Torsional Vibration and PSRU Design

Hi Kent,
Long time no talk. You still doing balance work?

Seemed to be a whirl mode much like that described for a radial with
a too-loose front propshaft bearing. This was with the 3 cyl Suzuki,
an engine with a natural wobble, again much like the radial. The PSRU
was a cantilever upper axle type, so just a little excess freeplay in
the bearing setup was enough to set it whirling at high power. The
torsional amplitude pulsed at about 2 hertz on the o-scope display.
Took awhile to realize what we were had. You could hear it in the prop
noise and see the whirl at night if you lit the prop disk with a flood.
Didn't explore it much as we had other stuff on the front burner.
Just got rid of the freeplay.

Dan

Ben,

I wasn't wrong about the front journal, it's just a matter of semantics.

When I said the front journal, I meant the one from which the drive pulley
is attached, obviously. Even if this is the rear journal on the engine as
mounted in the car, it is obviously the frontmost journal when a redrive and
prop is bolted to it.

And you are quite wrong about poly-v belts. They can indeed take upwards of
1000hp. I'm looking right now at the Hutchinson design manual and there are
belts available that will take 800 kilowatt, which is 1072 horsepower.

Also significant is the design rpm and duty cycle, both of which are very
high.

Cog belts are inherently inferior because the ribs acting against the pulley
sprockets produce more heat. They should only be used where you need a
synchronicity between the drive and the driven device. This is not the case
with a prop -- it does not need to be synchronized.

(Incidentally, "cog" is not even the correct terminology for these belts,
they are called synchronous belts. Cog belts are actually V-belts, with ribs
on their inside surface for heat dissipation, like fins on cylinder heads.)

Someone mentioned 200 hours for synchronous belts, which is nowhere near
what a properly desgned poly-v belt will provide.

Why are the "cog" belts so popular then? Who knows, but being popular does
not make it sound engneering.

Personally I would not use any commercially available redrive, gearbox or
belt, except for the ones offered by Rotax and Powersport, because they are
the only vendors who have actually done scientific torsional vibration
testing and measurement and have given prop moment of inertia ranges that
are known to be safe.

Regards,

Gordon.



"stol" wrote in message
ups.com...
Chain drives have the same effect.


Belt and chain drives do impose side loads on the front crankshaft
journal,
however, so that is a negative point.


Another issue is packaging

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

Well written explanation but this is incorrect. The sideload is imposed
on the rearmain journal of the crank, not the front one. Most redrives
that use the cog belt also incorporate an idler bearing outboard of the
lower sprocket, that prevents the sideload forces on the crank. Also I
think a multivee belt will not transfer 1000 hp, I don't think it would
evem work on my 330 hp+ auto engine powered plane. I will stick with my
cog belt, thank you.

Ben
www.haaspowerair.com

"Gordon Arnaut" wrote

Personally I would not use any commercially available redrive, gearbox or
belt, except for the ones offered by Rotax and Powersport, because they
are the only vendors who have actually done scientific torsional vibration
testing and measurement and have given prop moment of inertia ranges that
are known to be safe.

I remember you, now. I replied with BULL**** back then, and I'll do the
same, now. To say that nobody except Rotax and Powersport have tested their
drives is bull****, and if I were a drive manufacture that had, I would sue
you for slander.

Go back into your hole, where you have been for the past 10 or so months.
We don't need know-it-all like you, spouting off.
--
Jim in NC

P.S. Go ahead, be true to form, and make some personal remarks about me
now. I can take it. I consider the source, and take it for what it is
worth. Zero.

Hi Dan,
Yes, I'm still busy. Interesting example of the Suzuki.
I lost your email in my last CPU crash. And occasionally have wondered what
you was up to.
I'll write you later.


Btw, I'd to take this opportunity make an announcement to others in this
thread.

I've hung out on RAH since ohh maybe 1998-99 and I still check in a couple
times per week.
Anymore though, when I open the cellar door I hear too much scurrying to
want to come down the steps.

Last time I tried to contribute a little benefit of my experience about
balancing and vibration to a thread, some people quickly turned it personal,
inferred me and my friends were crooks and pieces of ****, or P.O.S. I think
was their term. Others here supported them.
In fact some same people contributed to this recent thread!
They know who they are. .
F Amateurs. Go Get a job.

Kent Felkins





"Dan Horton" wrote in message
oups.com...

Hi Kent,
Long time no talk. You still doing balance work?

