PTO Shaft to Gearbox Bolts Keep Breaking!

[old and tired]

...Gearbox is a low HP...

So 40 HP gear box? or is it lower? Were they pretty "affordable"?

 

[WI_Hedgehog]

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.

In general, understandable terms:

Bolts are [some sort of] steel (dependent on Chemical Composition, see attached).

Steel stretches (the modulus of elasticity).

Each material has a specified Tensile Strength, Yield Strength, Elongation, and Hardness.

Stretching a bolt is Strain, increasing the strain from basically zero to (some number) during rotation of the shaft causes the bolt to stretch (not much, but it stretches). Decreasing strain causes the bolt to contract.

Repeated stretching causes fatigue due to Work Hardening. Fatigue lowers the Tensile Strength and Modulus of Elasticity, so eventually the stretch changes from Elastic Deformation to Plastic Deformation (the bolt becomes increasingly weaker, less stretchy, and more brittle).

When the Strain exceeds the Tensile Strength elastic deformation becomes plastic deformation (the less-stretchy more brittle bolt breaks).

---

By pre-loading the bolt to a specified torque, the bolt is under fairly constant strain, resulting in "not much stretching," or at least "greatly reduced stretching."

Less movement results in less work hardening (less fatigue resulting in a less brittle bolt, so the bolt retains its toughness).

A bolt that retains its strength doesn't break, which is why bolts are pre-loaded.

---

Flex and Vibration, especially Harmonic Vibration, cause more problems in "high-speed" machinery than a person might imagine. PTO shafts should be run at approximately 540 RPM, but the equipment is heavy, so the forces involved are relatively large. (If you don't consider 540 RPM "high speed" try spinning the shaft and attached equipment at 540 RPM with a hand crank).

Large dynamic forces can cause a lot of stress, eventually resulting in part failure. Think about slide-hammering that bolt 540 times a minute, for hours straight--that's similar to 100,000 hammer blows in 3 hours--that bolt won't last forever.

---
The above uses terminology in a "brief" and "generally understandable" form, so if someone wants to be more accurate that's fine by me.

 

[wirlybird]

So 40 HP gear box? or is it lower? Were they pretty "affordable"?

Probably no more than that. How I ended up with it is this.
Years back I had a John Deere 755, still have it, and wanted to see if it could run a 5 foot cutter.
I knew I'd need a cutter with a low HP light duty gearbox and a light weight cutter.
I found this Big Bee on Craigslist with a damaged stump jumper for $150. Broken weld for the center spline part.
Seemed like a good one to try out.
Replaced the stump jumper with a heavy duty one and also new blades and blade bolts. Replaced all the other bolts on the cutter and got everything aligned and straight. Thing ran great and but great and was into it for about $400 total.

Fast forward - I found that with it on my JD 3032E with tail wheel up I could use my 14 foot trailer and Tahoe rather than the big trailer and 1-ton dually. Much easier to get into and out of places and cheaper on gas!
Took some time to get it dialed in but it cuts fantastic.
Not sure if they are made anymore. Agri-X may be the newer version.

 

[wirlybird]

In general, understandable terms:

Bolts are [some sort of] steel (dependent on Chemical Composition, see attached).

Steel stretches (the modulus of elasticity).

Each material has a specified Tensile Strength, Yield Strength, Elongation, and Hardness.

Stretching a bolt is Strain, increasing the strain from basically zero to (some number) during rotation of the shaft causes the bolt to stretch (not much, but it stretches). Decreasing strain causes the bolt to contract.

Repeated stretching causes fatigue due to Work Hardening. Fatigue lowers the Tensile Strength and Modulus of Elasticity, so eventually the stretch changes from Elastic Deformation to Plastic Deformation (the bolt becomes increasingly weaker, less stretchy, and more brittle).

When the Strain exceeds the Tensile Strength elastic deformation becomes plastic deformation (the less-stretchy more brittle bolt breaks).

---

By pre-loading the bolt to a specified torque, the bolt is under fairly constant strain, resulting in "not much stretching," or at least "greatly reduced stretching."

Less movement results in less work hardening (less fatigue resulting in a less brittle bolt, so the bolt retains its toughness).

A bolt that retains its strength doesn't break, which is why bolts are pre-loaded.

---

Flex and Vibration, especially Harmonic Vibration, cause more problems in "high-speed" machinery than a person might imagine. PTO shafts should be run at approximately 540 RPM, but the equipment is heavy, so the forces involved are relatively large. (If you don't consider 540 RPM "high speed" try spinning the shaft and attached equipment at 540 RPM with a hand crank).

