Balancing Science: ISO Tolerance — When Is Good Enough, Good Enough?

Balancing Science: ISO Tolerance — When Is Good Enough, Good Enough?

If you read our previous Balancing Science article, you’ll notice that we glossed over the acceptable tolerances for imbalance in a rotating assembly, deferring to a future article. (If you haven’t read that yet, go back and read it. It’s a great primer for this article.) This is that future article.

As Randy Neal of CWT Industries points out, “we are attempting perfection.” And that is the truth — as the saying goes, “the enemy of excellence is ‘good enough’.” However, in the real world, in an environment where things need to get done and time is a finite resource, there has to be something considered “good enough.”

Luckily for us, there is the ISO, or International Organization for Standardization. Their sole job is to come up with specifications and tolerances for, well, pretty much everything in the world. Specific to what we’re doing — balancing a rotating assembly for a gasoline engine — ISO has multiple tolerance grades within the specification.

20 years ago, the OEMs were balancing to 2 oz-inch. Now, the new engine in your grandma’s Camry is coming off the line balanced to below 0.20 oz-in. — Randy Neal, CWT Industries

History Of Good Enough

Anyone who remembers Bill Clinton in the White House is old enough to remember when hitting that 100,000-mile mark on a factory engine was an impressive feat. Much like all of technology, improvements in the past few decades have not only increased the longevity of our engines, but the efficiency, as well. Balancing tolerance of the crankshaft has played a significant role in that advancement.

“20 years ago, the OEMs were balancing to 2 oz-inch,” explains CWT’s Randy Neal. “In the old days, when we got an engine from GM, and it was measuring at 2 oz-in, we pulled a number out of the air. In a performance application, we wanted to be 10 times better. That being said, it was good. Our performance engines ran well and lived a reasonable amount of time.

Balance tolerances

Twenty years ago, this amount of imbalance would have been almost race-engine level in the aftermarket, twenty years ago. As you can see, according to the G6.3 ISO specification, it’s now more than twice the acceptable imbalance.

Fast-forward to today, and between advancements in understanding and manufacturing technology, OEM balancing is at a whole new level. “The new engine in your grandma’s Camry is coming off the line balanced below 0.20 oz-in,” says Neal. “Now, with grandma’s car being balanced to a 10-times tighter standard, we’re seeing that 2 oz-in in isn’t good enough for modern electronics and modern efficiency standards. The ISO standard has always been there, we just weren’t meeting it. Now we are.”

Neal points out that the primary balance of the crankshaft being an order of magnitude better, allows not only for smoother, quieter operation, but also improves both longevity and economy. If we look at the spin test report from the previous balancing article. We can see that a 10-times reduction in imbalance results in exponentially reduced forces on the main bearings (on the order of 100 times). That factor alone has helped contribute to the fact we can pull bearings out of a 100,000-mile LS engine and see a lack of wear we would have never believed 20 years ago.

balance coupling

In the previous article, we mentioned the two areas of the crankshaft being “in couple.” Notice how the two vectors are almost 180 degrees apart from each other — that is the ideal situation. Now, weight needs to be added to bring the total imbalance into spec.

The other area in which the improved tolerances have benefitted modern engines is efficiency. “Modern engines rely on knock sensors to be able to aggressively tune for economy,” explains Neal. “With a 2 oz-in balance, that aggressive tuning isn’t possible. You need that tight tolerance so the ECU can properly react to real-time knock sensor data.”

With a smoother, quieter (as far as the knock sensors are concerned) engine, the sensitivity of the sensors can be increased. This allows the calibration wizards to get far more aggressive in their operation calibrations, without increasing the risk to the engine itself.

ISO Balance Tolerance Calculator

Here, we can see the ISO Tolerance calculator built into CWT’s software. With G6.3 selected, the weight of the rotor is entered, as are the peak RPM numbers, and the number of correction planes. In this case, that results in a target of 0.225 oz-in or better. Also notice the radius distances. That is the outer location of the counterweight, so it tells you the amount of weight at the surface of the counterweight that is allowable.

