OK, I'll give it a try. First of all, when there is no misalignment between
the input shaft and the output shaft, both types impart a constant angular
velocity from the input shaft to the output shaft. The diffence occurs when
there is misalignment between the I/S and the O/S. When the I/S and O/S are
angularly misaligned, the four bearings in the U-joint's crosspiece do not
all rotate in the same plane. This "wobble" in the plane of rotation means
that the output shaft's angular velocity varies through one revolution. It
will slightly rise, then slightly fall, for four cycles per revolution.
This gets worse as the angle between the I/S and the O/S increases, which is
why UJs are not recommended for angular misalignments greater than 15
degrees, BTW. A CV is designed so that the plane of rotation of the
universal element of the joint is constant, thus imparting constant angular
velocity to the O/S no matter what the angular misalignment is, up to the
joint's mechanical limits. It is very difficult to describe the CV joint's
internal construction, but suffice it to say that it relies on contact
between balls and grooves, and the balls can move axially in the grooves
that follow an arc that is the same radius as the joint, except the
direction of movement of the balls is in an axial direction and they can all
move independently, I believe, except the mechanical design of the CV
constrains them to all be in the same plane that is coincident to the
bisector of the angle between the shafts. There, that was easy!
Interestingly, if a drive system consists of an I/S, a UJ, a tumbler shaft,
another UJ, and an O/S, then if the I/S and O/S are misaligned axially but
are parallel, the I/s and O/s will rotate with constant velocity, while the
tumbler shaft goes through the gyrations I described above. This is why two
UJs can be used per drive shaft in IRS - the output shaft from the diff and
the axle shaft are, for the most part, parallel, except at extremes of
suspension movement, or in the case of a vehicle with a lot of negative
camber. Even in that case, the angular misalignment is only a few degrees,
so the variation in angular velocity is minimal, but it does contribute to
wear in the UJs. I guess, for reliablity, I'd go with the CV joints,
whereas for cost I'd go with the UJs. There, does that help?
-----Original Message-----
From: sujit roy <sujitroy@hotmail.com>
To: triumphs@autox.team.net <triumphs@autox.team.net>
Date: Thursday, September 16, 1999 12:10 PM
Subject: UJ verses CV joints (What's the main difference?)
>
>I've seen after market half shafts advertised for the Stag with CV joints
>compared to the Standard UJs' What's the major difference between the two
>types of joints?
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