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Re: Crack repair

Author: clarnibass
Date: 2019-09-06 15:54

The thing about cracks is that nothing is 100% guaranteed...

Some cracks would be fine with no repairs, not growing or expanding. Some will "burst" any repair. Then there's anything in between.

I've seen a carbon fiber banded crack re-open. Not one I've done, but the repairer who had this happen had a 100% success rate for a long time until this happened.

Many now (including e.g. Yamaha) recommend just gluing cracks, without adding pins or bands. Over a long time I've found this is enough by far most of the time. I use other methods sometimes (pinning, carbon fiber banding). I'm sure some (if not most) repaired cracks would be ok with a less intrusive method... but you'd never know. The size of the crack is not necessarily a sign, since a large crack can just stop, while a tiny crack can gradually grow with extreme force.

There are several differences between threaded still pins and glued (non-threaded) carbon fiber pins (as shown in the video).
It's hard to have a direct comparison because it depends on the exact diameter of each... so this is a rough estimate.

Leaving material for now, the former relies on friction from the threads gripping the wood while the latter relies on the friction from the glue only. So it depends whether the glue can be as strong as the threading. I'd say it's not, even though most of the force is sheer which is good (the strongest for this type of glue).

Then there's the material. The weight difference is completely insignificant. You could use hardened tool steel which is stiffer than carbon fiber. Even regular mild steel is probably stiffer depending on the type of carbon fiber (e.g. how much resin is in it, etc.).

You could use smooth steel rod and then it could be a matter of how the glue is gluing to it (more friction).

When the wood is cracking, it is trying to stretch the pin, which is held by friction to both sides, resisting that force.
There's also the the tiniest amount of bending to the pin, but this is a minuscule amount that is almost nothing against the force of the crack. The video showed the person easily bending the pin by hand (not even on purpose) by more than that.
The pins put at an angle adds another direction to the bending, so a little more resisting by bending, but since the bending resistance is so little this makes next to no difference.
It is the "stretching" of the pins that is holding it.

In reality both methods are probably good enough for most cases. This is almost certainly why gluing carbon fiber pins is so successful. I guess a question is how many cracks are between them, so would re-open with glued pins but not with threaded pins. Who knows...

Post Edited (2019-09-11 08:45)

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