Page 241 - Introduction to Marine Engineering
P. 241

Steering gear  239
          Electrical  control

          The  electrical  remote  control  system  is  commonly  used  in  modern
          installations since it uses a small control unit as transmitter on the bridge
          and  is simple and  reliable  in  operation.
            The  control box assembly, which  is mounted  on  the  steering  gear, is
          shown  in  Figure  12.5 (a) and  (b). Movement of  the  bridge  transmitter
          results in electrical imbalance and current flow to the motor. The  motor
          drives, through  a  flexible  coupling, a  screw shaft,  causing it to turn. A
          screw  block on  the  shaft  is moved  and  this in turn  moves the  floating
          lever  to  which  a control  rod  is attached.  The  control  rod  operates  the
          slipper ring or  swash plate of the variable delivery pump. A cut-off lever
          connected to the  moving tiller will bring the floating lever pivot and  the
          lever  into line  at  right  angles to  the  screw shaft  axis. At this point  the
          rudder  angle will match the  bridge  lever angle and  the pumping action
          will  stop.  The  rotating  screw  shaft  will  have  corrected  the  electrical
          imbalance  and  the  motor  will  stop.  For  local  manual  control,  the
          electrical.control  is switched off  and  a  small handwheel is connected  to
          the  screw shaft.  A detent pin  holds the  handwheel assembly clear when
          not  in  use. Rotation of  the  handwheel will  move the  floating  lever  and
          bring about rudder  movement  as already described.



         Power units

         Two types of hydraulically powered  transmission units or steering gear
         are  in common use, the  ram and  the  rotary  vane.


         Ram type
         Two  particular  variations,  depending  upon  torque  requirements,  are
         possible  the  two-ram and  the four-ram.  A  two-ram steering  gear  is shown
         in  Figure  12.6.
           The  rams acting in hydraulic cylinders operate the tiller by means of a
         swivel crosshead carried in a fork of the rams. A variable delivery pump
         is mounted  on each cylinder and  the slipper ring is linked by rods to the
         control spindle of the telemotor receiver. The  variable delivery pump is
         piped  to  each  cylinder  to  enable  suction  or  discharge  from  either. A
         replenishing  tank  is  mounted  nearby  and  arranged  with  non-return
         suction valves which automatically provide  make-up fluid  to the  pumps.
         A bypass valve is combined with spring-loaded shock valves which open
         in the event of a very heavy sea forcing the rudder over.  In moving over,
         the pump is actuated  and  the steering  gear will return  the  rudder  to its
         original position  once the  heavy sea has passed.  A spring-loaded  return
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