Crankshaft Balancing Theory
Balancing goes hand-in-hand with performance engine building. Balancing reduces internal loads and vibrations that stress metal and may eventually lead to component failure.
From a technical point of view, every engine regardless of the application can benefit from balancing. A smoother-running engine is also a more powerful engine. Less energy is wasted by the crank as it thrashes about in its bearings, which translates into a more usable power at the flywheel. Reducing engine vibration also reduces stress on motor mounts and external accessories, and in big over-the-road trucks, the noise and vibration the driver has to endure mile after mile.
Though all engines are balanced from the factory (some to a better degree than others), the original balance is lost when the pistons, connecting rods or crankshaft are replaced or interchanged with those from other engines. The factory balance job is based on the reciprocating weight of the OE pistons and rods. If any replacements or substitutions are made, there’s no guarantee the new or reconditioned parts will match the weights of the original parts closely enough to retain the original balance. Most aftermarket replacement parts are "balanced" to the average weight of the OEM parts, which may or may not be close enough to maintain a reasonable degree of balance inside the engine. Aftermarket crank kits are even worse and can vary considerably because of variations within engine families.
If the cylinders are worn and a block needs to be bored to oversize, the larger replacement pistons may be heavier than the original ones. Some piston manufacturers take such differences into account when engineering replacement pistons and try to match "average" OE weights. But others do not. Most high performance pistons are designed to be lighter than the OE pistons to reduce reciprocating weight for faster acceleration and higher rpm. Consequently, when pistons and rods are replaced there’s no way of knowing if balance is still within acceptable limits unless you check it.
When building a performance motor, a stroker motor or an engine that’s expected to turn a lot of rpm's or run a lot of miles, balancing is an absolute must. No engine is going to survive long at high rpm's if it’s out of balance. And no engine is going to last in a high mileage application if the crank is bending and flexing because of static or dynamic imbalances.
Forces In Action
To better understand the mechanics of balancing, let’s look at the theory behind it. As everybody knows, a rotating object generates "centripetal force." Centripetal force is an actual force or load generated perpendicular to the direction of rotation. Tie a rope to a brick and twirl it around and you’ll feel the pull of centripetal force generated by the "unbalanced" weight of the brick. The faster you spin it, the harder it pulls. In fact, the magnitude of the force increases exponentially with speed. Double the speed and you quadruple 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.
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.
Centripetal force 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 the brick and the brick will fly off in a straight line (we don’t recommend trying this because its difficult to control the trajectory of the brick).
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.
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.
Okay, so how does all this scientific mumbo jumbo translate into the real world dynamics of a spinning crankshaft? A crankshaft, like a flywheel, is a heavy rotating object. What’s more, it also has a bunch of piston and rod assemblies reciprocating back and forth along its axis that greatly complicate the problem of keeping everything in balance.
With inline four and six cylinder engines, and flat horizontally opposed fours and sixes (like Porsche and Subaru), all pistons move back and forth in the same plane and are typically phased 180° apart so crankshaft counterweights are not needed to balance the reciprocating components. Balance can be achieved by carefully weighing all the pistons, rods, wrist pins, rings and bearings, then equalizing them to the lightest weight.
On V6, V8, V10 and V12 engines, it’s a different story because the pistons are moving in different planes. This requires crankshaft counterweights to offset the reciprocating weight of the pistons, rings, wrist pins and upper half of the connecting rods.
With "internally balanced" engines, the counterweights themselves handle the job of offsetting the reciprocating mass of the pistons and rods. "Externally balanced" engines, on the other hand, have additional counterweights on the flywheel and/or harmonic damper to assist the crankshaft in maintaining balance. Some engines have to be externally balanced because there isn’t enough clearance inside the crankcase to handle counterweights of sufficient size to balance the engine. This is true of engines with longer strokes and/or large displacements.
If you’re rebuilding an engine that is internally balanced, the flywheel and damper have no effect on engine balance and can be balanced separately. But with externally balanced engines, the flywheel and damper must be mounted on the crank prior to balancing.
Customers should be told what type of engine balance they have (internal or external), and warned about indexing the position of the flywheel if they have to remove it later for resurfacing. Owners of externally balanced engines should also be warned about installing different flywheels or harmonic dampers and how it can upset balance.
