New gears or gears which have been realigned should be given a wearing-in run at low power before being subjected to the maximum tooth pressure of full power.

For the proper operation of the gears, it is essential that the tooth contact (or total tooth pressure) be uniformly distributed over the total area of the tooth faces. This is accomplished by accurate alignment and adherence to designed clearances. Gear tooth contact is verified by the application of Dkem to the gear teeth and by jack-ing the gears. Then the gears are inspected to check for the Dkem impressions.

The designed center-to-center distance of the axes of the rotating elements should be maintained as accurate as practicable. In all cases the axes of pinions and gear shafts must be parallel. Non-parallel shafts concentrate the load in one end of a helix. This situation may cause flaking, galling, pitting, featheredge on teeth, deformation of tooth contour, or breakage of tooth ends. The designed TOOTH CONTOUR must also be maintained. If the contour is destroyed, a rub-bing contact will occur with consequent danger of abrasion.

If proper tooth contact is obtained when the gears are installed, there will not be much trouble as far as the WEAR OF TEETH is concerned. Excessive wear cannot take place unless there is metallic contact, and metallic contact will not occur if adequate lubrication is provided. An adequate supply of lubricating oil at all times, proper cleanliness, and inspection for scores will prevent the wearing of teeth.

If, after all precautions have been taken, the lubricating oil supply should fail and the TEETH DO BECOME SCORED, the gears must be thoroughly overhauled by a naval shipyard, as soon as possible. During the first few months that reduction gears are in service, PITTING may occur, particularly along the pitch line. Although slight pit-ting does not affect the operation of the gears, care must be taken to see that no flakes of metal are allowed to remain in the oiling system. Play between the surfaces of the teeth in mesh on the pitch circle is known as BACKLASH. It increases as the teeth wear out. However, backlash can increase considerably without causing any trouble.

ROOT CLEARANCE.The designed root clearance with gear and pinion operating on their designed centers can be obtained from the manufacturers drawing or blueprint. The actual clearance can be found by taking leads or by inserting a long feeler gage or a wedge gage. This clearance should check with the designed clearances. When the root clearance is con-siderably different at the two ends, the pinion and gear shaft are not parallel. Some tolerance is per-mitted, provided that there is still sufficient backlash and that the teeth are not meshed so closely that lubrication is adversely affected.

ALIGNMENT OF GEAR TEETH.When the gear and the pinion are parallel (axes of the two shafts are in the same plane and equally distant from each other), the gear train is aligned. In service the best indication of proper alignment is good tooth contact and quiet operation. The length of tooth contact across the face of the pinions and gears is the criterion for satisfactory alignment of reduction gears. To static check the length of tooth contact, coat about 5 to 10 teeth with either Prussian blue or red lead, then roll the gears together with sufficient torque to cause contact between the meshing teeth and force the journals into the ahead reaction position in their bearings. After you determine the tooth con-tact, remove all the coating to prevent possible contamination of the lubricating oil. If tooth con-tact is to be checked under operating conditions, coat the teeth with red or blue Dyken or with cop-per sulphate.

SPOTTING GEAR TEETH.All abnormal conditions which may be revealed by operational sounds or by inspections should be corrected as soon as possible. Rough gear teeth surfaces, resulting from the passage of foreign objects through the teeth, should be stoned smooth. If the deterioration of a tooth surface cannot be traced directly to a foreign object, give special attention to lubrication and to the condition of the bearings. Also consider the possibility that a change in the supporting structure may have disturbed the parallelism of the rotors. Spotting reduction gear teeth is done first by coating the teeth with Prussian blue and then by jacking the gear in its ahead direction of rotation. As the gear teeth come in contact with the marked pinion teeth, an impression is left on the high part of each gear tooth. Rotate the gear about 1/4 of a turn to a convenient position for stoning. Then remove all the high spots indicated by the marking with a small handstone. Normally, it will be necessary to replace the bluing on the pinion teeth repeatedly, since if the bluing is applied too heavily you may obtain false impres-sions on the gear teeth.

A satisfactory tooth contact is obtained when at least 80% of the axial length of the working face of each tooth is in contact and distributed over approximately 100% of the face width. Remember that the stoning of gears is useful only to remove a local hump or deformation, not to remove deep pitting or galling.