ENGINEMAN 1 & CThis is generally the result of very close oilclearances and the use of an oil having a viscos-ity higher than recommended. Early stages of theloss of lead, due to melting, will be evidenced byvery small streaks of lead on the bearing surface.loads on the main bearings because of the forcethat is necessary to retain correct alignment be-tween the bearing and the journal.3. Inadequate bond between the bearing metaland the bearing shell. A poor bond may becaused by fatigue resulting from cyclic loads, orit may be the result of defective manufacturing.A failure due to inadequate bond is shown infigure 3-20. In such failures, the bearing shellshows through the bearing surface clearly.A bent or misaligned connecting rod can bethe cause of a ruined crank-pin bearing. Misalign-ment between the connecting rod bore and thepiston pin bushing bore is indicated by the crack-ing of the bearing material at the opposite endsof the upper and lower-bearing shell. An indica-tion of a bent connecting rod is heavy wear orscoring on the piston surface.4. Out-of-round journals due to excessivebearing wear. As the bearings wear, excessiveclearance is created; this leads to engine pounding,oil leakage from the bearing, reduced flow of oilto other bearings, and overheating, with the con-sequent melting of bearing material. To preventbearing wear, the journals should be checked forout-of-roundness. Manufacturers require crankpins to be reground when the out-of-roundnessexceeds a specified amount, but the amount varieswith manufacturers. Always check the enginemanual for this type of data.5. Rough spots. Burrs or ridges may causegrooves in the bearings and lead to bearing failure.Removal of rough spots is done with a fine oilstone and a piece of crocus cloth. Be sure to placea clean cloth beneath the journal to catch all par-ticles. Apply a coat of clean lubricating oil to thejournal and to the bearing before a bearing isinstalled.7. Faulty installation, due to negligence orlack of experience. The paramount factor isinattention to cleanliness. Hard particles lodgebetween the bearing shell and the connecting rodbore, and create an air space. This space retardsthe normal flow of heat and causes localized hightemperatures. Such condition may be further ag-gravated if the bearing surface is forced out intothe oil clearance spaces and creates a high spotin the bearing surface. The result of a bearingfailure is illustrated in figure 3-21. Foreign par-ticles, excessive clearance, or rough surface maycause poor contact between a bearing shell anda connecting rod. Poor contact is indicated by theformation of a gumlike deposit (sometimes re-ferred to as lacquer or varnish) on the back ofthe shell.6. Misalignment of parts. Misalignment of themain bearings can be caused by a warped or bentcrankshaft. Such misalignment imposes heavyBearing failures may result from improper fitof the shell to the connecting rod. If the lockinglip of a bearing does not fit properly into the recessof the bearing housing, distortion of the shell andfailure of the bearing results.121.5121.4Figure 3-20.—Bearing failure due to inadequate bond.Figure 3-21.—Bearing failure resulting from wiping andexcessive temperatures.3-34
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