BALANCE!....Part one

Joe Sumen · Post by **Joe Sumen** » Thu Sep 11, 2003 4:41 pm

Hi Folks,
So, you saved the coin required to buy some real spiffy new parts in order to assemble that aircooled neck snapper huh?...Or maybe just a sweet running stocker or mildly built little spinner...Well, you've worked hard for that dough and dreamt night after night about how close you are to lovingly assembling your Teutonic marvel. The last of the parts have arrived, garage is all cleaned up, tools all organized, kids are on the PC or busy decifering Sponge Bobs escapades on the tube, wife is busy readying up a fantastic steak & shrimp feast for later, you have a cold libation of choice in your quiverring hand and it's finally time to commence with this long anticipated epic event. No watching T.V. or poker games tonight! No way!..Not even Heather Locklear nor Pamela Anderson asking for a full body massage is enough to sway you away from this! (well, for a little while maybe!)..At least, as long as it gives you time to get your "bottom end" buttoned up!..BUT!...Are you really ready?

Of course, what I'm referring to is "balance". Balancing of every component that goes round and round, up and down is EXTREMELY vital to longevity, and YES....Horsepower. Unless YOU personally have the expertise and equipment to perform these vital functions, you're at the mercy of the people you purchase from to take their word for it.
That's fine, as long as you're dealing with a reputable source, and even then how finely balanced is the component? What is acceptable for them?...

Let's take a look at the dynamics involved...

A rotating object generates "centripetal" AND "Centrifugal" force..
I'll get to that later...Centripetal force is the load/force generated "perpindicular" to the axis of rotation. This goes way back to the fable of David and Goliath...David, picked up a rock, stuck it in his sling and swung it above his head until releasing said rock and "beaning" Goliath squarely in the noggin...leveling the giant..O.K. enough of that..More understandably, we've all, at one time or another had some kind of "mass" tied to a rope and swung it round' and round' above our heads..When doing so, "we are the axis", the "Tug" we feel is the "centripetal" force. The faster we spin it, the higher the force. Matter of fact, the amount of force increases "exponentially" with speed. Double the speed and you've quadrupled the force. The centripetal force created by a crankshaft imbalance will depend upon the amount of imbalance and distance from the axis of rotation (which is expressed in units of grams, ounces or ounce-inches). A crankshaft with only two ounce-inches of imbalance at 2,000 rpm will be subjected to a force of 14.2 lbs. At 4,000 rpm, the force grows to 56.8 lbs.! Double the speed again to 8,000 rpm and the force becomes 227.2 lbs. WOW!...Now you can see how VITALLY important balancing is! This may not sound like much when you consider the torque loads placed upon the crankshaft by the forces of combustion. But centripetal imbalance is not torque twisting the crank. It is a sideways deflection force that tries to bend the crank with every revolution. Depending on the magnitude of the force, the back and forth flexing can eventually pound out the main bearings or induce stress cracks that can cause the crank to snap (I had posted last year the forces a crankshaft must endure, mysteriously that post, which required a LOT of time vanished, maybe someone copied and saved it?).

Centripetal force/load should not be confused with "centrifugal" force, which is the tendency of an object to continue in a straight trajectory when released while rotating. Let go of the rope while you?re twirling that mass and the mass will fly off in a straight line.

Back to centripetal force. As long as the amount of centripetal force is offset by an equal force in the opposite direction, an object will rotate with no vibration. Tie a brick on each end of a yardstick and you can twirl it like a baton because the weight of one brick balances the other. If we?re talking about a flywheel, the flywheel will spin without wobbling as long as the weight is evenly distributed about the circumference. A heavy spot at any one point, however, will create a vibration because there?s no offsetting weight to balance out the centripetal force.

The process of balancing begins by equalizing the reciprocating mass in each of the engine?s cylinders. This is done by weighing each piston on a sensitive digital scale to determine the lightest one in a set. The other pistons are then lightened to match that weight by milling or grinding metal off a non-stressed area such as the wrist pin boss. The degree of precision to which the pistons are balanced will vary from one engine builder to another, and depends to some extent on the application. But generally speaking pistons are balanced to within plus or minus 0.5 grams of one another.

