Help - 2 pc driveshaft out of balance

[mcintyrederek]

1986 C10 with TH400 and two piece drive shaft.  I replaced U-joints a while back and forgot to make a mark where the two drive shafts hook together right after the carrier bearing.  I stuck it back together and now get a slight wobble out of balance vibration that starts at 35 MPH and ends at 40.  There are several small metal weights stamped on to both drive shaft sections.  I wonder, at best guess, should all the weights be lined up on the same side?  They aren't now.  Just trying to find some logic on to how it should go back together without trial and error. 

[Irish_Alley]

in my mind if each piece has its own weight that means each piece was balanced independently. ive taken them apart before and not noted how to put it back together and didnt have a problem

[VileZambonie]

The trunnions should be in line for starters and driveshaft phasing is critical. Check and ensure they are all lined up and at this point you will have to use trial and error since you did not properly witness mark it when you took it apart.

[Irish_Alley]

The three most common causes of driveshaft vibration are: Driveshaft Imbalance, Critical Speed, and Universal Joint Operating
Angles.

Important: The critical speed of an assembly can be affected by driveshaft imbalance, improper universal joint operating angles,
or improperly phased driveshafts. (A properly phased driveshaft has the in-board yokes of the shaft in line with each other.) Each
of the above items will tend to lower the true critical speed from the values shown on the calculator.
so if the trunnions (the part the u joint caps go on) are inline then your phasing is correct

info is from spicer

[mcintyrederek]

I'm guessing if I can't get it right, I'll have to take it to a machine shop to get it done correctly.  The spline that the rear piece slides on to the front piece is 27 spline I think.  So that means I have a 1 in 27 shot at getting it back on the way it came off.  If I take the rear piece off and rotate it one spline notch each week until it doesn't vibrate anymore, it'll only take 6 months to hit every combination..

[Irish_Alley]

A properly phased driveshaft has the in-board yokes of the shaft in line with each other
what this means is if your front u joint is like this + your back u joint should be +. if your front yoke is + your back yoke shouldnt be like this x
a picture is worth a thousand words

it has nothing to do with how many splines you have. but which way the yokes are pointing

the green one has all the driveshaft yokes in the same direction the blue one doesnt. the green one is right the blue one is wrong

[Irish_Alley]

one more

[mcintyrederek]

Thanks tremendously, I have never paid attention to that.  Mine are out of phase, and that's what the problem is.

[Irish_Alley]

vile is the one who mentioned it, i just elaborated his ideal. but your welcome for what i did

[mcintyrederek]

I don't have any photos to show but I can describe.  The problem is now fixed.

The U Joints in fact WERE in phase.  The back section of the driveshaft was just fitted 180 degrees from what is was supposed to be.  When I installed the U Joints I did not pay attention to the orientation of the driveshaft..  I learned this when I pulled the rear section off the splined joint just behind the center carrier bearing.  The spline has a keyway in it and it only goes on one certain way.  I knew then that I must have put the back section in 180 degrees out.  There are three balancing weights..  one on the short driveshaft coming out of the transmission, and two on the longer piece hooking to the rear end.  Apparently it does matter how the weights are aligned for proper balance.  No more shakey shakes at 35 to 40 MPH..  Thanks for the good info and we got it all fixed now.