View Full Version : So I...

joe's gt
10-16-2009, 02:55 AM
finally learned something useful in one of my engineering classes. Vibrations to be exact.

It showed me how important a dampener is to an engine.

For those that care about the scientific theory, read on. For those that don't, just skip to near the bottom to the conclusion.

There can be 2 parts to vibrations. A natural frequency and a forcing function that causes a certain frequency. When you hit a crankshaft once with a hammer, that ringing you hear is a the shaft vibrating at its natural frequency. When hit it repeatedly with hammer you are forcing it to vibrate at a certain frequency.

So what the hell does this have to do with cars?

When a natural frequency and a forcing function (causing some frequency) end up being the same frequency a very dangerous effect called resonance occurs where the amplitude of the vibration keeps growing and growing.

So most of time the crankshaft is turning and vibrating at its natural frequency while at the same time the pistons are forcing the crankshaft to rotate and causing a different frequency.

This is all good and well until when certain points throughout the rpm range the frequency caused by the pistons turning the crankshaft matches the crankshafts natural frequency. Then resonance kicks in and the amplitudes (size) of these vibrations keep growing and growing to the point where internal parts can become damaged, suffer premature wear, or the crank pulley can flat out sheer off the crank.

Conclusion: At certain speeds, without the stock dampener/pulley, the engine rpm may be forcing the crankshaft to vibrate at its natural frequency. The longer that rpm is kept, the bigger those vibrations are gonna get causing premature wear and in severe cases, complete component failure.
The dampener on the stock pulley prevents this from happening. "Theoretically", if kept at the natural frequency rpm, the amplitudes could reach an "infinite" magnitude. But obviously that is not possible, but you can imagine how large these vibrations can get if the car is kept at this dangerous rpm for a long period of time.

So think it over before you just throw away the stock pulley to make the rotating assembly lighter. It serves a greater purpose than just a pulley.

Now on the other hand, I did a quick google search and while I have read about this happening, I haven't read a thread on a board or anything saying that this has happened. Luni mentioned the same thing in another thread.

So take this thread with a grain of salt. Although complete failure of components is probably unlikely due to vibrations, I do believe premature wear can result from not having a dampener, and in that case, it would be hard to pin-point "non-dampening" as the issue.

10-16-2009, 03:12 AM
Very cool didn't know the part about natural frequency. I knew that they had been tuned for a certian vibration to cancel. Thanks for the good info.

10-16-2009, 03:15 AM
ive seen airplane cockpits that say "Do not run engine at xxx speed due to resonance vibration damage." Its no joke it will bring down a plane mega fast. I keep all my stock crank pulleys, change the other ones.