PTO Shaft to Gearbox Bolts Keep Breaking!

   / PTO Shaft to Gearbox Bolts Keep Breaking! #71  
The break is not at either of the two points of shear, but outside the collar under the nut.
I disagree; my guess is that the geometry is:
1749172714037.png

@wirlybird can you post a pic with an unbroken bolt in going through, and one of a broken bolt in the hole?

My guess is that the bolt near the cap is about the right diameter, but the hole in the shaft is larger which allows the shaft motion on the bolt shoulder, and then the other edge of the collar is over threads which are very much not the right diameter (well not after you run it for 5 seconds).

Most likely the mismatch of the shaft hole and the collar hole sizes is mostly ok, but the collar being over threads is definitely going to do the bolt in
 
   / PTO Shaft to Gearbox Bolts Keep Breaking! #72  
Waiting for someone to suggest this!

1749174205220.jpeg
 
   / PTO Shaft to Gearbox Bolts Keep Breaking!
  • Thread Starter
#73  
I'll see about pictures tomorrow weather permitting.
Here is one thing I found though. Here is a rough sketch to illustrate.
In the sketch, the green arrow is the PTO collar that goes over the input shaft.
Red is the input shaft.
Bolt going through with bolt head at the right and nut at the left.
The bolt is breaking at the blue line which is inside the input shaft hole.
It is not breaking where the input shaft and collar meet.

1.jpg
 
   / PTO Shaft to Gearbox Bolts Keep Breaking!
  • Thread Starter
#74  
Thats a big boss on the clutch collar.
Now bolt photos make a bit more sense and they do not look like shear failure.

The break is not at either of the two points of shear, but outside the collar under the nut. And there are 2 mysterious untouched lands at the shear points. possibly due to previously yielded material around the shear point. But no bending in the bolt photos so not an issue now.

Cannot imaging what knocks the bolt heads off. Nice some more photos of the driveline arrangement. Also the grain at the fracture. Nuts would be great too but guess never find them.


For accurate shear limiting , the bolts need be loose. Tightening only introduces additional load ability via friction (& not a consistent value for this joint example: could be zero contribution ) at the expense using bolt strength needlessly.
The example cited earlier post here was for a bolt loaded in tension, not shear.


Not clear are :
- how far that shaft goes into the boss.
- whats the fit of the bolt into the shaft hole & into the collar holes. Looks like they are not parallel all the way. Or shaft hole bigger.
- you've confirmed yolks in phase ( fault when shaft bits are intermixed ) but the same problem is also caused by out of parallel pto and gearbox shafts. Are they parallel ? Assume there are only 2 universals in your system.
"The break is not at either of the two points of shear, but outside the collar under the nut."

Actually it isn't. The break is just inside the input shaft hole. I made a rough sketch on another post. Here it is.
Break is at the blue line in the sketch. Red is input shaft, green PTO collar.
I'll see about some pictures tomorrow if it is not raining.

1.jpg
 
   / PTO Shaft to Gearbox Bolts Keep Breaking! #75  
If break is internal and fitup is fine, then we should see the bolt shear at the head end as well.
But all options open ... something is not as it seems.
 
   / PTO Shaft to Gearbox Bolts Keep Breaking! #76  
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.
 
   / PTO Shaft to Gearbox Bolts Keep Breaking! #78  
My rotary cutter has a 1/2" shear pin. When it breaks, it shears neatly at both the top and bottom (flush with the shaft). What's left of the bolt looks unmolested. Your bolts look like they've spent 10 rounds in a cage match. There's a whole lot of shaking going on (and I don't know why).
1749176033493.png
 
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   / PTO Shaft to Gearbox Bolts Keep Breaking! #79  
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.
 
   / PTO Shaft to Gearbox Bolts Keep Breaking! #80  
My rotary cutter has a 1/2" shear pin. When it breaks, it shears neatly at both the top and bottom (flush with the shaft). What's left of the bolt looks unmolested. Your bolts look like they've spent 10 rounds in a cage match. There's a whole lot of shaking going on (and I don't know why).

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.
^^agree. That bolt is doing a bunch of gyrating in the hole somehow. ....and why?
 
 

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