U-joint phasing and drive shaft theory (long)

[jmbrodsky@domain.elided]

Bill,

     That's close about the speeding up and slowing down, except 
for one fact.  If you have the yoke in phase, meaning that if you 
draw a straight line from the center of one side of one yoke, 
down the entire shaft and to the center of another side of the 
opposite end's yoke, this line should not be drawn along the side 
of the shaft.  If you put a bar in one yoke as if to turn the 
driveshaft, a bar placed in the other end, should be at 90 
degrees when viewed down the axis of the shaft.  Thus the 90 
degree phase.

     Anyway, now that you're confused (I hope not though), the 
key to keeping the drive shaft from vibrating is the center of 
mass of the shaft.  The axis of the shaft actually moves ever so 
slightly when at an angle.  The problem is when the u-joints are 
not phased (bars are parallel when placed throught the yokes) the 
entire center of mass revolves around a radius that leaves an 
offset (eccentricity) somewhat deviated from the center line axis 
of the shaft.  If the yokes are phased, then the shaft ends will 
oscillate around this eccentricity, BUT, the center of mass, 
stays at the geometric center.  This allows the ends to cancel 
eachother out and minimize vibration.  The reason the Double 
cardon CV joint is out of phase at the ends, and in phase for the 
shaft itself, is that there is a ball in the center of the CV 
that keeps the eccentricity lined up with the centerline while 
rotating.

     I wish I had a piece of paper to show this.  If the joints 
are set a 0deg. phase the drive shaft will actually act most like 
a jump rope, all the mass revolves around a center line.  Where 
if the joints are 90 deg phased, then the shaft acts like it's 
suspended at it's center and the ends make circles, like two 
cones connected at their vertices.

     I really hope this helped, if not, I'll try to figure out 
another way to explain it.  I played alot with the Chev when I 
had it.  Since it was a 127 in. Wheelbase, and a mated T/C, the 
rear shaft was 6'6" long and the front was about 29".  The 
Travelall with the divorced T/C looks to have almost equal length 
shafts front and rear.  Also the Chev had a 3" rear shaft, so 
when it wiggled, you could really feel it!

     Thanks for the Trans info, I'm working on a game plan now. 

-Joel Brodsky

     '76 Scout II 345/tf727
     '75 T'all 150 4wd 392/tf727

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Date: Tue, 10 Feb 1998 14:33:36 -0700
From: "Bill Thebert" <bthebert@domain.elided>
Subject: Re: U-joint geometry, A/T removal

Joel Brodsky writes:

>      About the phasing, you are correct.  Whoever has rattles, 
> make sure the yokes on the ends of the drive shaft are 90 deg. 
> out of phase.  If they are in phase,then the whole shaft 'floats' 
> and rattles your fillings out. 

Joel:

Perhaps I missed the beginning of this thread, but this is contrary to
everything I understand about U-joint construction and operation.  

The motion of a bent u-joint is "non-uniform" -- that is, if the "driving"
shaft is rotating at a perfectly consistent 100 rpm, the "driven" shaft on
the other side of the bent joint will actually be speeding up & slowing
down twice per revolution.  Sure, the driven shaft still *averages* 100
rpm, but the motion is not uniform.

The purpose of the "equal and opposite" u-joint angle rule is to reverse
this process.  By inputting an irregular, non-uniform motion to another IN
PHASE u-joint at the very same operating angle, the driven shaft will have
a resulting *uniform* motion equal to that of the original "driving" shaft
in the paragraph above.

Putting the joints 90 degrees out of phase will compound (double) the
non-uniformity of the driveshaft motion rather than "neutralize" it.
<tranny info snip>

Good luck.

Bill Thebert
The Binder Bulletin
http://www.binderbulletin.com 

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