My failed bow's
Here my First failed Bow. Build 2008, broken 2009.
The error was, i've used too thick wood-core.
The error was, i've used too thick wood-core.
Here my second, Build 2009:
After 15 Month, after put the tepeliks i hear cracks and the sinew was torn
After removing sinew in warm water:
With the Horn of the Bow above i build my new Bow, see here:
http://198.170.107.188/phpBB2/viewtopic.php?t=2574
http://198.170.107.188/phpBB2/viewtopic.php?t=2574
is that a bamboo core? have you heat treated the bamboo before gluing?
and why covered the sinew with cloth?
and why covered the sinew with cloth?
No it's not Bamboo. It was maple.
It is need to heat treated the bamboo before gluing?
The cover is not need realy. I do it to protect fom dust.
It is need to heat treated the bamboo before gluing?
The cover is not need realy. I do it to protect fom dust.
Hi Abdal
Thanks for posting this! I've had a few failures recently..... Sometimes it helps to see that I'm not the only one that struggles!
Cheers and good luck!
J
Thanks for posting this! I've had a few failures recently..... Sometimes it helps to see that I'm not the only one that struggles!
Cheers and good luck!
J
Thank you for this helpful sharing..
The failures are the important things!
I am very interested in your last picture of the broken core. Exactly ther same thing happened to my last bow which was a real heartbreaker because the bow up until the core broke was absolutley perfect no twist, perfect bend...then I got it to around 24 and started hearing cracking noises. I saw the sinew had lifted so removed it and found the core cracked right acrosss just like yours but only in one place. I think mine went because my sinew was too thin over the center of the core.
Why do you think yours went like that?
I am very interested in your last picture of the broken core. Exactly ther same thing happened to my last bow which was a real heartbreaker because the bow up until the core broke was absolutley perfect no twist, perfect bend...then I got it to around 24 and started hearing cracking noises. I saw the sinew had lifted so removed it and found the core cracked right acrosss just like yours but only in one place. I think mine went because my sinew was too thin over the center of the core.
Why do you think yours went like that?
... I assume by bonding the tepelik to the core the pressure was simply too high, also because you tried to "follow" the curvature/radius of the tepelik from the beginning - instead of doing it steadily. See Adams book, page 115 .... a few centimeteres at both ends can be left....
hope this may help - Merry Christmas!
Reto
hope this may help - Merry Christmas!
Reto
Merry Christmas!
Mikekeswick is right: failures are very important for gaining an understanding of what is going on with these bows. It's all pretty subtle.
I am in the process of making a Magyar style bow, my first attempt at a horn bow. I will be starting the sinewing process soon, and am very apprehensive about limb failure when I get finally to the point of doing tillering.
From what I've been reading on this forum, core failures are quite common. While I don't claim to be an experienced bowyer, I am an engineer and do have a decent understanding of stress and strain. Unless there is a flaw in the wooden core (such as uneven grain, grain running off to the side or top, or unseen internal weak spot), it would appear to me that the remaining possibilities are i) the thickness and width of the core have bad proportions and thus cause too much stress to be concentrated over too small an area of the limb, or ii) the sinew isn't sufficiently doing its job of keeping the back side of the core from stretching to its point of failure.
I find this to be a little like pre-stress concrete, in which the reinforcing rods are stretched under many tons of pressure, the concrete poured, and then the pull on the rods released, thus squeezing the body of the concrete together and taking up much of the tensile bending stress. Likewise, it would seem that if the sinew is not sufficiently exerting a very strong pull on the back side of the core, the core is more on its own and will fail once the strain passes the allowable limit, which for wood is much less than for sinew. Just really a guess on my part.
Anyway, all these core failures suggest to me that it is safer to err on the side of making the core layer somewhat thinner and the horn and sinew layers somewhat thicker. Though I am ready to start glue sizing in preparation for sinewing, I might take a little bit more off the core thickness just to play safe
Mikekeswick is right: failures are very important for gaining an understanding of what is going on with these bows. It's all pretty subtle.
I am in the process of making a Magyar style bow, my first attempt at a horn bow. I will be starting the sinewing process soon, and am very apprehensive about limb failure when I get finally to the point of doing tillering.
From what I've been reading on this forum, core failures are quite common. While I don't claim to be an experienced bowyer, I am an engineer and do have a decent understanding of stress and strain. Unless there is a flaw in the wooden core (such as uneven grain, grain running off to the side or top, or unseen internal weak spot), it would appear to me that the remaining possibilities are i) the thickness and width of the core have bad proportions and thus cause too much stress to be concentrated over too small an area of the limb, or ii) the sinew isn't sufficiently doing its job of keeping the back side of the core from stretching to its point of failure.
I find this to be a little like pre-stress concrete, in which the reinforcing rods are stretched under many tons of pressure, the concrete poured, and then the pull on the rods released, thus squeezing the body of the concrete together and taking up much of the tensile bending stress. Likewise, it would seem that if the sinew is not sufficiently exerting a very strong pull on the back side of the core, the core is more on its own and will fail once the strain passes the allowable limit, which for wood is much less than for sinew. Just really a guess on my part.
