Tmoose
Mechanical
- Apr 12, 2003
- 5,633
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?