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gzig5
08-07-2017, 10:19 AM
A friend mentioned that the rear axle on his kid's go-kart broke, so I offered to take a look at it. Basically it is a 1" shaft reduced to 3/4" for 5" on each end, where the wheels mount. It broke at the 3/4>1" step. The smaller diameter has a key running all the way to the shoulder and on the side it broke on, there is a 1/4" keyway from the shoulder extending about 12". This 1/4" keyway has to extend to the end of the 1" section because the drive hub for the sprocket has an integral key. Clear as mud?

It would probably be faster to whittle a new axle from a new piece of material but I thought it would be fun to try and save the old one. So my initial reaction was that I would just bore the broken end 2" deep at .75" and press in and weld a new stub and then cross drill and pin the stub in with a 1/4" dowel to give some additional strength.. My concern is that I wont be able to maintain much of a weld fillet because the wheel hub tightens against that 1" shoulder. Also, that 1/4" keyway bottoms out at about where the counter bore diameter will be, leaving not much wall thickness. Reducing the diameter of the stub provides clearance to the keyway, but reduces strength. I'm sure the axle will see a lot of stress and I'd like to give it a fighting chance.

Another variation would be to make the stub from 1" material, leaving a full diameter collar about .5" wide between the full .75 section and the press fit section. This allows me to weld on the 1" diameter away from the shoulder and then re-cut the 1/4" key. I know this would be stronger but it is more work to turn down the stub from 1" material. I hesitate to put too much work into it because I'm doing it as a favor and the axle itself is not of good quality in general so may fail in another area.

Would the first method be sufficient? Any options I'm missing?
I'll post a pic if I can figure out how to have the site host it. >>>edit. Pic added

http://bbs.homeshopmachinist.net/attachment.php?attachmentid=1668&d=1502122988

ken
08-07-2017, 10:43 AM
http://azusaparts.com/Category.aspx?catID=6&subCatID=19&ssCatID=30

http://www.bmikarts.com/Axles-Key-Stocks_c_193.html

Mike279
08-07-2017, 12:44 PM
Go cart axles are no joke. The no suspension means they take a wicked beating. I made a couple before I realized how much I needed to buy a piece of expensive alloy and do it right. My kids cart had an eight horse motor and big soft tires but it would shake the crap out of you going across a fairly smooth lawn. They loved it. Mike

krutch
08-07-2017, 02:31 PM
Personally, I'd make a new shaft and taper from the 1" to 3/4" if possible.

Willy
08-07-2017, 03:23 PM
Personally, I'd make a new shaft and taper from the 1" to 3/4" if possible.

This, or at least a large radius where the shaft is reduced in diameter on both large to small and small to large junctures. Looking at the shaft it appears this is where the stress concentration zone initiated the fracture. A lot of stress on that axle, the sharp edge only concentrates and focuses that stress on very small area.




http://slideplayer.com/slide/4644806/15/images/9/Figure+7-9.jpg

cameron
08-07-2017, 03:25 PM
That axle looks like it was designed to demonstrate fatigue failure.

tom_d
08-07-2017, 03:40 PM
Would it be possible to swap out the wheels for ones with a 1" bore, and then just leave the shaft as a single diameter?

gzig5
08-07-2017, 04:04 PM
Personally, I'd make a new shaft and taper from the 1" to 3/4" if possible.

Taper is not possible, need the shoulder. With an appropriate radius of course.

Wheels are the wheels, and actually pretty nice. No reason to change them.

I know there are plenty of options to buy and modify or make from scratch, but in the near term I'd appreciate comments on my original question. How best to repair the existing shaft?

This particular axle is not the original but has lasted through some good use. Looks like some 1" shafting was re-purposed. The quality of the slotting and stub dimensions are horrible but that should not affect the ability to repair it.

