PTO Shaft to Gearbox Bolts Keep Breaking!

[zzvyb6]

Chamfer the hole in the gearbox so it doesn't act like a shear. Especially the edge where the bolts are consistently breaking.

 

[jmc]

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

But your other cutters might be SAE, not metric. 13 mm is .512 vs your .500 bolt. .012 clearance/hole is .024 total slop. (Actually, more clearance than that because rolled threads have undersized unthreaded sections.)

If not, and from your pics, Id try the next bolt length up that has a longer unthreaded section, even if have to saw off the extra threads.

 

[RandyT]

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.


View attachment 3546902

all 3 bolts show signs of slop hammering especially the one in the middle. You can see the mushroom where the collar meets the shaft. But all three have a wear shoulder at the collar/gearbox shaft junction.

 

[rScotty]

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?

Ok, so you are running an external slip clutch on the JD...that makes sense, but it is difficult to see how it could shear a shear bolt with that external clutch adjusted for less than bolt-shearing force. Those clutches do tend to need annual maintenance and readjustment. Maybe it needs that.

I see that the JD 3032e also has an internal slip clutch on the pto drive. So if you have an external one you have two of that type clutch. No problem with that I can think of & no idea how to adjust the internal one. But in the diagram there is also no evidence of an internal over-running clutch & I would have expected to see that feature.

Could that be the problem? Without an over-running clutch there is a stress reversal when the power is cut to the brush hog and the blades then try to turn the PTO shaft instead of vice versa. If it doesn't have that type of clutch, d\o you have room on the external PTO shaft to add an over-running clutch?

Here is the diagram I am looking at: ARIMain - WEINGARTZ.


Over to the Brush Bull... What type of PTO clutch setup does your Wood's Brush Bull have? I remember those came with a with a slip clutch that has to be maintained and the spring tension set. Don't know if they have an over-running clutch, too - but most people add that if it doesn't .

rScotty

 

[RayIN]

The harder bolts are less forgiving, they do not flex as much as ungraded bolts or Gr2.
You want the easiest to replace, cheapest part to break first. Gearboxes are not cheap. A new PTO shaft will cost over $250.
I carry spare Gr2 bolts in my tractor toolbox and wrenches to replace shear bolts.
I know why mine breaks shear bolts, the holes in the PTO end are wallowed out because the previous owner used 3/8" bolts instead of ½"
The gearbox shaft is hardened steel, leaving the PTO end to wear.

 

[Too many Kubotas]

Why the high RPM when engaging and disengaging the PTO?

Not that it should break fasteners, but I always use the lowest RPM possible.

 

[wirlybird]

Ok, so you are running an external slip clutch on the JD...that makes sense, but it is difficult to see how it could shear a shear bolt with that external clutch adjusted for less than bolt-shearing force. Those clutches do tend to need annual maintenance and readjustment. Maybe it needs that.

I see that the JD 3032e also has an internal slip clutch on the pto drive. So if you have an external one you have two of that type clutch. No problem with that I can think of & no idea how to adjust the internal one. But in the diagram there is also no evidence of an internal over-running clutch & I would have expected to see that feature.

Could that be the problem? Without an over-running clutch there is a stress reversal when the power is cut to the brush hog and the blades then try to turn the PTO shaft instead of vice versa. If it doesn't have that type of clutch, d\o you have room on the external PTO shaft to add an over-running clutch?

Here is the diagram I am looking at: ARIMain - WEINGARTZ.


Over to the Brush Bull... What type of PTO clutch setup does your Wood's Brush Bull have? I remember those came with a with a slip clutch that has to be maintained and the spring tension set. Don't know if they have an over-running clutch, too - but most people add that if it doesn't .

rScotty

Both PTO shafts are identical.
They use a spring plate instead of individual springs. Both are properly adjusted and if anything adjusted slightly on the loose side.
Thing is that this cutter is never under any extreme shock loads like you might see running over 2 inch saplings or getting a ground strike.
PTO is engaged/disengaged at about 1500 RPMs. Throttle is either applied slowly or reduced slowly before PTO engage/disengage.

The Woods have a much heavier stump jumper and blades, more moving mass, and is used in much harsher conditions but never breaks bolts.

Yea, the internal slip clutch setup is like a last resort protection.
I usually run grade 5 bolts so they will likely shear easier in conjunction with the PTO shaft slip clutch just for a little extra safety if the blades should get totally jammed up. Break a bolt instead of burn up a slip clutch!

Once the rain goes away I'll check out the shaft for anything odd.

 

[wirlybird]

all 3 bolts show signs of slop hammering especially the one in the middle. You can see the mushroom where the collar meets the shaft. But all three have a wear shoulder at the collar/gearbox shaft junction.

Yes, but the thing is on this one it is never under any real stress or shock loads. These are just snapping while going along in a straight line just mowing grass. Also not mowing close to the ground.
When I check the bolts on my other brush hogs, they also show a little bit of that, but they never break under normal use.

 

[wirlybird]

The harder bolts are less forgiving, they do not flex as much as ungraded bolts or Gr2.
You want the easiest to replace, cheapest part to break first. Gearboxes are not cheap. A new PTO shaft will cost over $250.
I carry spare Gr2 bolts in my tractor toolbox and wrenches to replace shear bolts.
I know why mine breaks shear bolts, the holes in the PTO end are wallowed out because the previous owner used 3/8" bolts instead of ½"
The gearbox shaft is hardened steel, leaving the PTO end to wear.

I understand all of that but I run slip clutches on my shafts and normally the manuals call for grade 8.
I normally use grade 5 for a little extra protection if the blades get stopped dead so I don't burn up the clutches.
The bolts fit tight but am going to take a closer look.

 

[RayIN]

I understand all of that but I run slip clutches on my shafts and normally the manuals call for grade 8.
I normally use grade 5 for a little extra protection if the blades get stopped dead so I don't burn up the clutches.
The bolts fit tight but am going to take a closer look.

Anything mechanical can fail to operate perfectly every time.