The ISO Specification

At this point, I’m sure some of you are just champing at the bit for the meat and potatoes of the ISO specification. The proper term for the standard is ISO 21940: Mechanical Vibration — Rotor Balancing. ISO is constantly updating the standard, but in its current iteration (as of 2021), it covers everything automotive (and then some, from ultra-high-precision gyroscopes and spindles all the way up to large, slow marine diesel engines).

Each type of assembly is assigned a different tolerance “grade” under the specification. That grade is essentially the level of allowable imbalance (or balance precision) being applied to the ISO calculation. For our purposes in a performance automotive gasoline internal combustion engine, grade G6.3 is used by CWT in their machine’s tolerance calculators. In the grand scheme of things, G6.3 is a tight tolerance to hold.

The ISO standard is not a fixed value — far from it, actually. Even if we assume a constant grade of G6.3, the exact amount of allowable imbalance changes based on rotor weight (we’ll discuss that more in a second) and rotor RPM. As the rotor weight increases, the standard allows for a little bit more imbalance. Conversely, as the desired rotational speed of the rotor increases, the standard tightens up. Basically, slow and heavy is allowed more imbalance than light and fast.

Here you can see on one of the spins, the weight of imbalance at the counterweight's surface was shown to be 13.6 grams. So, before welding, 13.6 grams of modeling clay is added, and the assembly respun.

The Rotor Weight Discussion

If you stop and think about the fact that the rotor weight affects the ISO balance standard, you realize that getting the weight right is important. However, what constitutes the rotor? “ Typically, guys will use the rotor weight mass of the crank only,” says Neal. “But what about the mass of the rods and pistons? And then the damper up front? And the flywheel?”

Before you start racking your brain thinking about all of that, the fact is, considering all of their masses only makes the tolerance looser. If you stick to the mass of the crank alone, you’ll get the lowest result from the ISO tolerance calculation. “Really, as long as you are meeting the ISO spec, you’re going to be solid.”

That’s not to say the rotating assembly should or shouldn’t be balanced with the flywheel and damper (in external counterweight situations it’s mandatory), but is simply a discussion of whether their mass should be included in the calculation of your final balance target.

imbalance tolerance

Here, we can see an incredibly low spin result. The rear of the crank is .007 oz-in out of balance (or about half a tenth of a gram) and .002 oz-in on the front (or about a tenth of a tenth of a gram). Currently, there is no reason to strive for this level of balance (more than 60 times below tolerance), except for entertainment.

The Quest For Perfection

One of the questions we posed to Neal early on was, if “good enough isn’t good enough”, then what really is good enough. Looking at the ISO standards, G6.3 offers some pretty tight tolerances – 0.123 oz-inch maximum in the case of our LS5.0 project (52-pound rotor wight at 8,000 rpm). Here, he seems to be of two minds. The first is that going below the spec is by no means bad, but it’s purely “entertainment” as he calls it.

“As of right now, there is no appreciable benefit that would warrant the extra time and effort to get ridiculously small numbers,” says Neal. “If you have the time and inclination to do that, God love ya.” However, Neal also recognizes that we are not at the peak of our understanding or ability.

“Looking at the first engines to where we are now, there are a lot of advancements we’ve made. Rather than patting ourselves on the back for what we’ve done, it shows me that there is a path to understanding what we still don’t know yet.”

Will we be at a point where we will regularly see performance engines balanced to the hundredth of an ounce-inch? Maybe, as long as we continue to see benefits of a smoother running engine in increased efficiency and longevity. We know that CWT is constantly striving to improve its own processes and products through industry collaboration and R&D, and are excited to see what is coming out of it all (like this recent PERA webinar With Dan Begle from MAHLE and Aaron Neyman of Fluidamper).

This webinar recently hosted by PERA dives deep into some of the latest areas of exploration regarding how primary balance affects the rest of the systems in the engine. If you have any interest in the subject at all, it’s absolutely worth the time investment.

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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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