Norman Bain
08-07-2017, 05:56 PM
I have never done this; but you could try friction welding. Of course do a few test pieces first. Basically hold the long shaft in the chuck and the 3/4" portion in your tailstock; spin up the lathe and bring them together.

From the YouTube videos I have watched that have successful results in home lathe setups the users put a taper on both pieces first so as to get the heat going early in the touch.

https://www.youtube.com/watch?v=FRDQwP4JcNA

If your tailstock holding chuck cannot handle the 3/4" portion then perhaps lathe up a new end for that with a small tang that fits your tailstock chuck.

Bob Engelhardt
08-07-2017, 07:31 PM
...
http://slideplayer.com/slide/4644806/15/images/9/Figure+7-9.jpg

Very interesting - I'd never seen any of them. "c" is puzzling - I can't see a radius at all, especially not one "into the small diameter". Maybe the drawing is flawed. I can see how a filet radius to match the bearing's "corner" radius would work.

Thanks for posting.

Bob

Video Man
08-07-2017, 09:12 PM
Seems like a lot of work for what would likely be a compromised repair. It's not what you asked, my 2c., but I'd take a nice length of 1144 and turn the stubs and cut new keyways and be confident it was a safe repair....

Erich
08-07-2017, 10:29 PM
I would remake the shaft from scratch from a length of flexproof shafting.

If I absolutely had to repair what is there, I would cut the shaft off several inches inboard of the sprocket.
Put a big V chamfer on the end of the shaft and a 1/4 inch or so counterbore.

The I would make a new end piece with the stepped shaft and a 1/4 inch pin and a matching big V chamfer.
This part I would turn on centers and make in one setup.

Press the new piece into the end of the existing shaft and weld up the big chamfer in several passes, ideally with a motorized rotation fixture.

Put it in V blocks in a press and bend it until you get the runout down to 0.005 inches or so.

Last step is to cut the keyways in the mill.

akajun
08-08-2017, 08:36 AM
Taper is not possible, need the shoulder. With an appropriate radius of course.

Wheels are the wheels, and actually pretty nice. No reason to change them.

I know there are plenty of options to buy and modify or make from scratch, but in the near term I'd appreciate comments on my original question. How best to repair the existing shaft?

This particular axle is not the original but has lasted through some good use. Looks like some 1" shafting was re-purposed. The quality of the slotting and stub dimensions are horrible but that should not affect the ability to repair it.

How much do you value a child's life? When your repair breaks when the kids at speed and he's not wearing a seatbelt or helmet how proud will you be of your work when he's in the hospital or a casket? Either buy or make a new shaft of proper material.

ezduzit
08-08-2017, 02:40 PM
...buy or make a new shaft of proper material.

This.

gzig5
08-08-2017, 02:48 PM
So, I've been beat into submission. We are going to make a new shaft. I couldn't convince myself that any of the repair methods would hold up for very long.

So what would be the best material? I have Speedy Metals local and in 1" they carry 4140 Cold Rolled annealed or HT, 8620, 41L40 Cold Annealed, 1144, 1045, 1117, and a couple lower grades of steel.

I need to turn down to 3/4" for 5" on each side, thread the ends for a nut, and cut 3/16 and 1/4" key ways. 4140 HT seems to be the strongest and should turn and thread OK? Maybe 8620?

Zahnrad Kopf
08-08-2017, 03:49 PM
If it were me, I'd be using 4140 or 4340. HT&T, too. I probably missed it, but how long a piece do you need? I may have some here.

