L6 crankshaft balance? 2

[kitabel]

I'm trying to get a better understanding of how an L6 crankshaft is balanced (not what it does).
Searches turn up nothing useful, including such gems as "in-line engines do not have balance weights" (what's that big thing opposite the pin?), and that L6 engines do not require balance factor compensation for reciprocating weight.
Obviously, the counterweights:
1. oppose the mass of rotating components (crankpin, rod big end)
2. position this opposing mass as closely as practical to the actual crank throws

Assuming that all component weights are matched between all cylinders (pistons, rods end-for-end and total, etc.), what determines the counterweight mass? 100% of the eccentric rotating mass + ???
Engine is the older Gen-2 Chevy "stovebolt" 235/261, 4 mains, 3.9375" stroke, 6.8125" rods.

 

[GregLocock]

"in-line engines do not have balance weights"

Well you can remove that bookmark.

You are talking, I think, about counterweights, not primary balance. They are there primarily to counteract the inertia of each piston/conrod assembly.

There are several different ways of designing the counterweights for an engine, one of the most common historically was to attempt to reduce the maximum main bearing loads down to an acceptable number.

This relies on being able to work out the dynamic stiffness of the crank, which is in turn affected by the counterweights. So it wasn't an easy job before computers, and even now is a bit esoteric.

Incidentally the crank on a BMW I6 is primary balanced at every web, not just the front and rear.

If you run an engine at low speed it no longer needs counterweights, but it'll still need to be balanced. If you run an engine with inadequate counterweights at high speed it'll run for a while, but expect to run into durability problems.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[SomptingGuy]

Maybe you're getting confused between primary and secondary balance? Big I4 (>2L) engines often have secondary balancer shafts, whereas I6 engines don't need them.

- Steve

 

[Lou Scannon]

Maybe it's been superceded, but the relevant textbook in my uni days was Martin's "Kinematics and Dynamics of Machines". Reciprocating engine balancing explained. (Caveat: this textbook hands you a few sardines for an appetizer, but mainly teaches you to fish).

 

[tbuelna]

kitabel,

An in-line 6 with proper indexing of the crank pins has no inertia force or couple unbalance. So solely with regards to these vibratory effects, no crankshaft counterweights are required.

But as GregLocock notes, counterweights also serve to reduce peak main bearing inertia loads, reduce crank bending between mains, and also can be used to mitigate torsional vibration effects along the crankshaft structure.

The optimum use of counterweight masses in an in-line 6 crank is an exercise in compromise, just like any other crank design. The trade off is between vibrations, weight, mass properties, shaft dynamics, bearing life, cost, fatigue life, etc. A limited life, low inertia, lightweight racing crank made from high strength alloy steel would probably not benefit from counterweights. On the other hand, a low cost, cast iron, production automotive crankshaft would greatly benefit from an optimized counterweight configuration.

At a very simplistic level that just considers inertias, the counterweight mass should balance 100% of the opposing crank pin and web structure plus approximately 50% of the reciprocating rod/piston mass. That should provide a good starting point. The counterweight should be designed to have a CG as far away from the crank axis as possible. Maximizing the counterweight MOI will minimize the total counterweight mass needed.

A good, easy to read reference is "Fundamentals of Automotive Engine Balance" by W. Thomson. 1978.

Hope that helps.
Terry

 

[kitabel]

100% of the opposing crank pin and web structure plus approximately 50% of the reciprocating rod/piston mass

But, since this is exactly the method for (completely different) 90° V8.....?