Seemed to be a whirl mode much like that described for a radial with
a too-loose front propshaft bearing. This was with the 3 cyl Suzuki,
an engine with a natural wobble, again much like the radial. The PSRU
was a cantilever upper axle type, so just a little excess freeplay in
the bearing setup was enough to set it whirling at high power. The
torsional amplitude pulsed at about 2 hertz on the o-scope display.
Took awhile to realize what we were had. You could hear it in the prop
noise and see the whirl at night if you lit the prop disk with a flood.
Didn't explore it much as we had other stuff on the front burner.
Just got rid of the freeplay.

Dan



*** Posted via a free Usenet account from http://www.teranews.com ***

Gordon Arnaut wrote:
However, the prop is a spring mass that acts to amplify the
excitations that comes from other sources...

No. Blade root bending within the plane of rotation can be modeled
as equivalent to a shaft stiffness. The prop itself has no special
ability to excite anything.

And yes, it is the inertia produced by the centrifugal force, not the
centrifugal force itself ....

Centrifugal force has nothing to do with it, period.

However I do not agree that the problem frequency will necessarily
have to fall within the operating range. Stiffness of the shaft will be
largely a function of its slenderness ratio, so using a material with a
high modulus, perhaps carbon, and a large diameter, could produce a
shaft that is light yet stiff enough to do the trick.

Overall system stiffness is cumulative. Every shaft or shaft
equivalent (crank twist, belt or chain elongation, flexible structure
between sprockets, blade root bending, whatever) contributes so that
the overall system is somewhat less than infinitely stiff. Even with a
hell-for-stiff carbon shaft I don't think you can push F1 up above the
operating range. You'll need an F1 above 220 hz to work with a 5500
RPM 4-cyl, or 300 hz for a 6-cyl with a 5000 RPM operating range. The
current average for short "hard" systems (think Blanton style) is about
50 hz. Good luck.

Dan

Dan,

The prop can indeed contribute excitations, if it is first excited to go
into resonance itself. As the prop oscillations grow in amplitude it will
overstress other components that are not as flexible, such as the crank or
gearbox.

This can happen to a non-counterweighted Lycoming with certain CS props if
you operate it continuously between 2050 and 2250 rpm, or thereabouts.

What happens is the prop has a resonant frequency at that rpm range, which
means it will go into resonance and its oscillations will continue to grow.
But long before the prop breaks, the crank will break, because it is far
less flexible.

Without the prop, the engine has no problem running all day at that rpm, so
it is the prop that is contributing the excitations. Further proof is that
the lighter MT prop is STC'd to eliminate the placarded operating
limitation.

So the prop does indeed contribute to the excitation side of the equation,
if only in a roundabout way.

Of course the full explanation is more complex because when you combine a
prop that has its particular resonant frequency and and and engine with a
different set of resonant frequencies, what you get is a whole new set of
resonant frequencies. By bolting these two items together, you now have a
new system with its own vibrational characteristics.

And if you add a gearbox too, then you get a whole new set of resonant
frequency, because now you have a new system again. And here again the
g4earbox itself can now also contribute to the excitation side of equation
in the same way as the prop -- by going into resonance and either breaking
itself or something else in the chain.

An interesting approach has been to use a spring-loaded clutch disk between
the crank and the gearbox -- this is used in the Ross gearbox for example.
Most people assume that this works because the springs compress to
"damp"some of the torsional vibration, but that's not how it works.

It works because the springs have a preload of a certain force and will
compress only when torsional oscillations reach a certain amplitude. What
happens then is not some kind of damping, but the fact that as soon the
springs are compressed the system now instantly has more springiness, which
means its resonant frequencies are now completely different from when the
springs were locked up solidly. You have in essence a drivetrain system with
variable resonant characteristics.

This means that as soon as the springs kick in, the system is no longer in
resonance because its resonant frequencies are now quite a bit lower, due to
the added flexibility of the entire system. So the oscillations stop. You
yourself alluded to this in your remarks about overall system stiffness.

This is illustrative of how the problem needs to be approached from a
drivetrain system perspective -- because each piece that you attach to an
engine will change the system as a whole. It also shows that each piece can
contribute excitations if any constituent piece is allowed to go into
resonance.









"Dan Horton" wrote in message
ups.com...
Gordon Arnaut wrote:
However, the prop is a spring mass that acts to amplify the
excitations that comes from other sources...

