Torsional Vibration and PSRU Design

Gordon, you can run down the rabbit hole of relativity with someone
else. The fact is your statement that "it is the inertia produced by
the centrifugal force," is wrong. Inertia is not the result of
centrifugal force, it is the reverse. You could even argue semi
successfully that centrifugal force was just another name for inertia,
but you can't say inertia is produced by the centrifugal force and be
correct.

Charles

Gordon Arnaut wrote:
Charles,

I guess the concept of relativity is just a "convenience" for doing the math
too right?

Please do not descend any further into absurdities. Even Newton was well
aware of the relativity of motion long before Einstein came along, which is
why his law of inertias takes into account frames of reference.

Please quit before you bury yourself in a sinkhole.

Regards,

Gordon.

Just a thought here.

If the question is about a longish shaft that may support 'windup' torsional
vibrations, there are small torque sensors that can be clamped to a rotating
shaft that transmit their data wirelessly to a handheld display unit. Some
of these should have enough bandwidth to show the amplitude of any torsional
vibrations. This is a testing issue but it may allow the tuning of a
redrive and disclose any RPM bands that should be avoided.

bildan

"Mark Hickey" wrote in message
...
"Dan Horton" wrote:

Gordon, you got that? Please show us the "preload" that "will
compress only when torsional oscillations reach a certain amplitude".

I wonder if this might add something to the conversation:

http://www.international-auto.com/in...id=2600&cid=41

I recently bought an Alfa Romeo, and was intrigued to find one of
these in front of the first drive shaft (can't figure out why they
NEED two driveshafts on a short sports car, but that's a different
thread).

The transmission end hooks up with three bolts, the driveshaft with
the other three. Oh, and the metal band isn't there once it's
installed.

It seems to me that a device like this would probably give the effect
Gordon's looking for (since there's no "bottoming" of the spring, and
it's clear that the thing is designed to work in the power range we're
discussing (the Alfa Spider has around 120hp).

I'm guessing that this was added to the Alfa drivetrain to cure some
sort of resonance.

Mark "Mr. Flexible" Hickey

Gordon Arnaut wrote:
So multiply the spring rate that you measured by the number of
springs.

The quoted data is for all springs combined, not that it would make
any difference.

Also you have taken a clutch disk from a fairly small 4-cylinder
engine as an example. I think the Ross gearbox uses a considerably
beefier clutch.

No change in operating principle, just torque capacity before
bottoming.

In any case, what's your point?

That your assertion ("It works because the springs have a preload
of a certain force and will compress only when torsional oscillations
reach a certain amplitude.") is bull****. If your assertion were true,
the initial part of the graph would be a vertical line.

Dan

Up until that point the springs are not compressed and the coupling
is in effect a solid coupling.

Good God, he still doesn't get it.

Gordon, plot the supplied data and study at it carefully. If your
assertion were true, the plot would not leave the Y axis until reaching
some elevated torque value. Note that the real plot begins at 0-0.

Dan

Bill writes:
small torque sensors that can be clamped to a rotating shaft that
transmit their data wirelessly

Yes, for torsion the classic setup is a wheatstone bridge strain
guage array feeding a telemetry transmitter. I shopped new digital
transmitters a few years ago but didn't buy, hoping for better prices
and bandwidth later. We'll see. In 99/early 2000 we used a wheatstone
bridge and a borrowed analog transmitter from Wireless Data. The
bridge was still on the propshaft at S&F that year. I was there all
week and nobody ever asked why the propshaft was wrapped in strapping
tape g.

Dan

Hello Mark,
was intrigued to find one of these in front of the first drive
shaft

Appears to be a Goetz brand soft element, which is what Rotax
installs in the C and E box. Lord makes a similar unit called a
Dynaflex LCD (or something like that). I like the Lovejoy Centaflex.
One of the bolt sets is aligned radially, thus no need for the
installation band, plus the machine you're designing only needs one
flange instead of two. The radial bolt set can connect directly into
the driving or driven shaft.

All of the above come in carefully graduated spring rates. They are
generally used to lower natural frequency. You select one based on
your need for a particular torsional stiffness, and then make sure it
also meets criteria for torque capacity, etc.

