PTO Shaft to Gearbox Bolts Keep Breaking!

[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.