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Harmonic Dampers, what are they and what do they do and why does a motor need one? 



Four years ago my wife and I started a new business called Harmonic Damper Rebuilds Ltd. We purchased an existing business and a proven process for quality custom rebuilding of harmonic dampers. This largely follows existing manufacturing processes.

We are rebuilding dampers for American and Australian made vehicles. I am also developing tooling for European and Japanese engines. Diesel, heavy commercial and marine engines. We are also developing a rubber process with a German company to improve the quality and reliability of our rebuilt dampers.

The inventor of the harmonic damper was Dr Frank Lanchester. He and two of his eight brothers started the Lanchester Engine Company in 1900. They had high standards for their engines and Dr Lanchester would tinker with each motor that went out the door to make it more perfect than the last one. He invented the Torsional Crankshaft Damper [harmonic damper] and the harmonic balancer to cut vibrations in the Lanchester 38 hp six which was introduced in 1907. In 1910 he invented the single and double counter-rotating balance shafts. The patents for which were purchased in the 1970s and used by Mitsubishi and licensed to other manufacturers to the present day. He also invented the steering wheel, amongst many other things.

Harmonic dampers are called by a wide range of varied names. In truth they are all torsional vibration dampers. Terms like crank pulleys, balancers, power pulley etc are to varying degrees erroneous. The term harmonic balancer comes from some manufacturers using the hub as a convenient location for external mass balance weights. The reason for this is that it made fine tuning of engine vibration simply done by adding or removing weight from the mass balance.

Though it is common belief that large steel parts such as crankshafts are rigid and inflexible this is not true, when a force acts on a crankshaft it bends, flexes and twists just as a rubber band would. While this elastic deformation is often very small it can have a significant impact on how an engine functions.

All objects have a natural frequency that they resonate or vibrate at when struck. An everyday example of this is a tuning fork. The sound that a particular fork makes is directly related to the frequency that it is vibrating at. This is it's natural frequency which is dictated by the size, shape and material of the fork. A crankshaft is a series of tuning forks in a line, it has a natural frequency that it vibrates at. When an object like a crankshaft is exposed to a heavily amplified order of it's own natural frequency it will begin to resonate with increasing vigor until it vibrates itself to pieces. This is called fatigue failure.

Crankshafts twist back and forth a small amount every time a cylinder has a power stroke. This motion is complicated because the amplitude of the vibration varies along the shaft. The crankshaft will experience torsional vibrations of the greatest amplitude at the point furtherest from the flywheel or load.

Engines have major and minor critical rpms and are different due to the fact that some harmonics assist one another producing large vibrations whereas other harmonics cancel each other out. Major critical rpms have harmonics that build on one another to amplify the torsional motion of the crankshaft. Conversely, minor criticals are at rpm that tend to cancel and damp the oscillations of the crankshaft. If the rpm remains at one of the Major criticals for any length of time fatigue failure of the crankshaft will result. These major critical rpms are dangerous and must be avoided or properly damped.

The oscillation of an undamped crankshaft at a major critical speed will commonly shear the front crank pulley and/or the flywheel from the crankshaft. I have seen hub keys sheared, flywheels loose, and pressure plates coming apart. These failures have often required crankshaft and or gearbox replacement and in extreme cases the diff.

Technically, 'the primary purpose of a harmonic damper is to cancel out third harmonic distortion by using intermodulation between the second harmonic and the fundamental in the space charge regions of the triodes.'

Put simply the harmonic damper works by relaying resonance and torsion energy out of the crankshaft and dissipating it as heat by flexing a heavy metal inertia ring which is mounted on a vulcanized rubber cushion. Essentially the damper is a simple spring of rubber plus a vibration and heat absorbing mass - the hub and inertia ring.

Harmonic dampers are made of cast iron for the primary reason that cast iron has the ability to absorb and dissipate large amounts of energy because of it's high specific gravity. The damper on each engine is designed for the specific range of vibrations that a particular engine produces. Keeping the damper cool is also of primary importance.

The key weakness of the harmonic damper is the rubber.

Rubber weakens with age and also deteriorates due to excessive heat, oil exposure, solvent exposure, and contraction. So what happens when a harmonic damper is malfunctioning? What are the symptoms?

 Symptoms of harmonic damper deterioration

1.Deterioration of the rubber between the hub and the outer ring. Shown as cracking, bulging or shrinking instead of forming a smooth layer level with the metal components.

2.Any wobble of the balancer or wobble in the drive pulley.

3.Excessive engine vibration.

4.Crankshaft cracking or failure.

5.Squeaking that does not stop when all other possible sources have been eliminated.

6.Excessive bearing wear.

7.Excessive gearbox wear or failure.

8.Failure of other rotating or reciprocating engine elements.

9.Loose flywheel or pulley bolts.

10.Slipping, slapping or noisy drive belts. Having to change belts frequently.

11.Loss of horsepower and lessened fuel efficiency.

12.Highly polished asymmetrical spons in the pulley grooves, means a warped inertia ring.

13.Elongation of the balancer key-way or wear on the balancer hub or crankshaft.

14.Failure in the alternator, power steering pump, A/C or water pump.

15.Engines that don't run properly when adjusting the timing.

16.Irregular idle.

When a damper wears, what happens, at a minimum, is that the outer ring will move so the timing mark will be in the wrong position. Or, at worse, the ring comes completely off while you're driving and becomes the worlds heaviest "Frisbee" under your hood. Your radiator is one of the favorite items your newly found toy likes to go through.

Causes of damper failure:- cracked hub keyways, incorrect damper for the engine or crankshaft, damage from improper mounting or removal. Bent or out of balance drive shafts, buckled or out of balance driving wheels, excessive heat and age.

If you have a high performance engine without fitting a correctly calculated high performance damper, a standard damper could completely self-destruct sending high speed shrapnel throughout the engine bay with destructive consequences that are not hard to imagine.

It is important to understand that while an engine from any manufacturer is designed by a team of qualified engineers, after market dampers are often designed by people who do not understand the very important principles of physics that are used to design harmonic dampers. Great care should be taken in the selection of any damper to be used as replacement for a stock or for use in high performance engines.

If a cast iron damper is replaced with a steel damper care should be taken that the mass of the steel damper is equal to the mass of the damper being replaced. Because steel is much lighter than cast iron this means the damper will be somewhat larger than the one being replaced.

I have seen after market dampers that have failed in spectacular fashion after brief use and have customers who are returning to a stock damper due to unsatisfactory results with after market replacements. As in any field of after market componentry there is a small sector that know what they are doing and a wide range of pirate imitators.

John Mallett
The Damper Dude           
29 Russell Street, Waihi 3610
 New Zealand