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Cardan shafts problem

Cardan shafts problem

Cardan shafts problem

(OP)
Hello to everyone,
i have really strange case about cardan shaft problems.
I attachment you find some photos about one of the cardan shaft failured.
This is paper mill company and have big trouble during last year. All motors are DC motors with one AC/DC drive per unit motor.
Some indications for you:
  1. in the last year rise the cardan shaft damage before 5 month
  2. In the plant there are more than 10 cardan shaft and alternatively, without apparent reasons, it will fail
  3. Same builder ever, same type of grease ever, and same alignment company provide alignment ever
  4. Same lubrication interval ever
Customer assumes that this problem may derive either from eddy currents or from the fact that a 4MW cogeneration unit was installed in the last year.

What your opinion?

RE: Cardan shafts problem

I don’t work with cardan shafts at all. But my understanding is the arms of that cross form the inner race of needle bearings that oscillate position (don't continue rotating through their full range like normal bearings). One of the photos (below) presents a distinct pattern where the damage is heavier on one end of the inner race than the other. For a simple needle bearing, that type of observation would suggest to me a cause of excessive misalignment… but I’m not sure if the same conclusion holds for cardan shaft bearings.

In terms of what changed the two captain-obvious uninformed swags would seem to be alignment (maybe a different guy at the same company) or parts (maybe inner ring material is more susceptible to fatigue).

Again I have zero cardan shaft experience to go with that. I’m sure someone else can provide more insight.





=====================================
(2B)+(2B)' ?

RE: Cardan shafts problem

Sorry can’t open the attachment because it’s a zip file can you make a pdf please?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Cardan shafts problem

I didn't realize there was a minimum misalignment for cardan shaft / universal joints, but it makes sense and seems supported by this blurb which includes a thumbrule (undoubtedly the manufacturer's recommendation is more relevant):

Quote (Pruftechnik)

The spacer shaft is set at a minimum angle of usually 4° to 6° in order to ensure sufficient lubricant circulation, which in turn prevents the universal joints from seizing

=====================================
(2B)+(2B)' ?

RE: Cardan shafts problem

You'd want at least half a rotation of the needle roller per rev of the shaft. Vibration and poor lube are risk factors. Also, one might find that a vendor has changed manufacturing plants and the new factory doesn't actually replicate the quality of the old one. Ahem.

Cheers

Greg Locock


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RE: Cardan shafts problem

Are you saying there have recently been multiple cases of u-joints with this same damage?

What is the brand name on the Cardan joints? Both old reliables, and recent troublemakers.

It looks like those 3 or 4 needles were edge loaded pretty badly.
I suspect (but do not know for sure) that needles in good quality cardan joints are very slightly crowned ( a few µm / .0001s inch) to make them WAY more tolerant of small misalignments between joint cross journals and cups.

I picture a few things might cause such damage.
1 - Missing needles. There should be a full complement, with no room for more. ( assembly error, manufacturing error )
2 - Poor ID OD concentricity of the Bearing cups. (Poor quality manufacturing)
3 - Bent or poorly machined Cardan cross
4 - Bent or improperly machined yoke ears. Damaged yoke ear bores. (beyond the control of the Cardan joint manufacturer)
5 - IMMENSE torque overload (But I'd still expect more even full length needle loading, and one side of each of the 4 journals on each Cardan to show indications.)

Investigating all those 100% would keep me busy for a while.
A dial indicator for 2
V-blocks and a dial indicator for 3.
A precision pin several inches long for 4.

RE: Cardan shafts problem

Hi electricpete

Thanks very much for the PDF 👍😀

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Cardan shafts problem

(OP)
Thank you electripete for the PDF.
Unfortunately i tried to attach photos directly on the message but lot of error appear ...

@greglocock
This is exactly my idea for lot of reasons.
If nothing is cahged for years and only in this period is changed i'm afarid that vendor could be changed manufacturer and new cardan shaft quality is changed.

@Tmoose
thank you for the very detailed information, i'll go to check them and lot of them seems to be related to manufacturing quality and /or assembly error.. this is strictly related about info of Greglocock

Other than mechanical evident failures, what left me puzzled is the fact that it is happening in all the cardans at the same time while all the other surrounding conditions have not changed ..... I think, but I can be wrong, that the obvious cause of the damage is only the effect of a more structured problem

i'm going to verify alignment of some of the cardan shafts, i'm going to verify if nothing is changed on the motor control finding some tips can reveal the righ way to follow

RE: Cardan shafts problem



That looks a lot like spalling to me. I have seen it caused by fatigue and I have seen it caused by hard impacts.

I would expect to see fretting corrosion type wear if it were a lack of oscillation. Do you see corrosion products in the lubricant?

Is your phasing correct?

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