In recent years, the auto makers have added balance shafts to many four and six cylinder engines to help cancel out crankshaft harmonics. The counter-rotating balance shaft helps offset vibrations in the crank created by the firing sequence of the engine.
On these motors, make sure the balance shaft is correctly "phased" or timed to the rotation of the crank. If the shaft is out of sync, it will amplify rather than diminish engine vibrations.
Balance shafts are not a substitute for normal engine balancing, nor do they reduce the vibration and stress the crankshaft itself experiences as it turns.
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.
On V6 and V8 engines, the 60 or 90 degree angle between the cylinder banks requires the use of "bobweights" on the rod journals to simulate the reciprocating mass of the piston and rod assemblies. Inline four and six cylinder crankshafts do not require bob weights. To determine the correct weight for the bob weights, the full weight of a pair of rod bearings and the big end of the connecting rod, plus half the weight of the little end of the rod, piston, rings, wrist pin (and locks if full floating) plus a little oil are added together (100 percent of the rotating weight plus 50 percent of the reciprocating weight). The correct bob weights are then assembled and mounted on the crankshaft rod journals.
The crankshaft is then placed on the balancer and spun to determine the points where metal needs to be added or removed. The balancer indexes the crank and shows the exact position and weight to be added or subtracted. The electronic brain inside the balancer head does the calculations and displays the results. The latest machines have graphical displays that make it easy to see exactly where the corrections are needed.
If the crank is heavy, metal is removed by drilling or grinding the counterweights. Drilling is usually the preferred means of lightening counterweights, and a balancer that allows the crank to be drilled while still on the machine can be a real time saver.
If the crank is too light, which is usually the case on engines with stroker cranks or those that are being converted from externally balanced to internally balanced, heavy metal (a tungsten alloy that is 1.5 times as heavy as lead) is added to the counterweights. This is usually done by drilling the counterweights, then press fitting and welding the heavy metal plugs in place. An alternate technique is to tap the hole and thread a plug into place. Drilling the holes sideways through the counterweights parallel to the crank rather than perpendicular to the crank is a technique many prefer because it prevents the metal from being flung out at high rpm.
After drilling, the crankshaft is again spun on the balancer to determine if additional corrections are required. If the crank is for an externally balanced engine (such as a big block Chevy), the balancing will be done with the flywheel and damper installed. On internally balanced engines, the flywheel and damper can be balanced separately, or installed on the crank and balanced as an assembly once the crank itself has been balanced.
Understanding Crankshaft Balancing
Since different rods and different pistons are different weights, it is impossible to make a crankshaft that is balanced "right out of the box" for any rod and piston combination. All crankshafts must be balanced to your specific rod and piston combination.
The first step in understanding crankshaft balancing is to understand the purpose of the counterweights. The counterweights are designed to offset the weight of the rod and pistons. You have the weight of the crankshaft and the pistons and rods. At any point in the assembly's rotation, the sum of all of the forces are roughly equal to zero.
If the counterweights are the correct weight to offset the weight of the rods and pistons, the crankshaft is balanced. If the counterweights are too heavy, material must be removed by drilling or milling the counterweights. If the counterweights are too light, weight must be added to the counterweights. This is usually done by drilling a hole in the counterweight and filling the hole with "heavy metal" or "mallory". This filler metal is denser and heaver than steel (but not stonger) so the weight of the counterweight will increase as a result.
Internal Balance & External Balance
When the counterweights alone can be made to balance the crankshaft, the crank is said to be "internally balanced". If the counterweights are too light by themselves to balance the crankshaft and more weight is needed, an "external balance" can be used. This involves a harmonic dampener or flywheel that has a weight on it in the same position as the counterweight that effectively "adds" to the weight of the counterweight on the crankshaft.
Since the harmonic dampener (front) or flywheel (rear) play a part in the balancing of the assembly, they must be installed on the crankshaft when it is balanced. This is unlike an internal balance configuration where the harmonic dampener or flywheel do not contribute to the balance of the crankshaft and are not required to be installed when the crankshaft if balanced. Both methods are used from the manufacturer.
An example of some factory internally balanced engines are Chevy 305 and 350 (2 piece rear seal only!), Chevy 396/427, GM LS-series, and Ford "modular" 4.6. Some examples of factory externally balanced engines are Chevy 400 and 454, Ford 302 and 351W.
