BACKLASH.- Backlash is the play between the surfaces of the teeth in mesh measured at the pitch circle. The backlash will increase with wear, and can increase considerably without causing trouble. On some gears that are recut, the backlash does not affect operation or

Figure 3-1.- Gear tooth nomenclature.

cause noise during ahead operation. However, a small increase in noise during astern and low-power operation may be apparent. Lack of backlash may cause noise, overloading, overheating, or failure of the gears and bearings.

DAMAGE BY FOREIGN PARTICLES.- In cases where both the pinion and gear teeth have been indented by foreign material, both should be relieved of all raised metal around the indentation. If a tooth has been dented or a foreign particle has been caught in the mesh, it will track on the mating teeth when the MRG is operated. You can hear a damaged tooth when the MRG is spin-tested. The frequency of the noise in hertz (cycles per second) will indicate which rotating element in the gear train has the damaged tooth. In double-reduction, locked-train MRGs, the damaged tooth may be on one of the four second-reduction pinions and/ or one of the four first-reduction gears. The frequency will help you eliminate one of the four sets. If the damage is small, you must examine all four rotating elements until the damaged tooth is found.

FINDING DAMAGED TEETH.- Sufficient damage may be done so that just a careful visual inspection can locate the damage. If the damage is small, it may be faster to find the damaged tooth by painting the pinion teeth with a thin coat of metal-marking compound, such as prussian blue. After rotating the gears with the turning gear motor, the high spots will be shining through the coating of prussian blue.

REPAIR OF DAMAGED TEETH.- When very small foreign particles get in the gear train, they can scratch the teeth. Large particles can bend, dent, or crack the teeth. One bent or dented tooth will track on all teeth-in mesh with it. These bent and/ or dented teeth can be repaired by stoning, filing, or scraping. The abraded portions of the teeth should be dressed enough to prevent the cutting of the meshing teeth. Dressing includes such actions as removal of a wire edge that is large enough to break off and pass through the mesh, and/ or removal of high spots. Gear teeth should not be touched with hand tools except in an emergency! Even during an emergency, only steel scrapers or a fine file should be used, and every precaution must be taken to remove all filings or abrasive material. You should NEVER attempt to remove deep pitting or galling.

TOOTH ROOT CLEARANCE.- The designed root clearance of gears operating on their designed centers can be found in the manufacturer's technical manual drawings. You can determine the actual clearance with the insertion of a long feeler gauge, a wedge, or by the use of leads. The actual clearance should be within a few thousandths of an inch of the designed clearance and should be about the same at each end of the gear. If the root clearance is materially different at the two ends, the pinion and gear shafts may not be parallel. A difference of a few thousandths of an inch can be accounted for by errors in observation and by slight errors in machining. The amount of clearance may change a limited amount one way or another. This change is acceptable provided there is sufficient backlash so the teeth are not meshed so closely as to cause tooth interference.

GEAR TOOTH CONTACT.- Gears in mesh that are rotating in parallel and have uniform tooth contact will operate satisfactorily. Active pitting, tooth breakage, and uneven tooth contact indicate that some corrective action is required.

Satisfactory tooth contact is defined as at least 80 percent of the axial length of the working face of each tooth is in contact, distributed over nearly 100 percent of the face width. You can determine gear tooth contact using one of the following two methods:

1. Static check- Apply a thin coating of prussian blue to the pinion teeth and roll the gears with the turning gear. The compound will transfer to the gear teeth.

-NOTE Some gears are cut with a very slight taper of the teeth (helix angle deviation) to offset the effects of torsion. In such gears, full contact across the face will not be obtained by static testing.

2. Operation-Use blue or red DYKEM or copper sulphate to determine tooth contact under operating conditions. Use DYKEM for dock trials, as it will show marking with light loads. Copper sulphate shows marking after much longer and higher power operating conditions than that required for DYKEM.

TOOTH WEAR AND FAILURE.- Wear is defined as the removal of metal from the gear teeth. Normal wear is the removal of metal at a rate that does not impair the satisfactory operation of the gear. If proper tooth contact is obtained when the gears are installed, little trouble should be encountered in respect to wear. Excessive wear cannot take place without metallic contact. Proper clearances, inspections for removal of high spots, and/ or adequate supplies of lubricating oil can prevent excessive wear. If the lubricating oil supply should fail and the teeth become scored, the gears must be overhauled at the first opportunity.

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.