Thread: phasing the propshafts

A phased shaft is essentially 90° opposed one end to the other.

Early defender/Disco front shafts are semi-phased - aligned approx 45° from each end

Phasing is the orientation.of.the holes of the shaft.

Depending on how you were taught a shaft is in phase if the holes are in alignment or if they are 90 degrees out.

Generally propshafts are assembled with the yokes aligned with each other but rotating the slip joint on the splines can sort vibration issues.

Phasing is the the alignment of the yolks. If the input and output shafts/drive flanges are pretty well parrallel, then the yokes must be inline. Series I, II, III, are all parallel.
My discovery 1 are not parallel. Because they are not parallel, a vibration will be set up through the rotation of the prop' shaft. Mis-aligning the yokes can counteract the vibration (the cause of it).

The theory,

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The poor excuse for a sketch above, is a representation of a prop' shaft with it's 2 Hook's joints (yes that is the actual name for a universal joint - named after the inventor). The drive flanges on the ends are shown as not being parallel. The drive flanges are the input and output shafts.
Each Hook's joint has an input and output shaft. In this case, the output shaft of one becomes the input shaft of the other.

What happens with a Hook's joint, is that, when the input and output shafts are in line, there is nothing happening. But put them at an angle, even a small one, and there is a change in velosities between the two shafts. The greater the angle, the greater the changes in velosity. As they revolve through the full 360 degree rotation, there are 2 speeding up and slowing down sections that alternate, which averages the input speed. So what we get is the output shaft speeding up and slowing down very quickly. This creates a catastrofic vibration. To counter this effect, we put a second Hook's joint into the drive line, that is 90 degrees out of phase with the first one. The input of this second one is the speeding up and down output from the first one. But, because it is out of phase by 90 degrees, it reverses the process completely, and the output shaft is revolving at the same even speed as the input shaft of the first Hook's joint.
Now this is where you get under your LR and have a look. The [ bit of the drive flange is the input yoke of the joint, and is 90 degrees out of alignment with the ] output yoke which is attached to the prop shaft. The yoke (input) at the second joint at the other end of the prop shaft, is inline (close enough for here) with the output from the first joint. But that one is 90 degrees out of alignment with the one on the drive flange (input) to the first hooks joint, and therefore reverses the changes in velosity.
When the drive flanges are no longer close to being parallel, the second joint cannot counteract all the changes in velosity. To help take out the vibration, it is common to, effectively, twist the prop shaft so that the miss timing of the 2 changes in velosity (1st joint accelerating - 2nd joint deaccelerating) counteracts more of the uncounteracted changes in velosity. This is often done by sliding the splines apart and rotating by a spline or 2 and sliding it back together. There are some vehicles with prop shaft yokes that are welded together at the "optimum" angle.


With my Disco 1, I set up a spread sheet to calculate the accelerations at every degree and graph it, so I could see what was happening. I found there was not much difference in change of velosity by changing the phase angle by either 1 or 2 splines, compared to the standard in phase yokes. So I left them in phase. But, that doesn't mean that other models of Disco are the same. I have no idea what my '14 Defender 90 is like.

I hope this clears up some things, and gets other thought processes happening. The forces affecting your prop shaft once you go to high lift, due to the greater angle of the joints, becomes quite servere and sometimes destructive at high speeds.

I still have the spread sheet if anyone would like to see it.


Cheers,

Phill.