Next the rods are weighed, but only one end at a time. A special support is used so that the big ends of all the rods can be weighed and compared, then the little ends. As with the pistons, weights are equalized by grinding away metal to within 0.5 grams. It?s important to note that the direction of grinding is important. Rods should always be ground in a direction perpendicular to the crankshaft and wrist pin, never parallel. If the grinding scratches are parallel to the crank, they may concentrate stress causing hairline cracks to form.

This brings us to another law of physics. Every object wants to rotate about its own center of gravity. Toss a chunk of irregular shaped metal into the air while giving it a spin and it will automatically rotate about its exact center of gravity. If the chunk of metal happens to be a flywheel, the center of gravity should be the the flywheel?s axis. As long as the center of gravity for the flywheel and the center of rotation on the crankshaft coincide, the flywheel will spin without vibrating.

But if there?s a heavy spot on the flywheel, or if the flywheel isn?t mounted dead center on the crank, the center of gravity and axis of rotation will be misaligned and the resulting imbalance will create a vibration.

This is quite a long EXPOSE` on balancing, and I've taken up a LOT of space!...This is PART 1 so hang on a bit and I'll post the second part within a couple of days![/i]

Marty · Post by **Marty** » Thu Sep 11, 2003 4:44 pm

HOLY COW!
He climbed out from under his rock!
I thought we were going to hear from Sadam first.

Good stuff!

A_67vdub · Post by **A_67vdub** » Thu Sep 11, 2003 5:30 pm

Thanks, Joe! I've been looking forward to this dissertation.

I already have a question. I've heard engine balance people say they balance the parts to within a certain number of grams (usually 1 or 0.5). For me to really understand how balanced my engine is, don't I need to know a distance, too? Like gram-meters, or gram-centimeters? What is the standard distance (if any)? You already touched on this when you said the centripetal force can be expressed in ounce-inches, but I'm wondering what the metric standard is for engine balancing.

Thanks again,

Steve

workingstiff · Post by **workingstiff** » Thu Sep 11, 2003 6:39 pm

Looking forward to the next installment. Is it possible for the moderator to tag the better well written, more informative posts so that they don't get deleted or lost?

nacerenza · Post by **nacerenza** » Thu Sep 11, 2003 9:10 pm

Good disertation!
I am Nicolás Acerenza, an engineering student from Uruguay. I realy liked your disertation and passion to share your knowledge, but I would like to discuss some points in disagree:
1) Centripelat force really exists (it is the force made by then rope you said done to the stone),but by Newtons Action-Reaction law centrifugal force isn´t really a force, it is a pseudoforce (imaginary force) which is the reaction to the force exerted by the rope.
2) Balance doesn´t actually build horsepower, two identical engines (one balanced and the other no) runing at the same frequency (eg: 2000rpm) make the same horsepower. It is the product of this balance, which we can make the engine run at higher crank speeds than a non balanced engine and therefore make more horsepower (horsepower is theorically a product of many variables, which includes the rotating frequency of the engine).
3) Objects tend to turn around their center of gravity depending on the external forces exerted on them. If the resultant of the sum of all the external forces cannot be simplified to one force on one aplication point then that affirmation is not always true.

Never mind my corections.....good article, and you are correct....BALANCE REALLY WORKS.

NAHPerformance

Post by **sparkmaster1** » Thu Sep 11, 2003 9:39 pm

Great reading. Can you e-mail me the whole thing?? Tim

andy4d · Post by **andy4d** » Fri Sep 12, 2003 1:04 am

great article jo!

but on the centripetal/centrifugal forces - centripetal (from the latin for 'centre finding') is the force exerted by the rope in your example, centrifugal is a product of the objects inertia-the rock wants to go in a straight line.

Joe Sumen · Post by **Joe Sumen** » Fri Sep 12, 2003 1:23 am

nacerenza wrote:Good disertation!
I am Nicolás Acerenza, an engineering student from Uruguay. I realy liked your disertation and passion to share your knowledge, but I would like to discuss some points in disagree:
1) Centripelat force really exists (it is the force made by then rope you said done to the stone),but by Newtons Action-Reaction law centrifugal force isn´t really a force, it is a pseudoforce (imaginary force) which is the reaction to the force exerted by the rope.
2) Balance doesn´t actually build horsepower, two identical engines (one balanced and the other no) runing at the same frequency (eg: 2000rpm) make the same horsepower. It is the product of this balance, which we can make the engine run at higher crank speeds than a non balanced engine and therefore make more horsepower (horsepower is theorically a product of many variables, which includes the rotating frequency of the engine).
3) Objects tend to turn around their center of gravity depending on the external forces exerted on them. If the resultant of the sum of all the external forces cannot be simplified to one force on one aplication point then that affirmation is not always true.

