Stress concentration in cuts...

LordFarkwad

Well-Known Member
Just thought-experimenting. I'm wondering why hinges don't often break earlier than intended, given the relatively small/flat/narrow volume over which the tension and compression forces act on the wood fibers as the stem hinges over, with the traditional arrangement of back cut and acute-angled notch apex in both the traditional and humboldt notches.

It seems like a more desirable arrangement for distributing stress along the hinge (thus heading off premature shearing/fracture) would be something as is displayed below, where the effective 'hinge' is made taller by a gap cut into the notch on the front side, and a vertical bore on the backside of the notch,
which is placed before the standard back cut is executed, coming in from the left and terminating at the vertical bore.

Traditional notch/backcut on the left, what I'm describing is on the right:



A couple of observations/questions...

Is the cut on the right anything close to a real cut? I've seen faces 'blocked' out like that (but never with the vertical bore terminating the backcut), but I don't know offhand what the purpose is of it, other than it being similar to an open-face, where the desire is to prevent interference between top and bottom of notch until after the stem has traveled through a larger arc than a 45deg humboldt/traditional would allow for (hinge acting longer for control advantages, etc.,).

It seems like it is at least possible that the upper and/or lower terminals of the vertical bore cut could very well become the nucleation sites for a vertical splitting (upwards or downwards), i.e., 'barber-chairing'. The same stress risers would not be present in a normal backcut (not terminating in an intersection with a vertical bore).

Maybe it is also the case that the vast majority of tree species during most conditions are flexible enough such that the stress that exists at the focal point (really not a 'point', but a plane that runs) between the notch apex and backcut terminus is not enough to cause premature fracture. Actually, that must be the case, or else the traditional arrangement of notch/backcut wouldn't as widely practiced.

Disclaimer: this is just a thought experiment; I'm not going to do anything beyond pondering this or going out in the backyard and cutting some tulip poplar sticks up to see how they behave. I have a copy of FOGTW. I light incense and say my "hail Gerry"'s thrice daily (peace be upon him).
 
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Fivepoints

Well-Known Member
This just seems like a bad idea. I would think you would have trouble with the log sliding backwards off the stump. Be very careful if you experiment with this. This also seems like trying to solve a problem that doesn't exist.
 

LordFarkwad

Well-Known Member
This just seems like a bad idea. I would think you would have trouble with the log sliding backwards off the stump. Be very careful if you experiment with this. This also seems like trying to solve a problem that doesn't exist.
I won't be experimenting with this beyond sticks, and I've far from mastered the basics to be trying anything non-typical, so absolutely no worries there. I'm just wondering more, probably, why things are they way they are, because I know folks much more intelligent than I have either come up with the things that are, or have studied the things that are, and there are probably good reasons for all of it.

So, yeah, you might be right about butt sliding off, etc. The 'problem that doesn't exist' might be something like wood that is incredibly brittle, I don't know (like I say, just thought-experimenting).
 

LordFarkwad

Well-Known Member
Back cut should be above the backmost part of top cut of scarf like every other back cut..
Edited image to move backcuts upwards. Is that a hard-and-fast rule (or rule of thumb), Chap, that the backcut should coincide in the horizontal plane with the upper apex of the notch ('upper' in the case where you have two apexes, as in the gapped-notch configuration)?
 

Chaplain242

Well-Known Member
In most of our applications the back cut is above the apex so the spar doesn’t slide back off the stump (especially if the top gets restricted or hung up during felling, and/or when the hinge snaps when the scarf closes.

The gapped notch can be considered when felling very big timber, or when one wants a timing increase (delay snapping hinge wood) without having very large angled notches. Also better when trying to preserve the spar when logging for maximum harvest.

In urban setting it’s much more important to make safe cuts and not let the stump slide off the stump when felling especially if gets hung up.

Hence to increase safety better to back cut one inch per foot spar diameter above the high apex. Also gives you some safety margin so don’t cut straight through hinge if you over cut the back cut.
 

LordFarkwad

Well-Known Member
Loud and clear, makes a lot of sense, especially the part about the gap increasing yield - you can use a shallower angle on the notch, but still have clearance for control through a wider arc of travel.

The ~1" per foot of spar diameter is new info to me. Never heard that. Thank you for the info, @Chaplain242.
 

