TM 10-3930-671-24 SECTION  6 ACCESSORIES FLEXIBLE GOVERNOR DRIVE An optional flexible retaining ring is available in the Model DB2 pump.    This  ring  serves  as  a  cushion  between  the governor weight retainer and the weight retainer hub.  It is  designed  to  absorb  torsional  loading  and  vibrations transmitted to the pump from the engine as well as those created within the pump itself.  In doing  so,  it  allows  for smoother  governor  operation  under  all  speed  and  load conditions  and  reduces  wear  of  the  pump  drive  shaft tang and governor parts. ELECTRICAL SHUTOFF Electrical  shutoff  devices  are  available  as  an  option  in both  Energized  to  Run  (ETR)  and  Energized  to  Shutoff (ETSO) models for the DB2 pump.  These solenoids are included  in  various  applications  to  control  the  run  and stop  functions  of  the  engine.    They  accomplish  this  by positively   stopping   fuel   flow   to   the   plungers,   thereby interrupting injection. TORQUE SCREW Torque  is  commonly  defined  as  the  turning  moment  or "lugging ability" of an engine.  Maximum torque varies at each speed in the operating range for two reasons:   (1) as  engine  speed  increases,  friction  loses  progressively increase  and,  (2)  combustion  chamber  efficiency  drops due to loss of volumetric efficiency (breathing ability of an engine),   and   due   to   reduction   of   time   necessary   to completely   and   cleanly   bum   the   fuel   In   the   cylinder. Since the torque increases with increased load conditions,   a   predetermined   point   at   which   maximum torque is desired may be selected for any engine.  Thus, as     engine     RPM decreases, the torque generally increases  toward  this  preselected  point.    This  desirable feature is called ’Torque Back-Up." In the Roosa Master pump, three basic factors affect Torque Back-Up. These are: 1. Metering valve opening area. 2. Time allowed for charging. 3. Transfer pump pressure curve. Of these, the only control between engines for purposes of   establishing   a   desired   torque   curve   is   the   transfer pump pressure curve and metering valve opening, since the  other  factors  involved  are  common  to  all  engines. Torque  control  in  Roosa  Master  fuel  injection  pumps  is accomplished in the following manner: The    manufacturer    determines    at    what    speed    for    a specific  application  he  wants  his  engine  to  develop  its maximum   torque.      The   maximum   fuel   setting   is   then adjusted  for  required  delivery  during  dynamometer  test. This delivery must provide acceptable fuel economy. The engine is then brought to full load governed  speed.  The fuel delivery is then reduced from that determined by the maximum   fuel   setting   by   turning   in   an   adjustment   or ’torque  screw’  (Figure  6.0),  which  moves  the  metering valve  toward  the  closed  position.    The  engine  Is  now running at full load governed speed.  When the engine Is operating at high idle speed, no-load, the quantity of fuel delivered  Is  controlled  only  by  governor  action  through the  metering  valve.    At  this  point,  the  torque  screw  and maximum  fuel  adjustment  have  no  effect.    As  load  is applied, the quantity of fuel delivered is controlled only by governor action and metering valve position until full-load governed    speed    is    reached.        At    this    point,    further opening of the metering valve is prevented by its contact with  the  previously  adjusted  torque  screw.    Thus,  the amount  of  fuel  delivered  at  full-load  governed  speed  is controlled  by  the  torque  screw  and  not  by  the  roller-to- roller   dimension.      As   additional   load   Is   applied   and engine   RPM   decreases,   a   greater   quantity   of   fuel   is allowed  to  pass  into  the  pumping  chamber  due  to  the increased   time   of   registration   of   the   charging   ports. During    this    phase    of    operation,    the    metering    valve position remains unchanged, still being held from further rotation by the torque screw.  As engine RPM continues to  decrease  under  increasing  load,  the  rotor   charging ports remain in registry for a longer time period allowing a larger quantity of fuel into the pumping chamber.  Fuel delivery    increases    until    the    predetermined    point    of maximum torque is reached. At this point, the quantity of fuel is controlled by the roller- to-roller dimension.  It must be remembered  that  torque adjustment on the Roosa Master pump may be properly carried  out  only  during  dynamometer  or  bench  test.    It should  not  be  attempted  on  a  unit  in  the  field  without means of determining actual fuel delivery. Figure 6.0 F-248