| Author |
Message |
|
Jerry Knox
Location: Palm Bay, Florida, USA Joined: 12 Jun 2007
Posts: 53
|
 Posted: Thu 17 Jan, 2008 10:03 am Post subject: |
 |
|
I apologize for my error. In my job, I mostly work with ductile materials.
With wood, which is brittle, the stress undergone by a rod stepped from 1.25" to 1.125" with an internal radius of the cut corner of 1/16" would be 1.7 times the stress of a simple 1.125 rod under the same load.
In ductile metals, the stress concentration factor is usually disregarded in less severe cases like this diameter reduction, but in wood (and in hardened steel, for that matter), it cannot be ignored.
I felt uneasy giving the previous reply, as it disagreed with my intuition. Glad I checked my texts.
As for a tapered shaft with an almost undetectable cut radius of, say 9/16", the stress would be only 1.1 times that of a cylindrical rod.
Of course, if the wood can take 1.7 times the stress of your application, you'll never know the diffeence.
|
|
   |
 |
Sean Flynt
|
| Posted: Thu 17 Jan, 2008 10:10 am Post subject: |
|
|
| Jerry Knox wrote: | | Of course, if the wood can take 1.7 times the stress of your application, you'll never know the diffeence. |
This is great stuff, Jerry! Thanks for the expertise! Now, I just wish we had some good destructive testing to look at. It would be much cheaper to do this properly than to, say, test historically accurate war bows against historically accurate plate armour, but to my knowledge nobody has systematically tested the variables of historic polearm construction. We just need a few excellent spear heads and some ash hafts. Donors? 
-Sean
Author of the Little Hammer novel
https://www.amazon.com/Little-Hammer-Sean-Flynt/dp/B08XN7HZ82/ref=sr_1_1?dchild=1&keywords=little+hammer+book&qid=1627482034&sr=8-1
|
|
 |
|
Jean Thibodeau
|
| Posted: Thu 17 Jan, 2008 10:58 am Post subject: |
|
|
| Sean Flynt wrote: | | Jerry Knox wrote: | | Of course, if the wood can take 1.7 times the stress of your application, you'll never know the difference. |
This is great stuff, Jerry! Thanks for the expertise! Now, I just wish we had some good destructive testing to look at. It would be much cheaper to do this properly than to, say, test historically accurate war bows against historically accurate plate armour, but to my knowledge nobody has systematically tested the variables of historic polearm construction. We just need a few excellent spear heads and some ash hafts. Donors? |
Thank Jerry for a bit of more precise information about exactly how much of a weakening effect there is with a step.
Sean: The way I visualize it, is that the shaft of higher diameter beyond the reduced diameter section is more rigid than if the shaft was of the same diameter as inside the socket and is thus more rigid: So any bending force gets concentrated on the weaker part instead of being spread uniformly over the entire length of the shaft.
If one used a comparison of a chain with one link smaller in diameter than all the other links it would be the proverbial " weak link " . The situation with a flexible solid bar or shaft would be similar but I suspect mathematically different with a lateral load as opposed to just a pulling load: The leverage factor concentrating forces even more.
You can easily give up your freedom. You have to fight hard to get it back!
|
|
|
|
|
Sean Flynt
|
| Posted: Thu 17 Jan, 2008 11:05 am Post subject: |
|
|
| Jean Thibodeau wrote: | | The way I visualize it, is that the shaft of higher diameter beyond the reduced diameter section is more rigid than if the shaft was of the same diameter as inside the socket and is thus more rigid: So any bending force gets concentrated on the weaker part instead of being spread uniformly over the entire length of the shaft. |
That's the way I understand it, too. Now we just have to figure out why this apparently didn't bother folks whose lives depended on the stability of their weapons.
-Sean
Author of the Little Hammer novel
https://www.amazon.com/Little-Hammer-Sean-Flynt/dp/B08XN7HZ82/ref=sr_1_1?dchild=1&keywords=little+hammer+book&qid=1627482034&sr=8-1
|
|
|
|
|
Jean Thibodeau
|
| Posted: Thu 17 Jan, 2008 11:12 am Post subject: |
|
|
| Sean Flynt wrote: | | Jean Thibodeau wrote: | | The way I visualize it, is that the shaft of higher diameter beyond the reduced diameter section is more rigid than if the shaft was of the same diameter as inside the socket and is thus more rigid: So any bending force gets concentrated on the weaker part instead of being spread uniformly over the entire length of the shaft. |
That's the way I understand it, too. Now we just have to figure out why this apparently didn't bother folks whose lives depended on the stability of their weapons. |
Well, if slopping or radiusing the cut can mean the stress being only being 1.1 instead of 1.7 more it may well be to small to matter if the shaft is sufficiently overbuilt and knowing were the head might snap might be better than having it let go anywhere ? Minor advantage maybe ?
