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View Full Version : Tenon with vs w/o a shoulder? Any strength differences?



Rick Cicciarelli
01-04-2012, 9:44 AM
In the design stages of my workbench build and I am looking at the stretchers for the legs. Is there any strength difference
between doing a tenon with shoulders vs without shoulders? I am thinking of any wracking in the legs. I would think any kind of
double pinned tenon should minimize wracking, but would shoulders add rigidity or are shouldered tenons just more aesthetic?

Rick Cicciarelli
01-04-2012, 11:06 AM
anybody? 42 views so far and no one has some input? I figured it was a silly question that exposed my lack of experience. Apparently not. I look forward to hearing what folks have to say.....

David Kumm
01-04-2012, 11:07 AM
If you size the tenons to fit well you will be fine. The strength difference is only relevant if set the house on the bench. Schwarz' Roubo design doesn't put shoulders on the stretchers either. Dave

Jamie Buxton
01-04-2012, 11:22 AM
If you really want high resistance to wracking, don't use a standard stretcher. The standard horizontal stretcher forms a rectangle with the legs and tops. A rectangle is fairly easy to wrack. Instead, use bracing on the leg to make a triangle. The triangle is nearly impossible to wrack.

Carl Beckett
01-04-2012, 11:33 AM
No opinions you say??? Thems fighten words on here.

;)

OK, a couple thoughts - although Im not sure I understand the design situation exactly, that wont stop me from voicing an opinion :D

I would be concerned about the 'moment' that is created across the joint from top to bottom. By having a shoulder, you have the full width of the board in contact with the face of the leg. Thus the resisting moment is that full width.

But by putting that board into a mortise slot, the top and bottom edges, if rotation force applied, will need to come in contact with the top/bottom edges of the mortise slot itself. Which you wont be able to get nearly as tight as you would pulling a shoulder down tight.

Now it might not matter, if the length of the tenon was longer than the height of the stretcher. Because then this moment arm would be longer than the shoulder moment arm (maybe to accomplish this it would be a lot closer to a bridle joint)

There you have it - my first thoughts - a shoulder is likely more resistant to rotation (Will have to draw a free body diagram to explain this more clearly)

Bill Haumann
01-04-2012, 11:49 AM
I've always heard that shoulders add strength to the joint. I believe there was an article in FWW or PWW some years back testing various mortise and tenon configurations. If you want to maximize the strength of the joint on a large piece like a workbench you also might want to consider a double tenon.

David Kumm
01-04-2012, 12:02 PM
While a shoulder will add strength, I am assuming you are looking at a tenon approx 1.25" thick x3-5" wide x 2-3" deep. If you glue and drawbore the joint it will take a collision of the planets to rack that joint with or without a shoulder. Dave

Stephen Cherry
01-04-2012, 12:07 PM
I had a bench base made from 4x4s for the ends, with old 2x8s for front and back stretchers (chopped it up a little to make space, but could rebuild in a short time). The tenons were the full width of the 2x8s, with top and bottom shoulders cut down maybe 1 inch each. So the tenon itself was about 1.5 (nominally 2) by 6. I had a wedge about a half inch from both top and bottom, pounded in hard and glued.

As for strength, I don't think that strength is the concern, I think you want rigidity. Related, but different properties. Even a shakey bench can be strong, but you really want one that is not going to move. That said, the above bench was plenty rigid for my use. I really could not make it move, and the best part is it was dirt cheap. Just a few 4x4's and some recycled 2x8's. It would be my advice to build a bench or two from cheap materials, just to learn what you want. Live with it for a while, and see how you use it.

If I were going to build my bench of a lifetime, I would be looking for recycled structural lumber. It pops up on craigslist regularly.

