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Dovetail grooves have long been a source of many bearing wipes. A dovetail groove is a slot cut into the bearing steel that was needed when there were no good chemical tinning agents as there now are.

The problem is the difference in thermal expansion between the bearing shell material and the Babbitt. Babbitt has a coefficient of expansion that is about twice that of steel or iron. When the bearing heats up, the thick Babbitt inside the groove expands but is restricted from moving by the steel walls and bottom surface of the groove. The Babbitt then expands upward out of the groove, rising to form a localized ridge above the dovetail slot. Depending upon the shape of the dovetail groove and the temperature of the bearing metal, the ridges may rise sufficiently to penetrate an oil film while the journal bearing is in operation. This leads to a burnish or smear, or a full-blown wipe. A most common time for this burnishing, smearing, or wiping to occur is during the coast down after the first operating period after a refurbishment outage, in this manner: During the first period of operation, an oil film is developed with the bearing cold and no ridges. Then, during operation, the bearing metal temperature increases and the ridges develop. When the unit is turned off and the shaft slows down, the thickness of the oil film decreases. When the oil film thickness reduces sufficiently, the journal contacts the tops of the ridges and the burnish/smear/ wipe occurs. This changes the geometry of the bore and reduces the ability of the bearing to create a proper oil wedge, and therefore, the performance of the oil film is degraded, sometimes wiping the Babbitt surface to the point that the bearing must be rebabbitted and rebored before it can go back into service.

The photo below has two close-ups. The lower left close-up shows the smeared area with a jagged edge. The burnished/smeared/wiped areas line up with the thicker Babbitt in the dovetail grooves which are shown in the close-up on the bottom right.

 

 

 

 

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