scored, the gears must be overhauled at the first
Pitting, particularly along the pitch line, may occur
in the first few months of service. This pitting (often
referred to as connective pitting) usually stops after a
short time, and no further trouble is experienced.
Corrective pitting requires only one precaution. You must
be sure that no flakes of metal are allowed to remain in
the LO system. Remember, very minor pitting does not
affect operation. Pitting in new gears is due to very slight
high areas. These high areas are removed by the pitting.
This condition is corrective and will stop. However,
pitting that continues can result in progressive
deterioration of the gear (fig. 3-2).
Scoring is characterized by transfer of metal from
one sliding surface to another. Scoring in gear teeth is
caused by contact of the tooth tips due to insufficient tip
relief or lack of lubrication.
Dirt tracks are caused by foreign particles passing
through the mesh. The gear teeth are marked in the same
location on each meshing tooth. Prominent high spots
caused by foreign particles require removal. Removal of
foreign particles avoids problems such as load
concentration, pitting, or tooth breakage.
Wire edge caused by plastic flow of metal results in
a fin at the outside diameter of the tooth. If the fin is
heavy, it must be removed. If not removed, it may break
off and pass through the mesh.
Cracked teeth are normally caused by fatigue, but
may be caused by shock. Cracked teeth like those shown
in figure 3-3 will break if operation of the MRG is
continued. The cracks are clearly shown by indicating
dyes used for inspection.
Tooth fatigue breakage is caused by repetitive
cycling at a load greater than the fatigue strength of the
material. Tooth fatigue is progressive. A short crack
appears first, and then propagates. Characteristic oyster
shell lines can usually be seen. Figure 3-4 shows a
typical broken tooth.
The gear train is in alignment when the gear and the
pinion are parallel. That is, the axis of the two shafts are
in the same plane and equal distance from each other at
Figure 3-2.Progressive destructive pitting.