Large dynamic forces can cause a lot of stress, eventually resulting in part failure. Think about slide-hammering that bolt 540 times a minute, for hours straight--that's similar to 100,000 hammer blows in 3 hours--that bolt won't last forever.

---
The above uses terminology in a "brief" and "generally understandable" form, so if someone wants to be more accurate that's fine by me.

I understand all of that and understand preload.
What I see though is the only "stretch" force on the bolt is centrifugal force of the spinning shaft which has to be pretty minimal.
Bolt is tightened so there is no sliding movement of the bolt. Just snugged not torqued. Odd thing is that this one breaks bolts fairly frequently but none of my other cutters do this. I have 6, 5 footers and 2, 6 footers.

When researching tightening the bolt the answer I get the most is to snug the bolt down to remove any play in it.

 

[Bentrim]

From the posted pictures it appears as if only one end of the bolt is taking the load. Normally a shear bolt shears both ends and leaves the center of the bolt in the shaft with the ends being "lost". Are both holes in the yoke the same size, or is one slightly larger? As for tightening the shear bols I have always tightened them tight and "given them another turn" as the old timers would say. Figuring the extra squeeze put on the yoke would take some load off the bolt and reduce any "hammering".
Some shafts I have seen use a key to drive and a bolt to retain the PTO on the gearbox shaft. Are you missing a key? This is usually used for a slip clutch.
Is the slip clutch too tight or rusted tight? A slip clutch is not only used for overloads but a "shock absorber" too. If tightened too tight or rusted it will have no "give". Tighten the clutch until it will just take the load after loosening it and slipping it to clean up the clutch surfaces.

 

[wirlybird]

From the posted pictures it appears as if only one end of the bolt is taking the load. Normally a shear bolt shears both ends and leaves the center of the bolt in the shaft with the ends being "lost". Are both holes in the yoke the same size, or is one slightly larger? As for tightening the shear bols I have always tightened them tight and "given them another turn" as the old timers would say. Figuring the extra squeeze put on the yoke would take some load off the bolt and reduce any "hammering".
Some shafts I have seen use a key to drive and a bolt to retain the PTO on the gearbox shaft. Are you missing a key? This is usually used for a slip clutch.
Is the slip clutch too tight or rusted tight? A slip clutch is not only used for overloads but a "shock absorber" too. If tightened too tight or rusted it will have no "give". Tighten the clutch until it will just take the load after loosening it and slipping it to clean up the clutch surfaces.

I agree it is odd that only one side is breaking. Probably a clue. When I used to run a shear pin style shaft on this cutter it would break both side just as you described.
I could tighten the bolt to just before breaking and it would not compress the PTO shaft at all where it goes over the gearbox input shaft. It is a very thick.
Slip clutch is fine. As mentioned, I inspect and maintain them.

 

[Too many Kubotas]

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.

I understand all of that and understand preload.

Bolt is tightened so there is no sliding movement of the bolt. Just snugged not torqued.

When researching tightening the bolt the answer I get the most is to snug the bolt down to remove any play in it.

Not a great comparison, but if you only snugged your lug nuts down, breakage would follow.

Based on the photos, those bolts have taken some serious shear forces.

 

[wirlybird]

Not a great comparison, but if you only snugged your lug nuts down, breakage would follow.

Based on the photos, those bolts have taken some serious shear forces.

That is the thing, they haven't as far as cutting goes. As mentioned, I am cutting field grass, maybe 2 feet high, on flat smooth land. It's just early spring growth, nothing even heavy.
The cutter doesn't hit anything, it doesn't bog down. No indication of any big shear stress. That is what is puzzling.

These bolts are breaking after 6 or 7 hours of cutting. Some of my other cutters will go the whole summer without breaking a bolt and some of those are being used in quite rough areas.

One thing is that the bolt is only breaking on the thread side and not shearing like you might expect from a typical shear force incident.
Once the rain quits I am going to pull the shaft and inspect the bolt holes and see if I can find anything.
Also am going to change to another PTO shaft and see.

 

[Too many Kubotas]

As mentioned earlier, it looks like the bolts have been "rattling" in their holes, until they give up due to the shear forces. Work hardened if you will.

I don't think it's one "shear force incident", but repeated abuse from not being tight.

 

[wirlybird]

As mentioned earlier, it looks like the bolts have been "rattling" in their holes, until they give up due to the shear forces. Work hardened if you will.

I don't think it's one "shear force incident", but repeated abuse from not being tight.

Could be but I do tighten so snug, no play, I just don't crank them down!
I agree they are getting banged up some.
Going to pull the one that is in there and see how it looks. It has about an hour on it.