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.

 

[DieselBound]

Pictures worked the other day. Suspect a system glitch: I'll follow up to make sure they show up for posterity sakes. The pictures really don't add any insight into the problem, just shows what the cutter looks like with the top link fully contracted.

This is a slip clutch shaft, so swapping ends ain't going to be possible without disassembling the U-joint on the slip clutch end.

Again, the previous owner appears to have had a problem as well: he had a Deere, probably 45hp-ish. If I am to believe him, and by the newish appearance of this shaft, he had recently replaced this shaft because he couldn't find U-joints for the previous shaft. Because of this I do not believe it's anything to do with my tractor or my use, but something else.

On the individual shafts, is the larger diameter piece supposed to go on the tractor end? That's how this one is set up. pmsmechanic might be on to something regarding the proper alignment of the joints: I'd never think this would be an issue, so perhaps that would be the case with most others as well; NAPA didn't seem to be concerned; I didn't, however, provide both shaft sections.

I'd like to check the rear (mower end) section, but am not sure how to get the plastic shield's cover off. There's these plastic (white) tabs that say "PRESS." I'd attempted to do so (on the other end) but was unsuccessful in getting the plastic housing to separate: I'm guessing that the tube disconnects from the collar- this is what I want so that I can expose the shaft to inspect for straightness. If I can get this off I'll upload a picture of all these bits (no assurances that this site will display it!).

 

[RickB]

Unless the person doing the assembly has a special talent for creative stupidity it is impossible to assemble a tri lobe or lemon shaped PTO shaft and get the joints out of phase.

 

[DieselBound]

Unless the person doing the assembly has a special talent for creative stupidity it is impossible to assemble a tri lobe or lemon shaped PTO shaft and get the joints out of phase.

To help clarify, I believe the issue is with the orientation of how the U-joints are installed (maybe the alignment of the yokes?) and not with the alignment of the shafts (which are keyed).

 

[RickB]

To help clarify, I believe the issue is with the orientation of how the U-joints are installed (maybe the alignment of the yokes?) and not with the alignment of the shafts (which are keyed).

Again, not possible under anything approaching normal repair methods. The yokes only fit the tubes one way, just as the tubes only fit together one way.

Google universal joint phasing and see if yours are phased properly.
You don't have to guess or speculate.

 

[DieselBound]

Again, not possible under anything approaching normal repair methods. The yokes only fit the tubes one way, just as the tubes only fit together one way.

Google universal joint phasing and see if yours are phased properly.
You don't have to guess or speculate.

I only had to look at the shaft to see, so yeah, got i!

Pretty much without a clue here. Previous owner had this issue and now I'm having it.

 

[MHarryE]

I read above yours should be a Bondioli Series 4 which has a 35 HP capacity at 540 rpm. The capacity rating is with u-joints straight and drops quickly with misalignment (I know j-joints are intended to transmit power around corners but they do it with a cost in driveline life).

PTO Driveshaft - BONDIOLI & PAVESI - Page ndeg 13 - PDF Catalogue | Technical Documentation | Brochure

the link is to the 250 page bondioli & pavesi pto driveshaft design manual.

Torque limiters (assuming yours has a slip clutch) can have a large variation when left sitting. Page 142 of the design manual covers friction clutches.

 

[Rock knocker]

Unless the person doing the assembly has a special talent for creative stupidity it is impossible to assemble a tri lobe or lemon shaped PTO shaft and get the joints out of phase.

To help clarify, I believe the issue is with the orientation of how the U-joints are installed (maybe the alignment of the yokes?) and not with the alignment of the shafts (which are keyed).

The shafts normally have a key way so while they may appear the same on all sides they can't go together unless the u joints are phased. And because the yokes are welded to the shafts it is unlikely this could get messed up.

Sorry tp reiterate what has already been stated.

 

[Xfaxman]

Here is a visual aid, because some shafts can be put together out of phase or a repair job may have welded the yokes on wrong.

 

[DieselBound]

Can I point out that it wasn't me that raised the issue of the "out of phase" U-joints? I just kind of jumped on it (before I actually looked) in more or less of a crapshoot hope thing.

According to the manual for this cutter it says that it's a class 4 drive line. Gearbox is rated 110hp. Bush Hog states that the MINIMUM HP is 40. No reference to what the "maximum" would be. And of course, there's no mention of whether this is engine or PTO HP they're talking about.

Harry, yes, it's a Bondioli: there were two different ones spec'd, but the one on this unit is a Bondioli.

In my mind I'd figured that this cutter would work on my NX. Apparently the shaft won't (and I don't want to try patching around this problem with a torque limiter), in which case is it possible to up-scale the shaft? I'd hate to have to get rid of this cutter, but if it ain't going to work it ain't going to work.

How can one bend a shaft? I'm really thinking that the thing was bent when I got it: always had a bit of a vibration to it. And most likely it was the bent shaft that caused the U-joint to fail (not my use).

 

[RickB]

Bush Hog has been making rotary cutters for more than 50 years. You don't have to re-engineer this driveline or up-spec it. You might have to buy some parts or maybe even a new shaft. There's nothing special or unique about connecting a 286 to an NX 5510. Find a Bush Hog dealer or a retailer of farm equipment parts that understands PTO shafts and get yourself fixed up. Do learn how to service the slip clutch and make sure it is done at least annually.

Download your rotary cutter operator's manual and parts manual here:

http://www.bushhog.com/uploads/documents/BHRotaryCutter280-285-286-287OM-09.pdf

http://www.bushhog.com/uploads/documents/BHRotaryCutter280seriesPM-11.pdf

All the parts for your shaft are available generically or from Bush Hog.