Balancing the engine, long
Gregory R. Travis
Charles...@dartmouth.edu (Charles K. Scott) writes:
>Note: Here is a question for Greg Travis: Are aircooled horizontally
>opposed engines balanced? I recall you mentioning that the cranks
>don't have counterweights so I wondered. It's possible that Lycoming
>and Continental and other manufacturers of aircraft engines do not
>consider balancing necessary due to the extremely slow operating rpms
>compared to auto engines. But I don't know this for sure.
Horizontally opposed engines (doesn't matter how they're cooled or if they're
in cars, planes, etc.) must be balanced for the same reasons that other
engines must per your quoted article. However, with an engine arrangement
that places the banks of cylinders exactly 180 degrees apart (a horizontally
opposed engine), it is not necessary to add ADDITIONAL static balance to
the crankshaft in order to ensure that the whole is balanced.
This is because, in an opposed engine, each pair of opposing crankthrows
and rod big ends balances each other. A "V" engine, where the angle between
the banks is not 180 degrees, you must use additional static weight on the
crankshaft to balance the engine.
Another consequence of this fact is that you cannot balance the crankshaft
of a "V" engine by simply spinning the crank itself in a balancing machine.
Because of its static counterweights, "V" engine crankshaft is not
balanced without the connecting rods installed.
If you were to mount a V8 crank, all by itself, in a fixture that could
spin it, it would wobble all over the place and try and jump out of the
fixture.
Therefore, when balancing the crank of a "V" engine, you MUST add bobweights
to each crankpin to simulate the weights of the connecting rods.
Bobweights act as proxies for the actual connecting rods while the
crankshaft is being balanced. Therefore, it's extremely important that
the bobweights are an accurate replacement for the connecting rods.
In contrast, the crank in an opposed engine is in static and dynamic
balance without the addition of bobweights (or it should be), thus you
can put it directly in a balancing machine and spin it. If everything
is OK, it should spin smoothly without any wobble or vibration.
This makes it easier to get a better balance on an opposed engine because
there isn't the possibility of error introduced by the bobweights. If the
crank itself is in static and dynamic balance (doesn't wobble or flop in
the balancing machine) and if the big end weights of all the connecting
rods are the same then the engine is balanced for the rotating parts.
To reiterate - yes, the rotating parts of opposed engines need to be in
static and dynamic balance just as do "V" engines. The principles are
exactly the same. However, the opposed engine has two advantages over the
"V" engine in that it doesn't need additional heavy static counterweights
(internal or external) to balance the mass of the connecting rod big ends
and this makes the act of balancing the opposed engine technically simpler.
The lack of need for static counterweights on an opposed engine has nothing
to do with the operating RPM or tolerance of pilots for vibration. You
just don't need them with an opposed layout.
Does this make any sense?
greg
--
Charles K. Scott
Following is an excerpt from an article written by Dan LaGrou who is
one of two engine people who's avocation is the Buick 215 cid aluminum
block V-8. Dan has worked for Detroit a number of years and offers a
number of products for the Buick engine from blocks to cranks, heads,
cams, crank seals, flywheels etc. etc. He also can do a complete
engine with whatever mods you feel it should have.
The article is long but I thought this one section of it might be of
interest to a few people. Following is the section from the article
concerned with balancing. Dan wrote the information to assist those
who had bought the engine and wanted to blueprint it.
Corky Scott
BALANCING AN ENGINE
Everyone is familiar with the effects of an unbalanced moving load. A
washing machine on the spin cycle will sometimes literally "walk"
across the floor if all of the clothes have moved to one side of the
drum. An unbalanced tire can cause the steering wheel to vibrate so
badly that it numbs your hands. These are occurring at only a few
hundred revolutions per minute. Now take an unbalanced engine spinning
at 6,000 rpm. The forces acting to tear the motor apart are
tremendous. As single ounce has a dynamic weight of over 700 pound
when it is located on a crankshaft counterweight. the importance of
accurate engine balance cannot be overemphasized. Imbalance will cause
premature failure of the crank, connecting rods, wrist pins, or
pistons. It can also cause very rapid bearing wear and failure.