Never mind my corections.....good article, and you are correct....BALANCE REALLY WORKS.
NAHPerformance

Hi Nicolas,
I'd first like to welcome you to the STF ( I see this is your 3rd post ). Secondly, I encourage you to continue with your education in engineering. It is a fascinating field to be in. Though I really don't have the time to delve into physics too deeply, let's take a look at your comments....

1) Centripelat force really exists (it is the force made by then rope you said done to the stone),but by Newtons Action-Reaction law centrifugal force isn´t really a force, it is a pseudoforce (imaginary force) which is the reaction to the force exerted by the rope.

When the subject of circular motion is discussed, it is not uncommon to hear mention of the word "centrifugal." Centrifugal, not to be confused with centripetal, means away from the center or outward. The use of or at least the familiarity with this word centrifugal, combined with the common sensation of an outward force when experiencing circular motion, often creates or reinforces a deadly student misconception. The deadly misconception, believed by many physics students, is the notion that objects in circular motion are experiencing an outward force. "After all," a well-meaning student may think, "I can recall vividly the sensation of being thrown outward away from the center of the circle on that roller coaster ride. Therefore, circular motion must be characterized by an outward force." This misconception is often fervently adhered to despite the clear presentation by a textbook or teacher of an inward force requirement. The motion of an object in a circle requires that there be an inward net force - the centripetal force requirement. There is an inward-directed acceleration which demands an inward force. Without this inward force, an object would maintain a straight-line motion tangent to the perimeter of the circle. Without this inward or centripetal force, circular motion would be impossible.

2) Balance doesn´t actually build horsepower, two identical engines (one balanced and the other no) runing at the same frequency (eg: 2000rpm) make the same horsepower. It is the product of this balance, which we can make the engine run at higher crank speeds than a non balanced engine and therefore make more horsepower (horsepower is theorically a product of many variables, which includes the rotating frequency of the engine).

This is a true statement Nicolas, though you've answered your own boggle here. Any motor, regardless of output is of little use if not capable of reaching the "cycling frequency" required to produce its maximum output. Maintaining that frequency without self destructing is what we're after.

3) Objects tend to turn around their center of gravity depending on the external forces exerted on them. If the resultant of the sum of all the external forces cannot be simplified to one force on one aplication point then that affirmation is not always true.

Yes Nicolas, I believe the centripetal/centrifugal dynamic explains this. Mass dynamics is a difficult subject to condense into an easy to understand format Nicolas, which is what I'm attempting to do here. In the second installment of this topic, I'll be posting additional material which is basic and understandable to the average enthusiast giving them a better understanding of how, what and why. To post on topics using highly technical/mathematical explanations would isolate too many folks. Much good luck in your future endeavors, Nicolas. Thank you for your querry.

mschilling · Post by **mschilling** » Fri Sep 12, 2003 5:24 am

The balancers that I have used (Garland-Toronto) balance "to zero" (not .5g or.1g, but zero) and I'm assuming they can claim that because their equipment is calibrated to under .1g, and also they don't balance pistons by grinding the piston itself, they grind material from inside the piston pins which then stay with their specific piston.

Billyisgr8 · Post by **Billyisgr8** » Fri Sep 12, 2003 8:20 am

mschilling wrote:The balancers that I have used (Garland-Toronto) balance "to zero" (not .5g or.1g, but zero) and I'm assuming they can claim that because their equipment is calibrated to under .1g, and also they don't balance pistons by grinding the piston itself, they grind material from inside the piston pins which then stay with their specific piston.

I thought it is very hard to balance to .02 of a gram as if you breath the wrong way it shows out of balance. you end up chasing it around at .02. Some companies will only balance to .04 of a gram unless you pay for the extra time it takes to get less than that.

This is what I was told by a balancer who owns his own machine at his house.

