Figure 8-5.—Locked-train-type gearing. ranges. A typical steam turbine reduction gear is shown  in  figure  8-5. The  use  of  reduction  gears  is  by  no  means limited  to  ship  propulsion.  Other  machinery,  such as ship’s service generators and various pumps, also have reduction gears. In these units, as well as  in  shipboard  propulsion  units,  engine  operating efficiency requires a higher rpm range than that suitable  for  the  driven  unit. Reduction gears are classified by the number of steps used to bring about the speed reduction and   the   arrangement   of   the   gearing.   A   gear mechanism consisting of a pair of gears or a small gear  (pinion)  driven  by  the  engine  shaft,  which directly drives a large (bull) gear on the propeller shaft, is called a single-reduction gear. In this type of  arrangement,  the  ratio  of  speed  reduction  is proportional  to  the  diameter  of  the  pinion  and the  gear.  For  example,  in  a  2-to-1  single-reduction gear,  the  diameter  of  the  driven  gear  is  twice  that of  the  driving  pinion.  In  a  10-to-1  single-reduction gear, the diameter of the driven gear is 10 times that  of  the  driving  pinion. Steam propulsion-type ships built since 1935 have  double-reduction  propulsion  gears.  In  this type  of  gear,  a  high-speed  pinion,  connected  to the turbine shaft by a flexible coupling, drives an intermediate  (first  reduction)  gear.  This  gear  is connected  by  a  shaft  to  the  low-speed  pinion  that, in  turn,  drives  the  bull  gear  (second  reduction) mounted on the propeller shaft. A 20-to-1 speed reduction might be accomplished by having a ratio of 2-to-1 between the high-speed pinion and the first-reduction   gear,   and  a  ratio  of  10-to-1 between  the  low-speed  pinion  and  the  second- reduction  gear  on  the  propeller  shaft. For  a  typical  example  of  a  double-reduction application,  let  us  consider  the  main-reduction gear   shown   in   figure   8-6.   The   high-pressure and  low-pressure  turbines  are  connected  to  the propeller  shaft  through  a  locked-train  type  of double-reduction   gear. NOTE:  This  type  of  reduction  gear  is  used aboard  many  naval  combatant  ships. First-reduction   pinions   are   connected   by flexible  couplings  to  the  turbines.  Each  of  the first-reduction pinions drives two first-reduction gears. A second-reduction (slow speed) pinion is attached  to  each  of  the  first-reduction  gears  by a  quill  shaft  and  flexible  couplings.  These  four pinions  drive  the  second-reduction  (bull)  gear  that is  attached  to  the  propeller  shaft. CLUTCHES  AND  REVERSE  GEARS Clutches  are  normally  used  on  direct-drive propulsion  engines  to  provide  a  means  of  dis- connecting  the  engine  from  the  propeller  shaft. In small engines, clutches are usually combined with reverse gears and are used for maneuvering the  ship.  In  large  engines,  special  types  of clutches  are  used  to  obtain  special  coupling  or control  characteristics  and  to  prevent  torsional (twisting)  vibration. Diesel-propelling  equipment  on  a  boat  or  a ship must be capable of providing backing-down power  as  well  as  forward  power.  There  are  a few  ships  and  boats  in  which  backing  down  is accomplished   by   reversing   the   pitch   of   the propeller.  Most  ships,  however,  back  down  by reversing  the  direction  of  rotation  of  the  propeller shaft.  In  mechanical  drives,  reversing  the direction  of  rotation  of  the  propeller  shaft  may be  accomplished  in  one  of  two  ways.  You  can reverse  the  direction  of  engine  rotation  or  use  the reverse gears. Reverse gears are used on marine engines to reverse the rotation of the propeller shaft during maneuvering  without  reversing  the  rotation  of  the engine.    They  are  normally  used  on  smaller engines. If a high-output engine has a reverse gear, the gear is used for low-speed operation only and does  not  have  full-load  and  full-speed  capacity. For   maneuvering   ships   with   large   direct- propulsion engines, the engines are reversed. The  drive  mechanism  of  a  ship  or  a  boat  is required to do more than reduce speed and reverse 8-4


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