No. Blade root bending within the plane of rotation can be modeled
as equivalent to a shaft stiffness. The prop itself has no special
ability to excite anything.

And yes, it is the inertia produced by the centrifugal force, not the
centrifugal force itself ....

Centrifugal force has nothing to do with it, period.

However I do not agree that the problem frequency will necessarily
have to fall within the operating range. Stiffness of the shaft will be
largely a function of its slenderness ratio, so using a material with a
high modulus, perhaps carbon, and a large diameter, could produce a
shaft that is light yet stiff enough to do the trick.

Overall system stiffness is cumulative. Every shaft or shaft
equivalent (crank twist, belt or chain elongation, flexible structure
between sprockets, blade root bending, whatever) contributes so that
the overall system is somewhat less than infinitely stiff. Even with a
hell-for-stiff carbon shaft I don't think you can push F1 up above the
operating range. You'll need an F1 above 220 hz to work with a 5500
RPM 4-cyl, or 300 hz for a 6-cyl with a 5000 RPM operating range. The
current average for short "hard" systems (think Blanton style) is about
50 hz. Good luck.

Dan

Dan Horton wrote:
Gordon Arnaut wrote:
However, the prop is a spring mass that acts to amplify the
excitations that comes from other sources...

No. Blade root bending within the plane of rotation can be modeled
as equivalent to a shaft stiffness. The prop itself has no special
ability to excite anything.

And yes, it is the inertia produced by the centrifugal force, not the
centrifugal force itself ....

Centrifugal force has nothing to do with it, period.


According to Ker Wilson, prop flutter has no real impact on torsional
vibration. He could be wrong, but he devoted more than a half century
to the subject. Blade passing frequency, however, apparently does come
into play in some systems. So does whirl, but that isn't the internet
topic of the year.

Centrifugal force is a result of the inertia, not the other way around.
At least in this reality.

Not only does the crank/rod/piston system have multiple resonant
torsional frequencies, they move during operation.

While we're at it, it sure would be handy to see a list of V8
crankshafts with a critical lower than the idle rpm gas excitation
forces, since that is apparently often the case.


Charles

"Gordon Arnaut" wrote

This means that as soon as the springs kick in, the system is no longer in
resonance because its resonant frequencies are now quite a bit lower, due
to the added flexibility of the entire system. So the oscillations stop.
You yourself alluded to this in your remarks about overall system
stiffness.

For pete's sake, give it a rest. Your incomplete knowledge is showing in
spades.
--
Jim in NC

Charles,

Actually there is more than one reality when it comes to centrifugal force,
namely the reactive centrifugal force and the fictitious centrifugal
force -- depending on what you want to use as your reference frame.

But this is quickly descending into ridiculous semantics. My original point
was that if you have a flywheel with enough inertia, it will be an effective
restraining force to act against excitations that would otherwise produce
vibration. Naturally, higher moment of inertia in a rotating object must
necessitate a higher centrifugal force. Saying that one causes the other is
quite meaningless, in a chicken and egg kind of way.

Regards,

Gordon.






"Charles Vincent" wrote in message
et...
Dan Horton wrote:
Gordon Arnaut wrote:
However, the prop is a spring mass that acts to amplify the
excitations that comes from other sources...

No. Blade root bending within the plane of rotation can be modeled
as equivalent to a shaft stiffness. The prop itself has no special
ability to excite anything.

And yes, it is the inertia produced by the centrifugal force, not the
centrifugal force itself ....

Centrifugal force has nothing to do with it, period.


According to Ker Wilson, prop flutter has no real impact on torsional
vibration. He could be wrong, but he devoted more than a half century to
the subject. Blade passing frequency, however, apparently does come into
play in some systems. So does whirl, but that isn't the internet topic of
the year.

Centrifugal force is a result of the inertia, not the other way around. At
least in this reality.

Not only does the crank/rod/piston system have multiple resonant torsional
frequencies, they move during operation.

While we're at it, it sure would be handy to see a list of V8 crankshafts
with a critical lower than the idle rpm gas excitation forces, since that
is apparently often the case.


Charles

Hello Charles,
According to Ker Wilson, prop flutter has no real impact on
torsional vibration. He could be wrong, but he devoted more than a
half century to the subject. Blade passing frequency, however,
apparently does come into play in some systems. So does whirl, but
that isn't the internet topic of the year.

Ahh, thank you, appreciate the confirmation.

Lucky dog, wish I had my own copy. I have to beg my local
librarian to get it from the UA library.

Dan