Strength of welds on gears
 

I don't know much about welding, but welding a gear to a shaft on only one side of it, can it take like 300 lbs of shear force at the weld? (wow that was poor grammar) Or would we have to weld it on both sides? It is a 45 T gear mated with a 14 tooth gear on a Chiaphua. The ID of the 45 tooth is .5" They are Martin steel gears and some sort of steel shaft.

I know you are going to say it depends on how good of a welder you have so I will tell you as much as I know: he is the welding instructor at a community college with about 20 yrs expreience. I'd just like to know in general terms if a weld on a single side will be good enough.

Is hot rolled or cold rolled better for this? Also, I measured the ID of the gear and the OD of the shaft it was going on and at most I could only find a +.001 difference. Will there be any problem with the gear axis not being parallel and colinear with the shaft axis?

Thanks.

Re: Strength of welds on gears
 

pulling out my ridiculously huge handbook for mechanical engineers I see that the allowable shear for a fillet weld is:

tau = 0.30 x electrode tensile strength

various other magic happens and you get:

allowable force per linear inch = 0.707 x weld size x 0.30 x electrode strength.

so... with a half in shaft, you figure the torque and diameter and all and multiply out a lot of constants and you can figure out how big the weld needs to be. I get:

torque thru shaft / (0.083291 X electrode strength) = min weld leg size

that's all in lbs and inches, by the way. So you ask the welder what kind of electrode he's using and you can figure out how big a bead he needs to make.

Electrodes are usually labelled in thousands of psi. an E60 has a strength of 60,000 psi. just tossing in some numbers... I'll assume the torque is 75 lb-in from your 300 lb figure and an E60 electrode....

I get a minimum bead size of 0.015 in. Which is ridiculously small, and took entirely too much work, but now you know that the weld should be plenty strong.

Re: Strength of welds on gears
 

i can see it looks like you are trying to use the white paper on the gearbox from andy brockway (as it seems). The best way to attach it (from my standpoint) would be to either heat shrink it on (in a kilm) or by using say liquid nitrogen to shrink the metal and let it warm back up to room temp for the metal to expand again and bind to the gear. Be forwarned through heating and cooling the metals may cause stress and strains that are not intended for the material use and may cause some problems if not done correctly.

[edit] fixed N2 error [/error]

Re: Strength of welds on gears
 

The main problem with welding a shaft to a gear isn't strength, it's distortion; you will end up with a shaft axis that's not aligned with with the gear axis, which will create very high point loads on the gears. Gears are typically strength limited at both the tooth root in bending and contact area in bearing, so you would be increasing the stress there. The weld heat affected zone is usually 2-3 times the thickness so a small pinion might have a strength problem where a larger gear wouldn't. Typical steel gear stock material is about the same strength as the weld rod (60-70 ksi tensile), but the teeth are frequently hardened or case carborized to increase their local strength.

Re: Strength of welds on gears
 

I believe welding gears onto shafts is plenty strong. Last year we welded on side of the gear to the 1/4" shaft for each stage of reduction on our chippy gearboxes. We had others gear problems, but not with the welds.
Best of luck,
Kevin Antaki
481

Re: Strength of welds on gears
 

ok... I think dez250's post was slightly confused, since liquid nitrogen shrinks metal. at any rate, that's not an option for you unless you get a new gear or shaft because your shaft already fits inside the gear. But I understand that thermal interference fits are a relatively standard thing in practice. Heat a hole up, cool down a shaft, stick'em together real quick and you're done. I'm not sure of the load carrying capabilities, though. probably depends a lot on the interference and roughness of the shaft and holes and such.

Re: Strength of welds on gears
 

What Kevin didn't mention is that I was the one welding...and if my welds stayed together you should have no problem :)

Seriously, though...I don't think you'll have any problem with the strength of the welds. And...we welded TINY gears without a problem...I thought the heat might be a problem. I really don't know about distortion, though...perhaps that will be an issue.

Eric

Re: Strength of welds on gears
 

Can anyone who has welded gears tell me about warping? Was it ever a problem? Thanks.

Re: Strength of welds on gears
 

My suggestion would be: don't weld gears to shafts in the first place. In FIRST competitions, things often break unexpectedly, and welds are very hard to disassemble if you need to replace something. Just use a keyway. It's much simpler to do, and can carry a sufficient amount of torque.

Re: Strength of welds on gears
 

I would suggest that you use a pair of nice big set screws set at 90 degrees from each other and/or key the shaft and gear, then use a keystock to secure them. But without overanalyzing the question, yes a weld will hold. I would be more worried about stripping the gears with that much force on them. What about using a pair of sprockets with #25 roller chain? With gears, you have 300lbs of shear force focused on a few metal teeth, with chain, this force is distributed along the circumference of the sprocket. But then again, I'm not an engineer.

Re: Strength of welds on gears
 

Was that the sound of Woody cringing? Set screws?

Re: Strength of welds on gears
 

... and wailing, and crying, and gnashing his teeth ...

Re: Strength of welds on gears
 

Two additional items to note when using electric stick welding on motors:

Motor has a permanent magnet. Strong electric fields from welding cables can degrade the magnetic field, high heat on the magnet will also degrade the field, as will a mechanical shock (dropping motor).

Never allow the current to pass through the motor bearing. (ouch!)

Re: Strength of welds on gears
 

Basically, dont weld with the motors on the gearboxes (*gasp* you mean someone would actually do this? :ahh: )

Cory

Re: Strength of welds on gears
 

ok first off...welding is a good idea, but you are almost better off getting a keyed shaft. if you need to move or fix somthing with the gear and shaft you can easily take it off and reset it. using keyed shaft is just as good as welding. also if you are looking for tensil strength and other good information on metals,plastics, ect... go here http://www.matweb.com/search/SearchSubcat.asp. and yes DON'T use set screws very bad.