Differences in balancing an engine at crank vs flywheel

[Qikbbstang]

Admitedly algebra equations give me tough time. At the crank I guess the diameter the weight added to balance the rotating assembly spins on is around 7-8 inches, perhaps 6" with a SCJ hatchet.  When compared to a flywheel the added weight to balance the rotating assembly spins on a 12-14" diameter path.

What I'm wondering about is are the variations in the balanced assembly in a static balanced, dynamic balanced in a range of RPMS.
    How does an assembly that's balanced closely on a small diameter path (tucked inside the crankshaft) vary much from an assembly that's balanced on a large diameter path (weight located at outside edge of flywheel) ?

 My WAG is the "balance of the assembly" stays true over a greater RPM band when it's tucked in tightly....

 How much of a difference that "in balance band's width" is,  I don't have a clue and how that applies to a typical FE with a 800-6,000rpm oprating band?

[WConley]

BB -

The answer to your question is found in the units of unbalance: ounce-inches (or even gram-inches sometimes!)

The unbalance force is a weight at a distance from the centerline of rotation.  This means a 1-ounce weight mounted 6 inches from the centerline behaves exactly the same as a 6 ounce weight mounted 1 inch from the centerline.

No difference at any rpm - Nada.  Correct balance in a given plane is correct balance in a given plane: Ounce-inches.

Now there is a whole other question when you start talking balancing the crank internally vs. balancing at the flywheel or damper.  This gets into the balancing planes.

You can think of each big rotating mass along the crankshaft as a balancing plane.  The damper, crank throws, and flywheel are the biggies on a crankshaft.  If you balance the whole deal using the end balance planes only (flywheel and damper like the SCJ), the inner crank throws still have significant unbalance forces at speed.  If the crank was infinitely rigid, this wouldn't be much of a problem and you wouldn't feel the inner unbalance.  Unfortunately cranks flex, so those inner unbalance forces do get transmitted to the main bearings of the block. 

This is why most ultimate performance engines are built with fully internally balanced crankshafts.  Each balance plane along the crank has closer to neutral balance, so the block absorbs less inertia (unbalance) force.  (Note that the balance can never be perfect on each cylinder - a one cylinder engine always has shaking forces - but the net imbalance is a lot lower across all cylinders.)  This gives the main bearings a little more headroom to absorb the punishing power strokes at high rpm.  Every little bit helps!

Engine balance is pretty complicated stuff, and the method that works best doesn't always agree with the books.  That's the nature of these crazy things we build with parts flying all over the place inside.  It's still a bit of a head-scratcher that these things don't fly apart more often.  Imagine what's going on in an F1 engine at 22,000 rpm???

[jayb]

Bill, thanks for that explanation regarding balance planes, I was unfamiliar with that concept.  I imagine that this is why the better crankshafts have center counterweights, while a lot of stock crankshafts don't?

[cjshaker]

Great explanation, Bill. Some of the tests that have been done to determine crankshaft flex in high performance engines makes me wonder how balance can even be achieved, but I guess it's just a "get it as close as you can" scenario.

It's still a bit of a head-scratcher that these things don't fly apart more often.  Imagine what's going on in an F1 engine at 22,000 rpm???

I've always been amazed that any engine holds together at any higher RPM. When you start doing the math to calculate certain events per second, I'm always truly amazed.

[Yellow Truck]

I'm not an expert, but I do have some math.

Recognizing that a crank is not in fact rigid, nor is the block, the internal harmonics can get pretty interesting. If the block was perfectly rigid you could assume that an out of balance plane between any two main bearing caps would have a fundamental frequency defined by the distance between the two caps. The fundamental frequency is the longest sinusoidal wave that can exist in that distance. This would occur at a specific RPM and would probably be the most destructive (or at least disturbing) frequency for the engine. The different crank segments have very similar length,  and the engine has other unbalanced planes, each contributing their own wave, and since the block and crank are not perfectly rigid, they are also transmitted across the block and crank.

Add in the periodic shockwaves from combustion, valves closing and causing their own shock waves, and you have an interestingly complex mathematical problem. I suspect that the firing orders used by different manufacturers are in some way an attempt to reduce the compounding or wave super-position effect (two waves with different frequencies can overlap to either cancel or amplify the wave height) and use it to offset the waves.

This is manifestly obvious when you see a flat plane crank engine fire. It has no adjustments to crank angle and position to reduce the effect.

Short version is that each engine will probably have an RPM at which the combined wave forms give it the most destructive flex and vibration, and they probably do their best to minimize it at that frequency. Once they have done their best, they rely on the age old engineering practice - add in extra iron - so it won't fly apart.

[Qikbbstang]

Never forget one of Hot Rod Magazines exposé's into Top Fuel Motors....I was decades ago but they explained the cams were ground off in zero degree, five degree, ten degree, fifteen degree etc chunks for each pair of cylinders to make up for the crankshaft's being twisted up under full power.
  The thought of a crank springing back and forth is I believe exactly why dampers are a science.  They dampen that twitching of the crank from all the "hits" it's taking. Hopefully it does not start getting the hit's where they add to the flexing and essentially turn it into a dribbling basketball. The damper to me prevents the crank from adding the results up and springing all to hell. Even a bone stocker if the hits start adding on each other will stress the hell out of the crank.
  If I recall they don't run dampers on Sprint Cars?............

[Yellow Truck]

Good point, didn't consider twisting into the harmonic distortion. When you think about it - damned lucky these things work at all...

[Heo]

We learned at school about harmonics and amplitudes
and abouth considering them when enginering
and theoreticaly everything have a amplitude
where it shake it self to pieces
Like if you build a bridge with same distans between
the supports thats equal with the bridges amplitude
the right wind can make the bridge start flapping and
it will accelerate till the bridge collaps
There is bridges that have collapsed due  to marching
troops.
So every crankshaft have a rpm thats not healty for it
could be 500 rpm or 15000 or whatever
Two identical cranks one cast one forged have completly
different amplitude

[cammerfe]

Bill, your comments often make me smile. This topic, and your contribution, is one of those occasions.

KS

[WConley]

Thanks Ken    I did my Masters work in machine dynamics and rotor balancing.  Imagine balancing 30-foot long helicopter tail rotor driveshaft made of woven carbon fiber, with no intermediate bearings!  The thing was running at over 5x critical speed (akin to a jumprope being held at each end with five waves in between).

Engines, like helicopters, are quite scary when you get to know them better.  It was sobering to first look at the accelerometer data of the pan rails moving on a dyno test engine at Ford.  Then you look at the main bearing caps walking around and wonder how the thing can live??

Been offline most of the week - Somewhat off the grid in Taos, NM for some vacation time.

Be back next week.

- Bill