Crankshaft balance

I had it tested at an FBO that does prop balance and they found about
0.2 ips at the prop (FAA acceptable) and about 0.4 (+?) ips at the
other end and told me it was the engine.

R1.... .2 ips is 'faa acceptable' but any tech should be able to get it
below 0.1 ips.
Sometimes I see a cross-effect in which the rear is running big because
of a prop being installed in an angle that is not optimun. Another factor
is the static weights on the prop itself. The adjustment may resolve an aft
imbalance. What was the phase angle difference?
However in most cases it is a crank or rod imblance that is probably
causing it.



But as Ben said I
could probably do the pistons/rods myself with a good scale. But
balancing complex rotating masses like a crankshaft requires a proper
machine.

R2. You need more than a scale for the rods. A rod is suspended by each
end and in turn the big ends and the small ends are weighed, the C.G. is
deduced. In other words the rotating and the reciprocating masses are
measured. Auto guys will then make up a bobweight of nuts and bolts
equal to the rotating part which they add to the crank throws and that is
then spun on a dynamic balance machine.



Sorry, I disagree that automotive people can do a better job.
How many auto shops check the nitriding and heat treatment?


R3: The issue isn't RPM unless you are relating to 'flexible or
non-flexible rotors'
Typically Auto cranks are balanced by drilling the counterweights a little
deeper or what ever.
I can't say about Franklins, but Lycomings and Continentals do not have
counterweight molding in
the casting because the arrangement is a 'balanced' engine in the 1`st order
by design.


In the 'approved' after market, Lyc and Cont cranks are further balanced
the same at the mfg does but to a finer degree.
in my experience, it hasn't been whether is the balance is better than '3
mils' as the factory print specifies, but rather it has been making sure
you do have not a sloppy part with some 40- 80 gram-inches imbalnce that
slipped out the door!

good luck

Kent Felkins

...Auto guys will then make up a bobweight of nuts and bolts
... equal to the rotating part which they add to the crank throws and that is
... then spun on a dynamic balance machine. ...

Yes. Thats true for V8's or V6, etc. But not needed for opposing cyl
engines.

.. I can't say about Franklins, but Lycomings and Continentals do not have
... counterweight molding in the casting

Its the same for the Franklin - its an opposing flat 6. Opposing flats
are inherently easier to balance.

On last comment... That's one reason why opposing flats are lighter
engines not needing the extra counterweights.

R2. You need more than a scale for the rods. A rod is suspended by
each
end and in turn the big ends and the small ends are weighed, the C.G.
is
deduced. In other words the rotating and the reciprocating masses are

measured. Auto guys will then make up a bobweight of nuts and bolts

equal to the rotating part which they add to the crank throws and that
is
then spun on a dynamic balance machine

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

This is correct to a point. I have balanced more engines then I care to
remember and I have never had a motor where I didn't weigh all the rods
as is. For instance if you have 8 rods and 7 weigh in at 580 or so
grams and one weighs 640, you can grind /machine /rub / pray,,, what
ever you want to do there is no way you can remove from the big end or
small end to get "that" heavy rod balance with the other seven. So the
point I was making is one could gram out the parts at home to find out
pretty darn fast is they have a bad match of stuff. Once the motor gets
to a quality auto machine shop they will put the rods on a fixture and
duplicate big end and small end weights.

"stol" wrote in message
ups.com...


R2. You need more than a scale for the rods. A rod is suspended by
each
end and in turn the big ends and the small ends are weighed, the C.G.
is
deduced. In other words the rotating and the reciprocating masses are

measured. Auto guys will then make up a bobweight of nuts and bolts

equal to the rotating part which they add to the crank throws and that
is
then spun on a dynamic balance machine

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

This is correct to a point. I have balanced more engines then I care to
remember and I have never had a motor where I didn't weigh all the rods
as is. For instance if you have 8 rods and 7 weigh in at 580 or so
grams and one weighs 640, you can grind /machine /rub / pray,,, what
ever you want to do there is no way you can remove from the big end or
small end to get "that" heavy rod balance with the other seven. So the
point I was making is one could gram out the parts at home to find out
pretty darn fast is they have a bad match of stuff. Once the motor gets
to a quality auto machine shop they will put the rods on a fixture and
duplicate big end and small end weights.


gee.... a fixture... What does a quality auto machine shop use? A CNC
machine?

Kent Felkins

R3: The issue isn't RPM unless you are relating to 'flexible or
non-flexible rotors'
Typically Auto cranks are balanced by drilling the counterweights a
little
deeper or what ever.
I can't say about Franklins, but Lycomings and Continentals do not have

counterweight molding in
the casting because the arrangement is a 'balanced' engine in the 1`st
order
by design.


/////////////////////////////////////////
Look close at the Lyc / Cont cranks and you will see balance marks on
the rod throw end. As you have pointed out they don't have counter
weights so thats the only logical place to correct a large imbalance
that happens during the crank forging process.

"stol" wrote in message
oups.com...

/////////////////////////////////////////
Look close at the Lyc / Cont cranks and you will see balance marks on
the rod throw end. As you have pointed out they don't have counter
weights so thats the only logical place to correct a large imbalance
that happens during the crank forging process.

Yes, you have noticed the factory and aftermarket balance marks. Have you
ever checked the depth of material removed? How deep is the nitride layer
usually?
What do you do about the bifilar dampers that most higher horsepower
crankshafts have?
It is a misnomer that these are commonly called crank counterweights.
You install new bushings and rollers? Do you balance with them
installed on the crank? What about a Lycoming ring gear?

btw, thanks your three posts in "setting me straight".

Kent Felkins
Felkins Aircraft Ltd.
FAA CRS WNKR918K
Tulsa Oklahoma

"stol" wrote in message
oups.com...

/////////////////////////////////////////
Look close at the Lyc / Cont cranks and you will see balance marks on
the rod throw end. As you have pointed out they don't have counter
weights so thats the only logical place to correct a large imbalance
that happens during the crank forging process.

Yes, you have noticed the factory and aftermarket balance marks. Have you
ever checked the depth of material removed?
How deep is the nitride layer usually?
What do you do about the bifilar dampers that most higher horsepower
crankshafts have?
It is a misnomer that these are commonly called crank counterweights.
You install new bushings and rollers? Do you balance with them
installed on the crank? What about a Lycoming ring gear?

btw, thanks for your three reply posts for "setting me straight".

Kent Felkins
Felkins Aircraft Ltd.
FAA CRS WNKR918K
Tulsa Oklahoma

Kent Felkins
Felkins Aircraft Ltd.
FAA CRS WNKR918K
Tulsa Oklahoma

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

I was just giving out my two cents worth. After all this is the Rec,
Aviation, " HOMEBUILT" site. I would never post something like this on
the rec. aviation "certified" site. After all the FAA knows all.I still
think Lycoming murdered 13 people by selling a inferior product and the
FAA was in bed with them the whole way. I have owed several certifeis
planes in my life and I never want to see a yellow tag again. Those
things have tried to kill me a couple of times and the funny thing is
the Feds didnt even investigate the repair stations involved. Now I
will head at and fly my homebuilt plane powered by a Firewall forward I
designed and built. The air is good and thick at -34f. Peace and Merry
Christmas from Jackson Hole Wy

Ben
www.haaspowerair.com