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Howdy Machine Man,
I don't expect to see "tight wire" and "oil stack up" terms get into these small engine manuals.
I am not familiar with either term. But I might know what oil stack up means.
I once read how an investigation into bearings came about because of very expensive catastrophic failures of propeller shaft bearings in steam ships.
For as long as there had been bearings it was assumed that shafts "floated" on a layer of oil supported by surface tension. As long as steam powered vessels operated at very low RPM, this assumption worked well. Make the bearings as round as possible and supply copious amounts of oil.
As steam propulsion technology progressed, shaft rotational speeds increased. As RPM increased bearings and expensive prop shafts would be destroyed by metal to metal contact. Clearances, oil viscosity and pressures, and babit materials were all specified to a tighter tolerances. Still the bearings failed.
What was not understood was why rotational speed would break the surface tension of the oil. Marine engineers put their collective minds to the problem and began to investigate just how bearing/shaft surfaces interact.
They learned that the area between the shaft and the journal squeezes the oil into a wedge along the leading edge of the direction of rotation. This wedge of oil prevents contact between the two surfaces. If either the rotational speed or the torque pressure exceeds the ability of the oil to flow in front of the shaft-to-bearing contact area, lubrication is lost. As RPM increases the shaft begins to overcome the cushioning oil wedge and fall off in front of the wedge; metal to metal contact.
Now that they understood the mechanism they had to figure out how to overcome the limitations. Tightening tolerances had not worked.
I assume that much head scratching ensued. At some point it was noted that slightly "bent" shafts seemed to live longer than more perfect examples. That was odd. Not only odd but unacceptable. Bent shafts describe a circle about their center at the business end. Power is lost to unwanted motion. Not only that, the entire ship's structure vibrates in sync with the massive shaft wobbling about. Not good at all.
The "fix" was to keep the shafts as true as possible but make bearings less round; more eccentric. This slight variance in bearing clearance promotes formation of the necessary wedge of oil along the leading edge of flow. It was counter intuitive to make the bearings "out-of-round" but it worked.
Is that what is meant by oil stack up?
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Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 52 year old fat man. ;-)
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