linderpat wrote: ↑Fri Oct 16, 2020 6:46 am
Lets talk for a minute about the technical aspects of the damper - specifically, what harm is caused by an old worn out one? Can it harm the internals of the engine, and how do you know?
The crankshaft has natural (resonant) frequencies (Fn), one in bending and one in torsion (twisting). Because it is well supported in bending via the main bearings, torsional Fn is normally the major concern – not that bending can be ignored. It’s just that, due to the physical design, the torsional Fn is normally much lower than the bending Fn. Lower Fn = greater deflections/displacement = greater stress. Greater torsional deflection in the crank (at Fn) also impacts the cam timing on the 928 as the cams are driven by the end of the crank that’s resonating/displacing.
The crank itself has very little internal damping - when it is exposed to a combustion forces at/near its Fn (which it will because the torsional Fn falls within the engine RPM operating range), it is going to “take off” and vibrate excessively at certain engine speeds, which increases stress in the crank significantly, due to the increased deflection. The Torsional Vibration Damper (TVD) is designed to dissipate this vibratory energy.
The TVD consists of a spring/mass/damper system tuned to the Fn of the crankshaft. As the cranks starts to vibrate at its torsional Fn, the vibration energy is transferred into the TVD, and the rubber portion, which acts as both the spring and the damper, dissipates vibratory energy, converting it through friction, to heat.
As the OEM TVD ages, the rubber (which is the spring/damper component) degrades and its properties change. Its damping capabilities (ability to dissipate vibratory energy) go down and its stiffness changes, which changes its Fn, so it is no longer tuned to the Crank. The result is that the crank sees increased stress which, over time builds up increasing fatigue, and ultimately failure in the crank. Another potential failure mode is due to what’s called cross axis vibration.
In a perfect world, when the crank is vibrating in torsion, that’s all it would be doing. But, because the various components involved are not perfectly square and balanced, the torsional vibration also imparts vibratory energy perpendicular to the crank rotation axis. So, not only is the crank twisting, but it is also bending at the front, external to the engine block. This external bending potentially causes premature wear on the crank seal and forward main bearing. Since the amount of cross axis vibration energy is basically proportional to the torsional vibration energy, employing a properly performing TVD reduces both, and the damage they cause. Because fatigue, by definition slowly builds up over time, I don't think there is a way to know the impact of a failing TVD, without systematic engine teardowns and inspections?