Thus  OK  is  the  radius  of  the  equivalent  eccentric  for  position  K  of
the  block  in  the  link,  assuming  EF  to  be  the  actual  link  to  a  reduced
scale.  If  valve  circles  be  drawn  on  OE,  OK,  and  O6,  and  the  lap
circle  be  added,  it  will  be  seen  that  as  the  link  approaches  mid-
position —  (1)  the  valve-travel  and  the  port  opening  rapidly  decrease
(compare  OK  in  Figs.  86  and  88)  ;  (2)  the  lead  decreases  ;  and  (3)
the  gear  may  be  designed  so  as  to  give  no  port  opening  when  the
link  is  in  mid-position.
In  setting  a  link  motion,  the  port  opening  or  lead  is  usually  set  to
be  the  same  at  both  ends  of  the  stroke  when  the  link  is  in  mid-gear.
The  Meyer  Expansion  Valve  Gear. — This  gear,  illustrated  in  Figs.
89  and  90,  consists  of  two  plates  sliding  on  the  back  of  a  main  valve


FIG.  89.
as  shown.  The  degree  of  expansion  is  regulated  by  varying  the
distance  apart  of  the  two  plates  as  required,  by  means  of  a  right  and
left  handed  screw.
In  Fig.  90,  MV  is  the  main  or  distributing  valve,  which  is  an
ordinary  simple  slide-valve,  with  the  addition  of  pieces  at  the  ends
to  form  ports,  p,  through  the  valve.  EV  are  the  expansion  valve
plates.  Consider  the  expansion  valve  EV  moving  to  the  left ;
then  when  e  reaches  n  cut-off  takes  place,  and  the  port  in  the  main
valve  MV  is  closed,  till  on  the  return  stroke  e  again  moves  to  the
right  of  n.  Evidently,  by  increasing  the  distance  between  the  two
plates  of  the  expansion  valve,  the  distance  8  between  e  and  n — when
both  valves  are  in  mid-position — is  decreased,  and  cut-off'  takes  place
earlier.
