Let’s Talk Engine Balancing With SCAT Crankshafts

Crankshaft balancing is just that, a balancing act. When an engine is running, there are many different forces working against each other that need to work in harmony. Unfortunately, the balancing process is something that many enthusiasts rarely give a second thought. After all, that’s why you spent extra for better parts and used an experienced engine builder instead of doing it yourself. But, the fact-of-the-matter is, if you’re building an engine, having a properly balanced rotating assembly should be one of the most important things on your build list.

You may be asking yourself why balancing a crankshaft is so important, or saying, of course balancing a crank is important. While it may be an obvious necessity, you might not understand the logic behind it, or know much about the balancing process itself. That’s why we sat down with Tom Lieb, owner of SCAT Enterprises to get some insight about crankshaft balancing.

SCAT

When balancing a crankshaft, holes can be drilled into the counterweights to either remove weight, or, to be filled with heavy metal to add weight.

The basic design of a crankshaft is simple. First, we’ll look at journals. Main journals are the locations where the crankshaft is “attached” to the block, and rod journals are where the piston and rod assembly attach to the crankshaft. The crank’s counterweights are designed to offset the weight of the rod and piston as rotation occurs. These counterweights is where balance is achieved. At its most basic level, balancing a crankshaft minimizes internal engine vibration, but it is a game of compromise.

When an engine is running and the crankshaft is rotating, the connecting rod and piston assembly are reciprocating (traveling up and down). Balancing a crankshaft is a matter of managing rotational and reciprocating movement. Both directions of movement need to be considered when balance is determined. But, how is balance achieved?

Generally speaking, it is done by taking away or adding metal to the counterweights. But, to find balance, you need to know bobweight. The bobweights are actual weights that are attached to a crankshaft’s rod journals when balancing, to simulate the weight of the rod and piston assembly. With the bob weights in place, the crank is ready to be spun on a balancing machine. While there are simple formulas to calculate the amount of bobweight needed, there is a lot that these calculations fail to consider.

Balancing a crankshaft is a matter of managing rotational and reciprocating movement. The crankshaft rotates, while the connecting rod and piston assemblies move in a reciprocating motion.

A running engine is extremely dynamic. Because of this, it’s almost impossible to perfectly balance a crankshaft. There are many factors that come into play when you start to look at the forces acting upon a crankshaft, connecting rod length, bearing friction, cylinder pressure, counterweight phasing, engine rpm, ring friction, stroke length, secondary vibrations, rocking couples, and static mass. These are all forces the machinist or engine builder cannot account for during the balancing process. This “imperfection” in the process has lead many machinists and engine builders to experiment with different styles of balancing.

Not balancing your crankshaft is like herding ants. – Tom Leib, owner SCAT Enterprises

If your talking about a stock-style engine for a passenger car that will almost always operate in a low rpm-range and not make a large amount of horsepower, the balancing process is not as critical. When you get into high-rpm, high-horsepower applications, precision balancing is everything.

Crankshaft balancing requires special equipment and specially trained machinists or engine builders who understand what needs to be done, and how to do it properly. SCAT Enterprises has 10 balancing machines that run for 10 hours a day. SCAT’s facility can turn out 50-60 balanced rotating assemblies daily.

“Not balancing your crankshaft is like herding ants. Everything inside the engine works together, and if you have cylinders fighting each other, you’ll have bad vibrations. It’ll shake everything apart,” Tom explained.

Balancing is a game of numbers, and most people have unrealistic expectations going into the process. For instance, many times, people are looking for a machinist to balance the rotating assembly (rods and pistons) to within 1-2 grams. Thankfully, that is usually taken care of by the manufacturer when rods and pistons are grouped together.

“The fact is, pistons are balanced to within roughly 2 grams.” Tom continued, “the rods are balanced to plus or minus 2 grams, end-to-end. Most people don’t know what a gram actually is. One gram is 1/28 of an ounce. The actual weight of a gram is roughly the weight of a dollar bill. When people talk about balancing within half a gram or balancing to zero, it’s nearly impossible to do that with 100-percent accuracy.”

