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.

 

[Teachu2]

What are you torguing the bolts to? My guess - and I confess I didn't read all 12 pages here - is that they were over-torqued and stretched. The breakage point is consistent with that. I'd try 40 ft/lbs and use nylock nuts. I'm not a pro, but I've broken a few bolts over the years.....

 

[ning]

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

Looks like they skip M13 and go M12 -> M14

 

[Too many Kubotas]

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

Excess bolt length is easily taken care of with a spacer.

 

[ruffdog]

What are you torguing the bolts to? My guess - and I confess I didn't read all 12 pages here - is that they were over-torqued and stretched. The breakage point is consistent with that. I'd try 40 ft/lbs and use nylock nuts. I'm not a pro, but I've broken a few bolts over the years.....

Post #39.....he is not over tightening the bolt.

 

[airbiscuit]

What I have been doing is to snug the bolt/nut down so the bolt doesn't slide in its hole.

What are you torguing the bolts to? My guess - and I confess I didn't read all 12 pages here - is that they were over-torqued and stretched. The breakage point is consistent with that. I'd try 40 ft/lbs and use nylock nuts. I'm not a pro, but I've broken a few bolts over the years.....

Post #39.....he is not over tightening the bolt.

BTW, I use a lock washer on my shear pin.

 

[RayIN]

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.

The shaft on my King Kutter gearbox is hardened, no increasing bolt hole size. I suspect other brands also have a hardened shaft. A simple hit with a file will prove hardness.
That reminds me; I have a pocket hardness tester somewhere in one of my bench toolboxes. I'll have to find it and test my gearbox shaft.

 

[Stevehogan]

I run a 5 foot brush hog on my JD 3032E. I have used this cutter for years for lighter duty work. Mostly just cutting field grasses on nice flat areas.
I do run a slip clutch PTO shaft. Clutch is adjusted correctly and is in good shape.
I usually run grade 5 or grade 8 bolts where the PTO shaft connects to the input shaft on the cutter.
This thing consistently breaks the bolts. I break one every week or two. I am not hitting anything like dirt clumps, sticks stumps etc.
The bolts all break at the same spot, which is about an 1/8 inch from where the threads meet the smooth shank.
The slip clutch fits snuggly on the gearbox input shaft so there is no wobble.
The gearbox is in good shape, spins freely and has no slop. Gearbox mount is solid and the gearbox is aligned straight.

I also run a 5 foot Woods BrushBull on the same tractor and have never broken a grade 5 bolt.

Any thoughts on this one?

My first thought is to remove the slip clutch from the shaft. Then inspect the shaft & clutch for any damage around the shear bolt hole. If there is any deformation or missing material from the shaft or excessive movement of the bolt within the bolt hole it’ll allow the bolt to be sheared more easily.
My two cents

 

[repete]

I use a long bolt and double nut it so I put no pressure on the coupling itself.

Make REAL SURE your PTO shaft is adjusted to the proper length. If the shaft is too long and bottoms out the forces must be exerted until the weakest point gives. Disconnect the PTO shaft from the tractor and raise the mower to where the shaft is it its shortest compressed length and then attach the PTO shaft to the tractor. If you cannot have enough clearance to get the shaft on and off then count your blessings it is the shear bolt that breaks and not the tractor PTO output.

 

[TMGT]

Looks like they skip M13 and go M12 -> M14

They are made but not common, they also make 13mm shoulder bolts which are more common.

Maybe even try a 1/2 shoulder bolt as its not undersized like a normal 1/2 bolt.

 

[SPYDERLK]

I am trying a longer bolt so we'll see.
However, the interesting thing is [[[ the sheared spot is actually about a 1/4" short of the shear zone ]]]. It is shearing just inside of the gearbox input shaft. Maybe that is just how it is breaking off?
I'm also going to try some grade 2 bolts to see if the higher grades are too brittle.

After some testing, I am also going to switch to one of my new PTO shafts to see how that does.

This suggests that the input shaft is wallowed to a "bellmouth" and the bolt is continually flexing at the root of the bell ... or-r the thread being in the stressed region is causing a similar problem favoring fatigue failure. From pics I see you are only getting single shear. Then I presume the freed bolt remainder works its way out - very destructive. I think your longer bolt is going to work.