Anyway, all these core failures suggest to me that it is safer to err on the side of making the core layer somewhat thinner and the horn and sinew layers somewhat thicker. Though I am ready to start glue sizing in preparation for sinewing, I might take a little bit more off the core thickness just to play safe
The core on my last bow broke.
After much head stratching I realized my error the sinew was only just over 1mm thick above the center of the rounded core. The wood was still being asked to do tension work.
I don't think the thickness of the core is the main consideration here it's more that you get sufficent thickness of sinew to as you say take up enough of the tension load.
If the sinew isn't thick enough the core is too far above the neutral plane to be able to take the forces applied to it.
Of course the grain is cut through all along the back of the core pre-sinewing and would therefore have no chance of taking a tensile loading.
I was gutted when my last one broke because until it actually broke the bow was literally perfect - no tillering, no instability.........well you live and learn!
The more I get to know about hornbows the more I realise I don't know!
After much head stratching I realized my error the sinew was only just over 1mm thick above the center of the rounded core. The wood was still being asked to do tension work.
I don't think the thickness of the core is the main consideration here it's more that you get sufficent thickness of sinew to as you say take up enough of the tension load.
If the sinew isn't thick enough the core is too far above the neutral plane to be able to take the forces applied to it.
Of course the grain is cut through all along the back of the core pre-sinewing and would therefore have no chance of taking a tensile loading.
I was gutted when my last one broke because until it actually broke the bow was literally perfect - no tillering, no instability.........well you live and learn!
The more I get to know about hornbows the more I realise I don't know!
I agree from my limited understanding of engineering the tension sinew backing is the "magic" thing about the composites: otherwise
- the horn and wood would break or
- the limbs had to be longer thus slower
(- horn is dispensible yet necessary to prevent string follow)
the common thought is that the wood is in the neutral plane - however I wonder if this is a proven matter or if in fact there is only compression owing to the non-stretching sinew ? - data anyone ?
PS Edmund, check "Arab Archery" for some very interesting quotes on the proportion of horn, wood, sinew and glue. (Its even more interesting now as we learned that its 200 years older than previously thought.)
- the horn and wood would break or
- the limbs had to be longer thus slower
(- horn is dispensible yet necessary to prevent string follow)
the common thought is that the wood is in the neutral plane - however I wonder if this is a proven matter or if in fact there is only compression owing to the non-stretching sinew ? - data anyone ?
PS Edmund, check "Arab Archery" for some very interesting quotes on the proportion of horn, wood, sinew and glue. (Its even more interesting now as we learned that its 200 years older than previously thought.)
Uli -
The way the stresses are distributed in one of these bows is probably very hard to determine with any degree of accuracy.
Theoretically, the best chances of preventing the core from failing in either tension or compression is to have the neutral plane running right down the middle of the core thickness. That way, both stresses will be more or less equal, or so the theory goes.
However, I doubt that is ever the case, because for it to be so, the compressive support from the horn and the tensile support from the sinew would have to be exactly equal, and I doubt anyone ever gets it perfect. Thus, some bows probably have the neutral plane closer to the horn layer and some closer to the sinew layer.
Plus, all this stuff about neutral planes, etc. is based on the structural theory of a beam. But a bow really isn't a beam, as it is more like a flat spring of variable curvature and cross section. Beam theory is based on small deflections (a few percent), whereas a bow bends over a far greater range. Thus, my guess is that the position of the neutral plane could vary quite a bit from brace to fully drawn positions.
I find it fascinating that while the ancients obviously didn't have any math or engineering theory to go on, they developed a very sophisticated empirical 'feel' for what worked and what didn't. We have a lot to learn from them.
The way the stresses are distributed in one of these bows is probably very hard to determine with any degree of accuracy.
Theoretically, the best chances of preventing the core from failing in either tension or compression is to have the neutral plane running right down the middle of the core thickness. That way, both stresses will be more or less equal, or so the theory goes.
However, I doubt that is ever the case, because for it to be so, the compressive support from the horn and the tensile support from the sinew would have to be exactly equal, and I doubt anyone ever gets it perfect. Thus, some bows probably have the neutral plane closer to the horn layer and some closer to the sinew layer.
Plus, all this stuff about neutral planes, etc. is based on the structural theory of a beam. But a bow really isn't a beam, as it is more like a flat spring of variable curvature and cross section. Beam theory is based on small deflections (a few percent), whereas a bow bends over a far greater range. Thus, my guess is that the position of the neutral plane could vary quite a bit from brace to fully drawn positions.
I find it fascinating that while the ancients obviously didn't have any math or engineering theory to go on, they developed a very sophisticated empirical 'feel' for what worked and what didn't. We have a lot to learn from them.