The smaller the step combined with a very gradual transition + thinning of the edge of the socket might mean an almost theoretical weakening to small to matter ?
You can easily give up your freedom. You have to fight hard to get it back!
|
|
|
|
|
Sean Flynt
|
| Posted: Thu 17 Jan, 2008 11:38 am Post subject: |
|
|
| Jean Thibodeau wrote: | | The smaller the step combined with a very gradual transition + thinning of the edge of the socket might mean an almost theoretical weakening to small to matter ? |
That sounds right. I think the difference is largely academic, given the diameter of the haft and the normal stresses expected in combat use, especially if the wood in question is ash rather than poplar. For all we know, a medieval hafter might say, "you can use a step with ash or oak, but not softer hardwoods or soft woods". No doubt they were extremely familiar with the variables of different materials and construction methods.
-Sean
Author of the Little Hammer novel
https://www.amazon.com/Little-Hammer-Sean-Flynt/dp/B08XN7HZ82/ref=sr_1_1?dchild=1&keywords=little+hammer+book&qid=1627482034&sr=8-1
|
|
|
|
|
|
Peter Johnsson
Industry Professional
|
| Posted: Thu 17 Jan, 2008 11:43 am Post subject: |
|
|
In those cases where the shaft remains it seems a neat square cut was the norm. A good fit between socket and spear head and a socket the is not overbuilt (with a wall that is too thick), provides a reliable combination. At least it does not seem that the cut was problematic for our ancestors: they kept doing it over the centuries.
I would think that the fit between socket and shaft is the most critical. If there is any wiggle room, ever so slightly, the head will act as a lever on the shaft. If there is a inlet cut, that does not sit neatly, there will then be a pinching effect together with the leverage: snap!
I would recommend using soot in finding out the perfect fit between head and shaft. Soot the inside of the socket and find out where the high spots are. Take away *only* the high spots and refit to find the next high spots. Repeat until head sits tight and straight.
Attached are some spear heads and shafts from the Nydam find. Late roman era.
Note the neat decorative knot work cut into the wood: this was before such decorative features were added to the socket itself with silver and niello. All these shafts have a sharp 90 degree inlet cut to make the socket flush with the shaft.
 Attachment: 69.53 KB
|
|
|
|
|
Sean Flynt
|
| Posted: Thu 17 Jan, 2008 12:16 pm Post subject: |
|
|
| Peter Johnsson wrote: | In those cases where the shaft remains it seems a neat square cut was the norm. A good fit between socket and spear head and a socket the is not overbuilt (with a wall that is too thick), provides a reliable combination. At least it does not seem that the cut was problematic for our ancestors: they kept doing it over the centuries.
I would think that the fit between socket and shaft is the most critical. If there is any wiggle room, ever so slightly, the head will act as a lever on the shaft. If there is a inlet cut, that does not sit neatly, there will then be a pinching effect together with the leverage: snap!
I would recommend using soot in finding out the perfect fit between head and shaft. Soot the inside of the socket and find out where the high spots are. Take away *only* the high spots and refit to find the next high spots. Repeat until head sits tight and straight.
Attached are some spear heads and shafts from the Nydam find. Late roman era.
Note the neat decorative knot work cut into the wood: this was before such decorative features were added to the socket itself with silver and niello. All these shafts have a sharp 90 degree inlet cut to make the socket flush with the shaft. |
Perfect! Thanks, Peter!
I will second Peter's comment about a tight fit between the haft and inside of the socket. It's all too easy to reduce the haft to a shape that fits tightly only at the base of the socket, leaving the head room to move at its upper end. The fit should be tight all the way to the tip of the haft. I use the soot method, although in my experience the inside of the socket is grimy enough that no further grime needs to be added. I just keep taking down the wood and trying on the head, then taking off more wood according to the marks left by the socket grime. When I can push the head down, using only my hands, to within about 3/16" of the step I stab the spear into a stump to seat the head firmly. Then I check to make sure the head is sitting equally flush against the step all the way around, using a craft knife blade to cut away a thin sliver here and there as needed. Then another stab into the stump, followed by drilling and riveting.