Rick Cicciarelli
01-04-2012, 2:58 PM
@Stephen: Actually, I've put off building a bench for so long because of lack of proper tools to justify the nice lumber...that I finally decided to do just what you said...I'm just going to build it using 2x lumber from the box stores. I figure there will likely be mistakes and modifications and I'll use this as the 'test' bench. That way I can see what I like/dislike, and when I am finally ready to do it properly with some better quality wood, I'll have the experience to know what I want. I am sure ANYTHING is better than the door-sitting-on-sawhorses that I have right now. Try to run a hand plane down something clamped down to THAT setup :)

Andrew Pitonyak
01-04-2012, 3:11 PM
Looks like it provides some use

Effect of shoulders on bending moment
capacity of round mortise and tenon joints

http://www.agriculture.purdue.edu/fnr/faculty/Eckelman/pdf/fpj56%281%2982-86.pdf

Stephen Cherry
01-04-2012, 3:14 PM
something like this for the bottom:
http://ithaca.craigslist.org/mat/2729378890.html

Actually, I think a used commercial fire door is a pretty nice surface. Super flat, heavy, etc. Or used bowling alley, or a used bench. If you look, they will start presenting themselves on a regular basis.

David Kumm
01-04-2012, 3:53 PM
Don't give up the door on the horse thing. Even with a great Roubo I find the solid core door and a couple of different height horses are used at least as much if not more. Dave

glenn bradley
01-04-2012, 4:07 PM
anybody? 42 views so far and no one has some input? I figured it was a silly question that exposed my lack of experience. Apparently not. I look forward to hearing what folks have to say.....

Since the Creek's population explosion it indeed has a high percentage of 'lookers' versus contributors. The shoulders resist racking and in this sense provide more strength than non-shouldered stick.

Paul Symchych
01-04-2012, 4:16 PM
How do you scare a 10x10 by 20 foot beam?

John Coloccia
01-04-2012, 4:20 PM
IMHO, the real question is "why use a tenon?" You can simply use drawbolts with no tenon and have a very strong joint. To answer your original question, though, yes the shoulders make the joint stronger. When the tenon tries to move, the longer shoulder acts as a pivot point, reducing the mechanical advantage of the stress to move the joint.

But drawbolts are totally the way to go, IMHO.

Rick Cicciarelli
01-04-2012, 4:33 PM
@Andrew: Someone after my own heart....managed to dig up a scientific journal article for an answer to the question :D

Carl Beckett
01-04-2012, 5:56 PM
Looks like it provides some use

Effect of shoulders on bending moment
capacity of round mortise and tenon joints

http://www.agriculture.purdue.edu/fnr/faculty/Eckelman/pdf/fpj56(1)82-86.pdf


Beautiful - exactly what I was trying to explain.

But after reading the other posts - if the tenon still has a shoulder (full length on the sides of the tenon, just not at each end where it fits into the mortise), then these moments should still hold true.

Johns point about just pulling it down with a bolt is very valid - the first workbench I did was 2x4's with drawbolts, and two layers of 3/4" plywood as a top. It worked GREAT! I believe it was a Shopnote article design.

John Coloccia
01-04-2012, 6:38 PM
FWIW, I happened upon this today.

http://eberhardt.bz/GME_Wood_Land/GME_Woodworking_Stuff/0_Techniques_Information/Joinery/3_Draw_Bolt_Joint.pdf

David Kumm
01-04-2012, 7:21 PM
The roubo designs use a doubled up stretcher so you get one shoulder. Since the legs and stretchers are tied together, racking puts force against the one shoulder no matter what. The doubled up stretcher also gives more support to a sliding deadman or leg vise if used. Dave

john lampros
01-04-2012, 8:01 PM
Well , here's my 2cents, Shoulders always ad strength and every joint should be chosen for the stresses it will likely bear. The shoulder in this case transfers the stresses to the end grain of the stretcher which can bear more stress without deformation . you wouldn't put a 6" stretcher in the back of a dresser to keep it from wracking, you would probably go with a full piece back for maximum result. so anything less than that is a compromise. And we accept these compromises on behalf of asthetics, weight and ease of construction. Draw bolts are great. I use them all the time but still a compromise, anything that tightens that easily will also loosen that easily.and I have a store bought cabinet makers bench at work that falls apart twice a year ( if we forget to tighten seasonally) because of that compromise. You obviously dont need a full back (although it will never rack) but think architecturally about the stress your going to put on your bench and get creative, maybe add a stretcher up top under the top slab or whatever. By the way, a storage cabinet base can eliminate these wracking problems as they usually have full backs.