crankshaft counterweights

OK, you smart people, just some curiosity questions:

There appears to be some lack of precision in terminology, where the term
"crankshaft counterweights" sometimes refers to the big weighty parts of the
crankshaft that are part of the single piece of metal, and other times refers to
some movable parts that are attached to the crankshaft. Am I right about this
lack of precision?

Is there a simple way to tell, say from the engine model number, whether an
engine is equipped with the movable type of crankshaft counterweights?

How are the movable type of crankshaft counterweights attached to the
crankshaft? Does anyone have a picture or drawing or explanation?

When an engine is equipped with movable counterweights, is there one per
crankshaft journal, or just one or two per crankshaft, or what? If not one per
journal, where are they placed?

In what plane (in the geometric sense) do the movable counterweights move? I
assume they are free to move in a plane perpendicular to the axis of crankshaft
rotation, is that correct?

How free are the movable counterweights? Is there an angular limit to their
motion, or do they rotate through a full 360 degrees (with respect to their
attachment point), or what?

How massive are the movable counterweights, typically?

What is the physics of the operation of the movable counterweights? How does
their presence reduce torsional vibration?

Thanks.

Dave

Dave Butler wrote:
snip

Is there a simple way to tell, say from the engine model number,
whether an
engine is equipped with the movable type of crankshaft
counterweights?

How are the movable type of crankshaft counterweights attached to the

crankshaft? Does anyone have a picture or drawing or explanation?
snip

http://www.sacskyranch.com/detune.htm

regards;

TC

wrote:
Dave Butler wrote:

crankshaft? Does anyone have a picture or drawing or explanation?

snip

http://www.sacskyranch.com/detune.htm

Thanks, TC, that's extremely helpful. Unfortunately it also raises some more
questions... forgive me.

In the first picture at the top of that web page I was thinking the
counterweights were the two objects that look like ears in the picture, with 3
round things in each (pins? bolts? through-holes?).

In the nest-to-last picture at the bottom of the page, those things are called
"plates". Are the plates the actual masive part of the weights?

The caption to that next-to-last picture says "Pin diameter determines the
pendulum length and thus the frequency." I'm having some difficulty visualizing
that. Can you clarify that?

In that next-to-last picture, do the weights (plates?) rotate around one of
those pins to do their job? If so, it looks like they can't move through more
than a very small angle. True?

Thanks again.

Dave Butler wrote:
Hi, I'm not TC, but you might try looking over http://www.enginehistory.org/
for a couple papers on crank dampers. Specifically I am thinking about
a story on the design of the P&W R2800 crankshaft that covers the various
modes of crank bending (which the counterweights are supposed to minimize).
Lycoming engines have dampers if their model number is "complex"
like O-540-J1BD and don't have 'em if it's simple like O-360-A4A.

The damper rolls around the inside of the hole in the crank counterweight.
Sometimes the plate on the outside moves too. That's the gist of the article.
When the holes get gummed up from oil coking the dampers don't move anymore
and you can have problems.
--
Aaron Coolidge

Aaron Coolidge wrote:
Dave Butler wrote:
Hi, I'm not TC, but you might try looking over http://www.enginehistory.org/
for a couple papers on crank dampers. Specifically I am thinking about
a story on the design of the P&W R2800 crankshaft that covers the various
modes of crank bending (which the counterweights are supposed to minimize).
Lycoming engines have dampers if their model number is "complex"
like O-540-J1BD and don't have 'em if it's simple like O-360-A4A.

The damper rolls around the inside of the hole in the crank counterweight.
Sometimes the plate on the outside moves too. That's the gist of the article.
When the holes get gummed up from oil coking the dampers don't move anymore
and you can have problems.

Thanks, Aaron, Gene and TC. I may have to get a firsthand look at one of these
things before the light comes on in my head. Pictures don't seem to be doing the
job. Maybe next time I'm on the field with an engine shop I'll ask whether they
have any engines opened up that I can look at.

Anyway, your info has been helpful. Aaron, that web site looks great just for
its entertainment value. Thanks for pointing it out.

Dave

Gene Kearns wrote:

Dave... email me off list... if you are local and I'll let you put
hands on and go through any documentation you'd like to see.... I
have samples of just about every/any thing you'd like to see...

Thanks, Gene! I tried the "email me" link on your web site, but nothing happens.
Maybe it has something to do with Unix and Mozilla, or the Adblock popup
blocker. I'll try again when I get home to my Windoze machine.

I think you're at GSO, right (you once said you could throw a rock and hit the
FSDO)? I'm at RDU. Would enjoy trying to drop in on you (figuratively, of
course) some time.

Dave

[for the benefit of non-locals GSO = Greensboro, NC, RDU = Raleigh-Durham, NC]

Gene Kearns wrote:
On Fri, 04 Feb 2005 13:15:26 -0500, "Gene Kearns"
wrote:



abeam the numbers of RWY 32 at GSO! http://tinyurl.com/6vpg7



Oh... geeze.. brainfart... make that: abeam the numbers of RWY 14....

Heh, yeah, I thought so. The building where Wings Weekend events are sometimes
held, right?

BTW, the email link on your web site is fine (but you knew that). It was my
adblock that was preventing it. Disable adblock and it works fine.

