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.

 

[Too many Kubotas]

Could be but I do tighten so snug, no play, I just don't crank them down!

Using the comparison to lug nuts again, it's not the fasteners that keeps the wheels secure. It's the resulting clamping force between the wheels and hubs that does the job.

Sure, untorqued lug nuts would keep the wheel from falling off...but not for all that long.

 

[wirlybird]

Using the comparison to lug nuts again, it's not the fasteners that keeps the wheels secure. It's the resulting clamping force between the wheels and hubs that does the job.

Sure, untorqued lug nuts would keep the wheel from falling off...but not for all that long.

You could tighten the bolt down to its max torque and it isn't going to tighten the PTO shaft collar around the input shaft. The collar is too thick. It isn't going to compress at all!
However, tightening will keep the bolt from getting sloppy in its hole.

 

[Jaygyver]

I would bet most cutters have different requirements for the bolts. Tightened or snug. Some use them as a safety measure if the clutch fails to give and the bolts are then the last gasp protection. I would bet your Bee is that way? So I would torque the bolts down a bit and let the clutch do it's thing.
As an expert non-expert I can't see the reason to have the bolts shear before the clutch slips, wouldn't it be harder on the equipment?

 

[wirlybird]

I would bet most cutters have different requirements for the bolts. Tightened or snug. Some use them as a safety measure if the clutch fails to give and the bolts are then the last gasp protection. I would bet your Bee is that way? So I would torque the bolts down a bit and let the clutch do it's thing.
As an expert non-expert I can't see the reason to have the bolts shear before the clutch slips, wouldn't it be harder on the equipment?

On my Deere (Frontier) and other cutters they call for grade 8 1/2" by 3" with the slip clutch.
I usually run a grade 5 as a safety for the slip clutch. I figure the clutch should slip but just in case!

 

[Too many Kubotas]

You could tighten the bolt down to its max torque and it isn't going to tighten the PTO shaft collar around the input shaft. The collar is too thick. It isn't going to compress at all!
However, tightening will keep the bolt from getting sloppy in its hole.

I can't quite picture how your setup looks, having nothing like it.

It just seems odd to me to have loose (okay, snug) fasteners in any situation where rotational forces are transferred through them.

 

[wirlybird]

I can't quite picture how your setup looks, having nothing like it.

It just seems odd to me to have loose (okay, snug) fasteners in any situation where rotational forces are transferred through them.

It is a pretty standard setup.
In the picture the bolt in question, green arrow, holds the PTO saft to the input shaft on the gearbox as you might assume!
You can tighten this bolt as much as you want and it will not make the collar, red arrow, on the PTO shaft any tighter on the gearbox input shaft.
Tightening the bolt simply keeps it from sliding in the holes. It does not "clamp" anything in place.

Now, on this type of setup all of the driven force from the PTO shaft is transferred to the gearbox input shaft via that bolt. The input shaft nor the PTO shaft are splined in this case.

 

[dicktill]

It is a pretty standard setup.
In the picture the bolt in question, green arrow, holds the PTO saft to the input shaft on the gearbox as you might assume!
You can tighten this bolt as much as you want and it will not make the collar, red arrow, on the PTO shaft any tighter on the gearbox input shaft.
Tightening the bolt simply keeps it from sliding in the holes. It does not "clamp" anything in place.

Now, on this type of setup all of the driven force from the PTO shaft is transferred to the gearbox input shaft via that bolt. The input shaft nor the PTO shaft are splined in this case.

View attachment 3549135

In my opinion, that bolt needs to be very snug in its hole to work properly as a shear bolt. Can you drill/ream it out to "the next size"? And as stated before, you need to have the shank of the bolt in the sheer plane, not the threads. Also, I do not believe in using Grade 8 bolts in such an application, as they are too brittle.

 

[wirlybird]

In my opinion, that bolt needs to be very snug in its hole to work properly as a shear bolt. Can you drill/ream it out to "the next size"? And as stated before, you need to have the shank of the bolt in the sheer plane, not the threads. Also, I do not believe in using Grade 8 bolts in such an application, as they are too brittle.

I'd agree on grade 8. That is what the manuals call for however.
Drilling out the input shaft would be difficult as it is hardened. Without taking the shaft out so you can secure it in a drill press, mill or whatever works, I think you'd make it worse!

The thing is, I don't have this issue with my other cutters.
Plenty to look at now and see if I can find the culprit.

 

[ning]

I don't see how a shear bolt like this gets "stretched and relaxed" during the use - we're not talking thousands of RPM here and the distance from the center of rotation is very small; the bolt head may have 15g outward on it, which isn't actually that much for a 1/2" steel bolt (consider, when you hold the threads of the bolt with the head down, it's got 1g on it; imagine the head of the bolt now weighing 16 times as much - what's that, a few ounces? - it's not stretching much from this), and at a constant 540RPM that 15g isn't changing until you disengage the PTO or spin the clutch.

Note that if you torque a bolt down, it gets narrower as it stretches, so a snug bolt gets slightly less snug and is more likely to get pounded. What is the point of torquing a shear bolt? We're looking for shear, not tensile, and I think torquing it is going to reduce the shear strength.

Shear bolt should have a nut on it to keep it in, but it doesn't take much. I use a nyloc and have a few extra on hand, though not as many extra as bolts, since I usually can sometimes find the old one.

IMO the bolts are getting hammered because they're too small at this point for the hole, for whatever reason. Increase the bolt size (like go the next slightly larger metric) and ream the hole out to so that whatever goes in is tight; make an effort to have the shoulder go all the way through; if the shoulder extends beyond, don't care, just snug the nut on it. *if* the bolt moves during the spin (I strongly doubt it will if it's not way too small) it's going to either have the cap against or the nut against.

 

[wirlybird]

I don't see how a shear bolt like this gets "stretched and relaxed" during the use - we're not talking thousands of RPM here and the distance from the center of rotation is very small; the bolt head may have 15g outward on it, which isn't actually that much for a 1/2" steel bolt (consider, when you hold the threads of the bolt with the head down, it's got 1g on it; imagine the head of the bolt now weighing 16 times as much - what's that, a few ounces? - it's not stretching much from this), and at a constant 540RPM that 15g isn't changing until you disengage the PTO or spin the clutch.

Note that if you torque a bolt down, it gets narrower as it stretches, so a snug bolt gets slightly less snug and is more likely to get pounded. What is the point of torquing a shear bolt? We're looking for shear, not tensile, and I think torquing it is going to reduce the shear strength.

Shear bolt should have a nut on it to keep it in, but it doesn't take much. I use a nyloc and have a few extra on hand, though not as many extra as bolts, since I usually can sometimes find the old one.

IMO the bolts are getting hammered because they're too small at this point for the hole, for whatever reason. Increase the bolt size (like go the next slightly larger metric) and ream the hole out to so that whatever goes in is tight; make an effort to have the shoulder go all the way through; if the shoulder extends beyond, don't care, just snug the nut on it. *if* the bolt moves during the spin (I strongly doubt it will if it's not way too small) it's going to either have the cap against or the nut against.

All very good points and I have to agree that tightening the bolt more than bolt head and nut touching the shaft collar doesn't accomplish anything. You certainly are not clamping anything down.
Going to be looking at hole size vs. bolt size at this point.