ENGINEMAN 1 & CBroken or bent crankshafts may be caused bythe improper functioning of a torsional vibrationdamper. Vibration dampers are mounted on thecrankshafts of some engines to reduce the tor-sional vibrations set up within the crankshaft andto ensure a smoother running engine. If a damperfunctions improperly, torsional vibrations mayrupture the internal structure of the shaft.The principle of operation is similar in mostdampers, yet their construction and theircomponent parts vary somewhat. If the engine isequipped with a vibration damper, the engine in-struction manual must be consulted for informa-tion on type, construction, and maintenance ofthe damper.In most engines, one end of the crankshaft isflanged to receive the damper, the damper beingbolted or doweled onto the flange. A damper mustbe fastened securely to the crankshaft at all timesduring engine operation; otherwise, the damperwill not control the crankshaft vibrations.Small dampers are usually grease-packed,while larger ones frequently receive lubricationfrom the main oil system. Dampers that are greaselubricated must have the grease changedperiodically, as specified in the manufacturer’sinstructions. If the assembly is of the elastic type,it must be protected from fuel, lube oil, grease,and excessive heat, all of which are detrimentalto the rubber.Excessive rumbling at certain engine speedsmay indicate that the damper is not functioningproperly. You must learn to distinguish betweenthis and the normal noise usually heard in someengines during the first and last few revolutionswhen the engine is starting or stopping. This noiseis normal, it is due to the large designed clearancesin the damper and is not a sign of impendingtrouble.Crankshaft breakage or bending may be theresult of excessive bearing clearances. Excessiveclearance in one main bearing may place practic-ally all of the load on another main bearing. Flex-ing of the crankshaft under load may result infatigue and eventual fracture of the crank web.(See figure 3-19.) Excessive bearing clearance maybe caused by the same factors that cause journalbearing failure. Furthermore, off-center and out-of-round journals tend to scrape off bearingmaterial. This leads to excessive wear and to theincrease of the clearance between the shaft andFigure 3-19.—Cracked crank web.121.3bearing. You can minimize the possibility of jour-nal out-of-roundness by taking measures to pre-vent improper lubrication, journal bearing failure,overspeeding or overloading of the engine,excessive crankshaft deflection, and misalignmentof parts.Crankshaft bending breakage (out-of-roundness) may also result from excessivecrankshaft deflection. Excessive shaft deflection,caused by improper alignment between the drivenunit and the engine, may result in a broken or bentshaft along with considerable other damage tobearings, connecting rods, and other parts. Ex-cessive crankshaft deflection may also be causedby overspeeding an engine. The amount of deflec-tion of a crankshaft may be determined by theuse of a straight gage.The straight gage is merely a dial-readinginside micrometer used to measure the variationin the distance between adjacent crank webs wherethe engine shaft is barred over. When installingthe gage, or indicator, between the webs of acrank throw, place the gage as far as possible fromthe axis of the crankpin. The ends of the indicatorshould rest in the prick-punch marks in the crankwebs. If these marks are not present, you mustmake them so that the indicator may be placedin its correct position. Consult the manufacturer’stechnical manual for the proper location of newmarks.3-32
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