Balancing Science 101 — Battling Primary Rotational Imbalance

Balancing Science 101 — Battling Primary Rotational Imbalance

When it comes to most enthusiasts, balancing a rotating assembly is something that is simply seen as another expense during the engine build process. Dropping all your components off at the balance shop and then picking them up the following week is about the extent of most enthusiasts’ experience with the balancing process.

While it’s not a process that the average person — or even shop — will ever undertake themselves, understanding the process will give you an advantage, both when selecting components and when selecting a balance shop. After all, an uninformed customer is an easily misled customer.

We recently made a trip to the Norcross, GA headquarters of CWT Industries. That name might sound familiar to you if you have followed the Engine Performance Expo series at all, as Randy Neal has been a major part of that event, thanks to his desire to share knowledge. “My mission is to eliminate the voodoo of balancing,” Neal says. “Engine building has always been a science. However, there are still a lot of things regarded as voodoo within it, and we need to eliminate that.”

This article is actually the beginning of a multi-part Balancing Science series, which will dive into some of the more advanced concepts of balancing a rotating assembly. However, in order to properly discuss those concepts, we need to ensure that we have a solid base of understanding. That’s what this article aims to do. So whether you’re a seasoned veteran or just interested in learning more about how balancing works, read on.

crank balancing

The process of balancing a crankshaft is relatively straightforward. You take the crank, add bobweights to the rod journals, and measure the rotational imbalance. From there you modify the crankshaft to come within tolerance.

What Is Balancing?

While it might seem basic, we need to start at the beginning. For the purpose of this article, we’ll stick to common 90-degree cross-plane V8 engines as an example. When you look at a modern V8 crankshaft, you’ll notice there are rod journals every 90 degrees. In addition to those, there are large chunks of metal strategically placed opposite those throws, called counterweights.

Their job is to act as a counterbalance to the piston and rod assembly hanging off of the rod journal and help the entire rotating assembly spin smoothly. In a perfect world, the crankshaft would be designed and manufactured with the correct weight of the pistons, rods, piston rings, bearings, and wrist pins accounted for. However, we are in the world of modification and customization — quite far from perfect.

CWT Balancing stands

The alloy the support stands are made from can affect the balancing process. Everything has its own natural frequency, so CWT engineers the stands – both design and material – to ensure the natural frequency of the stands isn’t within the frequency range of what’s being measured. This is done to avoid any stray resonance between the machine and what’s being measured.

While some crank manufacturers can design the counterweight arrangement around a common bobweight (the total weight of everything hanging off of the rod journal), even then, fine-tuning is needed to achieve acceptable “balance”. When the assembly is balanced, it will spin smoothly through the entire RPM range. When it’s out of balance, at best, you’ll notice a vibration as the engine runs. At worst, you will damage parts.

Compensating For Mass

While that might sound relatively simple, we need to realize that it’s not a matter of just hanging weight off the rotating arm of the crankshaft. While the crankshaft spins, the piston and rods reciprocate — two distinctly different motions.

“Cranks go round and round, pistons go up and down,” explains Neal. “When we understand that basic mechanical activity, we understand the influence of the two masses.” The reason that distinction is important is that, because of the different motions, we don’t compensate for the piston and rod’s mass in a 1:1 ratio on the crankshaft.

Reciprocating mass and rotating mass have different values in the bobweight calculation. That's why we weigh the big end, and the overall rod weight separately. That gives us the big end weight (rotating mass) and small end weight (reciprocating mass)

“We’re taking the mass of what’s going up and down, and we’re taking a compensating percentage so that it works in harmony with the rotation of the crankshaft,” Neal says. Generally, rotating mass is accounted for at 100-percent, while reciprocating mass is “valued” at 50-percent. The exact percentages can be varied slightly for various reasons (called overbalance or underbalance), but that is one of the more advanced concepts we’ll be covering in a later article.

We explain that, to explain the process of creating bobweights for the balancing process. We don’t actually hang a piston and rod off the journal while spin balancing the assembly. But rather, a precisely weighted bobweight, which is clamped to the journal. By using precision scales to measure the weight of every component being used in the rotating assembly, bobweights are built to match those weights.

The pistons, wrist pin, pin locks, and ring pack are all weighed in and calculated as reciprocating mass. The only rotating mass in the bobweight calculation is the rod bearing and the big end of the rod.

Detecting Imbalance

Once we have our components weighed and bobweights built and installed onto the journals, the real mind-blowing things start happening. First, we need to discuss how imbalance is expressed. We also need to make it clear that when an assembly is “balanced”, that doesn’t mean there is no imbalance in the system, but rather, the amount of imbalance has been reduced to below the maximum allowable level. More on that in a minute.

“Imbalance is measured as mass and distance from the axis of rotation,” Neal explains. In the U.S., we most commonly use the unit of ounce-inch (oz-in). Across the pond, they most often use the unit gram-millimeter (g-mm). The units measure the same thing, but in different units, and are easily converted with simple math.

sine wave

Here you can see an ideal voltage trace through 720 degrees of crankshaft rotation on top, and an imbalanced trace on the bottom. With a trained eye, you could get pretty close to locating the vector of the imbalance by simply reading this trace. measuring to the trillionth of a volt, these sensors are incredibly sensitive.

