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Dinan part 2 (archive)

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Posted by Dan Wang on September 15, 2000 at 13:09:07:

The explanation of the destructive nature of power pulleys begins with the two basic balance and vibration modes in an internal combustion engine. It is
of great importance that these modes are understood as being separate and distinct.

1) The vibration of the engine and its rigid components caused by the imbalance of the rotating and reciprocating parts. This is why we have counterweights on the crankshaft to offset the mass of the piston and rod as well as the reason for balancing the components in the engine.

2) The vibration of the engine components due to their individual elastic deformations. These deformations are a result of the periodic combustion impulses that create torsional forces on the crankshaft and camshaft. These torques excite the shafts into sequential orders of vibration, and lateral
oscillation. Engine vibration of this sort is counteracted by the harmonic damper and is the primary subject of this paper.

Torsional Vibration (Natural Frequency)
Every time a cylinder fires, the force twists the crankshaft. When the cylinder stops firing the force ceases to act and the crankshaft starts to return to the untwisted position. However, the crankshaft will overshoot and begin to twist in the opposite direction, and then back again. Though this
back-and-forth twisting motion decays over a number of repetitions due to internal friction, the frequency of vibration remains unique to the particular crankshaft.
This motion is complicated in the case of a crankshaft because the amplitude of the vibration varies along the shaft. The crankshaft will experience torsional vibrations of the greatest amplitude at the point furthest from the flywheel or load.

Simplified Flywheel and Crankshaft Assembly
Harmonic (sine wave) Torque Curves Each time a cylinder fires, force is translated through the piston and the connecting rod to the crankshaft pin. This force is then applied tangentially to, and causes the rotation of the crankshaft.
The sequence of forces that the crankshaft is subjected to is commonly organized into variable tangential torque curves that in turn can be resolved
into either a constant mean torque curve or an infinite number of sine wave torque curves. These curves, known as harmonics, follow orders that depend on the number of complete vibrations (cylinder pulses) per revolution.
Accordingly, the tangential crankshaft torque is comprised of many harmonics of varying amplitudes and frequencies. This is where the name "harmonic damper" originates.

Critical RPM's
When the crankshaft is revolving at an RPM such that the torque frequency, or one of the harmonic sine wave frequencies coincides with the natural frequency of the shaft, resonance occurs. Thus, the crankshaft RPM at which this resonance occurs is known a critical speed. A modern automobile engine will commonly pass through multiple critical speeds over the range of its
possible RPM's. These speeds are categorized into either major or minor critical RPM's.
Major and Minor Critical RPM's
Major and minor critical RPM's are different due to the fact that some harmonics assist one another in producing large vibrations, whereas other
harmonics cancel each other out. Hence, the important critical RPM's have harmonics that build on one another to amplify the torsional motion of the
crankshaft. These critical RPM's are know as the "major criticals".
Conversely, the "minor criticals" exist at RPM's that tend to cancel and damp the oscillations of the crankshaft.
If the RPM remains at or near one of the major criticals for any length of time, fatigue failure of the crankshaft is probable. Major critical RPM's are
dangerous, and either must be avoided or properly damped. Additionally, smaller but still serious problems can result from an undamped crankshaft.
The oscillation of the crankshaft at a major critical speed will commonly sheer the front crank pulley and the flywheel from the crankshaft. I have witnessed front pulley hub keys being sheered, flywheels coming loose, and clutch covers coming apart. These failures have often required crankshaft and/or gearbox replacement.

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