“There’s a point of being practical, and there’s a point in being fanatical. The key to balancing is having each end of the crankshaft be equal. If you’re balanced to within a gram or two, then you’re good to go.”

Machining Process

When it comes to balancing a crankshaft, you’re either removing or adding weight. While that sounds simple and remedial, both situations require machining of the crank, which requires skill and some specialized equipment.

If weight is being removed, the counterweight is drilled or cut to lighten it. The counterweight can be drilled in specific spots, or the crankshaft can be turned on a lathe and the counterweight can be cut down to achieve balance. If turning the crank on a lathe is possible, in a lot of ways, it is a better way to remove weight. Cutting removes mass, which changes inertia characteristics. Anytime overall rotational mass can be reduced, it’s beneficial. Unfortunately, this is not always an option, since many times, weight needs to be removed from a specific location.

Specialized equipment is required to machine crankshafts to either lighten the counterweights or prepare them for Mallory (heavy metal).

If you have to add weight, metal needs to be added to the counterweights. This requires the counterweights to be drilled in specific locations, and pieces of Mallory to be pressed in those holes. Mallory is tungsten, which weighs about twice what steel does. This allows engine builders to add very specific amounts of weight to exact locations to achieve balance.

The challenge with adding weight is that material needs to be removed before weight can be added. So, hypothetically, if 28 grams needs to be added, 14 grams will be removed in order to add 28. This gives you roughly a 14-gram increase in weight.

Heavy metal (Mallory) is nothing more than tungsten steel. It weighs about twice that of regular steel. This makes it a great option for adding weight to a crankshaft. The counterweight is machined and the piece of Mallory is pressed into the hole. This allows the machinist or engine builder to add weight in very precise locations.

Beyond the equipment, the operator is also extremely important. It is important to use a machinist who understand the balancing process and what needs to be done.

“When a crankshaft is too heavy in one place, that means it’s too light in another.” Tom continues, “being able to look at a crankshaft and understand where that weight is and make those decisions – not everybody can do that. Most balancing issues come from a lack of knowledge or not knowing how to use the equipment.”

Internal Vs. External

When it comes to an internal combustion engine, there are times when physical room within the engine is limited. Sometimes the counterweights of the crankshaft simply cannot be large enough to counterbalance the rod-and-piston assembly within the confines of the crankcase. This forces manufactures to externally balance some engines. In these applications, additional weight is added to the harmonic balancer and the flywheel or flexplate to provide the needed counterweight. These added weights are effective with less amounts of weight, due to their location at the extreme ends of the crankshaft. The downside is, it is a less precise job of balancing the rotating assembly. These weights also put their own twisting force on the crankshaft, which can be a negative.

In an externally balanced configuration, weights are added to the balancer and flywheel/flexplate to offset the crankshaft. This is done in situations where real estate inside the engine block doesn't allow for large enough counterweights to offset the weight of the connecting rod-and-piston assemblies.

“External weight is unsupported weight. The limiting factor becomes rpm. An externally balanced crankshaft picks up weight as engine rpm increases, and can cause crankshaft movement and breakage in extreme cases,” Tom explained.

Whenever possible, internally balancing an engine is the way to go. The bobweight of each counterweight is dialed in to match each set of rods and pistons. This ensures the smoothest operation at the desired rpm range. This will leave you with a smooth running engine developing minimal vibration. Minimal vibration will ensure a long, trouble free service life.

If you’re balanced to within a gram or two, then you’re good to go. – Tom Leib

Whether you’re building a mild street engine or an all-out race engine, a precise, quality balancing job is key. Do your homework and know what goes into the process. If you’re starting with new parts, SCAT offers custom, pre-balanced rotating assemblies for almost any level of performance. The company pulls and balances 50-60 rotating assemblies daily. Each order is custom and balanced for each customer, so you know you’re getting great parts that are assembled and balanced correctly for your application. Don’t guess when it comes to the most important part of your engine. Let the experts do it right the first time!

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About the author

Pete Epple

Pete Epple has been an automotive enthusiast for the better part of 30 years, and a racer for nearly as long. He's been writing about cars for nearly 10 years.
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