Date: Tue, 8 Dec 1998 10:42:04 EST
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: CV-Joints (was: Web Page Update / Crew Cab update)
** Reply to message from "Dr. Rainer Woitok"
<woitok@RRZE.UNI-ERLANGEN.DE> on Tue, 8 Dec 1998 09:55:43 +0100
Woof! This is easy to visualize if you already know.... I'll give
it a try.
First of all, the CV property means that the input and output shafts
rotate at the same angular velocity throughout an entire rotation and
without regard to how much the joint is flexed (within its design
limits).
A "regular" U-joint is simply a disk with four stub shafts at the 90
degree points. The input and output shafts each have a trunnion which
fits over two of the stubs at the 180 degree points. Now if you hold
this straight, you simply have an expensive shaft, input and output
velocities are the same. However, as you start to bend the joint the
output shaft velocities will start to vary up and down through a
complete rotation. For angles of a few degrees such as you get with a
front engine / rear drive vehicle, the variation is small and is taken
up in torsioning the (long) drive shaft components, and in other
system slop. But as you increase the bend angle, the variation
becomes pronounced, and at a certain angle the joint locks up
entirely. Thus this is no good for transaxle systems with their short
output shafts and significant bend angles.
Therefore a joint is needed which provides a considerable range of
angles while maintaining a constant output speed, i.e. a CV joint.
These are typically much like the one illustrated on Tom's website,
with diagonal ball races in the inner and outer hubs, and power
transmitted through the balls. At any angle other than 0, the balls
slide back and forth in the races during a revolution, which is why
they need superb high-pressure grease, and why they fail even though
they are made extremely hard (if you look at the picture of a complete
Lobro joint on Tom's page you can see the color pattern resulting from
the hardening of the inner portion of the outer hub).
In the Vanagon the inner and outer joints are identical (and
incidentally you can sometimes get some extra life from a joint by
turning the axle around, which puts the pressure on the opposite side
of the races), but on most cars the inner joint is of simpler
construction with a formed outer hub attached to the transaxle, and
three stub shafts with rollers on the output shaft, which slide in
longitudinal grooves in the outer hub. I don't know why, does someone
else here? Why can the inner joint be simpler and cheaper than the
outer?
david
>
> I know that the "CV" in "CV-Joints" refers to "constant velocity".
But
> what exactly does this mean? Under which conditions and where is
there
> a constant velocity? And why do normal U-Joints under the
same
> conditions not have this constant velocity property?
>
> Inquiring minds want to know :-)
>
> Sincerely
David Beierl <dbeierl@ibm.net>
401 274-5827 voice, -6349 fax
OS/2 V4, FP9, JVM 1.1.6, JSM 98.6.3
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