PTO Shaft to Gearbox Bolts Keep Breaking!

[4x4moses]

I had a Titan/Ironcraft with a shear-bolt configuration. It would snap the grade 2 shear-bolts every time I used it. I finally learned to engage the rotary cutter while it was somewhat off the ground, and my RPM were just high enough not to stall the tractor. Then, when the implement engaged, I would ramp up the RPMs and lower the cutter to the ground. Long-story-short: don't engage the cutter at full throttle and under load. As for your model (with a slip clutch); I'm not sure why it would break a Grade 8 (dark gray) bolt.

 

[airbiscuit]

I had a John Deere 850 that had an abrupt PTO clutch. I would Idle it down, and then only momentarily engage the PTO clutch, and then fully engage it so it wasn't an all at once start. I don't think that is the problem. Those bolts look like they have been spinning around for a while before failing.

I'm surprised we're 100+ posts in, and still no solution.

 

[KWentling]

My opinion (retired machinist/millwright). The one bolt failure is definitely cyclic.
I think the problem is loose hub to shaft fit and loose bolt fit. Driveline shafts have an acceleration/deceleration event twice every revolution. This is magnified as driveline angles increase. I believe the bolt is being hammered into submission due to the loose fits. If it were mine I would drill the bolt fit out to 9/16" (if that will give a good fit), clean the shaft and hub bore and assemble with Loctite 660. As others have mentioned use a bolt long enough to get the threads out of the shear zone even though this bolt isn't intended to be a shear bolt.

 

[JRH02]

Here are the last three that broke. 2 grade 8 and 1 grade 5.
This is a lighter duty cutter and I don't push it hard. I engage and disengage the cutter with engine RPM at about 1500.
I use it mainly for doing fields that are in good shape and just cutting the field grasses.
Most of the time it is areas that I do about once a month to maintain. Next step would be a finish mower or mid mount and mow a little more frequently, but the customers don't need it cut that often.


View attachment 3546902

It has been mentioned a few times that the shear bolt must be the proper grade so IT will shear before something else breaks. It has also been pointed out that each of the three bolts in the picture show signs of wear on the shank. The breakage is at the bolt's weak point. Its obvious the bolts are breaking, not shearing.

My vote is that the bolt is loose in the through hole of the shaft and maybe the collar too. The shear bolt shank diameter must fit trough the shaft and the collar with no play and, of course, the holes must be round and not 'egged out'.

I'm merely agreeing/voting with what has already been said.

 

[Too many Kubotas]

we always use a regular soft bolt, the bolts are supposed to shear to protect the shaft and other components. thats the whole reason behind not making it a splined shaft. use regular cheap bolts, we used to use a 3/8 bolt.

I would imagine that a bolt is used primarily because it's a lot cheaper to drill three holes than to machine two sets of splines. And if the splines aren't lubed, it'll get even more expensive down the line.

There's no reason not to use a bolt in this application, but for it to work the three holes and the bolt must be of the correct diameters. Seems they no longer are.

Also, as mentioned above, I'd like to have a slight press fit for the shaft, not a loose connection.

 

[ruffdog]

My opinion (retired machinist/millwright). The one bolt failure is definitely cyclic.
I think the problem is loose hub to shaft fit and loose bolt fit. Driveline shafts have an acceleration/deceleration event twice every revolution. This is magnified as driveline angles increase. I believe the bolt is being hammered into submission due to the loose fits. If it were mine I would drill the bolt fit out to 9/16" (if that will give a good fit), clean the shaft and hub bore and assemble with Loctite 660. As others have mentioned use a bolt long enough to get the threads out of the shear zone even though this bolt isn't intended to be a shear bolt.

Agree....even install a 1/2" shoulder bolt with the shoulder completely through the hub. It sure wouldn't rattle apart like what is happening.

 

[airbiscuit]

a 14mm bolt is slightly larger than 1/2" Maybe see how that fits (or doesn't)

 

[TMGT]

13mm would be a smaller step at .512.
14mm is closer to 9/16.

You could also try a pin instead of a bolt.

 

[Jaygyver]

My observational guess's?

Looking at the broken bolts you can see 4 impact points, not two. The two major ones from the mower shaft but you can also see the impact/movement from the clutch collar.
The bolt should be tight and shouldn't be a loose fit even with the 1/2 in thick collar, yes your not pinching them together but your just keeping the bolt tight in one connection. With the bolt tight it only has one rotating part hitting it, the mower shaft, and it won't wallow out -both- sets of holes. With the bolt tight the bolt is the one that takes the brunt of the torque not the clutch housing collar AND the bolt AND the mower shaft.

The bolt should have the shoulder as near the outer edge of the collar as possible and still be able to tighten the nut.

 

[airbiscuit]

Can you buy 13mm bolts? They aren't listed on the charts.