Print

Email

Engine Balancing

By Doug Kaufman

In the dark ages of the automotive industry, engine balancing was considered an art practiced exclusively by performance engine builders. Only those wizards of speed (it was widely believed) could understand the forces that caused drivetrain vibration - and more importantly, those that stopped it. Those few who could balance an engine were looked upon with admiration and awe.

Of course, in today's enlightened world, it's clear that balancing is beneficial to ALL rebuilt engines - and is often a necessary part of the process if you and your customers expect the engine to provide trouble-free service.

"In performance, balancing was always important. But with standard automobiles, balancing never used to be a factor," explains Randy Neal, CWT Industries. "Now, through better manufacturing processes - including balancing -- the OEMs have tried to reduce any force that causes wear. As a result, we're seeing engines that go 100,000 miles instead of 50,000 miles."

Neal points out that more precision at the OE level means a smoother running, longer lasting car, which we have all come to expect. The tradeoff comes at the rebuilding level, when you are required to not only meet the boring, alignment and surface finish needs of the engine but the balance as well. If standard engine rebuilders, who may not be used to thinking about performance and balance, don't pay attention to the weight differences between parts, they could be setting an engine up to be a real shaker.

"When we introduce aftermarket components into the engine, in all probability we vary the balance," says Neal. "There are wide variations in the parts manufacturing process. Even parts that are physically identical can have significant differences in weight. Mixing and matching connecting rods to make a set - even if they are all the same part number - may very well introduce a variation into the engine that will cause an unbalance.

"It's critical that we as quality aftermarket suppliers build the equivalent of OE quality or better," Neal says. "To do less is to undermine the industry in which we work."

A smooth running engine isn't just a nice thing to have - it translates directly into power. If the crankshaft, connecting rods and pistons aren't tying to tear each other apart and in the process shake the vehicle to pieces, more power will be transferred to where it's needed: the wheels.

Failure to balance the components properly can result in many different problems: piston rings failing to seal; main and rod bearing wearing early or irregularly; harmonic balancer deterioration; valve spring failures; timing chains stretching; bolts and fasteners working loose.

Don't ignore comfort, though: fewer vibrations also mean accessories will last longer, and the vehicle will be more comfortable to drive.

"Think of the mom waiting to pick her kids up from school. Sitting there with a cup of coffee while the van is idling," explains Tim Meara, Sunnen Products. "It's not performance, - she may not be turning 6,500 rpms going down the highway, but if the rebuilt engine isn't smooth and that coffee starts splashing across the dashboard, even she'll know there's a balance problem."

Babcox Technical Editor Larry Carley explains that understanding engine balancing is actually quite simple - well, as simple as high school physics. "As everybody knows, a rotating object generates 'centripetal force,' which 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," he explains.

"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," says the professor. "Let go of the rope while you're twirling the brick and the brick will fly off in a straight line.

"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," says Carley.

A crankshaft is a heavy rotating object. What's more, the piston and rod assemblies reciprocating back and forth along its axis 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," Carley explains. "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."

Carley explains that 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). In fact, the magnitude of the force increases exponentially with speed. Double the speed and you quadruple the force. 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," explains Carley. "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."

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.

The process of balancing begins by weight matching the individual parts including pistons, rods and rings. Bobweights are added to the crankshaft to simulate the reciprocating weight of the piston and rod assemblies and then the crank is placed in the balancer.

The crankshaft is spun to determine the points where metal needs to be added or removed from the counterweights. If the crank is heavy, metal is removed by drilling or grinding the counterweights. If the crank is too light, Mallory metal (a special very heavy metal) is added by drilling holes into the counterweight and adding Mallory as needed.

Benefits of Balancing
Bob Keziah, TNS Machines, suggests that new equipment options make engine balancers a "must-have" piece of equipment to today's engine builder. "I have found that it's difficult to depend on a competitive shop to do the balance work. Without a balancer the cosmetics of the job are all that can be double-checked. The assembling shop will get the blame if the engine vibrates or doesn't live up to your customer's expectations."

The solution? Says Keziah, it's your responsibility: do it yourself.

"Yeah, but that physics stuff..." you may complain. Don't worry. The machines available today do practically everything for you.

"Balancing has always been a science, but people thought it was more art," explains CWT's Neal. "The operator used to have to jump through hoops mentally to interpret what the machine was telling him. Now, we've eliminated the frustration level by having better equipment and computer software to guide him through the whole balancing process."

Today's equipment and software are up to the efficiency challenges posed by any rebuilder, according to Hines Industries' John Witt. "A rebuilder's need for balancing has really only changed to the extent of speed," he says. "The progressive rebuilder of today is looking for efficiencies in his equipment. New technology can reduce your balancing time by as much a 60 to 70 percent using an older soft suspension machine without on-machine correction."

Engine builders are recognizing the value balancers bring to their operations as evidenced by results of our annual annual Machine Shop Market Profile (Engine Builder, June 2004, page 21). According to shops surveyed, nearly seven percent of shop owners say they are likely to purchase balancing equipment this year. Four percent of shop owners said they purchased balancing equipment in 2002 and two percent report having made such a purchase in 2003. Overall, 31 percent of shops surveyed say they have at least one engine balancer.

So, does that mean YOUR opportunity to make engine balancing profits is disappearing? Hardly, say the experts. Not only does utilizing engine balancing equipment allow you to sell better engines to your customers, it allows you to make profits in other markets as well.

Gary Hildreth, G&H Balancer Service, supplies equipment and parts for all models of engine balancers. The list of potential balancing jobs is long, he explains.

"Balancing can be used for any type of shaft-driven part - motorcycle engines, kart motors, pumps, impellers, small armatures. It doesn't have to be used for V8 performance engines," he says.

"Keeping the machine busy is great. If you use your imagination you can get a lot of business during the slow times," says Hildreth.

Bob Keziah wistfully agrees that the simplicity of today's machines offers unsurpassed opportunity, but that rebuilders need to make a greater effort to capture that business.

"Sometimes it seems a shame that an operator today doesn't need to learn the theory of balancing as with the older machines to be able to effectively balance a part to necessary tolerance in a fraction of the time. The reason I say it is a shame is with the knowledge gained from making the older design work an operator can really do some neat things with the newer machines," Keziah says.

Richard Idtenshon, Schenck Trebel, says in addition to the popularity of aftermarket crankshafts, increased use of turbochargers has produced a natural opportunity for engine builders.

Sunnen's Meara says the opportunities extend into the marine dealer as well as the industrial facility down the street. "Balancers give engine builders the chance to do things outside the realm of the automotive field. They can make profits by balancing boat propellers, cutting blades, water pump impellers, impeller housings for industrial blasters...it's easier than ever."

Regardless of how you utilize the equipment, it's likely to be a profit center. CWT's Neal points to the bottom line: "Look at the dollars. Say the mean average price nationwide for a balancing job is $225. With a state of the art machine, it may take two hours. That's generating more than $100 an hour, which is a lot better than some machine services.

"And that's just for the basic balancing job. Some of the more exotic jobs can generate a lot more revenue," Neal says.