HELP welding a stub shaft

   / HELP welding a stub shaft #1  


Elite Member
Apr 21, 2012
Cascade Mtns of WA state
Kubota B-series & Mini Excavator
I bought this broken sickle mower thinking I could just weld on a stub shaft. But further discovery shows that this is a very highly stressed connection due to the cyclical loading of the wobble crank. Model is Massey Ferguson MF 31.

Here is a pic of my broken shaft.

Here is a pic of Flusher's (TBN) complete, working MF 31 mower. It's the adjustable sheave at right. I suspect it's adjustable so speed can be "tuned" for minimum vibration @ 540 RPM (?).


Here is a pic of a (damaged) MF 31 shaft from TBN's Barry1.

Here is a pic of 3 ways I might repair it.

A) Obtain an intact replacement shaft. This would be the best solution but it doesn't seem like there are any available. maybe all broken?

B) Find a larger tapered shaft and weld it on, with a sleeve over the weld. And find a sheave/hub that fits this tapered shaft.

C) Weld on a standard keyed shaft that is significantly larger than the original tapered keyway. And locate an adjustable sheave with a hole 1.25". I think this keyed sheave hub would have to tighten onto the shaft, there cannot be any slop whatsoever. Are there split sheave hubs that might tolerate this kind of cyclic loading?

I don't have a lathe. I have grinders and a welder, and the ability to do this kind of work if I can find the parts I need. I'd really appreciate any other suggestions, or if anybody has expertise in where to find the various parts that I'd need for any solution.


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   / HELP welding a stub shaft #2  
Perhaps a machine shop could turn another out for you.
   / HELP welding a stub shaft #3  
I would think that the shaft would for sure have to be remachined after welding to get it running true, especially given the cyclical nature of the load you mention. Is there another bearing that goes on the 1.48" diameter? Looks like a couple snap ring grooves there.
   / HELP welding a stub shaft
  • Thread Starter
Running true would be handy, but I wonder if it would be even noticable. I bet this thing is a shaker.

If I weld a stub onto the crank (similar to "C" above) and take THAT to a machine shop and turn it down to be like "A"…..and add the woodruff keyway and the threads. They would have to match the taper in the sheave. Any experienced guesses how much it might cost? If the machine shop bill is under $200 that might work out.

There is a main bearing at the ~1.48"
   / HELP welding a stub shaft #5  
I think welding would be a waste of time, try and find used gear box or shaft. if it viberates very much it will take out the gear box
   / HELP welding a stub shaft
  • Thread Starter
I would not even think of welding it if a used crank was available. All wobble cranks operating a sickle will vibrate. I suspect they will vibrate in a very violent manner. There is no gear box - the wobble crank moves the sickle back and forth.

Crafty folks do this kind of repair. As always it would be great if experience appears before I have to forge ahead on my own. (??) I'm often pleased with the clever repairs that members post on TBN!
   / HELP welding a stub shaft #7  
From what I can see it seems there is an inherent design flaw that renders all shafts to break over time. It would seem the stresses on the shaft when new are probably acceptable, but as the seemingly open bearing gets jammed with dirt the shaft takes the beating and cracks or snaps off, as would be the result in your case. Therefore if you seek a repair that will last a long time, IMHO, you would need to find a way to adapt a beefier shaft to the existing mechanism. If you take the pics and pieces of what you've shown here to a machine shop I believe they might be able to figure out a way to do just that.
Here's an idea. Think about boring the existing broken hub so that a rod with threads could be screwed into place and the mating surfaces could be welded? into place to have a 2 tier level of strengthening. I'm not a welder or machinist by any stretch of the imagination, but by thinking outside the box you could come up with an improved design to handle the flaws inherent in the original. JMHO.
   / HELP welding a stub shaft #8  
I do think that a split sheave hub with it's tapered OD would handle the loading but the key and keyway might be the weakest link. Probably would be hard to find an adjustable sheave though to fit the hub.
A "D-method", if the inboard part of the broken shaft isn't hardened would be to drill and tap it as large as possible in the area "under" the main bearing land. Then a replacement tapered end could be made with an integral threaded stud to match the tapped threads along with some Loctite. The direction of the thread (RH or LH) might best be determined by the direction of rotation so that it is self tightening. Could this be done for $200? Probably depends on if you can find a machine shop looking for work and it might be easier to just make a whole new shaft.
What is the purpose of the flats on the shaft next to the bearing land?
   / HELP welding a stub shaft #9  
Without a doubt the taper is there to prevent slippage that'd hammer any key on a taper-less shaft. (think small engine flywheel) Having a new shaft turned from 4140, 4340 etc would be my choice (PHT'd to say ~30 Rc machines easily (think gun barrel), and would combine o'all strength with toughness (threads, bearing seats, keyway), tho' Barry1's looks to be a forging).

Costly, perhaps but also may be the best result possible and IMO less likely than any welding mentioned to come undone and teach a lesson in two steps (do-overs?). btw: Matching the taper angle to hub would be 'cake' to any competent lathe hand, and all work easily done from a 'properly dimensioned sketch' if sent out.

It'd sure help to know a guy with the shop machines you don't have. (I bet many of us chip makers wish we knew more tractor guys...) I'm not so sure that a weld-up would hold up too long. Your welder should know which rod, how much stress-relieving could help that.

Not steer OT, but can I ask what OP's bar's cut width is and how/whether the mower will see use on the BX?
   / HELP welding a stub shaft
  • Thread Starter
I do think that a split sheave hub with it's tapered OD would handle the loading but the key and keyway might be the weakest link.

Thats why I'd want to increase to 1.25" diameter on the stub shaft……? Still a question a split sheave hub can handle the loading of the wobble crank.….

I don't know if it broke by fatigue or by backing into an "immovable object". Yes it looks like a forging, hard to imagine that a weld could survive where the original forging could not. The MF 31 sickle mower is 7 feet long and I'd use it on the BX24 until I get a (slightly) bigger tractor such as a B3030. I would mow weeds in a field maybe twice a year, it's not hard use and not much horsepower.

Here's a roughly dimensioned pic of Barry1's thrashed wobble crank showing where mine is broken

And a repeat picture of my mower "wobble box" looking at the broken shaft stub


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