Need to calculate Torque on a plug weld 1

[MECH-DESIGNER]

Can I get someone to point me in the right direction? Here is my problem. I have a driveshaft that is inserted into a square tube and than plug welded perpendicular to the axis. I was wondering if someone knew of a formula to help predict this(torque/stress)? Currently there is no documentation on our current process and I'd like to better document it for any future changes.

 

[3DDave]

If you know the section area of the weld and the shear strength of the material then it's going to be

shear area)*(shear strength/unit area)*radius = resistance to torque before it shears.

I would use a good size factor of safety; more if this is a cyclical load that can produce fatigue (or use the fatigue limit for the shear strength/unit area. You say a drive shaft, so that sounds like a cyclical load that will produce fatigue. It may be worse if there is also misalignment that produces bending, but a shaft coupling like a u-joint can lower that additional load.

 

[rb1957]

I think the easiest way to an allowable would be to test some, and maybe develop an allowable equation ?

Of course it'd be easier (more reliable, stronger?) to fill the hole with a thru-pin, or rivet.
Maybe upset the pin to fill the over-sized hole and shave flush both sides.

another day in paradise, or is paradise one day closer ?

 

[XL83NL]

Help predicting what exactly?

Huub
- You never get what you expect, you only get what you inspect.

 

[MECH-DESIGNER]

I'd like to change it to a pin, that would be more predictable at least. Unfortunately it is more time consuming than what's currently in place. The welding process can at some times be erratic with new personal. I was thinking that since the cross section isn't uniform being placed on the side of a shaft it would lend itself to some extra variable that would need to be calculated.

 

[dvd]

If that shaft is designed around the shear strength of the plug weld (most likely with some sort of fatigue de-rating), you might consider making it with a smaller square tube and machining flats onto the shafting round so that the torque is transferred by the flats and not the plug weld, or pin. The flatted shafting round would need to be relieved for the weld flash inside of the tube.

 

[3DDave]

I'd look at welding some small diameter rods to the part that is going into the tubing - pieces of the square tubing can be used to fixture it. This could be added for each of the four corners to carry the torque load and then the spots just keep it from falling out. I think the diameter of the filler rods should be about .171 X interior tube width or rod diameter

 

[desertfox]

Hi MECH-DESIGNER

I think a pin would be much better than a plug weld and more predictable from a design point of view. Also any heat applied to that shaft might well cause distortion of the shaft.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Scuka]

If these things need to transmit any appreciable amount of torque, they're usually also welded (fillet in your case, or V weld in case of a circular tube) on the end of the tube so you don't just rely on the (unpredictable) plug weld.

Also, considering the fact you're joining a circular thing to a rectangular thing, you're also going to get significant misalignments, which introduce more unpredictability into the system.

 

[Scuka]

You say a drive shaft, so that sounds like a cyclical load that will produce fatigue.

Not necessarily.

Shafts experience cyclical bending loads because the shaft rotates, and the external radial load direction is usually constant.

This plug weld transmits torque, but doesn't experience any bending, so if the shaft always goes in the same direction, the weld won't see any cyclical loads.
Fatigue is only of concern if the shaft changes directions regularly or if there's a significant amount of start / stop cycles.

 

[LittleInch]

Apologies for being thick here, but can you define where exactly the weld is please.

Does "plug weld" mean you weld inside that little hole welding the shaft to the square tube??

What a strange way to connect the two. A tight fitting pin would surely be much better? Or a screw in pin sized to be flush with the outside of tube?

More time consuming than a weld of dubious "erratic" reliability ?

given that the weld is straight onto shaft with curves in one direction and no real weld penetration or weld profile, then all you really have is something that stops the two sliding off each other. I wouldn't trust that to transmit any significant torque reliably. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rb1957]

yes, I took "plug weld" as filling the holes.

ok, you don't want to use a fastener. Why fit a round peg into a square hole ?

Can you make the tube a round tube, over the round rod ?
Can you make the rod from a square section (fill the square tube, machine the round where you need it) ?

can you use a larger diameter rod, and machine flats to fit the tube better, and to increase the torque resistance ?

How much torque to you need these plug welds to react ? or how much shear (the couple due to the torque) ?

another day in paradise, or is paradise one day closer ?

 

[MECH-DESIGNER]

Alright let me clarify a few questions up.
1. It can't be machined due to cost, to cut the flats are unrealistic due to it being over 132"lg and the extra mass won't be liked by our motor/drives. Our volume is too high for a machined solution.
2. This is a design with legacy, I don't like it either. The welding process reeks havoc on the shaft runout.
3. A sprocket is placed on this that requires the square profile, it goes every few inches down the shaft.
4. Currently our largest gearmotor can put out 7612 in/lb. The min dia for the sprocket is Ø2.25", tubing is 2.5"sq, rod is Ø2".

 

[TugboatEng]

Build a few samples of the joint. There is no need to build a full shaft. Test to failure. Inline dynamometers can be rented and a chain or lever hoist (also rentable) can be used to generate the force.

 

[3DDave]

OK - so is the round shaft 132 inches long and you are pushing that into a square tube that is close to 100 inches long to add a square section for the sprockets? And then there are a large number of plug welds along the length of the two?

It's like the blind men and the elephant here trying to guess what the elephant looks like.

 

[rb1957]

If you've got a design that works (legacy design) then maybe that's a way to calibrate your design eye-ball.

If you need more substantiation these days, then testing is the easiest route.

I hope I've found the elephant's trunk ...

another day in paradise, or is paradise one day closer ?

 

[Tmoose]

Do the square bore sprockets fit snuggly on the square tubing, and run directly over the welds? If not, I would be concerned that torque will tend to distort the square tube and possibly peel and pick at the weld like a scab.

Is distortion the only problem with the design? Maybe spot welding both sides at once pinching the square tube would reduce distortion and weld prep.

 

[hydtools]

Is it more complicated than this?

Ted

 

[dvd]

Machine round journals on the ends of a square shaft.

 

[MintJulep]

@hydtools
Fundamentally you have the solution.

However, the picture in the first post shows 2 holes on opposite sides, and there might be multiple instances of that section asking the length.