PTO Shaft to Gearbox Bolts Keep Breaking!

[wirlybird]

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?

 

[Straightgrain3016]

First thought is something is out of balance

 

[jmc]

Weird.

Fractured 1/8" into the threaded part, or the unthreaded part of the bolt? (Ideally, the unthreaded section would span both sides of the shear circle.)

Maybe the bolt hole on the problem cutter is metric and you're using an SAE bolt of similar size. (Or vice versa) Say, 10mm (.394) hole vs 3/8 (.375) bolt. A little clearance would cause a lot of stress.

 

[RandyT]

Weird.

Fractured 1/8" into the threaded part, or the unthreaded part of the bolt? (Ideally, the unthreaded section would span both sides of the shear circle.)

Maybe the bolt hole on the problem cutter is metric and you're using an SAE bolt of similar size. (Or vice versa) Say, 10mm (.394) hole vs 3/8 (.375) bolt. A little clearance would cause a lot of stress.

Or another thought is over time the shear bolt has wallowed out the shaft hole and has now created enough slop to hammer the bolt until it breaks

 

[oldnslo]

Does the PTO shaft telescope in and out smoothly?

 

[wirlybird]

First thought is something is out of balance

PTO shaft and everything else spin very smooth.

 

[wirlybird]

Weird.

Fractured 1/8" into the threaded part, or the unthreaded part of the bolt? (Ideally, the unthreaded section would span both sides of the shear circle.)

Maybe the bolt hole on the problem cutter is metric and you're using an SAE bolt of similar size. (Or vice versa) Say, 10mm (.394) hole vs 3/8 (.375) bolt. A little clearance would cause a lot of stress.

Using the standard 1/2" X 3" bolt which is what I use in other cutters.

 

[wirlybird]

Or another thought is over time the shear bolt has wallowed out the shaft hole and has now created enough slop to hammer the bolt until it breaks

The bolts seem to fit tight when installing but I will look at this further.

 

[wirlybird]

Does the PTO shaft telescope in and out smoothly?

Yes, it is in good condition.

 

[wirlybird]

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.