83,  the  crank  pin  and  eccentrics  have  rotated  through  one-half


176


THE  LOCOMOTIVE  UP  TO  DATE


of  a  revolution.  Therefore,  the  error  due  to  the  angularity  of
the  rods  varies  from  the  smallest  to  the  largest  amount  during
half  a  revolution,  or  while  the  crank  pin  passes  from  one  dead
center  to  the  other.  This  changing  angle  of  the  eccentric  rods
causes  the  link  movement  to  be  variable,  just  as  the  angularity
of  the  main  rod  caused  a  variable  motion  of  the  crosshead.
With  the  crank  pin  on  forward  dead  center,  the  distortion  of
the  link  motion,  due  to  the  angular  motion  of  eccentric  blades,
during  one  complete  revolution,  is  shown  in  Figs.  81  to  84,  which
show  the  relative  positions  of  the  crank  pin  and  eccentric  rods,  in
quarter  turns,  for  one  revolution.
Fig.  82.
Omitting  the  main  rod  errors.  Fig.  81  shows  the  pin  on  forward
dead  center.  The  radius  of  the  arc  of  the  link  is  x  c,  and  the
location  of  the  link  arc  when  its  center  is  on  the  center  of  the
axle,  is  shown  at  c.  The  center  of  the  link,  for  this  position,  is
shown  at  &.  In  Fig.  82,  the  relative  positions  are  shown  when
the  crank  pin  and  eccentrics  have  rotated  through  one  quarter
turn.  Here  the  link  center  has  moved  only  a  slight  amount,
represented  by  t'and  o,  in  the  full  line  diagram.  That  is,  the
link  has  moved  back  from  point  h  to  point  o.    In  the  next  figure.
