Harmonic Damper Design

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Aaron Neyman of Fluidampr Sheds Light on Torsional Vibrations


http://www.enginelabs.com/engine-tech/e ... ibrations/


EngineLabs: Basically, what is torsional vibration and how do you measure and analyze it?

Neyman: Torsional vibration is the speed fluctuation of a rotating shaft. Think of it as the vibration you feel in your steering wheel when you apply the brakes and have a warped rotor, but the steering wheel is continuously rotating at the same time. Torsional vibration can have two components, twist and rigid body motion.

Twist is the kind of vibration that is expected when you think of crankshaft torsional vibration. From one end of the shaft to another the vibration amplitudes change direction. When this happens there is a point where there is no vibration amplitude. This is called a nodal point. The material stresses are the highest at a nodal point. Rigid body motion is when the rotating shaft has torsional vibration but there is no nodal point along the shaft. The vibration does not change direction. When testing Fluidampr applications, torsional vibration is measured with a high-resolution laser tachometer, a high-resolution gear-tooth sensor, or an incremental encoder, on either end of the crankshaft. The data is recorded on a multiple picosecond FFT analyzer that converts the rpm fluctuation data into the angular domain. This provides us data in terms of degrees of twist or degrees of roll per rpm and can be broken down by vibration orders. Orders come into play because you have multiple cylinders firing causing multiple initial and harmonic torsional vibration events during each revolution of the crankshaft. Resonant frequencies can also be determined from the collected data and are used to properly size a damper for the application.



EngineLabs: What is the basic design difference between a viscous damper and one that utilizes elastomer construction?

Neyman: The basic difference is the way a viscous damper reduces torsional vibration amplitudes. Stock elastomer dampers function by going into resonance at a specific frequency and remove the vibration energy from the system by the opposing motion of the damping mass. The more accurate name is a tuned vibration absorber.

A tuned absorber only removes vibration energy at the specific resonance frequency band. It is designed with a specific amount of mass and a specific elastomer durometer to deal with the worst engine resonance and to withstand the heat that is generated from the opposing motion. Over time the heat cycles degrade the elastomer and reduce the efficiency of the tuned absorber. Viscous dampers remove vibration energy from the system by shearing a free rotating inertia ring back and forth through a highly viscous silicone fluid. This converts the vibration energy into heat, which is easily dissipated through the sealed inertia ring housing. The viscous damper housing is designed with enough heat transfer area to reach a stabilization temperature that the damper does not exceed. Because of this, the damper does not lose efficiency over time in automotive applications. Another advantage of the viscous damper design is that it allows the free rotating inertia ring to convert vibration energy to heat across a broad frequency range. Superior engine protection, broad range performance and durability are why you will find a viscous damper as original equipment in cars and trucks like the Dodge Ram with the 6.7L Cummins and Audi R8 V10. Plus, nearly all high power engines that we depend on to move our economy are protected with a viscous damper. In those industries and applications it is less about cost and more about quality and dependability.


EngineLabs: What do you mean by “broad band” versus “narrow band” protection?

Neyman: Just like it sounds. The frequency range that a tuned absorber is effective is simply a narrow band, usually 50Hz in range. The absorber is tuned to the harshest frequency range of the engine as it leaves the factory. Once anything is changed on the engine, especially the rotating assembly, the harsh frequency band will shift. A viscous damper operates across a broad frequency range because the inertia ring is not bonded to the damper housing. This allows the effective frequency range to be more of a broad bell curve and provide the necessary protection as engine modifications are made.



EngineLabs: What are the tradeoffs in choosing between a lighter or heavier damper?

Neyman: Dampers are not the same as pulleys and hubs. Pulleys and hubs are generally kept as light as possible to reduce the weight of the rotating assembly. Dampers should not be treated the same way. Dampers are heavier because they contain a critical amount of weight that is needed to perform the work required to remove vibration energy from the engine. The more inertia weight present, the more amplitude reduction potential is available. High cubic inch engines with long strokes should have larger dampers than short stroke, low displacement engines. There is always a balancing point with weight. It is critical to not overload the crankshaft nose with so much weight that it causes runout, but it is also critical to use enough weight to effectively control torsional vibrations. When selecting a damper it is always best to contact Fluidampr if you have questions about what size and weight to run.





I have a few thoughts about this article.

Keep in mind that the answers came from an employee of Fluidampr.

I'm impressed by the speed of the instrumentation that must be used to get real data. To put a picosecond into context ....
A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years.

I thought a damper was more for durability, than performance. The graph on the Honda engine show a 10-15 hp difference in some places.

I wonder who was the first to become aware of this problem.

 

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thanks for posting that!
btw heres a bit more related info

viewtopic.php?f=53&t=3554

viewtopic.php?f=52&t=966


CrankltHub.jpg
LtHub.jpg

lt1hubv.png

https://www.nookandtranny.com/Info_LT1.html
 
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