Hotsaw

Member
Couple things on the vertical bore that should be considered
1) Production time. That one cut adds significantly to time spent on a cut as well as time under target
2) Taking a full bar width of wood out where you need things to hold fast; I suspect if you were trying to swing a hard leaning limb that vertical bore would destabilize the needed holding wood and it would fail early
3) Punch that vertical bore in an find out you have a defect inside you may not have enough good wood left to salvage the cut and have it fall to the intended lay
4) How does one determine where to locate that bore in relation to hinge thickness? It certainly changes the dynamics of the hinge wood compared to the math used to calculate the appropriate hinge thickness for a horizontal bore cut.
The 1" per foot is solid advice. Its prob saved a few lives over the years
 

LordFarkwad

Well-Known Member
Super practical stuff there, @Hotsaw. The hinge seems like it would be more 'flimsy', perhaps leading to the destabilizing effect that you mention. Of course, in nature, things that are flexible/flimsy usually are more effective at distributing stress as well. Hmm...I suppose as in all combinations of material properties and design/architecture, there is a sweet spot - a 'just right' balance/blend of ability to distribute stress without losing the integrity required to support load. Good point.

The effect I was imagining taking place with a setup as is on the right side of the diagram seems like it would be akin to what one is doing when releasing pressure on a spring pole - by either making a series of small adjacent kerfs on the tension side, or by thinning the cross-section by shaving away material on the compression side. Effectively, the hinge is being lengthened so as to distribute the stresses in the wood over a wider area, so the fibers hold on longer and allow tension to be gradually released.

Yeah, that 1 inch per 1 foot thing is neat - I usually just make the backcut about 1 hinge thickness above the floor/apex of notch, so the hinge wood has a square cross-section when viewed from the side. That ends up coming out a little more conservative than the 1-inch-per-foot rule of thumb, I guess.
 

*useless info*

Well-Known Member
Tis truly such bait,
sorry both brain cells were already in overdrive in other knot distractions..
Slant is gradual change, squaring is more running off of cliff change
>>as far as connective stresses thru base to hinge.
.
Next consideration is if face closes earlier and at same firmness i'd think.
>>and if "inner face" closes and "outer" still traveling/at what speed etc.
>>or 'snipe' 'nose' to give back some more rotational 'clarence (important dude).
>>wrong depth to miss-match disagreement of forces could lead to split decision BC
The best machine or team as a machine ain't sh!t w/o proper timing of parts
>>NOT binding against each other, but only flowing towards target!
.
type/condition of wood taking internal stresses plays out to splitting etc.
.
rear center bore/punch i think maintains more outer fibers, for more leveraged side to side to side control
>>and assures anti-swivel patch at same time on each side too
i'm with the theory that center punch can also eliminate stiffest center fibers, relying on the younger outer
See ALL hinges in same weight/distance/angle to pivot w/o wedge/rope force additives
>>as folding forward with EQUAL forward resistance, the difference between before any close etc.
>>is in how the remaining fibers are distributed against side force influences
As hinge strength is just a response to weight/distance/angle to pivot loading
>>wedge push or rope pull TOWARDS TARGET just fake extra loading
>>giving earlier fold/thicker/stronger hinge
>>then extra loading of wedge push or rope pull releived and rest of scenario plays out w/stronger hinge, exercised in youth/birthing of hinge to stronger, before venturing out into the world.
.
i spent a lot of time drooling and humbled in front of Beranek's 7'@150' poster to know do-able
(note the Humboldt slide/delivery ramp tho..)
This is ALL engineer at a chess board , and the faithful perseverance to those targets drawn; with little room for err in this magnified power band:

.
Any time thought might be getting good, this poster putt me in my place!
Was there just coming out of john, hitting squarely in face.
In time when younger climbers came along, start running that jaw etc.
>>time in front of poster usually quelled
If not, and we will still have to listen to ya,
>>just ask that you talk about how good you are while staring at that friggin'poster..
(kinda like someone that can't do inconsistent bi-lateral motions like pat head while making circles on belly at same time)
.
While got'em off balance, point out cast on his foot, and what nose boring and placing spring boards is and upper tack on end etc. All out of my realm.
(punchline mechanix too, is about the destabilizing setup punch, and the knockout follow thru, the'ol 1-2 !)
Peace A-gain!
.
 
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Birdyman88

Well-Known Member
Take a thin strip of wood. Cut off a 1" piece of it, stand it on end, place your hand on the other end, press hard and try to crush it. Chances are it won't break, or even kick out. Now do it with a 2 foot long strip of that thim wood. The latter will probably bow and snap, as well as try to kick out of position. That thin strip of wood comprising the hinge acts pretty much the same. Longer equals weaker and less predictable, within certain parameters. Some length is required to mitigate kick back though.
 

