In an already present thread I started, there was some talk about balancing a bottom end.
But, I’d like some detail on this.
As in:
are there levels of balancing?
ie, a minimal job, a moderate job, and an extensive job?
or when one says “dynamically balanced” or “fully balanced” or “balanced and blueprinted” do they all mean the same?
and what does each one mean, and what exactly is done, and what is needed?
I figure this would be a good info thread…
I’ve been wondering about balancing for a while as well. Here’s what I found in the Machinery’s Handbook on dynamic balancing:
“Running or Dynamic Balance - A cylindrical body may be in perfect static balance and not be in balanced state when rotating at high speed. If the part is in the form of a thin disk, static balancing, if carefully done, may be accurate enough for high speeds, but if hte rotating part is long in proportion to its diameter, and the unbalanced portions are at opposite ends or in different planes, the balancing must be done so as to counteract the cetrifugal force of these heavy parts when they are rotating rapidly. This is known as a running balance or dynamic balancing. To illustrate, if a heavey section is located at H (fig. 3), and another correspondingly heavy section at H1, one may exactly counterbalance the other when the cylinder is stationary, and this static balance may be sufficient for a part rigidly mounted and rotating at a comparatively slow speed; but when the speed is very high, as in the case of turbine rotors, etc., the heavy masses H and H1, being in different planes, are in an unbalanced state owing to the effect of centrifugal force, which results in excessive strains and injurious vibrations. Theoretically, to obtain a perfect running balance, the exact position of the heavy sections should be located and the balancing effected either by reducing their weight or adding counterweights opposite each section and in the same plane at the proper radius; but if the rotating part is rigidly mounted on a stiff shaft, a running balance that is sufficiently accurate for practical purposes can be obtained by means of comparatively few counterbalancing weights located with reference to the unbalanced parts.”
The book then goes on to discuss the formulas for calculating counterweights and such. Basically, from my basic understanding, engines need to be dynamically balanced, and I’ve been told that when they are, the entire rotating assembly is balanced together, crank, rods, pistons, pins, etc. I’ve also heard it suggested that only the cranks are dynamically balanced on some engines, which I would imagine would be a bit easier than trying to balance the entire rotating mass, but I dunno, I’ve never done it. As for what is needed, I’d like to know too, I’m curious to know how they locate the heavy spots in rotating masses. Maybe someone will have more info (I hope). I need to get my bottom end balanced eventually, so hopefully if I can find a shop to do it, they’ll let me watch and ask questions 
interesting…
more info more info…haha
Well, dynamic balancing has been well covered. Nice job :D. Fully balanced is not really something a shop calls it. I mean, if you get your rods shaved to weigh the same, and the pistons, and the crank, but don’t have it dynamically balanced, I suppose you could call that “fully balanced”. But that’s up to interpretation. I call a block that’s been dynamically balaced “fully balanced”. And evey engine that I rebuild gets dynamically balanced.
Blueprinting: this is a term that a lot of people throw around. And when they use it they are usually wrong. Blueprinting is the process of making the block EXACTLY to the engineers blueprints/specifications. By doing this you can usually squeeze out a few more ponies. The minute you put aftermarket pistons in your motor, it can no longer be “blueprinted”. I hope this makes sense. Let me know if you have any other questions.
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