Broken PTO pump bolt - B4672 backhoe

torch

Well-known member

Equipment
B7100HSD, B2789, B2550, B4672, RC54-71B, 48" cultivator, homemade FEL and Cab
Jun 10, 2016
2,195
434
83
Muskoka, Ont.
I was having great fun digging holes when suddenly boom operation became rhythmically jerky. In my experience, sudden changes in operating characteristics never bode well, so I shut it down and started investigating.

I soon found most of a broken bolt lying underneath the PTO pump. Sure enough, it seems to have been securing the PTO adapter to the pump shaft. Once that fell out, the adapter started slipping on the pump shaft, causing the rhythmic action under load. And warming the adapter due to friction!

It's a socket cap head (ie: Allen head) machine screw, either 5/16" or 8mm (threads are gone so I'm not certain which). Harder than a grade 8 bolt, anyway. I'm not sure why it broke, but the rust vs the shiny bit on the broken end shows it was cracked most of the way through for some time before it failed completely, so maybe it was overtightened. It is not grooved like a shear bolt, and did not shear off at the shaft, it seems to have broken off at the nut.

I had some 1/4" gr. 5 bolts as spare shear bolts for my 3ph snowblower in the tool kit, so I threw one of them in there temporarily and finished up my hole.

The IPL on the Kubota website does not show the bolt. It shows a machine key on that pump shaft. I don't know if the bolt was missed on the drawing or the result of a design revision. I assume that at one point, the hydraulic relief valve was considered adequate protection, but have no way of knowing if the bolt was the revision, or the key. I would hope that Kubota has the latest version on the website, but who knows?

So my question is: should I replace the bolt with a grooved shear bolt?
 

Dave_eng

Well-known member
Lifetime Member

Equipment
M7040, Nuffield 465
Oct 6, 2012
4,640
642
113
Williamstown Ontario Canada
I was having great fun digging holes when suddenly boom operation became rhythmically jerky. In my experience, sudden changes in operating characteristics never bode well, so I shut it down and started investigating.

I soon found most of a broken bolt lying underneath the PTO pump. Sure enough, it seems to have been securing the PTO adapter to the pump shaft. Once that fell out, the adapter started slipping on the pump shaft, causing the rhythmic action under load. And warming the adapter due to friction!

It's a socket cap head (ie: Allen head) machine screw, either 5/16" or 8mm (threads are gone so I'm not certain which). Harder than a grade 8 bolt, anyway. I'm not sure why it broke, but the rust vs the shiny bit on the broken end shows it was cracked most of the way through for some time before it failed completely, so maybe it was overtightened. It is not grooved like a shear bolt, and did not shear off at the shaft, it seems to have broken off at the nut.

I had some 1/4" gr. 5 bolts as spare shear bolts for my 3ph snowblower in the tool kit, so I threw one of them in there temporarily and finished up my hole.

The IPL on the Kubota website does not show the bolt. It shows a machine key on that pump shaft. I don't know if the bolt was missed on the drawing or the result of a design revision. I assume that at one point, the hydraulic relief valve was considered adequate protection, but have no way of knowing if the bolt was the revision, or the key. I would hope that Kubota has the latest version on the website, but who knows?

So my question is: should I replace the bolt with a grooved shear bolt?
I do not see a true shear bolt offering you any benefits. Encouraging shear incidents through the use of a shear bolt may just make the adapter fit more sloppy leading to increased problems

I agree the relief valve is the protection device for pto driven pump.

I would be looking for a situation where the failed bolt was loaded in a way outside the original design concepts. A hose snagged on something when the hoe is being moved, something getting temporarily wedged between the hoe and pump as the hoe is moved...........

Dave
 

torch

Well-known member

Equipment
B7100HSD, B2789, B2550, B4672, RC54-71B, 48" cultivator, homemade FEL and Cab
Jun 10, 2016
2,195
434
83
Muskoka, Ont.
It would be pretty hard for anything to get in there at the bolt with the BH mounted. It's pretty well surrounded by heavy duty steel, with the PTO shroud above and beside and the pump torque plate below. It was hard to get my hand in there to fit the temporary replacement and the BH has to come off to remove the pump.

But anything is possible, it's a rotating part so it might not take much of a rock to cause the fracture in a hardened bolt. And it was mounted on a different model before I got it. As I said, the rusty surface suggests this was an old failure just hanging on by a thread.

Anyway, now that I have removed the BH and pump, I see there is a keyway in both shaft and adapter. The through-hole is centered on the keyway. I suspect it did originally come with a machine key, just like the IPL shows but that someone stripped a set screw and decided to drill through instead of fixing the threads.

Now I'm debating if I should fit a heli-coil to the hole and go with a key/setscrew combo or just replace the through-bolt.
 

Dave_eng

Well-known member
Lifetime Member

Equipment
M7040, Nuffield 465
Oct 6, 2012
4,640
642
113
Williamstown Ontario Canada
It would be pretty hard for anything to get in there at the bolt with the BH mounted. It's pretty well surrounded by heavy duty steel, with the PTO shroud above and beside and the pump torque plate below. It was hard to get my hand in there to fit the temporary replacement and the BH has to come off to remove the pump.

But anything is possible, it's a rotating part so it might not take much of a rock to cause the fracture in a hardened bolt. And it was mounted on a different model before I got it. As I said, the rusty surface suggests this was an old failure just hanging on by a thread.

Anyway, now that I have removed the BH and pump, I see there is a keyway in both shaft and adapter. The through-hole is centered on the keyway. I suspect it did originally come with a machine key, just like the IPL shows but that someone stripped a set screw and decided to drill through instead of fixing the threads.

Now I'm debating if I should fit a heli-coil to the hole and go with a key/setscrew combo or just replace the through-bolt.
Rather than a helicoil, is there room to fit larger set screw? Drilling and then tapping a new thread would provide me with greater confidence than a helicoil.

If the key and key way were not snug fits on both the shaft and female adapter, the drill through may have been requiring the through bolt to sustain loads beyond its ability instead of having the key transfer the loads.

Dave
 

torch

Well-known member

Equipment
B7100HSD, B2789, B2550, B4672, RC54-71B, 48" cultivator, homemade FEL and Cab
Jun 10, 2016
2,195
434
83
Muskoka, Ont.
I think you may have misunderstood. The bolt did not shear at the shaft. The threaded end broke off and took the nut with it. The bolt then fell out.

There was no key, just a keyway. The bolt took the full load. The through hole is drilled right through the centre of the keyway, which is what makes me suspicious that the hole originally housed a set screw.