Overstressing PTO U-joints?

[DieselBound]

Was mowing some brush today (with the NX5510) and looked back to see the PTO shaft had dropped from the rear of the tractor Yoke was still on the output, with one U-joint cap in it. The one cap is chewed up a bit, likely from when the shaft decoupled. The yoke is hammered and likely needs replacing.

I had just had this replaced (by a NAPA shop). I don't trust the U-joint that they used. And now it looks like I need to replace the entire tractor side half of the shaft (w/yoke). I'm looking for input to make things right again.

Some background:

Acquired this bush hog -Bush Hog 286- earlier this year (or late last year). The previous owner said that he had to replace the shaft because he couldn't find a proper replacement U-joint. When I started checking things out after I got the unit I noticed that the U-joint's caps appeared a bit cracked/broken. Oh, this is on the tractor end of the shaft. I took this in to my local NAPA shop and had them replace the U-joint. And now with perhaps 10 hours of total use the U-joint gave way.

I was aggressively going after brush when it failed. I had been lifting up the tail wheel up quite a bit (hydraulic top link). I'd suspect that this overstressed the U-joint, but given that the unit came to me with stressed U-joints (I don't believe that the previous owner had a hydraulic top link on his tractor) I'm wondering if there's some other root problem going on here.

 

[RickB]

Driveline should be a metric Bondioli and Pavesi style (E G is the same) metric Series 4. If it is original to the cutter. What made yours fail? Who knows. There's too many fingerprints on it. The driveline is properly sized for the cutter provided the slip clutch is not seized.

 

[KYErik]

Is it possible that he PTO shaft can't get short enough (it is a little long) and begins to bind when you raise it with the hydraulic top link? That would really stress the joints (as well as the PTO stub bearings in your tractor's rear axle)

 

[Gator6x4]

Is it possible that he PTO shaft can't get short enough (it is a little long) and begins to bind when you raise it with the hydraulic top link? That would really stress the joints (as well as the PTO stub bearings in your tractor's rear axle)

:thumbsup: My thoughts also, the PTO is to long and raising the rear Up up with hydraulic top link is really stressing the U joint. If the pto shaft is to long and caused the failure be thankful it did not damage the pto area of the tractor.

How to measure and cut a PTO shaft can be found here. everything attachment on how to cut and measure a pto shaft - Google Search

 

[Oldoak]

I'm thinking if it were raised high enough to create a sharp angle at the "U" joints while running at full PTO RPM, that would really put extreme stress on the joints and break them for sure.

 

[DieselBound]

Shaft overlap is about 6 1/2". Retracting the top link does not significantly change the distance: I was expecting to see a change, but surprised that there really wasn't any.

Looking closely at the shaft it appears that it's not straight. From the beginning there's been a pretty good vibration going on; I just figured that it's something that one can expect from a big mower.

How does one go about putting the shaft back on? The shaft that's attached to the mower is laying on the plastic shield, impeding pushing the other half on. I was wanting to do this just to check for length at full compression.

I'd be more inclined to blame this on myself BUT, it is quite apparent that the previous owner had issues as well.

 

[DieselBound]

Here's some pictures for reference.

Top link fully retracted (angle really isn't as bad as one would think given how far the tail wheel is lifted):
View attachment 510842

Here's what the shaft looks like when the top link is fully compressed and the shafts compressed are fully compressed:
View attachment 510845

Oh, and I figured out how to get the shafts back together (duh!).

 

[pmsmechanic]

It's very important that the pto shafts are assembled correctly. The two yolks on the same shaft should be lined up with each other. In other words if you lay the yolk on one half of the shaft horizontal the other yolk on the opposite end should also be horizontal. If they are out even by a little bit there will be a destructive vibration.

 

[Rock knocker]

Here's some pictures for reference.

Top link fully retracted (angle really isn't as bad as one would think given how far the tail wheel is lifted):
View attachment 510842

Here's what the shaft looks like when the top link is fully compressed and the shafts compressed are fully compressed:
View attachment 510845

Oh, and I figured out how to get the shafts back together (duh!).

Can't see your photos

 

[boggen]

x2 can not see pictures.

i do not remember the specifications. all i know is that there are different "sizes" of PTO shafts. larger more beefer the more HP / torque they can take.

also take into consideration when sizing PTO shaft, HP of tractor, and gear box of 3pt hitch implement. most likely and normally tractor PTO HP is smaller than 3pt hitch implement is.

i do not remember the "angle" there is a limit on U joints in how much you angle them. once you get past a certain angle. they will begin "jamming up" each 1/4 or is it 1/2 turn. as the U joint rotates about.

watching some youtube videos awhile back. with different drive shafts and different joints for drive shafts. there was a "clock wise" smooth, and then a "counter clockwise" vibration. it was like what in the world?!?! but it was what it was. swap end for end on the test rig, and you could see it again by reversed rotation. also in the videos, it showed, getting the joints at certain angles and the shafts would really begin to vibrate severely. to point you could see the shaft it self "bending" due to the U joints got to x amount of angle.

no enough overlap between the 2 halves, can cause vibration. kinda like using a 1"x1" lumber vs a 2x12 lumber for a main beam of a deck. the 1x1 just does not have enough structural support and will bend/bow. not having enough overlap = 1" x 1" lumber. having to much overlap = damaging your tractor, shaft, and 3pt hitch implement (which ever gives first, and extra wear and tear on the others)

U joints in legos of all things. you can play around with, and watch some of the plastic pieces bend when you get the U joints at to much angle. yet you have everything nice and tight within lego standards of connecting stuff.