Rotary Cutter PTO yoke broke

[johndeerezach]

i was bush hogging just a littl bit ago and was mowing a hill and went to back down a little way to cut the top of the hill and hit a medium sized log and i raised it right when i hit it and i thought there was no harm done then i moved to a different spot to mow grass and when i started the pto the u joint broke and the part that connects to the spline on the tractor broke off the u joint. what did i do wrong anything helps

 

[leonz]

Hello John ,

I think the actual impact on the log was the
point where the yoke started to break and
the added stress of raising the mower
added to it and it snapped when you restarted it.

 

[RickB]

Service your slipclutch if your cutter has one.

 

[tessiers]

if the cutter went up enough to close the PTO shaft to the bottom will break the yoke every time.

 

[Dead Horse]

This may be an object lesson to setting up an implement properly.

The PTO shaft has to be cut to the proper length for the specific tractor, implement combination.

When you lift up on an implement the distance between the two spindles shortens. (tractor spindle and implement input spindle) IF the shaft is too long to allow that full retraction it will BOTTOM out and something will break.

Often times the spindle lock bends, deflects, breaks, resulting in it being VERY hard to get the shaft off the spindle on the tractor (as usually the implement end is through bolted and as a result tougher than the lock at the tractor end of the PTO shaft).

So, the lesson here is make sure when you install a new implement that nothing binds through the full range of motion on the lift, before you set out for work.

 

[johndeerezach]

the shaft was cut to the right length have taken it out already and am going to replace the yoke. but im pretty sure that the gear box should have a shear pin shouldnt it?

 

[tessiers]

every bush hog I ever saw has a shear pin usually 1/2" where the driveshaft bolts on the gearbox, or an eccentric flange thereabouts. A broken yoke is more typical of binding or bending than from over torque, unless defective or damaged prior to this incident.

 

[johndeerezach]

i dont think that it was binding because i lowered the mower and was backing up to cut near the hill and hit the log thats when the yoke broke when i engaged the pto after i hit it

 

[johndeerezach]

so the shear pin should be what attaches the pto to the gear shaft?

 

[Dead Horse]

so the shear pin should be what attaches the pto to the gear shaft?

Yes unless the mower came equipped with a slip clutch. All 6 of my mowers have some sort of slip clutch mechanism. In fact i do not recall seeing a bush hog (that was not at least 30 years old) without one.

 

[jml755]

so the shear pin should be what attaches the pto to the gear shaft?

Your cutter is designed to have a "fuse" in the drivetrain, either a shear bolt OR a slip clutch. If you don't have a clutch (mounted at the implement end) before the gearbox, then the "fuse" is that bolt (usually 1/2") that tessiers mentioned. If you have a shear bolt make sure it is a grade 5 bolt and NOT a grade 8. You're not looking for an indestructable bolt in this application. You WANT it to shear (i.e. blow the "fuse"). I carry a half dozen extras on the tractor. I've seen a few yokes break but it's always because the drivetrain was put into a position that were beyond the mechanical limits, usually when the pto was running. Turning in too small a radius with a manure spreader will do it. Dead Horse has good advice to operate an implement through the full range of movement with the PTO running. Once you do that, make sure you record or mark the settings of the top link if you use it on multiple implements.

Hitting a log (or other obstacle) is pretty common so you've got to make sure that the shock to the system DOESN'T get transmitted to the tractor. If you blew a perfectly good yoke and you replace it without making sure something else gives first (clutch or bolt), you'll be doing it again.

 

[johndeerezach]

WELL I HAVE HAD THIS MOWER FOR AT LEAST TWO YEARS AND HAVE NEVER DONE THIS WITH THE MOWER. AND THAT SHEAR PIN EVERYBODY IS TALKING ABOUT IM PRETTY SURE ON MINE IS LIKE A GRADE 8 BOLT THAT WAS SAID, THE ONLY REASON I SAY THAT IS ITS NOT TIGHT AND THERE ARE 2 NUTS THREADED ON IT.

 

[skipmarcy]

Yep, that's the shear bolt. As already mentioned, it needs to be a grade 2 bolt - no markings on the head. I also keep at least 6 or so in my little toolbox on the tractor. Sometimes I can go a couple years without breaking one, then can break 4 or 5 in one day - just the nature of the beast. If you cut alot of saplings etc. regularly you might go to a grade 5 bolt but you run a heightened risk of the damage you already have had. A grade 8 bolt is just looking for trouble. Most the time when I break a bolt the end with the nuts on it usually land on top of the cutter close to the gearbox and I reuse the nuts for the next bolt. And yes, it should be a little loose in the hole - makes getting the broken piece out easier when it sticks in there. I have used 7/16" bolts before even in a pinch with no problems.

 

[johndeerezach]

yea skipmarcy i just bought a 506 jd mower and im pretty sure thats not shear bolt either the reason i think this is there are marking on the bolt head but i could be wrong and the drive is weird the shaft from the tractor comes back close to the gear box in 6 spline but then converts to alot of smaller ones i havent counted yet and that is driving a plate that is fit inside of the same thickness of plate with a bold through it and sides that are bent on 90 degrees around so it sits in and it is surrounded what type is that i will post photo later tomorrow or saturday