set and stress

[Manolovis]

something i m not clear with somehow ....
i understand that set happen where too much stress. But too much stress happen on a weak or stiff tension? when you look at it, if a limb is weak, it s going to be under stress when we ask too pull it to a certain length right ? on the other hand i see that a stiff limb will be under a lot of stress if being to pull at that same length cause it cant bend as much ?
maybe it s very trivial for some but not for dumb me!
could anyone explain me this ?
cheers

 

[Selfbowman]

I’m not sure about your question but it’s all about the mass in the wood and where you put it. This is why I like pyramid bows. The mass diminishes with the need for it. Leaving less to move forward. All that yields a fast bow. But not the only kind of bow for sure. Arvin

[bownarra]

If you asre tillering a bow and one limb is weak, the weak limb is getting a double wammy of strain. The other/stiff limb is acting like a lever (sort of) and throwing some of its bending load onto the weak limb.
Have a stiff limb or a stiff area on a limb automatically concentrates the bending load onto the weak area. If you keep pulling the strain gets more and more concentrated on that weak spot. This is why you should only ever pull a bow to the point where you can see something is 'wrong' and no further. Set always shows up where we are asking a bit too much of the wood.
If the whole bow is well tillered but you get lots of set the design was wrong eg. not enough wood to take the strain ofg the bow you are trying to make.
One limb takes more set than the other? It was weak overall.
Set in one area of a limb? It was weak right there/ or the rest of the limb is too stiff.
Set will always tell you how well you have done tillering. Set is the wood speaking to you - you've got to learn how to listen!

[Manolovis]

thanks bownara that make much more sense !

[simson]

Hi Manu!
Good explanation above by Bownarra.
When a stick is bent, there is compression on the belly side and tension on the back side. Somewhere in between is the neutral zone. This is not in the exact middle (depends also on wood species), because wood can better absorb compressive stress and can only be stretched to a very limited extent. This means that almost all the energy is charged via the belly side cells by pressure. If there is a weak point somewhere, the stick will bend too much there, as Bownarra has already explained. If no countermeasures are taken, there will be too much stress and set will develop. Set is caused by overstressing the wood cells, they lose their spring force and do not regain their original shape when stressed. This is accompanied by the fact that the energy charge can no longer take place as it should, at least the first few inches during draw out store significantly less.

[SLIMBOB]

I will add to the very fine explanations above. Your question seems to be a theoretical one. Does a thin limb take more set than a thick limb?  All things equal, no. The thin limb can bend into a tighter radius than the thick one. But things are not equal. A bow pulling 50 lbs, with one limb thinner than the other (weaker) is doing most of the work. The thick limb is bending less at 50 lbs, and does not feel the strain put on the thinner limb. The weaker limb will feel more strain, just like the weaker section on a limb will feel more strain than the stiff section as it is doing more work. “Thick limb” and “Thin limb” is relative to the strain applied.

[Manolovis]

thanks All for your explanations 

 

Hi Manu!
Good explanation above by Bownarra.
When a stick is bent, there is compression on the belly side and tension on the back side. Somewhere in between is the neutral zone. This is not in the exact middle (depends also on wood species), because wood can better absorb compressive stress and can only be stretched to a very limited extent. This means that almost all the energy is charged via the belly side cells by pressure. If there is a weak point somewhere, the stick will bend too much there, as Bownarra has already explained. If no countermeasures are taken, there will be too much stress and set will develop. Set is caused by overstressing the wood cells, they lose their spring force and do not regain their original shape when stressed. This is accompanied by the fact that the energy charge can no longer take place as it should, at least the first few inches during draw out store significantly less.

really happy to see you back in a workshop simon ! love your last bow