PTO Shaft to Gearbox Bolts Keep Breaking!

[ruffdog]

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.

^^Agree, it would be worth a try.

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.

Are you tightening the shear bolts or leaving them a tad loose with a nyloc nut?

 

[wirlybird]

^^Agree, it would be worth a try.


Are you tightening the shear bolts or leaving them a tad loose with a nyloc nut?

I have a 3.5 inch one in there now so we'll see on that part.
I tighten them until the head of the bolt and the nut make contact with the PTO shaft sides. So, no play but not tight at all.

I have wondered what would be better, tighten or leave a little loose?

 

[Too many Kubotas]

I took for granted that they need to be tight.

 

[dicktill]

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

A) I don't know anything about PTO's.
B) It appears as if all three of these are sheared in the threads.
C) If so, you should try a longer bolt so that the "shear area" would fall on the shank itself; use washer(s) as required to prevent the nut from bottoming on the threads instead of clamping the parts. If necessary, trim some of the end of the threads off if they are too long.

 

[ruffdog]

I have wondered what would be better, tighten or leave a little loose?

I have been using a self locking nut, snug it and back it off 1/2 turn.

 

[oldnslo]

Have you noticed if the PTO engagement is abrupt when starting?

Does the mower spin down or stop fairly soon after disengaging the PTO?

Those are the two times you can introduce shock when not mowing.

As a test start and stop the mower a few times. Then check the bolt to see if it showing signs of shearing. Might want to put a mark of some type for orientation so you know if turns or which way it is shearing.

 

[wirlybird]

Have you noticed if the PTO engagement is abrupt when starting?

Does the mower spin down or stop fairly soon after disengaging the PTO?

Those are the two times you can introduce shock when not mowing.

As a test start and stop the mower a few times. Then check the bolt to see if it showing signs of shearing. Might want to put a mark of some type for orientation so you know if turns or which way it is shearing.

No, it actually starts up very smooth. Pull the PTO switch, electric, and after a second or so it is a kind of a gradual spin up. Like engaging a clutch with a smooth action.
When turning off the PTO I first slowly throttle down to about 1500, wait a minute, then switch off. The cutter and shaft spin down somewhat slowly.

I'll see about trying a few times with a new bolt.

 

[Windblown]

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.

 

[AndrewInOz]

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...

 

[Gord Baker]

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