Main bearing potential to damp torsional vibration
Main bearing potential to damp torsional vibration
(OP)
For decades Many engines have sported torsional dampers on their cranks' noses.
Older Chevy sixes had main bearing journals all vary in diameter. The journal near the flywheel is largest, and going forward each journal is about 0.03" smaller than the previous one.
In a recent discussion an acquaintance proposed that this diameter variation results in some level of torsional damping for free that would not exist if all the main journals were the same size. I think part of his argument is a claim the smaller shaft diameter results in greater shaft deflection and lower surface speed, with the result the small journals run more eccentrically in the bearing thereby generating circumferential pressure differentials of greater magnitude within the fluid film, which he felt would result in "damping."
I know models of rotor dynamic analysis of turbines credit hydrodynamic bearing allow including a variety of damping factors and coeffients, but as best I recall they are typically radial damping, not torsional.
My questions - Is viscous torsional damping from crankshafts winding/unwinding < 1 degree ignored in engine design, since there is going to be a big old damper on the snout anyway?
And in any case, does that exist to a significant degree?
Older Chevy sixes had main bearing journals all vary in diameter. The journal near the flywheel is largest, and going forward each journal is about 0.03" smaller than the previous one.
In a recent discussion an acquaintance proposed that this diameter variation results in some level of torsional damping for free that would not exist if all the main journals were the same size. I think part of his argument is a claim the smaller shaft diameter results in greater shaft deflection and lower surface speed, with the result the small journals run more eccentrically in the bearing thereby generating circumferential pressure differentials of greater magnitude within the fluid film, which he felt would result in "damping."
I know models of rotor dynamic analysis of turbines credit hydrodynamic bearing allow including a variety of damping factors and coeffients, but as best I recall they are typically radial damping, not torsional.
My questions - Is viscous torsional damping from crankshafts winding/unwinding < 1 degree ignored in engine design, since there is going to be a big old damper on the snout anyway?
And in any case, does that exist to a significant degree?





RE: Main bearing potential to damp torsional vibration
RE: Main bearing potential to damp torsional vibration
RE: Main bearing potential to damp torsional vibration
Cheers
Greg Locock
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RE: Main bearing potential to damp torsional vibration
Mike Halloran
Pembroke Pines, FL, USA
RE: Main bearing potential to damp torsional vibration
Chevy used it at least as early as 193X on the first [three main bearing] Chebbie 6. They pumped up the main diameters pretty rapidly at first, but kept the stepped journals when they increased the number of main bearings right thru 1963.
RE: Main bearing potential to damp torsional vibration
RE: Main bearing potential to damp torsional vibration
Why do you feel the flywheel end of the crank is "the loaded end?"
Regards,
Dan T
RE: Main bearing potential to damp torsional vibration
This means that there will be less torsional flex from the crankshaft to dampen the impulses from combustions that are closest to the flywheel.
Picture a lever being used to lift a weight. The main bearings are the fulcrum in this lever. The load that the clutch couples to the flywheel represents the dead weight that we need to lift. The combustion represents the force we apply to this lever in order to lift the dead weight.
The combustions near the flywheel deliver a more concentrated impulse to the bearing, because it is not being dampened by torsional flex in the crankshaft. Distributing that impulse over a larger surface in the engine's case allows you to control where the stress is distributed, without making the case so rigid that stress is significantly more concentrated to the crankshaft.
That was the most simplified version of this concept. There are more sources of load, such as the rest of the crankshaft that the combustions still need to spin. Then there are pumping losses to deal with, which leads into the subject of engine braking. I haven't even touched on end-play or bending forces yet.
The combustions that happen on the side farthest from the flywheel are also dampened by increased torsional flex from the length of crankshaft which couples those combustions to the load. Reducing those bearing sizes serves to decrease drag and rotational mass, which also helps the PTO-side main bearings survive.