cause noise during ahead operation. However, a smallincrease in noise during astern and low-power operationmay be apparent. Lack of backlash may cause noise,overloading, overheating, or failure of the gears andbearings.DAMAGE BY FOREIGN PARTICLES.— I ncases where both the pinion and gear teeth have beenindented by foreign material, both should be relieved ofall raised metal around the indentation. If a tooth hasbeen dented or a foreign particle has been caught in themesh, it will track on the mating teeth when the MRGis operated. You can hear a damaged tooth when theMRG is spin-tested. The frequency of the noise in hertz(cycles per second) will indicate which rotating elementin the gear train has the damaged tooth. Indouble-reduction, locked-train MRGs, the damagedtooth may be on one of the four second-reductionpinions and/or one of the four first-reduction gears. Thefrequency will help you eliminate one of the four sets.If the damage is small, you must examine all fourrotating elements until the damaged tooth is found.FINDING DAMAGED TEETH.— Sufficientdamage may be done so that just a careful visualinspection can locate the damage. If the damage issmall, it may be faster to find the damaged tooth bypainting the pinion teeth with a thin coat ofmetal-marking compound, such as prussian blue. Afterrotating the gears with the turning gear motor, the highspots will be shining through the coating of prussianblue.REPAIR OF DAMAGED TEETH.— When verysmall foreign particles get in the gear train, they canscratch the teeth. Large particles can bend, dent, orcrack the teeth. One bent or dented tooth will track onall teeth-in mesh with it. These bent and/or dented teethcan be repaired by stoning, filing, or scraping. Theabraded portions of the teeth should be dressed enoughto prevent the cutting of the meshing teeth. Dressingincludes such actions as removal of a wire edge that islarge enough to break off and pass through the mesh,and/or removal of high spots. Gear teeth should not betouched with hand tools except in an emergency! Evenduring an emergency, only steel scrapers or a fine fileshould be used, and every precaution must be taken toremove all filings or abrasive material. You shouldNEVER attempt to remove deep pitting or galling.TOOTH ROOT CLEARANCE.— The designedroot clearance of gears operating on their designedcenters can be found in the manufacturer’s technicalmanual drawings.You can determine the actualclearance with the insertion of a long feeler gauge, awedge, or by the use of leads. The actual clearanceshould be within a few thousandths of an inch of thedesigned clearance and should be about the same at eachend of the gear.If the root clearance is materiallydifferent at the two ends, the pinion and gear shafts maynot be parallel. A difference of a few thousandths of aninch can be accounted for by errors in observation andby slight errors in machining. The amount of clearancemay change a limited amount one way or another. Thischange is acceptable provided there is sufficientbacklash so the teeth are not meshed so closely as tocause tooth interference.GEAR TOOTH CONTACT.— Gears in mesh thatare rotating in parallel and have uniform tooth contactwill operate satisfactorily. Active pitting, toothbreakage, and uneven tooth contact indicate that somecorrective action is required.Satisfactory tooth contact is defined as at least 80percent of the axial length of the working face of eachtooth is in contact, distributed over nearly 100 percentof the face width. You can determine gear tooth contactusing one of the following two methods:1. Static check—Apply a thin coating of prussianblue to the pinion teeth and roll the gears with the turninggear. The compound will transfer to the gear teeth. -NOTESome gears are cut with a very slight taper of theteeth (helix angle deviation) to offset the effects oftorsion. In such gears, full contact across the facewill not be obtained by static testing.2. Operation-Use blue or red DYKEM or coppersulphate to determine tooth contact under operatingconditions. Use DYKEM for dock trials, as it will showmarking with light loads. Copper sulphate showsmarking after much longer and higher power operatingconditions than that required for DYKEM.TOOTH WEAR AND FAILURE.— Wear isdefined as the removal of metal from the gear teeth.Normal wear is the removal of metal at a rate that doesnot impair the satisfactory operation of the gear. Ifproper tooth contact is obtained when the gears areinstalled, little trouble should be encountered in respectto wear. Excessive wear cannot take place withoutmetallic contact. Proper clearances, inspections forremoval of high spots, and/or adequate supplies oflubricating oil can prevent excessive wear. If thelubricating oil supply should fail and the teeth become3-3
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