Engine Bearing Failure - Is it Round?
A journal bearing and its mating shaft must have a clearance fit that is not too tight or loose to maintain a hydrodynamic fluid film between the surfaces. A bearing bore or journal might appear to be round but if magnified or measured carefully found to have deviations from a true round form. These deviations, made up of low and high spots called "lobes", carry most of the load of the shaft when it runs in the bearing. Lobing is mostly the result of machining and grinding and is difficult to prevent.
Consider the crankpin bearing below. The horizontal dark lines are contact marks where the bearing pressed against high spots (lobes) in the connecting rod bore.
The bearing, being somewhat soft and malleable, takes the shape of the bore so that the crankpin bearing journal rides on top of these high spots, reducing clearance and hindering the formation of a hydrodynamic oil film. These high spots also "take the load" and might cause localized bearing over-load failure as shown below in the picture of the connecting rod bearing failure.
Babbit bearing failure to connecting rod bearing.
When taking an engine apart, a close inspection of the bearings, both front and back, tells you if the journal and bearing "like each other". Any bearing distress caused by size or geometric form (roundness, parallelism, circularity, etc) large enough to adversely affect the running of the engine will usually be seen in the bearings. If our bearings have normal wear, then we can proceed with the normal micrometer inspections of size. Problems arise when we send the crankcase or crankshaft out for machining or grinding. This is when lobing occurs and most likely the machining or grinding shop does not have the expensive CCMM's (Cylindrical Coordinate Measurement Machines) needed to measure roundness. Having someone machine your crankcase or crankshaft is like having a tooth drilled and filled. You wouldn't do it to a perfectly good tooth as it's never as good as the original. But go ahead, if the tooth is damaged and you need to save it.
When we measure a crankshaft journal, typically we check it at several points 90 degrees apart. If the journal is the same size (constant diameter) in all measurements the mechanic would state that the journal was "round" or had zero OOR (out-of-round). This is adequate for measuring wear but is not a measure of roundness and will not detect most forms of lobing.
Consider the 3- lobed figure above. It could represent a crankshaft journal or bearing bore with greatly exaggerated lobes. A micrometer inspection will result in zero OOR based on the fact that its diameter does not vary. Even though the figure is obviously not round, its diameter is the same no matter where we take our measurement.
Circular or roundness is based on every point on the surface being equidistance from the center axis. A good approximation for field measurements is to use vee-blocks and carefully rotate the shaft and report on the amount of deviation. Crankcase bores can be measured with a 3-point micrometer using a rotating technique. We would detect the 3-lobed pattern above.
Lobed shaft measures 1 inch no matter where we measure it!
Now consider the piston pin. Can we conclude that it's round based on a micrometer inspection? Might it not have a lobed pattern of a few thousandths of an inch that we cannot detect using our micrometer?
Based on our micrometer inspection we might erroneously conclude that there is nothing wrong with parts because they have zero oor. Yet, the solid line figure and the picture of the same form are clearly not round. The cause of our bearing failure above would remain a mystery!
What is actually being measured above is a measurement of the diametrical variation of the cross section. We cannot conclude that zero diametrical variation of a cross section is a circular form. In fact, our measurements of the figure above would result in a diameter equal to the dotted line but our shaft extends beyond the dotted line. This is one reason a shaft might not fit into a hole even though their diameters show adequate clearance between the parts!
Both pictures below show a shaft that measures 1 inch in diameter. The washer installs onto the round shaft but will not install onto the lobed shaft; even though they are both the same diameter.
Now imagine if this were a crankshaft journal and we wanted to be sure the bearing had adequate running clearance in the bearing. We cannot be assured of proper clearance by just measuring bore and journal diameters. Better would be to measure actual clearance.
How might we better measure roundness?
Bearing Condition Slideshow
Video
Why using a micrometer is not a valid method of measuring shafts for roundness
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