| sammyg2 |
04-21-2015 12:17 PM |
Quote:
Originally Posted by john70t
(Post 8586863)
Mr. SammyG, it's a question well above my pay grade...but... :
I thought the roller bearings in universal joints were always supposed to move/deflect to avoid flat spotting.
(I'm guessing it has something to do with rpm and peak load???)
pic:
http://forums.pelicanparts.com/uploa...1429585754.jpg
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On a single cardan (pictured) the two shafts (input and output) need to be parrallel or ... the output shaft will speed up and slow down twice during each revolution. Same problems can show up if the angularity is larger on one joint than the other. That can cause some hellacious harmonic vibration.
Interestingly enough, if you were foolish to align a cardan shaft perfectly with zero offsets, it would most likely fail like a big dog throwing parts all over creation. It needs an offset to keep to from whipping.
BTW, in 35 years in the industry I have seen or worked on exactly ONE single or double cardan shaft that was not part of a vehicle or crane.
We just don't do it that way. It causes too much vibration and problems and there are better, safer ways to do it.
The most common is with a short spacer flex coupling.
EDIT: just re-read your question. on a u joint if the flex is controlled and consistant, the roller bearing inside the U joint will roll back and forth, back and forth, the same path and same distance over and over and over. If the needle bearings only roll a little tiny bit, the wear will be concentrated on that little tiny bit. Flat spot.
That's part of the reason CV joints are so much better. As they roll around hte ball bearings can spin distributing the wear around the entire ........ what'st he word i'm thinking of, 3D version of perimeter.
U know what I mean.
While we're on the subject of things going boom, here's a crankshaft from a very large quitiplex pump we pulled yesterday for "vibration". (5 cylinder reciprocating plunger pump).
http://forums.pelicanparts.com/uploa...1429647424.jpg
The connecting rods you can see the bottom of are about 30" long and have 5" crank journals.
New crank costs $35k.
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