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.