There are two major forces acting on the engine's internal parts. The
first is rotational force which acts primarily on the crankshaft con
rod big end bearings. Reciprocating force is the result of the
straight line travel-stop-reverse travel motion experienced by the
pistons, rings, wrist pins, and con rod small end bearings. The
connecting rods see both movements as they attach the reciprocating
pistons to the rotating crankshaft. Balancing attempts to bring these
forces into harmony, primarily by equalizing the weight of the various
reciprocating components (each piston, ring, wrist pin, and con rod
small end bearing combination equalized with the other seven in the
engine) and by adjusting the weight of the crankshaft counterweights or
balancing components.
The Buick/Olds/Rover 215/3.5L engines are internally balanced as
opposed to the Buick 300 which is externally balanced. (So if your are
using the Buick 215 block but the 300 crank to stroke the engine, you
would be balancing the crank externally) Internal balancing means that
there is sufficient mass in the crankshaft counterweights to balance
out the reciprocating forces of the pistons, wrist pins, etc. Balance
is achieved by adding or subtracting weight from the counterweights.
To remove weight, material is ground or machined from the
counterweight. To add any needed weight the counterweights are drilled
and a plug of Mallory metal is installed in the drilled hole. The
crankshaft pulley can be changed without altering the engine balance,
assuming it is in the correct balance.
Note: Here is a question for Greg Travis: Are aircooled horizontally
opposed engines balanced? I recall you mentioning that the cranks
don't have counterweights so I wondered. It's possible that Lycoming
and Continental and other manufacturers of aircraft engines do not
consider balancing necessary due to the extremely slow operating rpms
compared to auto engines. But I don't know this for sure.
Some engines, particularly those with large crankshaft strokes, must be
externally balanced. This is because the crank counterweights cannot
be made large (i.e. heavy) enough due to interference with the block or
camshaft. In order to effectively balance the reciprocating and
rotational forces, weight is added or subtracted to the flywheel and
harmonic balancer. Obviously, these two items cannot be changed
without necessitating a re balancing of the engine.
It is sometimes possible to internally balance an originally externally
balanced engine by reducing the weight of the reciprocating components
i.e. lighter pistons, aluminum con rods, etc. The feasibility of this
would have to be discussed with the engine balancing technician.
When you take your motor into the machine shop for balancing they will
need the:
¥ Ready to install crankshaft
¥ Pistons
¥ Wrist pins
¥ Prepared connecting rods
¥ One set of rings
¥ One set of rod bearings
¥ Harmonic balancer, bolt and washer
¥ Flywheel, or flexplate, and bolts
The basic steps that the technician will go through to balance the
component are:
1. Weigh all of the pistons, wrist pins, and connecting rods. The
connecting rods will actually have their big ends and small ends
weighed separately. One end of the rod is supported horizontally on a
hanger or fixture, while the other end is on a weighing scale. Then
the rod is reversed and the opposite end weighed. All weight s are
taken in grams (28.35 grams = 1 ounce)
2. The lightest piston and the wrist pin will be determined and the
weight of all the other pistons will be reduced to match them.
3. The same process will be performed on the connecting rods, first the
big ends, then the small ends.
4. Because the crankshaft cannot be spun with the con rods, pistons and
pins attached, the technician clamps special weights, called bob
weights, to the crank's con rod journals. The bob weights replicate
the weight of the reciprocating components imparted on the con rod
journals.
5. The crankshaft is mounted in a balancing fixture and sensors are
installed on the crankshaft on the crankshaft ends.
6. The crank is spun and any vibration causes the crank ends, and the
sensor, to oscillate. this oscillation is measured by the sensors and
tells the technician where weight needs to be removed or added to the
crankshaft counterweights in order to achieve correct balance
(internally balanced engines) or weight.
When you receive the balanced crank and other components, the machine
shop should include a record of the weights of the individual pistons,
wrist pins, and con rod big and small ends as well as the bob weights.
