What Does a Harmonic Balancer do?

March 5, 2019 / by John Baechtel

Harmonic balancers are a staple ingredient of any engine build, but what do they really do? Do you have to have one? Is bigger really better? We clear the air about the right balancer to hang on your crankshaft. 

Harmonic balancers are misnamed. They do not balance an engine, rather they absorb and remove unwanted vibration due to torsional twisting of the crankshaft. They are in effect vibration dampeners and are frequently called dampeners. Dampeners are like torsional shock absorbers used to dampen torsional twist and vibration in an engine. Torsional vibration is a twisting vibration caused by the firing pulses of each combustion event. The force of the combustion process causes the crank to deflect ever so slightly in the direction of the force, and when that force goes away the crank ever-so-slightly springs back. At certain frequencies the crank can resonate, making the vibration much worse. With eight cylinders firing, these forces are moving back and forth and right through each other constantly. This is where a harmonic dampener works its magic.

OEM dampers consists of an outer and inner ring bonded together with rubber. They are very effective in production cars but can deteriorate and fail over time due to exposure to the elements.

During the combustion process, each piston is forced to move down the cylinder as a result of a pressure rise within the combustion chamber. This stroke imparts a sudden rotational force to the crankshaft. Even though it is a very stout component, a crankshaft is not perfectly rigid. So, during these combustion events, the crank will twist slightly in response to each application of pressure which can be many thousands of pounds.

Crank twist is analogous to a simple torsion bar with a lever arm at one end. Now assume you hit that lever arm with a hammer. You can imagine there will be some slight twist when you first hit the lever arm, but that would be followed by the arm springing back into place, maybe even vibrating for a short time before coming to rest.

So, we have we have torsional twist followed by torsional vibration (during the spring back event). Torsional twist is a function of part length (inline engines will have a longer crankshaft than V-configurations) and thickness, material shear modulus (think: material stiffness), and Torque (force from combustion x crank throw). Similarly, torsional vibration is a function of part length, torsional stiffness, and polar moment of inertia (think: object’s ability to resist torsion).

As the engine operates, harmonics from combustion resonate through the crank. The job of the balancer is to help eliminate these harmonics before the cause problems.

 The circular device, made of rubber and metal, is bolted at the front end of the crankshaft to help absorb vibrations. It's usually connected to the crank pulley, which drives accessories like the air conditioner. The rubber inside the pulley is what actually absorbs the vibrations and keeps them at a safe level. In essence, the device is designed to help prevent crankshaft failure.  The rubber material can deteriorate over time. So, if your harmonic balancer is going bad, you could get rough engine vibrations, a cracked crankshaft, or even a serpentine belt that gets thrown off its track.

The damper is composed of two elements: an inertia mass and an energy dissipating element. Most often made of rubber, this element may be composed of a synthetic elastomer, a clutch, a spring or fluid. The mass counteracts the torsional crank motions and in concert with the energy dissipating element absorbs the harmonics vibrations. Dampers must be interference fit to the crankshaft in order to function properly.

In certain engine applications, the harmonic balancer is actually part of the engine balancing strategy. Weights are added to the balancer that offset the weight of the pistons and rods. this is called external balancing.

An OEM damper consists of an outer mass bonded/vulcanized to an inner hub. An aftermarket performance damper consists of a mass which is attached/mounted to a housing (steel, aluminum, titanium, etc.) based on the different types of damper and where the mass is controlled differently. The first three use older technology; First is the liquid type damper which surrounds the mass immersed in the housing which is then bonded or welded together. Second is the O-ring type which surrounds the mass with a number of O-rings as it sits in its housing. Third is the friction type which has clutches and spring acting on the mass inside the outer housing. Fourth is the newest type in which the mass sits over and is attached an elastomer ring which is then attached to the outer housing.

Certain engines, like the traditional V8, have a firing order prone to excessive harmonics by nature necessitating the use of a damper. The flat plane V8, traditionally used in more exotic engines, does not suffer from the excessive harmonics and therefore may use a solid undampened device. 

ATI Super Dampers use high durometer O-rings operating between the inner and outer ring to dampen crankshaft harmonics.

Without the harmonic balancer there is a high risk your crankshaft would crack or a serpentine belt would be thrown off track. A simple way to think of the harmonic balancer is like a massive stress reliever. It buffers and dampens engine parts and engine performance by absorbing the violent torsion of the modern-day engine.

Some SFI approved dampers such as the ATI Super Damper use steel inertia weights and they are tunable and rebuildable. A Fluidamper uses a heavy viscous fluid to dampen firing pulses and a BHJ damper uses a dry friction element. All are effective at dampening crankshaft torsional twist.

The performance benefits of using the proper harmonic damper are considerable. A properly functioning damper also stabilizes camshaft and ignition timing. A reduction in twist at the end of the crank also stabilizes the valvetrain. The result is more power and longer engine life.

Topics: featured, ENGINE TECH, Theory and How To

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Written by John Baechtel