Most of the troubles listed in figure 3-23require the replacement of an antifriction bear-ing. The cause of damage must be determined andeliminated so that similar damage to the replace-ment bearing may be prevented.Dirty bearings may be made serviceable witha proper cleaning, providing other damage doesnot exist. In some cases, races abraded on theexternal surfaces can be made serviceable, but itis generally advisable to replace abraded bearings.Dirty frictionless bearings must be thoroughlycleaned before being rotated or inspected.AUXILIARY DRIVE MECHANISMSAuxiliary drive mechanisms are used in inter-nal combustion engines to maintain a fixed anddefinite relationship between the rotation of thecrankshaft and the camshaft. This is necessary inorder that the sequence of events necessary forthe correct operation of the engine may be car-ried out in perfect unison. Timing and the rota-tion of various auxiliaries (blowers, governor, fueland lubricating oil pumps, circulating waterpumps, overspeed trips, etc.) are accomplished bya gear or chain drive mechanism from thecrankshaft. (Some small engine auxiliaries maybe belt-driven.)GEAR MECHANISMSThe principal type of power transmission fortiming and accessory drives in most diesel enginesis a system of gears similar to those shown infigure 3-24. In some of the larger engines, theremay be two separate gear trains, one for drivingthe camshaft and the other for driving certainaccessories.The type of gear employed for a particulardrive depends upon the function it is to perform.Most gear trains use single helical spur gears, whilegovernor drives are usually of the bevel type;reverse and reduction gear units employ doublehelical gears to balance fore and aft componentsof tooth pressure.Small gears are usually made from a singleforging, while larger ones are quite often built upin split sections. (See the crankshaft gear in figureChapter 3—ENGINE MAINTENANCE121.8Figure 3-24.—Relative arrangement of the gears in an auxili-ary drive mechanism.3-24.) Most gears are made of steel, although castiron, bronze, or fiber are sometimes used.The timing gear train shown in figure 3-24 isused on some two-stroke cycle diesel engines. Thecamshafts rotate at the same speed as thecrankshaft. Note that two idler gears are necessaryto transfer crankshaft rotation to the camshaftgears. The idler gears are used because the cam-shafts and crankshaft are displaced a considerabledistance. If idler gears were not used, thecrankshaft and camshaft gears would have to beconsiderably larger.A similar timing gear train may be found insome four-stroke cycle engines, except that thecamshaft gear or gears will have twice as manyteeth as the crankshaft gear to permit the cam-shaft to rotate at one-half the crankshaft speed.3-37
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