The successful Suzuki drive used a Shore 50 Centaflex CF12.
Running in parallel with a viscous disk damper, maximum measured
vibratory torque during steady-state resonant operation was about 115
ft-lbs at 1500. Without damping maximum amplitude was about 180
ft-lbs. We did not install telemetry on the previous bad drive (a hard
system with no soft element) but we did model it. Predicted vibratory
amplitude was around 10 times the above, at a critical RPM of 2200.
Having flown it, I had no trouble accepting the model results. It
sucked g

Dan

Dan,

You're right about one thing. I don't get what your objection is.

Are you saying the springs immediately begin to compress at the first sign
of torque? Hence the plot beginning at 0-0?

I don't see how this is possible unless the springs were installed without
any preload at all. My understanding is that the springs in a clutch disk
are under preload, so the torque has to rise to a certain level before they
will compress. Until that point it is a solid coupling.

If the springs had no preload, it would never be a solid coupling. It would
contantly be compressing and decompressing. How could that kind of clutch
even be usable in a car? It would be lurching all the time.

Also glad you mentioned the rubber torsional coupling brought up by the
poster in reference to the Alfa driveshaft.

The Rotax boxes you mention use this for the same reason the Ross box uses
the clutch disk with the springs. But as I was trying to point out, the
springs are not used to actually isolate the vibrations but to introduce
variable stiffness into the system.

Regards,

Gordon.







"Dan Horton" wrote in message
oups.com...
Up until that point the springs are not compressed and the coupling
is in effect a solid coupling.

Good God, he still doesn't get it.

Gordon, plot the supplied data and study at it carefully. If your
assertion were true, the plot would not leave the Y axis until reaching
some elevated torque value. Note that the real plot begins at 0-0.

Dan

Charles,

I did in fact say that the inertia is produced by the centrifugal force,
which is just as wrong as saying the centrifugal force is produced by the
inertia.

That is ridiculous, yet you cling to it. At least I have the good sense to
go back and see that I did make an error in phrasing. I thought I had said
that inertia is proportional to the centrifugal force, which is correct.
The
two are irrevokably linked.

But one does not cause the other. The cause of both is the energy that
produces the rotation to begin with and the centripetal force that keeps
the
rotating object from flinging off into space.

If you have no centripetal force, you have no centrifugal force. You also
have no rotational inertia.

And there is nothing rabbit-holish about inertial reference frames. It
applies fully to this discussion. Much of our understanding of modern
physics has been built on the underpinnings of Newtonian relativity.
Inertial reference frames are an important and very real concept, and we
can
thank Newton and Galileo before him for making us aware of the importance
of
those reference frames in understanding the physics of motion.

Amazing how you can be so cavalier with your wording and then lecture me
on
the meaning of semantics.

Regards,

Gordon.

Come on. Lighten up and count to ten before you pound your keyboard.

Inertia exists are a property of mass, which is a property of matter. You
can have all the inertia that you please with out centrifugal, or
centripetal, force--provided that the motion of the mass is linear rather
than curved.... Inertia is the source, while centrifugal force is one of
the possible products.

There are even those who claim that centripetal and/or centrifugal forces
are fictions used for illustrative purposes. I have no way of knowing
whether some such remarks were serious, intended for a liers club meeting,
or just what happens when physicists are bored and alcohol is involved.

Peter

"Dan Horton" wrote in message
oups.com...
Bill writes:
small torque sensors that can be clamped to a rotating shaft that
transmit their data wirelessly

Yes, for torsion the classic setup is a wheatstone bridge strain
guage array feeding a telemetry transmitter. I shopped new digital
transmitters a few years ago but didn't buy, hoping for better prices
and bandwidth later. We'll see. In 99/early 2000 we used a wheatstone
bridge and a borrowed analog transmitter from Wireless Data. The
bridge was still on the propshaft at S&F that year. I was there all
week and nobody ever asked why the propshaft was wrapped in strapping
tape g.

What was this rig-up on?

What is it you do, Dan? It sounds interesting.

It is refreshing to hear from someone who knows his stuff, unlike some other
poster, as of late! g
--
Jim in NC

"Dan Horton" wrote

The successful Suzuki drive used a Shore 50 Centaflex CF12.
Running in parallel with a viscous disk damper, maximum measured
vibratory torque during steady-state resonant operation was about 115
ft-lbs at 1500.

So, do you have a marketable Susuki flying? What association are you with
the project?

The Suzuki looks like it is about the right setup for my *future* needs.
Note the future. I'm talking around 5 years down the road. It still is fun
to figure and plan, or is it scheme? g
--
Jim in NC