Well as one of the 5, and someone who has studied mechanical vibrations at school and in the work place I can try to give you some background information as to why it is a bad idea to replace the crank pulley with a lighter improperly ballanced or solid pulley. And it is true, sarcastic as it was, what billy said about driving around with no air filter. Hey, look at the gains and it will run, but it sure isnt good for anything. And people should stop compairing daily drivers with race engines as it has been said already. If you are going to rebuild that often, who cares if it only last several thousand miles when that engine is being run for a couple hundred.......
Well, I can post up some of what I did before and some more to try and show that while our engines are designed by Honda Engineers, these crank pulleys are created and installed by people who do not understand some very important principles of engineering. The natural frequency (and its harmonics) are of greatest interest to the designer of a dynamic mechanical system as they define the frequencies at wich the system will resonate. First lets get some basics under our belts.
Elastic Deformation
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 crank it bends, flexes and twists just as a rubber band would. While this movement is often very small, it can have a significant impact on how an engine functions.
Natural Frequency
All objects have a natural frequency that they resonate (vibrate) at when struck with a hammer. 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 its "natural frequency," that is dictated by the size, shape and material of the instrument. Just like a tuning fork, a crankshaft has a natural frequency that it vibrates at when struck. An important aspect of this principle is that when an object is exposed to a heavily amplified order of its own natural frequency, it will begin to resonate with increasing vigor until it vibrates itself to pieces (fatigue failure).
Fatigue Failure
Fatigue failure is when a material, metal in this case, breaks from repeated twisting or bending. A paper clip makes a great example. Take a paper clip and flex it back and forth 90° or so. After about 10 oscillations the paper clip will break of fatigue failure.
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.
The first mode is 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.
The second mode is 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.
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.
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. There have been many reports of front pulley hub keys being sheered, flywheels coming loose, and clutch covers coming apart. These failures have often required crankshaft and/or gearbox replacement.
Harmonic Dampers
Crankshaft failure can be prevented by mounting some form of vibration damper at the front end of the crankshaft that is capable of absorbing and dissipating the majority of the vibratory energy. Once absorbed by the damper the energy is released in the form of heat, making adequate cooling a necessity. It is also important to note that while the ballancer shafts absorb some of the torsional impulses conveyed to the crankshaft, they are not harmonic dampers, and are only responsible for a small reduction in vibration.
One of the key components in determining the natural frequency of a moving system is mass, the other the stiffness, ie sping constant or dampening constant. The designer has a degree of control over resonance in that he can tailor the systems mass and stiffness to move its natural frequency away from any required operating frequencies. A common rule of thumb is to design the system to have a fundamental natural frequency at least 10 times the highest forcing frequency expected in service, thus keeping all operation well below the resonance point. This is often difficult to acheive in mechanical systems. One tries to acheive the largest ratio of natural frequency/operating frequency possible nevertheless, even if it is less than 10.
The equation for calculating the natural fequency is: (excuse the lack of symbols here and the wrote out formula)
natural frequency = the square root of the fraction stiffness over mass
At first it seems that it would be beneficial to have the system members to be both light (low mass) and stiff to get high values for the natural frequency driving it farther away from the operating frequency. Unfortunately, the lightest materials are seldom the stiffest. Aluminum is one-third the weight of steel but is also about a third as stiff. Titanium is about half the weight of steel but also about half as stiff.
Steel is used in the cranks for these reasons as well as many others. This means that the mass of this system is fixed and there is a need to keep the the natural frequency away from the operating frequency. Thus the harmonic dampener is put on the end of the crank system to help control the stiffness of this part of the system. The mass of this pulley is also important as it was designed to be incorporated into the entire stystems dampening in the harmonic dampener.
Wow, I cant believe I wrote that much. It is always good to blow the dust off the old books and look up some of this stuff...
Hopefully I have put a little bit of info out there. Hopefully we can FAQ this one so that I dont get cramps in the fingers again.
Linear Mode
