ALIGNMENT  OF  GEAR  TEETH.—When the gear and the pinion are parallel (axes of the two shafts are in the same plane and equally dis- tant 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 satisfac- tory 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 at- tention  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. Main Thrust Bearings A ship is moved through the water by an ax- ial thrust that is developed by the propeller and transmitted  to  the  ship’s  structure.  This  axial thrust is transmitted by the shaft through a thrust bearing which is located either at the forward end or at the after end of the main reduction bull gear or in the propeller line shafting aft of the gear. Pivoted-segmental shoe bearings (Kingsbury type) utilize a wedge-shaped film of oil in their opera- tion. The source of lubricating oil for thrust bear- ings depends on the location of the bearings. In some installations oil is provided by the same system which furnishes oil to the reduction gears. In   other   installations,   a   separate   lubricating system  is  provided. Kingsbury-type thrust bearings consist of a collar mounted on the shaft and revolving between one or more sets of babbitt-faced segmental shoes. The  backs  of  these  shoes  rest  against  round hardened steel pivots which permit the shoes to assume a tilt and change their angle with respect to the shaft collar. Bearings in which the thrust is  always  exerted  in  the  same  direction  are equipped with shoes on one side only, but since provision  must  be  made  in  most  marine  applica- tions for thrust in two directions, it is more com- mon to find shoes on each side of the collar. The shoes are free to adjust themselves at an angle to the collar. Rotation of the shaft collar drags a film of oil into the space between the shoes and the collar, and as the oil film forms, the shoes adjust themselves to the angle most efficient for the load conditions and the oil viscosity. Additional  information  on  Kingsbury-type thrust bearings and other types of bearings is pro- vided   in   the   NAVSHIPS   Technical   Manual, chapter 243. Detailed information on allowable tolerances  and  procedures  for  taking  thrust  bear- ing readings can be obtained from the manufac- turer’s technical manual. End play checking of a Kingsbury thrust bear- ing must always be done with the upper half of the housing solidly bolted down, otherwise the base rings may tilt and provide a false reading. Chapter  4—REDUCTION  GEARS  AND  RELATED  EQUIPMENT 4-7