Crankshaft balance

Its not much flying time around here in SD with temps averaging 0F.

I got my engine (Franklin 6) apart trying to cure a vibration problem
(0.4 IPS) - not extreme but anoying. Is it OK to balance the crank in a
automobile machine shop or are there some tricky things about aircraft
cranks? I know automobile cranks go to 6000 rpm and my aircraft crank
only goes to about 2800. So auto shops should be able to do a more
precise job?

If it is in an experimental plane then a good auto machine shop can
probably do a better job then most "certified" aircraft shops. You can
check most of the weights, ie; pistons, rings,pins bearings, rods,,
yourself with a good gram scale. If you do get it balanced please post
back here to let us know how far out it was.

Ben.
www.haaspowerair.com


abripl wrote:
Its not much flying time around here in SD with temps averaging 0F.

I got my engine (Franklin 6) apart trying to cure a vibration problem
(0.4 IPS) - not extreme but anoying. Is it OK to balance the crank in a
automobile machine shop or are there some tricky things about aircraft
cranks? I know automobile cranks go to 6000 rpm and my aircraft crank
only goes to about 2800. So auto shops should be able to do a more
precise job?

I'm curious about how you have 0.4 ips vibration problem?
Are you sure its the crankshaft?

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

Pistons and rings are not part of crankshaft balance.
Pins bearings?

Kent Felkins
Tulsa



"stol" wrote in message
oups.com...
If it is in an experimental plane then a good auto machine shop can
probably do a better job then most "certified" aircraft shops. You can
check most of the weights, ie; pistons, rings,pins bearings, rods,,
yourself with a good gram scale. If you do get it balanced please post
back here to let us know how far out it was.

Ben.
www.haaspowerair.com


abripl wrote:
Its not much flying time around here in SD with temps averaging 0F.

I got my engine (Franklin 6) apart trying to cure a vibration problem
(0.4 IPS) - not extreme but anoying. Is it OK to balance the crank in a
automobile machine shop or are there some tricky things about aircraft
cranks? I know automobile cranks go to 6000 rpm and my aircraft crank
only goes to about 2800. So auto shops should be able to do a more
precise job?

I'm curious about how you have 0.4 ips vibration problem?

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.

Are you sure its the crankshaft?

I didn't actulally say it was (only) the crankshaft. 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.

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

The issue is balance not other engine work. Auto shops balance engines
to 6000+ rpm redline, whereas aircraft engines redline only to about
2800. Search through this group and you will find some interesting
discussions about lax approach to balance by Lyc. etc.

Pistons and rings are not part of crankshaft balance. Pins bearings?

???

The whole engine balance is made up of pistons, rods, bearings, rings,
crankshaft, moving masses which need to balance together.

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.

I'm curious about how you have 0.4 ips vibration problem?
Are you sure its the crankshaft?


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


Pistons and rings are not part of crankshaft balance.
Pins bearings?


Kent Felkins
Tulsa

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

Let me set some things straight for ya. Any good racing machine shop
can balance a motor far better then the vast majority of FAA approved
places. In fact Lycoming can't even built a crank for some models they
produce that won't break, and they have killed people this way. So much
for the FAA keeping a close eye on quality... As for nitriting and heat
treating, this is an every day detail in the racing market and is down
to a science, not black magic... Pistons and rings and circlips are
part of the rotating/reciprocating assembly and need to be balanced
too. Good shops even add in a factor for the weight of oil that clings
to the balanced mass to get it perfect.

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.

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.