While match weighting the pistons and connecting rods for my latest engine build I got to thinking (not always a good thing) about how the bob weight for the rotational balance on a crankshaft is determined. I had always been told that on a 90* vee type engine the bob weight was 1/2 the weight of the piston and piston end of the connecting rod plus the weight of the crankshaft end. I can see where that makes sense since half of the piston end weight is supported by the cylinder wall. What I am wondering about is how the piston end weight is determined. They guy at the balance shop, and everyone else that I have talked to, stated that he would weigh the piston end of the connecting rod and then add the weight of the piston. It occurred to me that would only be accurate if the center of gravity of the piston was on the centerline of the piston pin. As a result I weighed both ends of an assembled piston and connecting rod. Then I weighed both ends of a bare rod and added the piston weight. As I expected the assembled piston end weight was higher than when adding the separate components together. Obviously the crankshaft end was lighter. After comparing the resulting bob weights the "traditional" method of weighing the parts individually resulted in a bob weight 31 grams heaver than when the components were assembled. So does this make any difference in the end? Or have I been sniffing too many solvent fumes while cleaning pistons? I am aware that the balance shop has been doing it by adding the component weights for several decades with good result. So let me know what your thoughts are on the subject, Norm
Easier with a pic. Small end is suppported to measure big end weight, big end weight is subtracted from total weight to get small end weight. Source http://www.fordmuscle.com/archives/2008/04/RotatingAssembly/index2.php
Yeah, I got that. I'm asking about adding the piston weight. I used a similar set up to measure each end with all of the components assembled together; piston, pin, rings, rod, rod cap, bolts, nuts and bearing shells. And yes, I got a different weight with all the parts together than I did weighing the parts separately and adding their weights together.
With the components assembled or un assembled - with one end fixed, the fixed end is being supported and will not have the same total mass as being weighed in the free state. You mentioned using the fixture, to get the weight of both ends. For rod weight you measure one end them subtract from the total. I may be misunderstanding, Ill follow along, hope you get it figured.
Yes the total will be the same BUT when the piston is installed on the rod the center of gravity of the piston is outboard of the pin centerline resulting in more weight on the pin end of the rod. This is due to the ability of the assembly is able to pivot on the crank end causing some of the crank end weight (actually mass) to shift to the piston end. The result is that while the total weight does not change the proportion on either end does. So back to my original question....is this important in a real engine?
But you're titling it about the crankshaft? The rods/piston/etc, can be made to the same weight, doesn't mean the reciprocating mass is the same though. Well, maybe it is/isn't...but if I had two rod/pistons side by side, if they are both the exact same weight...are they balanced? Or if I put the assembly on a transit and made sure my big ends and little ends were the same... If not, the CS balance could be off when assembled...but What are you building here?
Because the mass of the piston/rod assembly needs to be known in order to determine what bob weight is needed to balance the crankshaft. Since I get a different bob weight depending on whether I calculate the mass or measure it on the scale I'm curious which method is more correct. I'm currently working on a '74 vintage Pontiac 455 although the same theory should apply to any 90* vee type engine. As long as the engine is installed in an upright position that is. Some aircraft engines were installed upside down. The discussion on how that affects the balance is perhaps better left for another time, and forum.
There's more to balancing an engine than just "balancing" the crankshaft. There are two different formulas used and added together to figure the bob weight, rotating weight and reciprocating weight. Rather than go into it all here, it's probably easier just to do a google search on "engine balancing". I am Butch/56sedandelivery.
When you weigh the stuff static, you are not really seeking the weight. You are seeking the mass. So you don't let gravity interfere. When the engine is spinning gravity is irrelevant. It is inertia vs. mass that matters.
Well, let's hope that 455 isn't doing vertical 180's Thing is, bob weight isn't generally need on the crank...not for 95% of even "hot builds"...so, what I was trying to say before...let;s look at an extreme. So take two rods...each weigh say 700 grams...just saying...identical weight. Now lets say the the big end on one weighs 400 grams, little end weighs 300 grams...and vice-versa on the other rod. See what I mean? For all practical street purposes that's never a consideration, so rods are generally just weight balanced off a fulcrum...but not really matched like the expensive stuff is...and not even needed. So now you want to balance the crank...to what spec?...probably to get it where you may think you want it, you're going to be adding that fancy stuff..mallory? metal. Gonna get real pricey. So now that you said you're building a 455 Pontiac, what are you're expectations of it, and maybe you're over thinking the build? I'm just asking.
