In another thread, Trav posted
Originally Posted By: Trav
Shannow, is there any correlation between a cast crank and lightly reinforced block that are subject to flexing and this..
Quote:
Hydrodynamic journal bearings become unstable with small shaft eccentricity. It's the reason that high speed or relatively low loaded hydrodynamic bearings are to be designed less stiff.
Would the slight flexing be considered eccentricity?
Thought it would be timely to explain what the term eccentricity is, and how bearings work in general.
For those unfamiliar with hydrodynamics, the relative movement of surfaces, with a lubricant between them creates hydrodynamic forces that keep the parts separated...think a kid on a skim board down on the shoreline, he can't stand on the board and have the water support him, it doesn't work in deep water, but when the film of water is thin enough and he's going fast enough, there is enough force built up to keep the board and sand apart.
Same with a journal bearing
The oil that is in contact with the shaft is dragged along with the shaft, and the oil in contact with the bearing is stationary. The oil dragged along with the shaft is being dragged into the diminishing gap. As a result, it creates hydrodynamic forces which push the shaft and bearing apart.
The "eccentricity" that Trav has mentioned is the "off centredness" of the shaft, which creates the diminishing gap, that creates the forces that keep the shafts apart...it's the little "e" in the picture.
The vector diagramme to the right is the sum of forces that are generated. The load is shown as vertical down. There is a force perpendicular to the minimum film thickness point which is the direction of the internal pressure of the oil film, and the third is the equivalent drag that's produced by the turning shaft on the oil at MOFT (Minimum Oil Film Thickness).
Originally Posted By: Trav
Shannow, is there any correlation between a cast crank and lightly reinforced block that are subject to flexing and this..
Quote:
Hydrodynamic journal bearings become unstable with small shaft eccentricity. It's the reason that high speed or relatively low loaded hydrodynamic bearings are to be designed less stiff.
Would the slight flexing be considered eccentricity?
Thought it would be timely to explain what the term eccentricity is, and how bearings work in general.
For those unfamiliar with hydrodynamics, the relative movement of surfaces, with a lubricant between them creates hydrodynamic forces that keep the parts separated...think a kid on a skim board down on the shoreline, he can't stand on the board and have the water support him, it doesn't work in deep water, but when the film of water is thin enough and he's going fast enough, there is enough force built up to keep the board and sand apart.
Same with a journal bearing
The oil that is in contact with the shaft is dragged along with the shaft, and the oil in contact with the bearing is stationary. The oil dragged along with the shaft is being dragged into the diminishing gap. As a result, it creates hydrodynamic forces which push the shaft and bearing apart.
The "eccentricity" that Trav has mentioned is the "off centredness" of the shaft, which creates the diminishing gap, that creates the forces that keep the shafts apart...it's the little "e" in the picture.
The vector diagramme to the right is the sum of forces that are generated. The load is shown as vertical down. There is a force perpendicular to the minimum film thickness point which is the direction of the internal pressure of the oil film, and the third is the equivalent drag that's produced by the turning shaft on the oil at MOFT (Minimum Oil Film Thickness).