PTO Shaft to Gearbox Bolts Keep Breaking!

[ruffdog]

The bolt is not being sheared. It is being killed by countless rapid lateral shock loads that fatigue the metal at the weakest point...the bottom thread. I wonder how close a fit the hub and shaft are? There should be no slop there. What actual diameter are the holes in the hub and shaft?

 

[thclimer]

Gust a SAWG, but I suspect a bolt hole is larger either on the PTO collar or the hole through shaft from the gearbox which is allowing the bolt to wobble eventually resulting in its fracture. I'd guess its the hole in the shaft from the gearbox.

Can you remove the shaft from the PTO collar and take a picture of just the bolt in the shaft showing the clearance between the bolt and the shaft hole?

Is it easy for the bolt to slide into the hole on the shaft?

 

[wirlybird]

That's really close to where I'd expect a shear with the threads under the collar.
I mean, I'd expect it to be right at the blue/green joint, but hey close enough.
I'll bet if you found the other piece of the bolt the threads would be mashed to smithereens and strangely the fulcrum is at the transition from one diameter (mashed threads) to another (bolt shoulder); it's probably been flexing back and forth at that spot, aided by the shaft hole being slightly larger.
Not a pure shear break so much as a work-weakening.
Just get a bolt with a longer shoulder and be done with this.

You're probably right.
Longer bolt in there now.

 

[IXLR8]

You said you were going to test with a bolt with a longer shank, have you had a chance to try that yet? Would be interesting to see what that looked like after a few hours of use. I'm thinking the threaded section is slightly smaller and allowing movement between the shaft and the collar on that side. At least with a shank that goes from collar to collar would give a better fit.

 

[wirlybird]

Does it matter which side of the collar you insert the bolt head in - i.e. is the bolt head always on the right, or is the orientation of the shaft, collar, and bolt head random?

Also, it looks like the bolt is spinning a lot in the hole. It "machines" a step on one side, and a groove on the broken side. Not sure what that means.

That is something I thought about. I could be inserting it the same direction each time simply because I'd have the nut/ratchet in one hand and wrench in the other so I could be always inserting from the left, so to speak.

 

[wirlybird]

You said you were going to test with a bolt with a longer shank, have you had a chance to try that yet? Would be interesting to see what that looked like after a few hours of use. I'm thinking the threaded section is slightly smaller and allowing movement between the shaft and the collar on that side. At least with a shank that goes from collar to collar would give a better fit.

I had put a longer bolt in it and it has about an hour or so on it.
Going to take a look once the storms pass.
Thing is though I use the 1/2 by 3 bolts on all of my cutters and this is the only one having issues!
May also swap out the shaft and see what happens.

 

[dicktill]

I had put a longer bolt in it and it has about an hour or so on it.
Going to take a look once the storms pass.
Thing is though I use the 1/2 by 3 bolts on all of my cutters and this is the only one having issues!
May also swap out the shaft and see what happens.

It would be great if you could take and post some good closeup photos of the broken surface of the bolts, i.e. the actual fracture surface. Might give us a better clue as to what sort of failure it is. Right now, it is not looking like a shear failure.

 

[shirleyujest]

Gust a SAWG, but I suspect a bolt hole is larger either on the PTO collar or the hole through shaft from the gearbox which is allowing the bolt to wobble eventually resulting in its fracture. I'd guess its the hole in the shaft from the gearbox.

Can you remove the shaft from the PTO collar and take a picture of just the bolt in the shaft showing the clearance between the bolt and the shaft hole?

Is it easy for the bolt to slide into the hole on the shaft?

I think it is similar to using a cold chisel to cut a steel rod. repeated blows cutting the loose bolt. The grain unloading drive on my JD 9500 only uses a 6mm bolt.

 

[wirlybird]

It would be great if you could take and post some good closeup photos of the broken surface of the bolts, i.e. the actual fracture surface. Might give us a better clue as to what sort of failure it is. Right now, it is not looking like a shear failure.

Here is a grade 5 on the left and grade 8 on the right.

 

[airbiscuit]

I'm wondering if it's a combination of variables:

1. The hole in the collar or shaft is slightly oversized or wallowed
2. The bolt isn't long enough (collar rests on threads)
3. The bold isn't tight enough - allowing it to spin and degrade
4. There is some unusual vibration at the gearbox
5. There are demons at play