PTO Shaft to Gearbox Bolts Keep Breaking!

[wirlybird]

Firstly is it possible that the shaft is being asked to collapse to less than its min ? Consider odd situations : mower rear raising up when reversing into a bank ; mower front lower than normal when tractor runs up a rise ; offset mower dragging to center.

Otherwise check the clutch "correct adjustment" .... manufacturers often spec some small displacement off the springs bottoming out. Clearly this is a clutch max, not a setting to protect your unspecified driveline.
Quick check is to measure torque limit. 25hp @ 540 rpm is 240 ft.lbs
so factor for whatever Stillson / bar you have:
200 lbs standing 14 inches out from shaft center or
100 lbs out at 28 inches.
Drive end disconnected from tractor for safety and to eliminate any transmission brakes.

If too high , polish the plates by loosening tension and deliberately slipping clutch.
Adjust back to something more that that required to mow and less than equipment limit. I wouldn't be worried about a bit of clutch slip. All my failed friction disks have been from rust seized plates shattering the disc (if Im lucky) . Not from wear.

Assuming you have a single bolt on a round shaft ( not an external bolt on an ear ) then even a 0.5" gr 5 bolt in single shear is 10k lbs threads included. Use single shear as loose geometry allows one side to see load. Say 1.375 inch shaft the torque is over 500 ft.lbs to shear. Maybe less with bad setup, but not looking like the weak link if load is smooth, light 30hp engine load. Longer bolt with excluded threads will up the shear load once sure the clutch works.

If there is a round shaft in the mix, any chance the shaft is assembled out of phase ?ie both yolks need be assembled in the same plane, not at 90 deg to each other. That will flog out a shear pin worsening the more offset between the input and output shafts. Or if not parallel.

Photos of assembled machine would be great...

For the first part, I'm only running on flat areas. Just pretty straight forward, flat with field grasses and just this year's spring growth.
Running about a 4" cut height with the rear at about a 2" rise from the front of the cutter.
PTO shaft has plenty of remaining collapse room.
I disassemble all of my clutches in the spring, clean and test, then tighten appropriately for the implement.
Clutches are also checked periodically throughout the season so should be ok. I do mark the clutches so if I get a hard hit I can see if the clutch moved/spun at all.
I'll check the PTO shaft assembly to make sure that u-joints/yokes are in sync. Hadn't thought about that as most of my shafts are the triangle Bondioli type but these couple are the lemon Walterscheid type.

 

[wirlybird]

It appears that the bolts are getting hammered on the shank? Gearbox lube full?

Yes and gearbox spins freely and runs cool while working. Cutter runs very smooth, no vibrations to speak of. PTO shaft is straight.

 

[olenjohn]

It appears the threads are right at the edge of the shear zone. Try using a longer bolt so the unthreaded portion is completely outside the shear zone plus 1/4”. Use stacked washers to fill the gap so the nut will torque down tight.

 

[wirlybird]

It appears the threads are right at the edge of the shear zone. Try using a longer bolt so the unthreaded portion is completely outside the shear zone plus 1/4”. Use stacked washers to fill the gap so the nut will torque down tight.

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.

 

[Little Red Tractor]

Two quick thoughts:
Is your slip clutch actually slipping? If it's stored outside, you may have rust between the plates and that prevents it from doing its job. My other thought, (only because I'm not familiar with your tractor) is to ask if your PTO engages with a manual clutch or electric. I had a tractor with an electric clutch that slammed into engagement and had a brake when disengaged. I broke shear bolts with that all the time. I had to engage and disengage only at low RPMs.

 

[WI_Hedgehog]

As mentioned in a post above all the bolts in the photo show signs of getting hammered on their sides due to not being torqued down. That should not be happening. Torque them down tight when installing them. The nuts are only there to ensure they don't come lose.

To me @Windblown's assessment is accurate, the images suggest the fracture to be Fatigue Failure from insufficient preload during install. @wirlybird confirmed he was not torquing the bolts, further supporting the assessment.

In my opinion, Shear Force is not the issue, a lack of tension is causing the bolts to be hammered along their length as the shaft rotates, much like a slide hammer is stretching the bolts to their point of failure.

 

[wirlybird]

Two quick thoughts:
Is your slip clutch actually slipping? If it's stored outside, you may have rust between the plates and that prevents it from doing its job. My other thought, (only because I'm not familiar with your tractor) is to ask if your PTO engages with a manual clutch or electric. I had a tractor with an electric clutch that slammed into engagement and had a brake when disengaged. I broke shear bolts with that all the time. I had to engage and disengage only at low RPMs.

PTO shafts are stored inside and are serviced before first use in the spring and then at regular intervals throughout the season.
Electric PTO. I engage/disengage at 1500 RPM's.
PTO engages slow and smooth.
To disengage I throttle down slowly and when cutter speed is stable, I disengage. Cutter then spins down to a stop. Would seem minimal braking effect on PTO in tractor.

 

[old and tired]

What make is the "5 foot brush hog" that is breaking it's shaft???

I know your other one is good quality... Woods.

 

[wirlybird]

To me @Windblown's assessment is accurate, the images suggest the fracture to be Fatigue Failure from insufficient preload during install. @wirlybird confirmed he was not torquing the bolts, further supporting the assessment.

In my opinion, Shear Force is not the issue, a lack of tension is causing the bolts to be hammered along their length as the shaft rotates, much like a slide hammer is stretching the bolts to their point of failure.

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This is something I have asked before and also researched but never got a good definitive answer. Mostly just speculative opinions!
Some say "bolt" should be left loose and other say tighten up good.
What I have been doing is to snug the bolt/nut down so the bolt doesn't slide in its hole.
I am going to tighten them down now and see how that does. Only thing I see this doing is just loading the bolt. It won't make the PTO shaft any tighter on the gearbox input shaft or reduce any movement between input shaft and PTO shaft.
Worth a try.

 

[wirlybird]

What make is the "5 foot brush hog" that is breaking it's shaft???

I know your other one is good quality... Woods.

It's an old Big Bee. However, I have rebuilt it some time back with new stump jumper, blades and blade bolts. Gearbox is tight and spins fine. All bolts on tail wheel and A-frame are good and proper sizes and everything is straight. Gearbox is a low HP one so it doesn't sap much power from the tractor.
I use this one for certain jobs as it is light weight, tail wheel is easy to swing up out of the way so I can use a short trailer that has a gate.
It is not the prettiest one in the fleet, but it cuts fantastic. I can mow these fields and it is almost as good as a finish mower.