screws are only firmly seated deep in the wood, so as the bridge pulls on them, it's actually bending them slightly. The threads closest to the head aren't biting into wood (much or at all) otherwise they wouldn't just pop out with the force that's pulling perpendicular to their orientation. Although I scoffed at the thought at first, I think I would revisit it as a problem with the holes -- I'd carefully glue and fill them with hardwood slivers -- something more substantial than toothpicks. I can't think of any other way the screws would be subjected to any force that far down.
Click to expand...

I was just getting ready to say this exact same thing all I got was "The fact that." then I saw your post. :) in all seriousness this seems like the most logical theory I vote yes for this one.
 
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