Torsional Vibration and PSRU Design

Dan Horton wrote:
Dan, based on my studies, I don't think they (clutch springs) are
there to drop total
system resonance below idle speed......I am pretty sure that the
frequency of the driveline system is already low.

Good argument. Consider me corrected, with a caveat. I think
you're right about driveline frequency already being low, even if there
were no clutch. The caveat? The clutch springs are one of the
stiffnesses in that system, and contribute to that low frequency. Take
them out, frequency goes up. Design them in, frequency goes down.

I wasn't disputing the effect you had described, it would be hard to
disagree. My point was more along the lines of --- "are you sure that
is why they are there?" Crankshaft counterweights also lower the
system frequency, but thats not why they are on the crank. It is also
why I am tentative in this. I don't know for a fact, so I may
speculate, but will label it as speculation. I had researched it
before because I had heard the story about them being detuners which I
never found any support for in the literature and your numbers put to
bed. The only things I have found attributed to them in engineering
texts ( not websites or enthusiast pubs like Hot Rod) was shock loading
and the excerpts I quoted before.

Note the use of the term "damper" in the quoted text. Are you sure
the text wasn't speaking of something a bit larger than our light duty
clutches? Not much sign of a frictional damper in the Subaru clutch.


I am pretty sure, given that the image next to the text was the classic
clutch disc we are talking about. For further insight look up patent
2,674,863 It talks about the limitations of the friction mechanism in
standard clutch disc dampers. I have found though that in automotive
practice they seem to take a lot of liberties with the terms dampers and
absorbers. Look at the term "shock absorber". And they call the
detuner a balancer. I don't have a clutch disc around here at the
moment to look at. I have one in a storage building across town, so
may go look at it in more detail.


Regarding the rubber elements someone mentioned in their driveline,
"Automobile Engineers Reference" makes mention of these as well, saying
that they can provide similar damping to the clutch damper

Rubber elements do have a damping value, although it is very, very
small. We got the actual value from Lovejoy when we were doing the
modeling, but logic alone tells you it ain't much. If it had much
damping value, it would melt g

If you stick one in the driveline of an auto it cant help but get hit
with torsional vibrations and they can be found there. We are not
talking about damping at system resonance. The damping is dependent on
the hysteresis of the ruber which obviously creates heat, but it is
exposed to ample cooling air in a drive shaft if the amplitudes and
frequencies are mild I would expect. The rubber is going to heat up if
you are using them to correct for axial misalignment as well, since you
will be doing the same thing to it, alternately stretching and
compressing it. In any event, not my suggestion, just right out of the
manual "Autombile Engineers Reference Book" by Molloy (the book does not
use these terms, but the pictures make it obvious that the Layrub is a
rubber in compression unit and the Rotoflex is a rubber in shear.):

"A normal rear axle with a hotchkiss drive has probably adequate overall
flexibility in the drive shaft aft of the gearbox, and any flexibility
provided by such couplings as the Layrub and Rotoflex is probably
desirable only for local effect, i.e. to reduce gearbox chatter in some
cases. There does, however, in some cases seem to be a marked and very
welcome quieting effect in the car from their use and this may arise
from their effect in reducing the transmission of road noise. Their
torsional flexibility also becomes more desireable overall where more
positive and therefore more rigid drive and braking torque resisting
means are provided on the axle, and even more when independent rear
suspensions or a De Dion axle is used and the flexibility of the
half-shafts is largely subtracted too. This aspect, torsional
flexibility, can therefore be of increasing importance in the future"

and

"In both these cases, the axial flexibility is sufficient to make it
possible to dispense witha sliding joint in the propellor shaft..."


The comment about local effect is interesting and goes back to the
original discussion about the springs in the clutch plate.


Charles