It is important to ask what tolerance the company is doing your pcs to.

Kevin

Marty · Post by **Marty** » Fri Sep 12, 2003 10:23 am

Yeah, I would say its pretty much impossible to balance to "zero". I bet if they took a freshly ballanced assembly (balanced to "zero") off the machine, let it sit overnight and then put it right back on the same machine, it would not show that it is still balanced to "zero". I think they use "zero" as a term to describe that its as balanced as they can possibly get it.

Joe Sumen · Post by **Joe Sumen** » Fri Sep 12, 2003 1:19 pm

Well fellas, in reference to how close to "ZERO" can a component be balanced?...Amazingly close. There are different machines for different balancing jobs. Most shops just don't have the equipment to "precision" balance. Now what do I mean by precision balance?..How about "hundredths" or even "thousandths" of a gram. Problem is, the average shop will only have average equipment...Numeric instead of graphical displays, standard drill stands and grinders instead of articulated "mills" required for true precision work. Now these are fine for MOST. For others who want or NEED more precision, this simply won't do. Porsche, BMW, Honda which produce high horsepower, extreme R.P.M motors as well as independants like John Force etc. MUST rely on finely "precision balanced" components.
Their components are balanced on $30k+ equipment Software and hardware are combined to allow differences to be reported at each end of the crankshaft (including any rotational positioning or vector position of the unbalanced force). Because the position and unbalance amounts are reported correctly the technician can make changes to the crankshaft with confidence that he will not over shoot the correction. In most cases the required cycles of analysis and correction are reduced by over 90%. These systems are "windows" software based so vital data pertaining to "specific" crankshafts can be stored and retrieved at will. Graphic displays along with automatic vectoring "pinpoint" the EXACT location for balancing procedures.

Now, these machines vary in capacity. For example, precision balancing of say, turbo impellers requires a much more sensitive unit...Get this. A new Seimens unit manufactured in Germany has the ability for an operator to balance impellers as well as any mass 5 Kg and lower to an almost unbelievable value of +/- .0001 gr/mm. Now THAT is balance! I, personally use a specialist who does NOTHING but "precision balancing".
His clients include Porsche N/A, HMC and TRD. He does NO outside work, but because of rapport established while at ANDIAL he (fortunately) does my balancing..Is he cheap? Relative to what you get elsewhere, I should say so! About $200 for a VW or 4 banger BMW. $350 for Porsche. $750 for the 12 cyl BMW's. On the "EXTREME" balance jobs add an extra $1-200. These he'll bring down to "hundreths" of a gram. "Polishing" not grinding is required for such fine balance.

Tony Z · Post by **Tony Z** » Fri Sep 12, 2003 3:03 pm

I would just like to add my bit

"Balance doesn´t actually build horsepower, two identical engines (one balanced and the other no) runing at the same frequency (eg: 2000rpm) make the same horsepower. It is the product of this balance, which we can make the engine run at higher crank speeds than a non balanced engine and therefore make more horsepower (horsepower is theorically a product of many variables, which includes the rotating frequency of the engine)."

This is not entirely correct. Yes both engines will amke the same Hp in the cylinders, but the imbalance will increase the amount of friction as well as heat up the bearing and oil, thereby transferring the rotaional energy to wasted products. Basically said, the engine becomes less efficient and less HP is eseen at the flywheel. Take it to an extreme, weld a 500 gram weight to a web, lets say the web nearest the centre main, at the farthest pont from the centre of rotation (radius of gyration), run the crank at 5000rpm and then look at the bearing and the bearing saddle

mschilling · Post by **mschilling** » Fri Sep 12, 2003 3:26 pm

Interesting about the precision balancing. Obviously a shop that specializes in balancing has the right equipment, and worth it to seek out. I've seen interesting stuff in to get balanced like giant elevator wheels, and motorcycle road race knife edged cranks, along with my ACVW fan and even the alt pulley (just like factory).

Marty · Post by **Marty** » Fri Sep 12, 2003 4:24 pm

I digress.
You CAN get fine balancing IF you search it out and IF you are willing to pay for it.
I DONT believe that most of the shops that cater to our VW stuff do it to that degree of precision. You MUST look outside to get it.
Jake may have the proper equipment and takes the time to do it but I dont know of anyone else within "the circle".