Shear bolt keeps busting

[rmckibben]

As some of y’all know I usually only buy used so my problems could probably be eliminated if I had newer equipment. Anyway…

The sheer bolt on my bush hog keeps busting for no apparent reason. I’ll be going along just fine and then BAM it pops. It has happened probably half a dozen times and I can’t figure out what’s going on. It’s not hitting rocks or roots and the grass isn’t that high.

The only thing I can think is that the hole on the yoke is slightly elongated and looks a little worn out. Could it be allowing the bolt to move enough to bust that bolt?

Thanks,
Mack

 

[CADplans]

I have a mower that does that,, I got sick and tired of replacing bolts, and upped the bolt to Grade 5,,
I am satisfied that the Grade 5 bolt is the answer.

 

[sandman2234]

Good chance the elongated hole is allowing the outer ring to get a running start before hitting the bolt, allowing it to shear prematurely.
You can purchase a new section of the drive shaft to replace the old one, or maybe turn it 90 degrees and drill a new hole (only if you are experienced in metal working). Another way would be to drill the outer existing hole larger and add a bushing.
Going to a stronger bolt isn't a good idea in my opinion because the stuff you are trying to protect in the rear end of the tractor hasn't gotten any cheaper!
Another way is to change over to the slip disc clutch type, which is a driveshaft changeover, but still cheaper than the gears in your tractor!
David from jax

 

[ruffdog]

If your pto shaft u-joints are out of phase, it will try to speed/slow the cutter twice per revolution and is hard on shear pins. Also, use a nyloc nut and don't tighten.

 

[DieselBound]

Also, make sure that the blades/stump jumper aren't hitting, aren't loose (that'll present some wobble force). Generally a good idea to make sure things under there aren't loose! (I slung out the assembly from my 6' RC- out in the woods and no one was hurt; big mass to be ejecting!). Used stuff always needs a little more attending to at first, but if you get things up to snuff and keep up on necessary maintenance then you'll likely be fine.

 

[Slowpoke Slim]

My thought is that it's the elongated hole also. It's allowing the yoke to "run at" the shaft. It doesn't have to be so tight you have to drive the shear pin in like a nail, but it should be fairly snug when it goes in. If it's possible to replace the yoke, I'd start there.

 

[tomplum]

If you decide to re bore the hole, you may be able to switch from std. to metric or visa versa to lessen the amount of change. My shear bolt is a 307A, wouldn't be afraid to use a grade 2 in a pinch myself.