crankshaft counterweight mass vs. main bearing life questions.

[NismoMe]

I am in the middle of designing a single plane V8 crankshaft and am trying to figure out how to determine what mass of a counterweight I need to have as to not wear the main bearings quickly.

Naturally a crankshaft of this type is "balanced" because the opposing side of the crankshaft cancels any weight on the other side of the crankshaft.

I have many of the paramaters of the crankshaft worked through already but I am not quite sure about the counterweights and am looking for some direction here.

I am working in Solidworks 2007 by the way.

Thank you!

 

[patprimmer]

Ummmm

Naturally a crankshaft of this type is "balanced" because the opposing side of the crankshaft cancels any weight on the other side of the crankshaft.

Does this V8 have a 180 deg V angle?


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[ivymike]

without thinking too much about this, I'd guess it's like a long 4-cyl crank and the firing is uneven (bank angle whatever you like)

 

[NismoMe]

The engine is a standard 90 degree V8:

Engine layout:
-2-4-6-8
1-3-5-7-

My firing order:
1-8-5-4-7-2-3-6

The crank is "balanced" as far as having an opposing throw on the crank for every throw. I understand that the whole rotating assembly is not balanced to the second order and should have roughly 60% more vibration than a standard 4 cylinder engine.
It is the basically the same as an inline 4 crankshaft but has two rods per crank throw.

I have worked through several versions of oiling systems along with a professional engine builder but need some insight into the counterweight and bearing wear situation.
Attached is a study of the crankshaft that I am modeling.(minus the front snout and rear flywheel mounting flange, fr now.)

 

[evelrod]

For a time, the 180 cranks were very popular with the closed circuit crowd up at tracks like Irwindale. Small block V-8's sound like a screaming inline 6. I know that Henry Velasco of Downey, CA did a billet 180 for a BBC back in the 80's. I saw it when I went to pickup one of my cranks so he might be the person to ask if he is still around.

Rod

 

[tbuelna]

NismoMe,

You've put forth a relatively vague set of requirements. A planar 4-pin V8 crank does not really require any counterweights at all, as long as you're willing to accept high vibration and short main bearing life. The crank configuration itself is naturally balanced, but some counterweight mass is usually added, even for weight critical racing cranks.

Counterweights serve several functions: They help to alleviate main bearing loads due to the inertias generated by the reciprocating masses, they can help reduce engine vibrations due to secondary couples/forces when the counterweights are located correctly, they can help reduce torsional vibration problems, and they can help reduce bending between main bearings due to reciprocating mass inertias.

There is no ideal counterweight mass. It is usually a compromise between added weight and reduced vibration. A good starting point is to apply a counterweight with mass properties equivalent to about 50% of the reciprocatin part mass applied about the crankpin. You can go up or down from there based on experimentation, testing or analysis.

http://images.google.com/imgres?img...worth+dfx+crankshaft&gbv=2&ndsp=18&hl=en&sa=N