The oz-in unit of measurement means that 2 ounce-inches of imbalance is equal to both a 2-ounce mass 1 inch away from the axis of rotation, as well as a 1-ounce mass 2 inches away from the axis of rotation. To get the actual force generated by the imbalance, RPM must be factored in. Suffice it to say, the force generated by each of those scenarios is equal at a given RPM.

Centrifugal vs. Centripetal Force

Most of us are familiar with the term “centrifugal force”, thanks in large part to the popularity of centrifugal superchargers like ProCharger and Vortech. However, there is another force that is very closely related, called centripetal force. The cliff notes version is that centripetal force is an inward force that keeps a rotating body in motion, forming a fixed 90-degree vector of force to the direction of motion. The difference is important, as you’ll see when we start talking about the vector of imbalance, as it is centripetal, not centrifugal force that’s responsible for it.

With our crankshaft and bobweights assembled on the balancing machine, an electric motor drives the crankshaft at a set speed, and the stanchions the crankshaft is resting on custom made sensors built into them. By measuring both the amount of force generated on those sensors, as well as the point in the crank’s rotation it’s occurring, we can extrapolate not only the amount of imbalance, but also its vector within the crankshaft.

bobweight

Constructing the bobweight is as simple as weighing all the components and screwing them down onto the threaded rod. The trick is to have the exact bobweight across all four journals.

For example, on CWT’s Multi-Bal 5500 (which we used to balance LS5.0’s crankshaft), the sensors on the stanchions measure to the trillionth (9 digits to the right of the decimal) of a volt in a 0-5v range. This allows for extremely precise measurement of any imbalance in the assembly, as well as generating an extremely precise vector for the imbalance.

Measured at two points on the crankshaft — usually the numbers one and five main journals on a standard V8 crank — we are concerned with two things, the amount of imbalance as well as the vector of each imbalance. That will not only tell us how much correction to apply, but where to apply it.

Correcting Imbalance

Once we know how much imbalance is present, we can finally apply corrective action to the crankshaft. To easily understand this, think about a seesaw. If one end is too heavy, you lighten it. Conversely, if one end is too light, you add weight. Since we generally don’t want to add or remove weight from the piston and rod assembly, that means all of the corrections will be done to a counterweight.

“What we’re truly looking for is equilibrium,” says Neal. “This is a mechanism that has to work in concert with all the other moving parts in the engine. Anything that is not in concert, is probably not appreciated by the system.” Just like most things in an engine, that equilibrium is relative to the specific application.

drilled counterweight

One of the methods for removing weight from a counterweight is to simply drill it out. There are other methods as well, which we’ll be getting into in a future balancing article.

For a given rotating-assembly mass and its specific operating environment, there is a calculable ISO standard of allowable imbalance. (We’ll also be diving deeper into that subject in another article.) For simplicity’s sake, we’ll use our LS5.0 assembly as an example. The ISO calculated tolerance for that specific setup is 0.123 oz-in, which is a pretty tight tolerance. Our initial spin showed that with all of the aftermarket components in the rotating assembly, we were at 3.156 oz-in in the rear and 2.494 oz-in in the front.

Luckily, the vectors for the front and rear imbalances were 180-degrees apart from each other, which is said to be “in couple”. When the imbalances are in couple, they work harmoniously. The farther out of couple they get, the more they excite each other, compounding the imbalance issue.

When you have to add weight to the counterweight, you can add Tungsten (or "Mallory metal") slugs, which are denser (thus heavier) than the steel they replace (left), or you can weld up existing drill holes, if you happen to be working with a crankshaft that has been balanced previously.

There are a number of ways to correct the imbalance, which, again, we’ll dive deeper into in a future article. For simplicity’s sake, you either add or remove weight from the counterweight. Removing weight is fairly straightforward. CWT’s software will tell you the location (vector), size, and depth of the required drill points to remove the correct amount of weight from the crankshaft.

Adding weight can seem a little bit trickier. If this were a brand-new aftermarket crankshaft and we needed to add weight, we’d be forced to drill the crankshaft and add heavy metal (Tungsten, aka “Mallory” – which is denser than the steel it replaces). CWT’s software would tell you exactly where and how much to add to the counterweight.

However, this is an OEM crankshaft that was previously balanced for the OEM rotating assembly. So to add weight, we simply welded up the previous drill sites, adding weight back into the crankshaft, and then spinning it again until the software was happy with the amount and location of the replaced material.

And with that, you have the basics of crankshaft balancing. If you are at all interested in diving deeper into the subject of balancing and exploring just how much it can affect the entire engine, make sure to bookmark this article as a reference, and stay tuned, as we have plenty more in store for you.

Spin test report

This is a spin test report. You should get something like this back with your balanced rotating assembly. The key points here are looking at the difference between having a 2.807 and 3.805 oz-in balance, versus a .024 oz-in balance. If you hadn’t balanced this particular assembly, you would be slamming the main bearings with 243 pounds of force 100 times a second at 6,000 rpm, versus balanced exerting 1.5 pounds of force at the same RPM.

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About the author

Greg Acosta

Greg has spent seventeen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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