Joining tube with solid - weld or weld and pin?

GregSY · Apr 20, 2012

I want to permanently attach a solid, threaded end to a piece of tubing, as shown. The tubing is DOM 15/16" OD and 11/16" ID.

I will TIG weld the solid end in place...but is it a good idea to also drill a 5/16" hole through the pieces and press in a cross pin? I would then weld the pin in place.

This piece will see almost all loads in tension - meaning the tendency will be to pull the solid end out of the tube.

T. Jost · Apr 20, 2012

I don't see how it could hurt to add a pin to the assembly. It may not be necessary but hey, there is no shame in overkill, there is only shame in preventable failure.

HuFlungDung · Apr 20, 2012

The cross section of the stub shank will be considerably reduced drilling a 5/16" hole through it, in fact the pin will have a greater cross section than the remaining shank. I would think that the weld alone will impart the best strength joint, provided it is properly done. And the stress will be evenly distributed around the tube and joint if welded, whereas pinned connections tend to tear out right where the pin holes are. So drilling and pinning is merely sabotaging the welded joint farther up, IMO.

dave powelson · Apr 20, 2012

GregSY said:
I want to permanently attach a solid, threaded end to a piece of tubing, as shown. The tubing is DOM 15/16" OD and 11/16" ID.

I will TIG weld the solid end in place...but is it a good idea to also drill a 5/16" hole through the pieces and press in a cross pin? I would then weld the pin in place.

This piece will see almost all loads in tension - meaning the tendency will be to pull the solid end out of the tube.

The circumferential TIG groove weld can slightly cock the threaded end, creating a runout condition, unless one's careful.

2 plug welds might be an option in addition to the above weld. This would distribute stress farther into the DOM tube.

David J. · Apr 20, 2012

I'd skip the pin, I usually "fishmouth" the end of the tube recieving the insert.
Sometimes add 2-3 rosette/plug welds if not fishmouthed.
David

GregSY · Apr 20, 2012

The fishmouth idea is interesting. This part will see no rotation so the concentricity is no real issue.

As for weakening the solid part, the pin would be placed 'upstream' of the weld, so it would nothing but a positive effect on retention. Of course, it could be argued the hole-and-weld would weaken the tubing, but that's why I'm asking.

oldbrock · Apr 20, 2012

Weld only works fine on a automotive drive shaft. Make a press fit and provide weld bevels. Peter

Close Work · Apr 21, 2012

GregSY said:
I want to permanently attach a solid, threaded end to a piece of tubing, as shown. The tubing is DOM 15/16" OD and 11/16" ID.

I will TIG weld the solid end in place...but is it a good idea to also drill a 5/16" hole through the pieces and press in a cross pin? I would then weld the pin in place.

This piece will see almost all loads in tension - meaning the tendency will be to pull the solid end out of the tube.

Aircraft method is to "V" the tube to a length about twice the diameter of tube or close to the length of the solid rod, then TIG weld along the V cuts on each side of the rod and of course the end edge of the tube. Common method of attaching spherical bearing rod ends to tubes.

GregSY · Apr 21, 2012

Yeah, I think I am going to do some sort of v or fishmouth. I only have two to do so I can get fancy. Thanks for the good ideas.

Andrew-W · Apr 21, 2012

This shows how a pin can weaken a shaft.... http://www.practicalmachinist.com/vb/general/wrecked-drill-press-today-under-5-seconds-245381/

RobBinIN · Apr 21, 2012

I would pin it also. Maybe 2 times. Or at the very least use a 3/8 endmill to cut the tubing and spot weld a few times.

Ciszewski · Apr 21, 2012

Hello GregSY
No reason why a weld alone wouldn't work.

Briantodd · Apr 21, 2012

leave .008 under diam on the threaded part then silver solder the job you will never shift that joint.

magneticanomaly · Apr 21, 2012

If your welding is any good, the weld cross-section will be the same as the tube, and welding alone is the way to go, consider, as another member commented automotive driveshafts, in which i have seen many failures by breaking of the U-joints and twisting of the tubes but never a (factory) weld failure. Fishmouthing to increase total weld length will, IMHO, only increase total heat input and further anneal the parts. To the extent that more heat will slow cooling, if the parts are hardenable, it may reduce brittleness.

Silver solder is an excellent option if embrittlement of hardenable components is a possible problem. I think .004" total clearance would be better than .008, and length of joint equal to diam of plug will produce a joint stronger than the tube.

GregSY · Apr 21, 2012

Driveshafts see torsional force which is in keeping with a radial weld. This part will see tension forces.

Ciszewski · Apr 21, 2012

Hello GregSY
What kind of tension forces are you talking about? I wouldn't think it would be that much if you're only using a 0.125" wall tube. As stated above, the cross-section of the weld will be the same if not more than the tube.

HuFlungDung · Apr 21, 2012

Ok, so imagine this joint being pulled apart. As the yield point of the tube is reached, it will begin to neck down from the ring weld until it fractures. The entire cross section of the tube can be used to compute the ultimate tensile strength.

If you drill a pin hole and weld the pin, then there is no yield going on between where the ring weld is and the weld on the pin. All the stretch will occur on the free side of the pin and put each end of the pin in shear, and transfer that stress force to the stub shank, which is now much reduced in cross section because of this gigantic hole through it. So the stub yields readily because it is weak due to low cross sectional area.

GregSY · Apr 21, 2012

I don't know the forces but they are at least 'significant'. This is a strut rod for the front suspension of an automobile. All rearward motion of the front tire is prevented from happening by this rod, in tension. When you slam on the brakes and hit a big pothole at the same time, the tire and wheel wants to quickly move rearward - this rod sees all of that load and stops it from taking place.

Do I think a weld alone will work? Yes - I just want to add some safety factor if I can.

APD · Apr 21, 2012

If you are building this for someone else, I would suggest asking them how they want it built, and get it in writing.
If it does fail, or is even suspected of causing and injury, guess who they will sue?
And "All the guys on the internet said it would be strong enough" is not a good defense in court.

If its for your own project, ignore all that and have fun.

Mike C. · Apr 21, 2012

As said above, I wouldn't touch it if it was for a road going vehicle and I'd want EXACT instructions for how THEY wanted it done if it was for a race car or other off road application. If it were mine, I'd TIG weld it up solid and leave it at that. Aerobatic airframes have all welded joints and are subject to incredible stresses. Never seen one pinned.

Joining tube with solid - weld or weld and pin?

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