PTO Shaft to Gearbox Bolts Keep Breaking!

[Teachu2]

What are you torguing the bolts to? My guess - and I confess I didn't read all 12 pages here - is that they were over-torqued and stretched. The breakage point is consistent with that. I'd try 40 ft/lbs and use nylock nuts. I'm not a pro, but I've broken a few bolts over the years.....

 

[ning]

Can you buy 13mm bolts? They aren't listed on the charts.

Looks like they skip M13 and go M12 -> M14

 

[Too many Kubotas]

The bolt should have the shoulder as near the outer edge of the collar as possible and still be able to tighten the nut.

Excess bolt length is easily taken care of with a spacer.

 

[ruffdog]

What are you torguing the bolts to? My guess - and I confess I didn't read all 12 pages here - is that they were over-torqued and stretched. The breakage point is consistent with that. I'd try 40 ft/lbs and use nylock nuts. I'm not a pro, but I've broken a few bolts over the years.....

Post #39.....he is not over tightening the bolt.

 

[airbiscuit]

What I have been doing is to snug the bolt/nut down so the bolt doesn't slide in its hole.

What are you torguing the bolts to? My guess - and I confess I didn't read all 12 pages here - is that they were over-torqued and stretched. The breakage point is consistent with that. I'd try 40 ft/lbs and use nylock nuts. I'm not a pro, but I've broken a few bolts over the years.....

Post #39.....he is not over tightening the bolt.

BTW, I use a lock washer on my shear pin.

 

[RayIN]

My opinion (retired machinist/millwright). The one bolt failure is definitely cyclic.
I think the problem is loose hub to shaft fit and loose bolt fit. Driveline shafts have an acceleration/deceleration event twice every revolution. This is magnified as driveline angles increase. I believe the bolt is being hammered into submission due to the loose fits. If it were mine I would drill the bolt fit out to 9/16" (if that will give a good fit), clean the shaft and hub bore and assemble with Loctite 660. As others have mentioned use a bolt long enough to get the threads out of the shear zone even though this bolt isn't intended to be a shear bolt.

The shaft on my King Kutter gearbox is hardened, no increasing bolt hole size. I suspect other brands also have a hardened shaft. A simple hit with a file will prove hardness.
That reminds me; I have a pocket hardness tester somewhere in one of my bench toolboxes. I'll have to find it and test my gearbox shaft.

 

[Stevehogan]

I run a 5 foot brush hog on my JD 3032E. I have used this cutter for years for lighter duty work. Mostly just cutting field grasses on nice flat areas.
I do run a slip clutch PTO shaft. Clutch is adjusted correctly and is in good shape.
I usually run grade 5 or grade 8 bolts where the PTO shaft connects to the input shaft on the cutter.
This thing consistently breaks the bolts. I break one every week or two. I am not hitting anything like dirt clumps, sticks stumps etc.
The bolts all break at the same spot, which is about an 1/8 inch from where the threads meet the smooth shank.
The slip clutch fits snuggly on the gearbox input shaft so there is no wobble.
The gearbox is in good shape, spins freely and has no slop. Gearbox mount is solid and the gearbox is aligned straight.

I also run a 5 foot Woods BrushBull on the same tractor and have never broken a grade 5 bolt.

Any thoughts on this one?

My first thought is to remove the slip clutch from the shaft. Then inspect the shaft & clutch for any damage around the shear bolt hole. If there is any deformation or missing material from the shaft or excessive movement of the bolt within the bolt hole it’ll allow the bolt to be sheared more easily.
My two cents

 

[repete]

I use a long bolt and double nut it so I put no pressure on the coupling itself.

Make REAL SURE your PTO shaft is adjusted to the proper length. If the shaft is too long and bottoms out the forces must be exerted until the weakest point gives. Disconnect the PTO shaft from the tractor and raise the mower to where the shaft is it its shortest compressed length and then attach the PTO shaft to the tractor. If you cannot have enough clearance to get the shaft on and off then count your blessings it is the shear bolt that breaks and not the tractor PTO output.

 

[TMGT]

Looks like they skip M13 and go M12 -> M14

They are made but not common, they also make 13mm shoulder bolts which are more common.

Maybe even try a 1/2 shoulder bolt as its not undersized like a normal 1/2 bolt.

 

[SPYDERLK]

I am trying a longer bolt so we'll see.
However, the interesting thing is [[[ the sheared spot is actually about a 1/4" short of the shear zone ]]]. It is shearing just inside of the gearbox input shaft. Maybe that is just how it is breaking off?
I'm also going to try some grade 2 bolts to see if the higher grades are too brittle.

After some testing, I am also going to switch to one of my new PTO shafts to see how that does.

This suggests that the input shaft is wallowed to a "bellmouth" and the bolt is continually flexing at the root of the bell ... or-r the thread being in the stressed region is causing a similar problem favoring fatigue failure. From pics I see you are only getting single shear. Then I presume the freed bolt remainder works its way out - very destructive. I think your longer bolt is going to work.