Dave

Gene Kearns wrote:
On Mon, 07 Feb 2005 11:22:47 -0500, Dave Butler wrote:



BTW, the email link on your web site is fine (but you knew that). It was my
adblock that was preventing it. Disable adblock and it works fine.


I just caught this..... why does adblock flag my site? I don't set
cookies, I host my own site, and do nothing obtrusive.... why is
adblock picking on me????

Probably something to do with your site opening a new window with the email
link. If you want to play with it, it's http://adblock.mozdev.org/ and the
filter set I am using is from http://www.geocities.com/pierceive/adblock/

I haven't been using it long, and your site was the first time I had had any
trouble. Since then, I've found there are other sites that it blocks when I
don't want it to. Fortunately (I now know) it's just a couple of clicks to
disable it, get what I need, then re-enable it to stop the ads.

I wish it would be more discriminating in what it blocks, but now that I know to
disable it whenever I can't get something I think I should, I'm OK with it. I
plan to keep using it. You can play with the blocking rules to customize it, but
it looks a little fussy, so I doubt I'll bother with that.

I'm using it on Mozilla 1.6 and Solaris here at work, and on Firefox and Win XP
at home.

Dave

Aaron and all -

Maybe more than you wanted to know................

That is an interesting site having a lot of the history of tuned
absorbers.

The bifilar aborber described makes the mass act like a simple one-axis
pendulum. If you look at any high school physics book, the resonant
frequency of a pendulum is inversely proportional to the square root of
gravity (remember the l/g term under the square root sign). On the
other hand the radial acceleration of a rotating mass is proportional
to the square of the rotational speed. Mathematics and good Karma make
the two effects cancel such that the tuned torsional natural frequency
of the so-called "counterweight" (a misnomer) becomes an integral
harmonic of the crankshaft rotating frequency - but only if the radius
of motion of the mass is held within very narrow limits.

For small motions, the effective radius of relative motion of the mass
on the pins is determined by the difference between the pin OD and the
wear bushing ID. That's why both wear limits are critical. Wear will
always make the hole bigger and the pins smaller. The net radius of
relative motion then becomes incorrect such that the resonant frequency
is no longer the exact desired harmonic of the crankshaft rpm. That's
when the back of the crankshaft starts vibrating a lot in torsion and
the propeller also sees a lot of torsional vibration.

The torsional natural frequency of most crankshaft-propeller
combinations is on the gross order of about 220 Hz (almost middle C on
a piano) so the pendulum length has to be very short to reduce the
crankshaft stress effectively. The torsional motions are small, but a
crank is so stiff that a lot of stress can be built up with even a
small amount of motion. The propellor will vibrate in a slight "S"
shape such that there are high stress excursions in the leading and
trailing edges of the prop.

One way to describe the vibration mode would be to imagine you are
sitting on the rotating spinner with X-ray vision. You will get a
reasonably smooth ride but the prop tips will seem to go to and fro
while the back of the crankshaft goes fro and to. (Read that again)

Obviously Piper was able to get by the need for RPM restrictions by
stiffening the crankshaft slightly (i. e. the solid crank version) so
that a damper wasn't needed on those 4 cyl installations. I'm
surprised that small amount of increase in stiffness of the whole
rotating structure could be enough, but at any rate, any field mixing
errors in prop-engine-tachometer combinations could create a bad
situation by not making original certification spec.

Some times more than one frequency is needed to be absorbed, especially
in 6 cyl engines. Hence the slightly different pin and bushing
requirements for say the 5th and a 6 th order damper.

The mass of the damper doesn't seriously enter into the tuning
parameters but it is very important that the pin and bushing diameters
don't become mixed up on assembly. Otherwise tuned absorbers (tuned
masss dampers) are a very powerful way to supress the damaging effects
of torsional vibration.

Tuned absorbers are a hobby of mine. I designed the ones used to
stabilize the John Hancock Tower in Boston and the CitiCorp Center in
NYC against wind induced vibration. My dampers are bigger than
yours..............!

nrp wrote:
: That is an interesting site having a lot of the history of tuned
: absorbers.

Yes, I was most impressed with it. I can't find the article that I was
actually referring to anymore, though.. I especially liked the difficulties
with the 2:1 prop reduction drive!

: Obviously Piper was able to get by the need for RPM restrictions by
: stiffening the crankshaft slightly (i. e. the solid crank version) so
: that a damper wasn't needed on those 4 cyl installations. I'm
: surprised that small amount of increase in stiffness of the whole
: rotating structure could be enough, but at any rate, any field mixing
: errors in prop-engine-tachometer combinations could create a bad
: situation by not making original certification spec.

They probably added mass & stiffness to get the resonant frequency up
high enough that you'll never get there. Interestingly many medium-speed
marine diesels (that is, about 500 to 800 RPM redline) have prohibited
RPM ranges as well. Those engines surely have a very heavy crankshaft.

: Tuned absorbers are a hobby of mine. I designed the ones used to
: stabilize the John Hancock Tower in Boston and the CitiCorp Center in
: NYC against wind induced vibration. My dampers are bigger than
: yours..............!

Wow, I am truly impressed. One of the things that drove me into the
engineering field (I am an EE) was a set of pictures in my 9th grade
science book showing the tuned mass damper in the Hancock tower at rest
and in motion!
--
Aaron C.