PTO Shaft to Gearbox Bolts Keep Breaking!

[airbiscuit]

Maybe use a lock washer and tighten it down more?

 

[ruffdog]

As mentioned earlier, it looks like the bolts have been "rattling" in their holes, until they give up due to the shear forces. Work hardened if you will.

I don't think it's one "shear force incident", but repeated abuse from not being tight.

Agree^^ on the "rattling" marks on the bolts. It looks like those bolts are being hammered from all directions...a lot. My cutters will only show the bolt with the one mark just showing it is trying to slightly shear. The OP's bolts have heavy wear all over them. I'm still wondering if the PTO shaft u-joints are out of phase. If that is the case, the pto shaft will be trying to speed up and slow down the gearbox TWICE per RPM. That would stress the bolt and the clutch wouldn't help it one bit.

Shear bolt keeps busting

 

[Too many Kubotas]

Okay, that was not the fastener application I thought. Now things make sense.

I'd start measuring hole sizes and find a bolt that is a loose press fit in the largest of the three, then drill/ream the other two to that size. Which could be metric.

 

[ruffdog]

It is a pretty standard setup.
In the picture the bolt in question, green arrow, holds the PTO saft to the input shaft on the gearbox as you might assume!
You can tighten this bolt as much as you want and it will not make the collar, red arrow, on the PTO shaft any tighter on the gearbox input shaft.
Tightening the bolt simply keeps it from sliding in the holes. It does not "clamp" anything in place.

Now, on this type of setup all of the driven force from the PTO shaft is transferred to the gearbox input shaft via that bolt. The input shaft nor the PTO shaft are splined in this case.

View attachment 3549135

I can see why they recommend a grade 8 bolt there. I'm thinking that bolt is to retain the clutch hub to the shaft, not a shear bolt, as that is why the clutch is there. If the clutch is functioning, something ODD is going on to break the bolts. Take a picture of the pto shaft for us to see.

 

[wirlybird]

Agree^^ on the "rattling" marks on the bolts. It looks like those bolts are being hammered from all directions...a lot. My cutters will only show the bolt with the one mark just showing it is trying to slightly shear. The OP's bolts have heavy wear all over them. I'm still wondering if the PTO shaft u-joints are out of phase. If that is the case, the pto shaft will be trying to speed up and slow down the gearbox TWICE per RPM. That would stress the bolt and the clutch wouldn't help it one bit.

Shear bolt keeps busting

I looked at the u-joints and they are ok. In phase. After thinking about it, I don't know how you'd get them out of phase as some shaft halves can only go together one way. They are keyed, triangle type. Most probably don't realize they are keyed and that is why they fight with them trying to get the halves together!
I think you'd only get out of phase joints is the shafts were made wrong.

Here is an example of a triangle shaft. See how the hump at the arrow is more flattened as opposed to the other two which are more rounded. This style can only go together one way.
On a lemon style shaft (other picture), it can only go one of 2 ways, and each would 180 out from the other so it would be in phase also.

Good point on joints being out of phase though. Definitely something to check as you never know!

 

[old and tired]

For what it's worth, I've sheared many bolts, it's always both side that are cut (sheared). Usually have to punch out the middle part of the bolt. If somehow you are putting all the power to only break (crack?) one half of the bolt... I don't know what to say....

 

[old and tired]

Do you have more photos of the slip clutch, the other side? Are there springs on the other side?

 

[wirlybird]

For what it's worth, I've sheared many bolts, it's always both side that are cut (sheared). Usually have to punch out the middle part of the bolt. If somehow you are putting all the power to only break (crack?) one half of the bolt... I don't know what to say....

It is odd.

 

[wirlybird]

Do you have more photos of the slip clutch, the other side? Are there springs on the other side?

This one does not have springs on each bolt like some do. It has a spring plate called a Belleville spring. It does the same thing as the individual springs do, it applies the pressure to the clutch disks.

 

[AndrewInOz]

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.

 

[ning]

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:

@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

 

[airbiscuit]

Waiting for someone to suggest this!

 

[wirlybird]

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.

 

[wirlybird]

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.

 

[AndrewInOz]

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.

 

[ning]

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.

 

[ruffdog]

Waiting for someone to suggest this!

View attachment 3551232

Using a 6** type Loctite and a grade 8 bolt would stop this problem. Heat would be needed to get it apart later.

 

[airbiscuit]

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).

 

[airbiscuit]

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.

 

[ruffdog]

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?