*useless info*

Well-Known Member
Compression sine/side force presses Equal & Opposite pair further incorrectly / deforming more and more out of support column line;
whereas tension sine/side force pulls towards more correctly inline, can wrinkle buckle tho (as opposed to compression side bow buckle)as pulling to center goes to far.
.
Compression buckling from side force/not linear support column fail is a very usual compromise of device/material that tries to hold in place;
and doesn't spin/flip out to give outer visual of force, making same fight as internal war.
Length increase multipliers towards that fail.
Wood, actually very stable against said buckling;
compared to most materials, but has it's limits as any other.
.
But in any case, buckling is from a side/sine force val; generally starting in least elastic sections x distance from ends (ends as each other's pivot)of resistance to sine.
 

LordFarkwad

Well-Known Member
Take a thin strip of wood. Cut off a 1" piece of it, stand it on end, place your hand on the other end, press hard and try to crush it. Chances are it won't break, or even kick out. Now do it with a 2 foot long strip of that thim wood. The latter will probably bow and snap, as well as try to kick out of position. That thin strip of wood comprising the hinge acts pretty much the same. Longer equals weaker and less predictable, within certain parameters. Some length is required to mitigate kick back though.
Good model. Makes your point well.
 

southsoundtree

Well-Known Member
Back cut should be above the backmost part of top cut of scarf like every other back cut..
The hinge breaks free along the vertical plane.

The bottom of the hinge is low, at the bottom of the gap, not at the height of the back cut.

Full Gap face provided more hinge height for more hinge flexibility, especially useful on dead trees.

It's an extension of a sizwheel, in a way, with greater hinge flexibility.

Going higher exacerbated the trouble if a spiral grain not running 'true' with the hinge.
Easy to partially cut off the hinge, accidentally, in strong spiral-grained species.



Two horizontal cuts, and breaking out the face cut will break it along the grain, not cutting along a plant that hopefully matches up.


A full Gap face is easy on big trees, short bar.
 

Daniel

Well-Known Member
Just thought-experimenting. I'm wondering why hinges don't often break earlier than intended, given the relatively small/flat/narrow volume over which the tension and compression forces act on the wood fibers as the stem hinges over
The short answer to that question is flexibility. You can't see it happen, but you can hear it as it happens and see the resulting severed wood fibers. All that creaking and moaning as the tree begins to fall, are the wood fibers bending and tearing out. You can tell how long a hinge held by LISTENING to it on video, which is best played in slow motion.

The taller fibers on the front of the hinge (made with a plate cut or block cut) allow the fibers more movement, which tends to spread the forces of tension, trying to rip the hinge apart, across a greater amount of area within the hinge. Otherwise, these forces are concentrated on the back of the hinge, and then as the fibers at the back of the hinge break, the concentrated forces just cut like a knife, back to front. Spreading those forces out across a greater surface area makes a huge difference. You can see the tall fibers in the front of the hinge in the below video collapse a little, which allows the front to sink as the back of the hinge lifts, which spreads out the tension forces.

I don't get over here much, but glad I did for this thread. Definitely something I like to think about. I'm interested in the practical applications of these principles. Hinge dynamics could be a college course.


 

LordFarkwad

Well-Known Member
Is this what you're referring to when you say that the hinge breaks free along a vertical plane and thus terminates at the bottom of the gap? That appears to be consistent with any video I've ever shot myself or seen - with a high back-cut, the hinge actually is from the back-cut level down to the floor of the face-cut.



The hinge breaks free along the vertical plane.

The bottom of the hinge is low, at the bottom of the gap, not at the height of the back cut.

Full Gap face provided more hinge height for more hinge flexibility, especially useful on dead trees.

It's an extension of a sizwheel, in a way, with greater hinge flexibility.

Going higher exacerbated the trouble if a spiral grain not running 'true' with the hinge.
Easy to partially cut off the hinge, accidentally, in strong spiral-grained species.



Two horizontal cuts, and breaking out the face cut will break it along the grain, not cutting along a plant that hopefully matches up.


A full Gap face is easy on big trees, short bar.
 

LordFarkwad

Well-Known Member
The hinge breaks free along the vertical plane.

The bottom of the hinge is low, at the bottom of the gap, not at the height of the back cut.

Full Gap face provided more hinge height for more hinge flexibility, especially useful on dead trees.
And, this actually leads me to the conclusion that the vertical bore in this case isn't that critical, as that is in a very weak plane anyway - the consequence being that the rear vertical face of the hinge will form as necessary, just as it does on any other back-cut not having a vertical face bored into it. Yeah?
 

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