JekRankin
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| posted on 29/12/17 at 12:12 PM |
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2 piece prop alignment
A bit confused about how to properly align a 2 piece prop for BEC applications.
My diff and engine output flanges are parallel, but offset from each other by a small amount. So if I had a one piece prop, it would run between them
with the angles at both uj's being equal/opposite, and non zero. For a 2 piece prop, I understand this is the desired arrangement.
But.....I have a two piece prop with 3 uj's and a centre bearing.
The 3rd uj is between the diff and the bearing mount. How do I align this?
If I run the prop as if it were a 2 piece and position the bearing such that the front and back sections of the prop are in line with each other,
I'd end up with the central uj running with zero angle. I've read that uj's should idealy run with a small angle to avoid
brinelling. But...if I introduce a small angle to the central uj, then I'll throw the rest of the prop out of balance. Any thoughts?
Jek
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CosKev3
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| posted on 29/12/17 at 03:39 PM |
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Can't you alter the height of the centre bearing mounting?
That's all I've done on mine,so I've got slight angles on all UJ's as you say they shouldn't be run straight 
Have you got a pic of prop?
Can't understand what the third UJ is all about?
My two piece prop has 2 UJ''s, one on the flange that couples to prop adapter on engine,and one where the prop couples to the diff input
flange.
[Edited on 29/12/17 by CosKev3]
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adithorp
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| posted on 29/12/17 at 06:18 PM |
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Try altering the centre bearing height so that each section has angles in one plane. If thats not possible don't worry too much about it.
CosKev3, have you not got a UJ between the 2 sections where the centre bearing is? Never seen a 2 piece prop like that.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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CosKev3
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| posted on 29/12/17 at 06:32 PM |
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quote:
Originally posted by adithorp
Try altering the centre bearing height so that each section has angles in one plane. If thats not possible don't worry too much about it.
CosKev3, have you not got a UJ between the 2 sections where the centre bearing is? Never seen a 2 piece prop like that.
Not sure now you've said that!!!! 
Yeah just looked at a pic, I've got 3 UJ''s! One right behind the centre bearing as well as front/rear.
[Edited on 29/12/17 by CosKev3]
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JekRankin
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| posted on 3/1/18 at 02:07 PM |
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Thanks for the responses. I’ve done a little more research into this one and the only way I can see to keep the shaft in tune is to have two of the
uj’s cancelling each other, and the third one operating at a working angle of zero degrees.
It’s going to be easiest to set it up with the central uj working at zero degrees.
I know that ideally, uj’s should operate at a small working angle, but I think it’s probably preferable to run one straight instead of having a shaft
with out of phase velocities.
I’m being quite cautious about this having seen the results of a few prop failures. I’ll be fitting catchers too.
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