Date: Mon, 7 Dec 1998 12:52:57 -0600
Reply-To: Blue Eyes <lvlearn@MCI2000.COM>
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: Blue Eyes <lvlearn@MCI2000.COM>
Organization: Vexation Computer
Subject: Re: CV-Joints question
Content-type: text/plain; charset=us-ascii
Oh fun, another chance for me to fail in communicating a concept. Out of 700+
List members, my pleas for the merits of gearing steady state highway cruising
to match the engine's lowest BSFC rpm range has not generated EVEN ONE voice
in agreement. But despite that silence, I'm still confident that I'm right,
and to fail to follow that design path is comparatively inefficient. Let's
see if I can fail to communicate so poorly again.
This time I'll describe the simple 90 degree link Universal Joint, often
simply called a U joint. They are NOT CV joints, and to understand why CV
joints were made, you need to understand how these tough, old fashioned,
reliable flex couplings fail to maintain constant angular velocity agreement
between the driving and the driven shaft.
To see how the angular rotation velocity of a driven shaft is affected by a
"universal joint," it is very helpful to exaggerate these transmission
irregularities by examining extreme angles of transmission. So gain access to
a U joint that you can hold in your hands if you don't have an unusually good
dynamic 3D imagination. An old unit, temporarily saved from a mechanic's
trash barrel, which has been degreased would be a great help for instruction.
But a socket set U joint will do if you must stay clean. <G>
Now, define one end as the driving end, and the other as the driven end.
Misalign the driving and the driven rotation centers so they are pointing
widely apart. This forces the center coupling to flex through an extreme
angle range as the two ends are rotated,. Next, imagine affixing an analog
clock's minute hand to each U joint end, with a clock face for each. At the
beginning of our demonstration, align both minute hands parallel with each
other and with one of the coupling link's two pivoting axis directions. Then
spin the clock faces so zero seconds on each aligns with its minute hand.
Analog clock time seconds always correspond with 6 degrees of rotation. There
are 360 degrees in a circle, but only 60 seconds per minute, so 360 / 60 = 6
degrees per second. So any analog face clock can serve as your visual
protractor which you can mentally apply anywhere you go. This enables you to
estimate observed angles in degrees quite well. Teach your kids how to
mentally apply this and it will serve them well through their whole life. I
helped a friend's six year old girl to become able to look at angles and
describe them within 6 degrees after just a few days of "calibrating her eye
ball." Work on it. If you're caught staring off into space, you can always
explain that you're calibrating your eye balls.
Now we know that when the driving shaft second hand is pointing at 0 seconds,
the driven shaft will be pointing at 0 seconds, because we just set them that
way. I can tell you that at an indicated 30 seconds, they will both be
aligned again. But between these two points of rotation, they will not
agree. I just got done writing out a word description of the angular velocity
changes of the driven shaft compared to the driving shaft, broken out by 1/8
turns of rotation for the first 1/2 turn (30 seconds or 180 degrees). There
was no need to repeat that explanation, because it is exactly repeated in the
second half turn. But I deleted it because it got too messy for an audience
this wide. I suspect few who do not already understand this would track it
well. The main thing to remember is that when periodicity of rotation is
transferred through a simple 90 degree link U joint, the driving and the
driven shaft do not have the same velocity most of the time, even though after
any total number of complete rotations, they have rotated the same number of
total degrees.. It's just that they took different paths in terms of
rotational velocities.
Matched U joints pairs can compensate for each other:
But this single 90 degree link U joint effect can be revered by correctly
aligning a second U joint so it does the same thing, BUT IN THE OPPOSITE
DIRECTION. I have seen some incorrectly assembled drive systems where these
effects were not mutually canceling. I remember a shortened drive shaft for a
project car (blush) which got welded back together (don't ask) with the yoke
spun 90 degrees from how it should have been. On plenty of vehicles, just
sliding them in the splined shaft coupling out of sync by 90 degrees will do
the same thing. Rather than canceling each other, the U joint angular velocity
swings were additive! Talk about a nasty shaking dog putting pulses through
the drive system when ever it would flex! If you look at the drive systems in
some old carnival rides, a slower version of this same effect is sometimes
intentionally employed to cause riders to experience continuous cycles of
acceleration and deceleration. But that's another story.
If there is any flexing done by correctly assembled 90 degree link U joint
link drive system pairs, the center coupling shaft will accelerate and
decelerate many times per second as it goes flexing down the road, but the
input and output shaft will always be in angular velocity agreement. But you
MUST align those U joints to correctly to cancel each other's non-CV effects.
If you align them so these effects are additive, the inertia loads will hammer
the drive train when they flex. Even correctly assembled, the connecting
link's inertia induces some minor pulsing loads, but they don't cause much
trouble. (That last sentence clause may cause some disagreement, as this is a
controversial judgment call.) Remember, when there is no drive system
flexing, and in a well designed drive system, that's true most of the time, U
joints cause no non-CV effects. If a drive system is designed to require
flexing with every rotation, some guy with big black rubber ears named Mickey
was an influential member of the design team. Just my opinion.
Having described the effects of the simple, comparatively reliable 90 degree
link U joint design, I will leave it to others description of how nicely CV
joints handle all this rotation velocity agreement. As far as I'm concerned,
that's the only thing they do well in real applications. I'll also let them
explain why they are not nearly as reliable. CV joints are, on paper, more
elegant than the simple 90 degree link U joint system I just described. But
your vehicle is not just a paper drawing. You won't find CV joints in drive
systems where maximum service life is the top design goal. If they were as
reliable, they wouldn't be a constant topic of maintenance discussion.
Has anyone on the List but me ever owned a 600 cc BMW Isetta car? They had
rubber rings with four holes cut through them as their flexing drive
coupling. Two across would attach to each drive or driven shaft. I have a
book showing the mechanics of early cars. In one they had a paddle wheel with
center mounted blades driving rubber blocks captured inside a bowl with blades
coming in from the outside. The more rubber blocks, the closer it would be to
a CV joint. Even a CV joint doesn't have an infinite number of loaded ball
bearings, so it's not quite CV is it?.
John
Maybe we should try cement tires and rubber roads.
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