PTO Shaft to Gearbox Bolts Keep Breaking!

[IllinoisTC33oper]

Coming to this discussion late so I have not read the entire thread. Like dicktill said, can you get more pictures similar to post #89 showing the bolt fracture surfaces clearly? There are detail clues on those fracture surfaces that might help sort out what stresses are causing it. (lighting from a couple of different angles can reveal some subtleties on each of the fracture surfaces.) There appear to be differences in the way the Grade 5 bolts are failing and the way the Grade 8 bolts are failing (are these slightly different appearances consistent between all Gr5 and Gr8 failed bolts?), although the differences might not be that great. This also would not be surprising given the difference in strength between the two. There are some features on the fractures here that are suggestive of overload stresses (possible combination of bending, tension, and shear) and there are some features that are suggestive of bending fatigue. (post #36 from WI Hedgehog shows a great example of classic, "one-way" bending fatigue.) Once the fracture mode is known, can work backward from there on "how can the machine impose those loads/stresses on the broken parts?"
Initially I like IXLR8 suggestion in post #84 of using a bolt with an unthreaded body that goes all the way through the assembly, with a threaded section only long enough for assembly washers and the nut. (and, I like washers under both the head and the nut.)

 

[Trackerjoe]

1) Seems to me that if it was breaking from hitting something the bolt would be shearing in two spots. Could you be overtighening the bolt? 2) unrelated to your issue - - I think a grade 8 bolt is too strong to be used for a shear bolt.

 

[airbiscuit]

I have a 3.5 inch one in there now so we'll see on that part.
I tighten them until the head of the bolt and the nut make contact with the PTO shaft sides. So, no play but not tight at all.

I have wondered what would be better, tighten or leave a little loose?

He is not over tightening the bolt.

 

[chim]

What if the edges of the holes were VERY SLIGHTLY radiused? Not a whole lot, just enough so they wouldn't act like a bypassing shear. And why would there be a concern about having a bolt that was "too robust? There's a slip clutch on the PTO.

 

[dadohead]

Any thoughts on this one?

Mowing grass can pull more  steady power than just about anything. A couple things come to mind:

Use a longer bolt that moves the threaded portion away from the shear path.

Do you store the cutter outside? Even if the clutch is properly adjusted, rust can "stick" the plates together. I'd loosen the clutch tension some allowing it to slip just a little then re-tension. Hope this helps.

 

[wirlybird]

Have you made any witness marks on the slip clutch to confirm that it is slipping?

Maybe try loosening each tension bolt on the clutch an eighth of a turn and check it again for slippage and/or excess heat.

Yes, always do. thing is I haven't hit anything. Just mowing flat fields that are done on a regular basis.

 

[wirlybird]

Are you buying your Bolts from the same place? They may not be Grade 5 even though clearly marked on the head. There are a lot of counterfeit grade 5 bolts made in China on the market
and other than buying from a reliable vendor I don't know how to tell.

I have thought about this. Current bolts come from tractor supply.

 

[wirlybird]

Thanks for all of the suggestions and commentary. I haven't had a chance to look at things for a bit as I have been going sun-up to sundown with a break in the storms. Lots of mowing to do.
I hope to get back to this on the weekend and see if I can eliminate some of the items suggested.

 

[Too many Kubotas]

I have thought about this. Current bolts come from tractor supply.

Maybe try something from the opposite end of the spectrum?

After the hole diameter and potential clutch adjustment issues are taken care of.

 

[RayIN]

Looks like they skip M13 and go M12 -> M14

That's because 13mm=.5118".