scored, the gears must be overhauled at the firstopportunity.Pitting, particularly along the pitch line, may occurin the first few months of service. This pitting (oftenreferred to as connective pitting) usually stops after ashort time, and no further trouble is experienced.Corrective pitting requires only one precaution. You mustbe sure that no flakes of metal are allowed to remain inthe LO system. Remember, very minor pitting does notaffect operation. Pitting in new gears is due to very slighthigh areas. These high areas are removed by the pitting.This condition is corrective and will stop. However,pitting that continues can result in progressivedeterioration of the gear (fig. 3-2).Scoring is characterized by transfer of metal fromone sliding surface to another. Scoring in gear teeth iscaused by contact of the tooth tips due to insufficient tiprelief or lack of lubrication.Dirt tracks are caused by foreign particles passingthrough the mesh. The gear teeth are marked in the samelocation on each meshing tooth. Prominent high spotscaused by foreign particles require removal. Removal offoreign particles avoids problems such as loadconcentration, pitting, or tooth breakage.Wire edge caused by plastic flow of metal results ina “fin” at the outside diameter of the tooth. If the fin isheavy, it must be removed. If not removed, it may breakoff and pass through the mesh.Cracked teeth are normally caused by fatigue, butmay be caused by shock. Cracked teeth like those shownin figure 3-3 will break if operation of the MRG iscontinued. The cracks are clearly shown by indicatingdyes used for inspection.Tooth fatigue breakage is caused by repetitivecycling at a load greater than the fatigue strength of thematerial. Tooth fatigue is progressive. A short crackappears first, and then propagates. Characteristic “oystershell” lines can usually be seen. Figure 3-4 shows atypical broken tooth.AlignmentThe gear train is in alignment when the gear and thepinion are parallel. That is, the axis of the two shafts arein the same plane and equal distance from each other at270.7Figure 3-2.—Progressive destructive pitting.3-4
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