-Sean
Author of the Little Hammer novel
https://www.amazon.com/Little-Hammer-Sean-Flynt/dp/B08XN7HZ82/ref=sr_1_1?dchild=1&keywords=little+hammer+book&qid=1627482034&sr=8-1
|
|
|
|
|
|
Jeroen Zuiderwijk
Industry Professional
|
| Posted: Thu 17 Jan, 2008 2:23 pm Post subject: |
|
|
| Sean Flynt wrote: | | I'm not yet convinced that a 1/8" step down from a 1.25" spear haft poses significant risk to the haft. I sure would love to get a close look at the sockets of unmounted historical heads now that I know what I would want to look for (a very thin edge quickly sloping up inside to a thick socket wall). |
Well, I've got a broken shovel shaft here (ash), with a 3.7cm haft, and a 2mm step. It snapped clean at the step.
| Quote: | | As for thinning the edge of the socket--there might be more danger in this than in a straight step because a too-thin wall could be deformed or torn around the rivet. There is also the case of the spear shown below, which clearly was intended to withstand tremendous stress in the socket walls around its rivet. Now, this is a replacement haft, but one made or at least approved by the Royal Armouries, Leeds. It's flush mounted, with what must be a very thick socket and little, if any, thinned edge. |
The rivet doesn't do anything strengthwise. All the force of the thrust gets directed into the wood. The only thing the rivet does is prevent the spearhead from falling off. There are examples were just a wooden dowel was used to keep the spearhead onto the haft.
Thanks for the examples Peter! Hmm... odd though. Seems like they did particularly chose for a weaker construction. It's not the first time though. Like most swords, which have a large step between blade and tang, often with sharp corners. Perhaps it's just strong enough with or without sharp step, that it doesn't matter.
|
|
|
|
|
Gavin Kisebach
|
| Posted: Thu 17 Jan, 2008 3:22 pm Post subject: |
|
|
I've had shovels break at the same point, right at the step. Of course correlation is not causation; I still suspect that the slight gap between the step and the socket breaks first because it is the narrowest point in the haft that isn't encased in metal.
It could be that the ancients weren't as concerned about a spear breaking, or that they (unlike most of us) changed out a haft when they recognized the signs of fatigue. I certainly wouldn't know what to look for, but I know when my car is having an issue.
Another mitigating factor is proper usage. If a spear is only for thrusting, then the forces on the haft should be mostly compression, not shear. A glancing blow is still almost entirely compression. Shear force would only occur if you slash with the spear, or if the spear is trapped. The spears that Peter shared don't look lik e slashing spears at all.
|
|
 |
|
M. Eversberg II
|
| Posted: Thu 17 Jan, 2008 8:58 pm Post subject: |
|
|
Agreed. I doubt we're having the head work as much of a lever. Besides, wood was the cheap resource then (which is why spears where so economic I'd say), and I'm sure the haft was far far cheaper than the head.
Also, where do you all normally get your shafts? I'm contemplating making up a few of these spears and selling them at a small mark-up here for members who cannot make them themselves. I've found ash dowels that are the correct 1 1/8" diameter, but none that where the recommended 6' long.
M.
This space for rent or lease.
|
|
   |
|
Gavin Kisebach
|
| Posted: Thu 17 Jan, 2008 10:26 pm Post subject: |
|
|
Augh, don't get me started on hafts, it's an entire issue unto itself.
Arms and Armor offers ash poles, but the shipping is painful. Cold Steel used to offer 7 foot waxwood staffs, but I don't know if they still do.
Alternately you can make your own. For my danish axe I'm buying oak railing and then shaving it down to size; this is the cheapest way I know of to get oak long stock. Hardware stores might sell oak dowells, but invariably they are no more than about three feet long. I tried googling oak closet rods, and hardwood closet rods, but to no avail.
|
|
|
|
|
M. Eversberg II
|
| Posted: Thu 17 Jan, 2008 10:47 pm Post subject: |
|
|
 Problem (sort of) solved!
http://www.arms-n-armor.com/pole118.html
Though, shipping = price of item in this case D:
So if I where to make one for sale, it would be in excess of 100USD :|
M.