Here's a lengthy (three part) article on balancing, but it's nonetheless a good read if you have the time: http://www.onallcylinders.com/2016/03/17/the-basics-of-crankshaft-balancing/
You guys sound like a bunch of kids trying to impress each other..with the exception of Butch (56 SD) the other posters are full of the stuff that's found on the bottom of a bird cage...why not discuss a brain operation too??
Thanks for the link. I carefully read all three parts and found it very informative and interesting. The first section discussed the basics and reiterated weighing the piston and rod end separately. I eventually ciphered out that the reason that it is done that way is because the piston weight (mass) is entirely reciprocating, no rotational motion. The second section on the dynamic forces involved due to cylinder pressure, etc. was something that I had considered but have no way to measure or experiment with. As a result I am at the mercy of the experience of the balance shop. In the second section it also explained the "why" of the 50% used in a 90* vee engine. I thought that it had to do with gravity; it turns out it has to do with the interplay of forces between the cylinder banks. The third section discussed some fairly esoteric stuff including phasing the counterweights. I remember reading articles back in the day that extolled the benefits of "knife edging" the crankshaft counterweights in order to reduce the friction of the crank cutting through the air/oil mist in the crankcase. Now I'm thinking that reducing the air friction had less to do with the outcome than the change in position of the counterweight center of gravity (phasing). It also seems to me that removing material with a grinder is a pretty crude way to change the crankshaft. Thanks to all who contributed; I learned something today and as Gene Winfield is fond of saying "every day is a school day". Norm
A GMC 6 crank is internal balanced with no bobs so that's the only thing the crank shop does. The rest is up to me. With my 3 beam scale an a couple of fixtures I balance all the rest. A Super Damper on the nose and 3 dowels on the flywheel with shoulder bolts and balenced clutch finishes it off.
Hi just to throw something else at you guys. When the bob weight is determined (by what ever way) the guy also has the option to under balance or over balance the crank. this is determined at what speed the engine will spin at. eg street or race or constant speed Have a great day Dave
Friend of mine bored a 283 out .060", (to 292) pistons were cast, he weighed them and they were close to weight of the stockers. An acquaintance screamed and yelled about the 'larger bore size...' "It'll throw the engine into an imbalance you won't believe!" Well... Obviously, it didn't. Our 'critic' asked why the friend just 'threw the engine together'! I replied, "We wanted to throw YOU into a stage of imbalance YOU wouldn't believe." True story. P.S. That 292 ran smoother than my 292 Y block. (something Chevys seldom did)
Inline engines aren't balanced using bobweights like "V" engines are, so there are no bobweights to determine. On inline engines, the counterweights on the crank oppose each other like a seesaw, and the weight of the rods, pistons, wrist pins,etc.. is irrelevant to the balance of the crankshaft, as long as the total combined weight of those components are the same as the others, because they also oppose each other like a seesaw and balance themselves. A "V" engine is completely different, any weight that is added to the rotating weight in the way of heavier/lighter rods, pistons, wrist pins beyond a certain amount(sometimes as little as 30 grams)from what the crank was originally balanced to will require a re-balance to compensate for the heavier/lighter components via adding bobweights that are calculated from a formula to determine the amount needed. Also, grinding or lightening a "V" crank from knife-edging or turning down counterweights or drill the rod pins will also naturally require re-balancing the crank from its original balance even if the stock rods or pistons haven't been changed. But still, bobweights are required for this in the "V" engine, but not in an inline engine. These are two completely different animals in respect to how they are balanced and how heavier/lighter rotating assemblies affects them differently.
Crankshops has never asked for anything as they know what I am going to do. I've witnessed the spins. I balance each piston, pin,rod,ring set,to within 1/4 gram to each other as a unit when done. Never had a problem nothing comes loose.
One thing that seems like it would screw up the balance is the way the threaded rods stick way out of the bob weight assemblies when they're bolted to the crank journal. It seems to me that a (say) 28.5 ounce bob weight with studs that stick out 1/2" will affect the balance differently from a 28.5 ounce bob weight with studs that stick out 3" or 4", once the crank is spun up. But maybe I'm overthinking this.
The guy who balanced my 289 combo uses bolts not threaded rod. Everything makes a difference. He built his own balancer on a swing system with dial indicators for direct reading. It works beautifully. He built race engines for many years.
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