PTO Shaft to Gearbox Bolts Keep Breaking!

[dicktill]

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


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View attachment 3552393

Thanks for the photos.

The one on the left looks like a classic high-cycle fatigue failure: see all the striations. The one on the right looks more like a massive impact failure, but I am not sure.

And I am not sure what is causing this but am curious to learn!

 

[airbiscuit]

One other question. So when the nut end breaks off, does the cutter continue to function, or does the bolt head end eject itself? If there is a substantial force on the gearbox, I would expect the bolt head end to also shear off when half the holding power (on the other half of the collar) has disappeared.

 

[wirlybird]

One other question. So when the nut end breaks off, does the cutter continue to function, or does the bolt head end eject itself? If there is a substantial force on the gearbox, I would expect the bolt head end to also shear off when half the holding power (on the other half of the collar) has disappeared.

The bolt comes out also and is usually on the deck of the cutter. How long it stays in after breaking I don't know. Nut end is gone or I haven't found one yet.
I've learned to tell when it breaks or when the bolt comes out. It starts to sound like you ran over some cable or wire and got it tangled in the blades.
That is the first sign then the PTO shaft will come off the gearbox if you don't shut it off quick enough.

 

[ruffdog]

That is the first sign then the PTO shaft will come off the gearbox if you don't shut it off quick enough.

If that bolt were meant to be a "shear bolt", there would be a snap ring to keep pto hub from coming off the gearbox.

 

[wirlybird]

If that bolt were meant to be a "shear bolt", there would be a snap ring to keep pto hub from coming off the gearbox.

There is a slot for it on the input shaft but it is a monster pain to get the snap ring on and off.

 

[ruffdog]

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

There has to be something going on with the gearbox to have these quick impacts stressing the bolt. Is there a missing gear tooth? A bad bearing? Debris getting in the teeth? Stump jumper not attached solidly?

 

[etpm]

The bolt comes out also and is usually on the deck of the cutter. How long it stays in after breaking I don't know. Nut end is gone or I haven't found one yet.
I've learned to tell when it breaks or when the bolt comes out. It starts to sound like you ran over some cable or wire and got it tangled in the blades.
That is the first sign then the PTO shaft will come off the gearbox if you don't shut it off quick enough.

It might be that the nut backs off far enough that the load is now on the narrow part of the bolt and then it breaks. I think you need to find the nut end to see if the nut has backed off quite a ways or even come completely off.
Eric

 

[wirlybird]

There has to be something going on with the gearbox to have these quick impacts stressing the bolt. Is there a missing gear tooth? A bad bearing? Debris getting in the teeth? Stump jumper not attached solidly?

Gearbox is good, spins freely and runs cool. No issues I am aware of and short of taking it apart.
Stump jumper is good and tight. Reinstalled this spring with new nut and cottar pin. Also new blades and bolts.

 

[wirlybird]

It might be that the nut backs off far enough that the load is now on the narrow part of the bolt and then it breaks. I think you need to find the nut end to see if the nut has backed off quite a ways or even come completely off.
Eric

I use all metal lock nuts so I'd hope they are not backing off!!
I would like to find that part though to see what it looks like.

 

[chim]

In post 17, you say "They use a spring plate instead of individual springs. Both are properly adjusted and if anything adjusted slightly on the loose side."

First, I have no familiarity with slip clutches. There seems to be a consensus that movement is either at fault or a contributing factor. If the bolt tension is on the loose side, it may allow the rotating parts to move a little relative to each other. If the bolts are tightened to spec (or overtightened) , I'd think the only time there's a chance of that sort of movement is when the equipment encounters something that causes the slip clutch to slip.