This space for rent or lease.
|
|
|
|
|
Gavin Kisebach
|
| Posted: Thu 17 Jan, 2008 10:52 pm Post subject: |
|
|
|
I think the way to deal with shipping is to see if you can ship three or four at once and save money. If polearms are your thing then it will pay off in the long run.
|
|
|
|
|
J F. McBrayer
|
| Posted: Fri 18 Jan, 2008 5:49 am Post subject: |
|
|
|
You could try this site. A bit cheaper, but I don't know about shipping.
|
|
|
|
|
Sean Flynt
|
| Posted: Fri 18 Jan, 2008 6:13 am Post subject: |
|
|
| Jeroen Zuiderwijk wrote: | | The rivet doesn't do anything strengthwise. All the force of the thrust gets directed into the wood. The only thing the rivet does is prevent the spearhead from falling off. There are examples were just a wooden dowel was used to keep the spearhead onto the haft. |
Yes, but I referred to the specific spear shown, whose rivet is a robust hook. My point was that this particular rivet and socket wall were expected to withstand a great deal more stress than the typical rivet, and therefore the socket wall could not be thinned without compromising the weapon.
-Sean
Author of the Little Hammer novel
https://www.amazon.com/Little-Hammer-Sean-Flynt/dp/B08XN7HZ82/ref=sr_1_1?dchild=1&keywords=little+hammer+book&qid=1627482034&sr=8-1
|
|
|
|
|
|
Jeroen Zuiderwijk
Industry Professional
|
| Posted: Fri 18 Jan, 2008 11:08 am Post subject: |
|
|
| Gavin Kisebach wrote: | | Another mitigating factor is proper usage. If a spear is only for thrusting, then the forces on the haft should be mostly compression, not shear. A glancing blow is still almost entirely compression. Shear force would only occur if you slash with the spear, or if the spear is trapped. The spears that Peter shared don't look lik e slashing spears at all. |
There are a lot of ways the spear can get a sever sideload. One of them if the spear strikes a hard surface (shield, armour) at an angle. Another is when the spear gets deflected by a shield or other spear. Naturally there's also a big difference between a javelin and a handheld spear. If the javelin breaks, it either has already it its target, or missed. In the first case, no bother if it's broken. In the second, if it's broken it can't be thrown back, so even better!
|
|
|
|
|
|
Jeroen Zuiderwijk
Industry Professional
|
| Posted: Fri 18 Jan, 2008 11:22 am Post subject: |
|
|
| Sean Flynt wrote: | | Jeroen Zuiderwijk wrote: | | The rivet doesn't do anything strengthwise. All the force of the thrust gets directed into the wood. The only thing the rivet does is prevent the spearhead from falling off. There are examples were just a wooden dowel was used to keep the spearhead onto the haft. |
Yes, but I referred to the specific spear shown, whose rivet is a robust hook. My point was that this particular rivet and socket wall were expected to withstand a great deal more stress than the typical rivet, and therefore the socket wall could not be thinned without compromising the weapon. |
Ah, I get ya. In that case I'd agree.
|
|
|
|
|
M. Eversberg II
|
| Posted: Fri 18 Jan, 2008 1:23 pm Post subject: |
|
|
Wait, if the interior of the socket is conical, why on earth are we stepping it anyways?
M.
This space for rent or lease.
|
|
|
|
|
Jean Thibodeau
|
| Posted: Fri 18 Jan, 2008 1:53 pm Post subject: |
|
|
| M. Eversberg II wrote: | Wait, if the interior of the socket is conical, why on earth are we stepping it anyways?
M. |
I would compare it to a secondary bevel on a sword blade: The bevel on the inside of the socket would thin the wall only back a fraction of an inch into the main " bevel " or in this case conical taper.
The wall thickness of the socket would not be affected over most of it's length and this is important if one was concerned about weakening the socket by thinning the socket wall too much. ( At least this is what I imagine is the case ).
Oh, one could taper the end of the shaft at the same angle as the socket and have it continue to taper up to the full diameter of the shaft that would have a bigger diameter than the diameter of the socket opening: This would be another way to have a stout shaft instead of cutting a step to a higher diameter. One could also make the socket longer until the full diameter of the shaft is reached: But then we are back to a non-stepped shaft but of bigger diameter. ( Sort of back to the beginning.   ).
You can easily give up your freedom. You have to fight hard to get it back!
|
|
|
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
|
All contents © Copyright 2003-2024 myArmoury.com — All rights reserved
Discussion forums powered by phpBB © The phpBB Group
Switch to the Basic Low-bandwidth Version of the forum
|
This is a standard topic