Don65Stang
Well-known member
One of the recommended upgrades in the 351C engine is the crankshaft harmonic damper. Why? The cast iron ring of the stock dampener was capable of exploding when run hard. I found a white paper by BHJ Dynamics that demystifies crankshaft torsion and dampers. I've attached the pdf at the bottom of this post.
Here is the first page text as a teaser...
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1. Torsion - The hidden problem with the crank system – and why it needs a damper.
First - Special words to know
Frequency, RPM, Orders, Modes, Nodes and Inertia
• Frequency - is how many things happen in a period of TIME = (cycles/second - called Hertz and abbreviated (Hz)). The crank natural frequencies are defined this way and are constants, they do not change with RPM.
• RPM - is revolutions of the crank per minute.
• Orders - are defined as events per crank revolution. Because of this, their frequency is variable and goes up and down with RPM. Also, because the orders are in nice neat increments to each other, they tend to be confused with harmonics.
This is where the term Harmonic Damper creeps in, but is not quite technically correct. Harmonic is normally a time-based term (usually multiples of a base frequency) and orders are an event-based term. The damper is tuned to ONE frequency and damps the various orders as they excite the SAME frequency in the crank at different RPM.
• Mode – describes a pattern of twist in the crank. Basically it describes the number of places the twisting motion changes direction from clockwise to counter clockwise.
• Node – the name of the places where the crank vibration motion is zero as the motion changes from one direction to the other. (i.e. Second Mode motion has 2 Nodes).
• Inertia – (also known as the polar moment of inertia) is the tendency of a mass to resist changes in rotation speed (RPM). It is mathematically the mass times the square of the distance of the CG of the mass from its center of rotation (m x r_). It is different from weight because the radius_ factor makes it very sensitive to the diameter of the part. A thin flywheel has a much higher inertia than a long, small shaft of the same weight.
BHJDynamics_Damper_Info.pdf
Here is the first page text as a teaser...
***************************************
1. Torsion - The hidden problem with the crank system – and why it needs a damper.
First - Special words to know
Frequency, RPM, Orders, Modes, Nodes and Inertia
• Frequency - is how many things happen in a period of TIME = (cycles/second - called Hertz and abbreviated (Hz)). The crank natural frequencies are defined this way and are constants, they do not change with RPM.
• RPM - is revolutions of the crank per minute.
• Orders - are defined as events per crank revolution. Because of this, their frequency is variable and goes up and down with RPM. Also, because the orders are in nice neat increments to each other, they tend to be confused with harmonics.
This is where the term Harmonic Damper creeps in, but is not quite technically correct. Harmonic is normally a time-based term (usually multiples of a base frequency) and orders are an event-based term. The damper is tuned to ONE frequency and damps the various orders as they excite the SAME frequency in the crank at different RPM.
• Mode – describes a pattern of twist in the crank. Basically it describes the number of places the twisting motion changes direction from clockwise to counter clockwise.
• Node – the name of the places where the crank vibration motion is zero as the motion changes from one direction to the other. (i.e. Second Mode motion has 2 Nodes).
• Inertia – (also known as the polar moment of inertia) is the tendency of a mass to resist changes in rotation speed (RPM). It is mathematically the mass times the square of the distance of the CG of the mass from its center of rotation (m x r_). It is different from weight because the radius_ factor makes it very sensitive to the diameter of the part. A thin flywheel has a much higher inertia than a long, small shaft of the same weight.
BHJDynamics_Damper_Info.pdf
