They probably don't fit very well when you reassemble them. I would use epoxy. They make a material that is supposed to permanently swell the wood, but I haven't tried it.
If the ones you are reapiring are the ones that are popping out, then you may not be getting glue to wood contact. This can occur for 2 reasons:
1 - Either there is a lot of glue left on the rung and/or in the hole (wood glue won't adhere very well to old glue) or
2 - The rung has shrunk so much that it isn't making good contact with the other surface. Elmer's woodworker is not made to fill gaps.
Try this:
1 - Sand both the dowel end of the rung and the inside of the hole down to bare wood.
2 - Use a thin kerf saw and cut a groove in the dowel end of the rung.
3 - Insert a shim in the groove to expand the dowel end enough to get a snug fit.
4 - Apply glue, reassemble and clamp lightly.
If the joints you're gluing are what's coming apart, it's probably because you didn't clean the rungs or holes well enough. Glue-to-glue adhesion may not work very well - you want bare wood.
Lee Valley sells a glue called "chair doctor". While I've not used it, it should work fairly well because it's designed specifically for this. It's a somewhat thinned wood glue with a syringe cap. You don't have to take the joint apart... Stick the needle in the gap, and squirt.
I've used them for holding screws in stripped out holes; worked pretty well. Oddly I haven't needed them in 10 years; I must have gotten more careful!
Titebond and Elmer's woodworking glues are the standard glues for wood and very good brands. The wood must be clean and free from finish and old glue for the joint to bond well. Use a web clamp and allow the glue to fully cure, undisturbed, for 2 days before use. You might consider a "fox joint" on the end of a rung which swells and the rung is driven into the mortise. More often, chairs are mass-produced and not made properly. Very dry weather can make dowels shrink; high humidity is equally bad.
It has been about 12 years ago, but when I moved into my current home I had several chairs (purchased in the 1920's, that were a little loose. I bought a product at my local hardware store intended for that use. I followed the instructions totally and I have not had one of the loosen up yet. The key I believe is in the prep work. Remove all the existing glue (without removing any wood, and use a product designed for the job. Not just any great wood glue will work on this job, you need the proper one. It is not just strength that is needed.
Or if you'll like to get fancy, try this variation:
Make the shim wedge-shaped, so that if you were to fully drive it into the groove, the dowel end would be a little bigger than a snug fit. Then put the wedge in the mortise (hole) and start inserting the tenon (dowel), with the groove in the tenon lined up with the wedge. As you get close to bottoming out the tenon in the mortise, the wedge will expand the tenon to fit tightly against the mortise.
You'll have to use a mallet or clamp to get the tenon to fully seat. Just be sure not to make the wedge so big that you split the piece of wood with the mortise. Also, you should arrange the wedge so that it is perpendicular to the grain of the mortised piece, to reduce splitting.
My answer on my chairs. Pop the rung out. From the inside of the hole, drill through with a 1/8 inch bit. Glue, and put back together. Drill in through the hole you made, drill into the end of the rung. Thread in a drywall screw. That assists the glue to help keep things together.
Gross, yeah. But the only people who notice are obsessive compulsive carpenters, and drunks laying on the floor.
Harbor Freights has it but much cheaper. I've used it on stripped bolts and screws - steel to steel and steel to concrete - provided much added pullout force for such a cheap and quick fix.
I guess I should have been more specific... when I think shim, I think wedge shaped. I'm not sure how you could expand a a dowel end without damaging it with flat piece of stock.
After reading many of the replies, I would like to suggest that this is a special problem. Using the "strongest" glues is not a fix all solution. The grain direction, shrinking and swelling etc of the joint calls for a flexible glue and that is often not the "strongest" glue. That is why I suggest looking for a product designed for this type of joint and not just choosing an otherwise great glue for this job.
In the old days horse glue was used but than it was rare to have 300 pound plus person putting unnecessary stresses on old furniture that was designed for a 150 pounder.
re: Discussion subject changed to "GLUEING CHAIR RUNGS Special note" by Joseph Meehan
Just curious as to why you feel this response is "special" enough to warrant a change in the subject line. While there is certainly nothing wrong with it, it doesn't appear any more special than the other responses in this thread.
I changed the heading because I believe my response was a response to many of the responses and not a response to the original question. Also I did not want to respond to all those responses specifying one or another kind of adhesives.
I would like to that this opportunity to say that most of those responses I replied to offered suggestions that should work and should last for a long time. I would still suggest that while their suggested materials may be sufficient much of the time, the use of the "right" material should work longer and under more conditions.
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