Chapter 3—ENGINE MAINTENANCEReadings are generally taken at the four crankpositions: top dead center, inboard, near or atbottom dead center, and outboard. In someengines, it is possible to take readings at bottomdead center. In others, the connecting rod mayinterfere, making it necessary to take the readingas near as possible to bottom dead center withouthaving the gage come in contact with the con-necting rod. The manufacturer’s technical manualfor the specific engine provides information con-cerning the proper position of the crank whenreadings are to be taken. When the gage is in itslowest position, the dial will be upside down,necessitating the use of a mirror and flashlight toobtain a reading.Once the indicator has been placed in positionfor the first deflection reading, do NOT touch thegage until all four readings have been taken andrecorded.Variations in the readings obtained at the fourcrank positions will indicate distortion of thecrank. Distortion may be caused by several fac-tors, such as a bent crankshaft, worn bearings,or improper engine alignment. The maximumallowable deflection can be obtained from themanufacturer’s technical manual. If the deflec-tion exceeds the specified limit, take steps to deter-mine the cause of the distortion and to correctthe trouble.Deflection readings are also employed todetermine correct alignment between the engineand the generator, or between the engine and thecoupling. When alignment is being determined,a set of deflection readings is usually taken at thecrank nearest to the generator or the coupling.In aligning an engine and generator, it may benecessary to install new chocks between thegenerator and its base to bring the deflectionwithin the allowable value. It may also benecessary to shift the generator horizontally toobtain proper alignment. When an engine and acoupling are to be aligned, the coupling must firstbe correctly aligned with the drive shaft; then, theengine must be properly aligned to the coupling,rather than the coupling aligned to the engine.CAMSHAFTSIn addition to the camshaft and bearingtroubles already mentioned, the cams of acamshaft may be damaged as a result of impropervalve tappet adjustment, worn or stuck camfollowers, or failure of the camshaft gear.Cams are likely to be damaged when a loosevalve tappet adjustment or a broken tappet screwcauses the valve to jam against the cylinder head,and the push rods to jam against their cams. Thiswill result in scoring or breaking of the cams andfollowers, as well as severe damage to the pistonand the cylinder.Valves must be timed correctly at all times,not only for the proper operation of the enginebut also to prevent possible damage to the engineparts. You should inspect frequently the valveactuating linkage during operation to determineif it is operating properly. Such inspections shouldinclude taking tappet clearances and adjusting,if necessary; checking for broken, chipped, orimproperly seated valve springs; inspecting pushrod end fittings for proper seating; and inspect-ing cam follower surfaces for grooves or scoring.JOURNAL BEARINGSEngine journal bearing failure and their causesmay vary to some degree, depending upon thetype of bearing. The following discussion of thecauses of bearing failure applies to mostbearings—main bearings as well as crank pin bear-ings. The most common journal bearing failuresmay be due to one or to a combination of thefollowing causes:1. Corrosion of bearing materials caused bychemical action of oxidized lubricating oils.Oxidation of oil may be minimized by changingoil at the designated intervals, and by keepingengine temperatures within recommended limits.Bearing failures due to corrosion may be identifiedby very small pits covering the surfaces. In mostinstances, corrosion occurs over small bearingsareas in which high localized pressures andtemperatures exist. Since the small pits caused bycorrosion are so closely spaced that they formchannels, the oil film is not continuous and theload-carrying area of the bearing is reduced belowthe point of safe operation.2. Surface pitting of bearings due to highlocalized temperatures that cause the lead to melt.3-33
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