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. I n
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 manufacturers 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 checkApply 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
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