Some engines are a combination of both being internally balanced in the front and externally balanced in the rear. The most common example of this is the Chevy 350 (1 piece rear seal) including LT1. Regradless of how an engine is balanced from the factory any balancing method is acceptable as long as the required harmonic dampener and/or flywheel is available.
"Is my crank balanced?"
Since different rods and different pistons are different weights, it is impossible to make a crankshaft that is balanced "right out of the box" for any rod and piston combination. All crankshafts must be balanced to your specific rod and piston combination. When a crankshaft is listed as "internal balance" or "external balance" this is stating how this crank is intended to be balanced. It can be balanced otherwise, but it is much more difficult to do so.
Eagle crankshafts, for example, are listed with a "target bobweight". This is an approximation (+/-2%) of the bobweight the crankshaft is roughly "out of the box". Because of the tolerance (+/-2%) the crankshaft cannot be considered balanced. For instance, for a crankshaft listed as having a 1800 target bobweight.
The actual range of bobweights one of those cranks might have is from 1764 (1800-2%) to 1836 (1800 +2%). It might even be at the high end of that range on one end and the low end of that range on the other! This is not usually a problem because Eagle crankshafts are designed to have a target bobweight higher than most typical rod and piston combinations. Therefore, in most cases you will only need to remove material to balance the crankshaft instead of adding material.
The main benefit of the target bobweight is to help the machine shop know what to expect before balancing so that a more accurate price estimate can be made. Eagle will balance a new crankshaft at the time of purchase. You will need to provide the bobweight you want it balanced to, which must be below the target bobweight listed for the crankshaft.
When a crankshaft is balanced, the actual rods and pistons cannot be used in the balancing machine, so they must be simulated. This simulated weight is called the "bobweight". Once the bobweight is calculated, weights are bolted onto the rod journals to simulate the weight of the rods and pistons during the balancing process. Due to the configuration of a "V" type engine, just adding all the weights together does not work.
There are also some dynamic considerations to be made when balancing the crankshaft. Explaining those is beyond the scope of this discussion. If you want to study those topics further, contact a crankshaft balancing machine manufacturer and they can go into greater detail.
To calculate the bobweight of a particular assembly, the following formula and balance card is used:
For example, let's say we are balancing a Chevy 383 with the following component weights:
•Rod big end 458g
•Rod smal end 186g
The rod weight is seperated into "big end" and "small end". This is necessary because the small end has a reciprocationg (back and forth) motion and the "big end" has a rotating motion. This split weight is figured on a special scale fixture that supports one end of the rod while weighing the other end.
There are several things to note about this calculation. The "oil" value used on the left side of the calculation is an approximation of the weight of residual oil "hanging around" on the assembly. The number used here is a matter of preference. There is no solid "rule of thumb" for this. Eagle uses 5g for small block assemblies and 15g for big block assemblies. Since it is impossible to accurately represent this value, it is just an estimate. The actual amount of oil can change constantly and can even be different from cylinder to cylinder! We have found through experience that the numbers we use estimate this property well.
The second thing to note is the 50% value used for the reciprocating factor. This number deals with the geometry of the engine itself. A 90 degree bank angle "V" engine will use 50% here. A V6 or a narrow or wide bank angle "V" engine will use a different value (again, consult the balancer manufacturer). Some engine builders will perform what is call "underbalancing" or "overbalancing". They will use slightly differnet values here such as 48% or 52%. This is done to help compensate for dynamic effects at extremely high or extremely low rpm operation (again, beyond the scope of this discussion). Eagle uses 50% because this value is required for almost all common street or racing engines.
Balanced Rotating Assemblies
Most Eagle rotating assemblies are sold unbalanced so that engine builders can balance it however they wish. Eagle (and other manufacturers) do offer fully balanced assemblies balanced. But it must be ordered specifically as a balanced assembly. Part numbers for balanced assemblies will begin with the letter B. For instance, if you want assembly part number 12006 balanced and in +.030" bore size, you would order assembly number B12006-030.
All Eagle forged 4340 steel crankshafts are designed for internal balance. An internally balanced kit will not include a harmonic dampener or flywheel because they are not required for balancing – use whatever brand you like. Externally balanced kits will include a harmonic dampener and/or flexplate as needed. If a harmonic dampener and flexplate is provided, it will be an O.E. style replacement, not SFI approved. If you’re building a high horsepower engine, internal balance is preferred. Internal balance is better for longevity of parts and fatigue life.