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I Can't Believe This Happend AGAIN!

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#43
I have had brass screws break from the torque (twisting) due to the threads having so much friction in the wood. What I mean here is that when you are turning the screw in with a screwdriver, the threads are binding up in the wood due to friction and your twisting actually twists the shaft of the screw and weakens it. Not sure if that's the case here since these holes were already threaded (and 20 years old) ... but it is something I saw a lot with brass screws so I got in the habit of waxing or soaping the threads of a brass screw before turning it into hardwood and then I never had another brass screw break.

What makes me suggest this is the way the screws all broke down a ways on the shaft, below where the edge of the bridge plate hits it. That is exactly where they break when the shaft got twisted/weakened while turning into the hard wood with too much friction.
 
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#44
AndyPanda said:
I have had brass screws break from the torque (twisting) due to the threads having so much friction in the wood. What I mean here is that when you are turning the screw in with a screwdriver, the threads are binding up in the wood due to friction and your twisting actually twists the shaft of the screw and weakens it. Not sure if that's the case here since these holes were already threaded (and 20 years old) ... but it is something I saw a lot with brass screws so I got in the habit of waxing or soaping the threads of a brass screw before turning it into hardwood and then I never had another brass screw break.

What makes me suggest this is the way the screws all broke down a ways on the shaft, below where the edge of the bridge plate hits it. That is exactly where they break when the shaft got twisted/weakened while turning into the hard wood with too much friction.
Click to expand...

I actually put a little wax on the replacements when I installed them and all three broke at different places but I suppose it's possible.
 
#45
Killed_by_Death said:
Sounds to me like your bridge has sharp edges where it meets the screws.

Before filing them you might try just going to your local hardware store with a sample screw & ask them for their hardest screw of the same size & thread pitch.
Click to expand...

Except for the fact that the factory installed and replacements broke 1/8" to 1/4" under the bridge. And why did the originals all of sudden fail when they had never been removed since it was made?
 
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#50
It's worth noting that Home Depot sells garbage unless it's name branded like SPAX or the like. Their general off the rack screws are the cheapest of the cheap. A screw in shear only has 60-70 percent of its tensile rating. Screw failures are 80 percent caused by fatigue over time and not pure strength failures. The original spector screws were probably good quality and lasted a good long time. The HD screws, not so much. I agree with change to stainless and think that looking for sources or work fatigue. Could the bridge be moving in some way?

citation:Invalid Link Removed
 
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#51
Here's your problem, at least with the new screws. If you notice each one failed at a different place. What you have is mushy holes where only the lower portion of the screw is actually biting into the wood. The remainder of the screw, that part in the mush and through the bridge are putting all your string pressure on those points where the mush ends and the tight hole begins.

What you have is your bridge screws floating for most of their length, which allows all the string pressure to be concentrated on the spot where the screw actually bites. As long as you have some threads left in the bottom, it will bite and act like it's all hunky dory, but it's not. The bridge depends on the full length of it's screws to be firmly held by the body wood or they can do exactly what you see happening to your bridge.

I'm not a gambler, but I'd bet if you rehab your bridge holes the problem will go away.
 
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#53
Id get the old screws out with a hollow tube screw extractor (try Rockler Hardware) that cuts a new hole as it extracts the screw. Glue in a new dowel, or better yet a threaded brass insert for machine screws. No way thats failing again with good screws.

[Invalid or Expired Link Removed]
 
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#55
Which one of these stainless steel screws would be better for me to use? Pic 1 is closer to the original style Spector used with the threads starting about a 1/4 " down the shank. It's also zinc plated. Pic 2 is threaded all the way and not zinc plated. Both are the same size (#8 x 1 inch) although they don't look like it in the pics.

1.jpg


2.jpg
 
#57
leemelone said:
Which one of these stainless steel screws would be better for me to use? Pic 1 is closer to the original style Spector used with the threads starting about a 1/4 " down the shank. It's also zinc plated. Pic 2 is threaded all the way and not zinc plated. Both are the same size (#8 x 1 inch) although they don't look like it in the pics.

View attachment 705121

View attachment 705122
Click to expand...

I'm going with B though it may require a bit more force because of the extra threads.
 
#58
leemelone said:
Which one of these stainless steel screws would be better for me to use? Pic 1 is closer to the original style Spector used with the threads starting about a 1/4 " down the shank. It's also zinc plated. Pic 2 is threaded all the way and not zinc plated. Both are the same size (#8 x 1 inch) although they don't look like it in the pics.

View attachment 705121

View attachment 705122
Click to expand...
Oval heads.
image.jpeg
 
#59
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
Click to expand...
leemelone said:
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.
Click to expand...
fhm555 said:
Here's your problem, at least with the new screws. If you notice each one failed at a different place. What you have is mushy holes where only the lower portion of the screw is actually biting into the wood. The remainder of the screw, that part in the mush and through the bridge are putting all your string pressure on those points where the mush ends and the tight hole begins.

What you have is your bridge screws floating for most of their length, which allows all the string pressure to be concentrated on the spot where the screw actually bites.
Click to expand...
i think i like this answer.
 
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