PTO Shaft to Gearbox Bolts Keep Breaking!

[airbiscuit]

It must get your attention when the PTO/Slip Clutch backs off the gearbox.

 

[ruffdog]

It must get your attention when the PTO/Slip Clutch backs off the gearbox.

That is what I was thinking too.........

 

[wirlybird]

In post 17, you say "They use a spring plate instead of individual springs. Both are properly adjusted and if anything adjusted slightly on the loose side."

First, I have no familiarity with slip clutches. There seems to be a consensus that movement is either at fault or a contributing factor. If the bolt tension is on the loose side, it may allow the rotating parts to move a little relative to each other. If the bolts are tightened to spec (or overtightened) , I'd think the only time there's a chance of that sort of movement is when the equipment encounters something that causes the slip clutch to slip.

The bolt simply holds/connects the PTO shaft onto the gearbox input shaft.
The collar on the PTO shaft that goes over the input shaft is so thick that you'd break a bolt before ever getting it to "clamp" down on the input shaft.
You can see in the picture, red arrow, how thick the collar is!
The bolt does take all the stress.
On a slip clutch type setup, such as in the picture, the bolt is meant only to connect the shafts. It should be strong enough to not shear under a large stress. The slip clutch is the safety in this case.
In a shear pin style, no slip clutch, the bolt is the safety mechanism. The use of a grade 2 or ungraded bolt connects the shafts but also is designed to "shear" under a potentially damaging load condition.

 

[wirlybird]

It must get your attention when the PTO/Slip Clutch backs off the gearbox.

That it does!

 

[chim]

Oh boy was I confused. When I saw the picture of multiple bolts, even though you did explain it I thought the broken bolts were for the plates. Big DUH for me

 

[wirlybird]

Oh boy was I confused. When I saw the picture of multiple bolts, even though you did explain it I thought the broken bolts were for the plates. Big DUH for me

It's all good and sure can get confusing!

 

[AndrewInOz]

Fracture photos add weight to non-shear failure. Too bad the fractures out of focus.
If not shear, how can a snug bolt in a boss fail in tension ?

All I can think of is out there with the aliens but was an interesting exercise.

In focus is the shank : showing the bolt has been rotating under load. Not just rattling but failing in bearing. Like the bolt walking heavily around the hole. ( surely couldn't self tighten the nut ? ).

What could make the bolt forcefully rotate......would have to be axial movement at the same time as torq applied. Hallmarks of bad alignment.

A single non CV universal joint transmitting thru an angle for a constant input speed will output a jumpy uneven speed. That uneven speed is restored to the original constant speed when transmitted back thru the second universal so long as the yolks are in the same plane AND the input (pto) is parallel with output ( gearbox input).

Off alignment will apply a series of +/- acceleration pulses around each rotation, just the thing to rotate our bolt.

Since this mower is a bitzer we should consider odd geometry.
Photos of drive arrangement and shear holes would help narrow speculation....

 

[shirleyujest]

Another possibility is the Shaft outside diameter is much smaller than the Collar inside diameter. This would allow excessive play in the joint.

 

[dicktill]

You said that the gearbox rotates smoothly. With the PTO shaft in place, the tractor not running, and the PTO disengaged, can you try rotating the PTO shaft from the tractor end to see if you can feel any anomalies?

 

[DarkBlack]

One remote scenario ,I don’t think has been mentioned, but add it to all the others…during use, your nut is coming off, allowing the bolt to start backing out. It gets hung up with the threads catching on the bolt head side holes. At that moment you at the weakest point of the bolt, and the shaft’s rotational torque is all behind applied to that one point rather than two points on a bolt, so it snaps off.

Re-insert your found remaining bolt heads from your photo. Is the remaining length exactly the distance to the nut side hole, to prove the bolt was fully in place when the end snapped off?
If so, that would likely rule out this scenario

 

[bigtiller]

Have you made any witness marks on the slip clutch to confirm that it is slipping?

Maybe try loosening each tension bolt on the clutch an eighth of a turn and check it again for slippage and/or excess heat.

 

[Roadking52]

Check to see if your pto drive shaft is bottoming out when you lift the mower. It may need shortening. You should also never run a grade 8 bolt in that application. You want that bolt to be the weak link. It’s a lot cheaper than the other parts you can break.

 

[Zork]

A few things I can think of.
1- Is it a shear bolts? Slip clutches do not require shear bolts in the shaft as the clutch does the protecting of the gearbox.
2- Is the shaft too long and the back-forth movement is shearing bolt?
3- Are you over torquing the bolt when you tighten it?

 

[bill karrer]

You say the slip clutch is adjusted properly, but is it rusted together? Has the clutch ever slipped? Loosen the slip clutch until it does slip and gets a bit warm to the touch. This will assure the discs are free of rust and corrosion. Readjust clutch. Not a bad idea to do this at the beginning of every season. Use only grade 8 or 10.8 metric bolts and let the slip clutch take the pounding.

 

[boatmoter]

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?

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.

 

[Pinecrest]

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?

Are you buying your Bolts from the same place? They may not be Grade 5 even though clearly marked on the head. There are a lot of counterfeit grade 5 bolts made in China on the market
and other than buying from a reliable vendor I don't know how to tell.

 

[henerythe8th]

The bolt is not being sheared. It is being killed by countless rapid lateral shock loads that fatigue the metal at the weakest point...the bottom thread. I wonder how close a fit the hub and shaft are? There should be no slop there. What actual diameter are the holes in the hub and shaft?

After reading several pages of issues THIS needs further investigation!
The fit of the flange to the shaft should be "near perfect".
In other words, no slop between the OD of the shaft and the ID of the flanged piece.
If there's "wiggle" then that's likely the issue!

 

[Gord Baker]

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?

Take note of clutch plate spacing and loosen off bolts. Be sure the clutch is not Stuck. Run the machine level, Front to rear and side to side. Keep sides 4" or more OFF THE GROUND.

 

[RESINC]

1
Find the hardest grade and largest diameter bolt you can buy that you fan fit in. You may need to go to a bolt specialty shop.
2. When you engage the Bush cuter, engage at lowest rpm possible. 1500 IS TOO HIGH.
I have an 8ft JDeere. I had the same problem. I took it to dealer. They said nothing was wrong with the cuter. They said do 2.
I Took the initiative to do 1. Problem solved. Good luck.

 

[RESINC]

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?

In addition, good clutches and clutching very important. Also lower RPM when engaging