EDIT -

Holy Fire Sale, Batman! For the money, I honestly would not bother if I were in your shoes, having just seen what they charge for completed axles and knowing what Speedy charges for 1144TG&P. ( for comparison ) You can buy an axle already DONE for $50-ish. 4 foot of 1144 TG&P is $45 at Speedy. And then you still have to machine the ends, the threads, and the keyways. Unless you are looking for an excuse to use electricity and gasoline, buy the axle and use the time you save to enjoy running the cart. (!)

wdtom44
08-08-2017, 04:53 PM
The pre made shaft may be nothing special, for the price I would suspect it wasn't. If you get 4140 or some such you will know you have good steel.

tom_d
08-08-2017, 05:20 PM
I agree that repairing is not the best option here. As far as replacement material:

http://www.aircraftspruce.com/catalog/mepages/4130rod.php

4130 is used extensively in aircraft construction because of its mechanical and welding characteristics. It might be a good choice in your application if you find the need to weld a sprocket or wheel hub onto the shaft.

gzig5
08-09-2017, 11:00 AM
If it were me, I'd be using 4140 or 4340. HT&T, too. I probably missed it, but how long a piece do you need? I may have some here.

EDIT -

Holy Fire Sale, Batman! For the money, I honestly would not bother if I were in your shoes, having just seen what they charge for completed axles and knowing what Speedy charges for 1144TG&P. ( for comparison ) You can buy an axle already DONE for $50-ish. 4 foot of 1144 TG&P is $45 at Speedy. And then you still have to machine the ends, the threads, and the keyways. Unless you are looking for an excuse to use electricity and gasoline, buy the axle and use the time you save to enjoy running the cart. (!)

I've not been able to find a pre-made axle that wouldn't need to be modified due to the length of the ends. Plus I'd have probably at least $15 for shipping. I don't have it in front of me but I think it is 40" overall. They are asking $50+ for one that size that I'd have to modify.

I'm pretty sure that they are not using TG&P shafting for these. I know the old one isn't. I was planning on using standard cold rolled finish, which is typically 1-2 thou undersize but the pillow block bearings are pretty tolerant of that. The commercial ones seem to be available in "standard steel" and "Flexpruf", both are apparently technical terms created by the karting industry but there are no specs . Speedy has 4140 HT TGP $55 for 4', which is probably overkill for this app. The 4140 cold roll HT is $33 for 4', which is what I think I'm going to go with.

Willy
08-09-2017, 01:08 PM
4140 is probably is a bit overkill for a kids low hp gokart, but if the price and convenience of using it is ok with you and your friend then use it.
You certainly don't have to worry about it being suitable or not.

Just for yucks here's a quick reference pic of commonly used automotive drive axle shaft materials, both production and high performance.
1040 and 1050 is probably the most commonly used drive axle material used in cars and trucks, just for reference.

http://image.fourwheeler.com/f/39363358+w600+cr1/131_1210_15%2Baxleshaft_facts%2Bapproximate_materi al_strength_ratings

HWooldridge
08-09-2017, 04:27 PM
So, I've been beat into submission. We are going to make a new shaft. I couldn't convince myself that any of the repair methods would hold up for very long.

So what would be the best material? I have Speedy Metals local and in 1" they carry 4140 Cold Rolled annealed or HT, 8620, 41L40 Cold Annealed, 1144, 1045, 1117, and a couple lower grades of steel.

I need to turn down to 3/4" for 5" on each side, thread the ends for a nut, and cut 3/16 and 1/4" key ways. 4140 HT seems to be the strongest and should turn and thread OK? Maybe 8620?

Obviously, the rider didn't die the first time it broke so a repair still might be a valid consideration. I once repaired the end of a disc harrow shaft that had broken in similar fashion by machining the end details on a longer repair stub so the original break was moved behind the bearing. I drilled a pilot hole in the original shaft, machined the end of the stub for a press fit and left a wide enough root which I could fill in with weld metal.

Putting the splice inboard of the bearing puts the stress back onto a solid piece of shaft. You just have to make sure the piece remains concentric so it doesn't wobble after assembly.

gzig5
08-09-2017, 10:41 PM
I considered that as well, but the more I looked at the poor machine work on the axle, it just started making more sense to start from scratch. This thing has some pretty big turf tires on it and no differential so the axle sees quite a bit of torque during a turn plus the rest of the beating and banging.