Store this record in case a piston or con rod has to be replaced. The
weight of the replacement part can then be adjusted to the proper
weights to maintain the engine balance.
tomdal...@vnet.ibm.com
In <5fnc02$6...@gtravis.ucs.indiana.edu>, gr...@gtravis.ucs.indiana.edu (Gregory R. Travis) writes:
>Charles...@dartmouth.edu (Charles K. Scott) writes:
>
tomdal...@vnet.ibm.com
In <5fpag7$1a...@ausnews.austin.ibm.com>, tomdal...@vnet.ibm.com writes:
>In <5fnc02$6...@gtravis.ucs.indiana.edu>, gr...@gtravis.ucs.indiana.edu (Gregory R. Travis) writes:
>>Charles...@dartmouth.edu (Charles K. Scott) writes:
>>
>>>opposed engines balanced? I recall you mentioning that the cranks
>>>don't have counterweights so I wondered. It's possible that Lycoming
>>>and Continental and other manufacturers of aircraft engines do not
>>>consider balancing necessary due to the extremely slow operating rpms
>>>compared to auto engines. But I don't know this for sure.
>>
and balanced it as part of the rebuild. The crankshaft needed no balancing. It was almost
perfect as from Lycoming. The piston balance was within 1/2 gram from Superior. The rods
were very close, on overall weight, but big end and small end balance needed quite alot of
work. When I put the cylinders on the flow bench they were so different in the flow at all
valve lifts that at first I was not sure I could safely remove enough material to ever match
them. This engine had had several cylinders overhauled by different shops, and I attribute
the flow differences to the different methods or reworking the cylinders and perhaps the
cylinders being replaced with ones from different castings etc. The lesson learned is that
for a smooth running engine, I will always replace cylinders with matched sets if finances
will allow. I do not know whether my crank balance was policy from Lycoming, or just
luck with my particular crank.
Tom Dalrymple
Gregory R. Travis
tomdal...@vnet.ibm.com writes:
> I do not know whether my crank balance was policy from Lycoming, or
> just luck with my particular crank.
Lycoming, back when they made crankshafts, balanced them with the exact same
make and model of equipment that GM uses to balance their automotive cranks.
In fact, if I recall correctly, the equipment was actually MADE by GM.
As I indicated in my previous post - balancing the crankshaft on an opposed
engine is a lot easier and faster to do than it is for a crank for a
V engine. And even a very small effort here gets you a finely balanced
crankshaft.
As Tom indicated, the place where Lycoming and Continental DON'T spend
a lot of time is in balancing connecting rods and pistons. At Lycoming,
incoming rods are simply sorted into four different weight classifications
and engines are built so that any one engine gets all of its rods from
the same weight bin. The same is done for pistons.
Those of you used to micro-balancing rods and pistons by actually removing
metal are probably blowing your lunch right now. There is no way you're
gonna get the kind of micro-gram balance you might want by crudely
separating rods and pistons into weight classes.
Lycoming's attitude is that, with the low-RPM nature of their engines, it's
a waste of time to actually balance each rod by machining it. It would
also add tremendous amounts of labor to the manufacturing process (again,
their attitude). They're also against machining rods in the field because
of the possibility of weakening the rod. There are no weight pads on
Lycoming rods so people usually end up machining in the transition area
from the big end to the rod or by removing metal from the "H" section.
Both can be safe if done correctly but the rod WILL fail if you screw it up.
DAlexan424
Hi Greg,
I'd like to take the time to say thay I enjoy your writing. I dabble a bit
with writing articles about 2-stroke performance myself.
If memory serves me correctly, the con-rods are balanced end for end and
then for over-all weight as a final step. In high performance app's, the
rods that I do get polished after the semi-final balance check, rechecked
and then shot-peened to compress the outer skin on the metal. As I'm sure
you are aware, this is an old trick to stop cracks from developing in the
outer surface. Would this be of any use in a Lycoming where as you mention
there isn't a lot of material to work with in the first place?
As well, pistons, rings, pins and clips get weighed and suffled around
until the most favorable combination is found that reduces the amount of
material that needs to come off of any particuliar piece. Trouble starts
when something has to be pre-maturely replaced and the replacement piece
is way off weight wise from the original.
I don't have too much trouble accepting the Lycoming makes some effort to
balance their engines with the old pick and swap method as long as it is
kept to a reasonable tolerance level. The end result is the same, the most
effecient (money-wise) balance needed to bring about a running engine.
Anything else would bring the price of an engine up to the point that
would price them out of the OEM market as you stated.
Dale Alexander
San Bruno,CA. 94066
Dalex...@aol.com