Engineering question: U-joint needle size

[Extreme-Duty]

In case of our tractors MFD drive axle, we need two 27 x 71 mm or 1.063 x 2.795'' u-joints on each side. The genuine ones got 32 needles of 2 mm or 0.079" O.D. in each cap. Now I have been looking at some greasable options by "GKN", which are not too bad. They are slightly different, having 26 needles of 2.5 mm or 0.984".

How could this tiny difference affect the max. torque and angle speed a u-joint can handle?

 

[mechtech2]

Unless you are writing a paper for school, just get your new ones and keep them greased, if that is an option.

My opinion is that the greater number of needles is better.

 

[punisher]

From a purely torque carrying ability, you want as large a diameter trunnion as possible (meaning more smaller diameter needles), with no grease channels/fittings. Real world, the differences are insignificant. More important is the quality of the materials used than the small differences in design.

 

[Eddie]

I think the smaller dia needles will carry more load due the increased surface area. This info I get from working on 2 cyc racing out-board engines.

 

[RH+G]

The greasable u-joints are never as strong as the non greasable ones. Stick withthe non greasable ones

 

[onion]

Here you go with that 'over-thinking', ED.

I doubt you would ever notice any performance difference between the two. But all other things being equal, more needle bearings mean more surface area... so they should be stronger and more durable.

Also greasable u-joint crosses necessarily have grease channels drilled in them. That makes them weaker than a solid cross.

In my experience, I would prefer a high-quality non-greasable u-joint over a cheaper greasable u-joint for most applications. I would probably only prefer the greasable u-joint if it's likely to see water regularly.

 

[Extreme-Duty]

A special thanks goes to Dr. Onion for reminding me of my OCD. I thought I was healed, buuuuut... here I go.

As opposed to many other sealed-for-life u-joints, these here do indeed have grease channels in all four "arms". I see a potential to drill a hole and tap a thread in one of the caps to place a zerk in there.

So the question about the strongest cross is harder to answer by now, I guess. The greasable joints got slightly longer crosses because of the thinner cap bottoms, which does not mean anything at all. What might be noteworthy is that their needles and arms are shorther, meaning the actual "body" is bigger. Then I have to add the diameter of the arms and the cap I.D.:

U-joint .................... Greaseable .... Sealed

Cap I.D. ..................... 22.95 mm. .... 22.50 mm.

"Arm" O.D. ................... 17.85 mm. .... 18.45 mm.

Cross body width
(where the thinnest part starts). 41 mm. ....... 38 mm.

Picture: Bottom = greasable, top = sealed.

 

[onion]

Just put a different brand on each side and see which one breaks first. Then you'll have your answer.

 

[The_Eric]

What happened to cause the other to fail? How long did it last?

If it only lived a few hours of operation, then maybe try the grease-able part.

 

[Extreme-Duty]

The original U-joints got 12,000 hrs of fairly LIGHT use on them. Call me crazy or not, I can feel a little more wear than I want, considering there are no plans to sell this tractor, unless my parents have to quit farming because of the price situation (pork vs. swine feed). The joints are not totally worn and have not failed, they are just leaking grease and obviously, the cross and the needles got some dark spots that can't be wiped off with acetone. To me it looks like a mix of EP-additives and surface fatigue, maybe even solid lubricants, I don't know.

No matter what, I will be installing some zerks in the caps, as the yokes do not leave the room for a center mounted zerk. 1 - 2 shots of fresh grease every 3 - 400 hrs. would be my suggestion.

The genuine u-joints might be the way to go because of the superior needle setup. Their downside is the smaller body + the longer arms. I may add that a genuine Carraro joint costs 55% of what the greasable high quality GKN joint costs.

That's just me thinking, or over-thinking.