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Double Cardan Propshaft


Aragorn
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I've come across something about these that had me a little intrigued...

A standard defender/disco (and infact most RWD cars) propshaft is known in engineering terms as a double cardan shaft. This is because the UJ's are phased to correct the changing angular speed that a UJ creates, the phasing cancels this out, to allow the input and output to run at the same speed.

Now if we consider the front prop on a D2, which has a double cardan joint at one end things get a bit odd. The double cardan joint itself is in effect a CV (constant velocity) joint which is then attached to a shaft with a UJ at the other end. This means the intermediate shaft (ie the prop itself) is already running at a constant velocity, instead of the usual pulsing that a single UJ would generate. When this constant velocity reaches the axle flange, the single UJ will have changed it into a pulsing velocity, which would really mess with the axle internals...

So what have i missed here?

Is it related to the difference in angle between the diff flange and TXB flange?

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Simple answer - the amount on pulsing is proportionate to the angle of the joint; for a rear prop the two joints cancel each other out. For the front prop on a coiler, the flanges aren't parallel, so the angle across the single UJ at the axle end is small, so it nearly acts as a constant-velocity joint on its own, and that balances the DC joint at the gearbox end. Otherwise, the angles aren't the same and problems arise. Previous to the DC prop, the two UJs were installed out of phase to minimise this effect.

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IIRC regardless of the operating angle of the double cardan joint, ideally the single UJ at the other end should be set at an angle not exceeding 3 degrees. You could set it up with zero angle but the needle rollers would Brinnel into the bearing surface on the U joint cross, so they need some angle to keep working efficiently.

Bill.

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A double cardan is a NEAR constant velocity joint, so doesn't eliminate all pulsing. The phasing only works well when both flanges (diff and TC) are parallel, which is not the case in a (lifted) Rover. By replacing the UJ at the largest angle with a double cardan, the difference is greatly reduced, and so the vibrations diminish as well.

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way too technical for me... but.. i lifted my disco 200tdi 5" on the suspension and the prop used to catch on the bolts at the transfer box end... it smashed the transfer box to bits!!

lesson learnt! replaced the box and put one of the td5 double cardan props on with an adaptor flange and hey ho it works!

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