H  Y  D  R  A  U  L  I  C   S  Y  S  T  E M TM   5-3805-258-24-1 S  Y  S  T  E  M  S   O  P  E  R  A  T  I  O N tion. The pilot oil at the left end of the lift valve spool goes back to lift stem (7) and on to hydraulic tank (18). The implement oil in lift cylinders (21) is held by lift valve  spool  (22)  and  movement  of  the  cylinder  pistons and rods stops. The lift arms and bucket are held up until the lift control lever is moved again. LOWER  Operation When  the  lift  control  lever  is  moved  to  LOWER position, lower stem (6) moves to LOWER position. There  is  no  detent  for  this  position.  Pilot  oil  is  now free  to  go  through  lift  and  tilt  pilot  valve  (5)  to  the right end of lift valve spool (22). The lift valve spool moves to the left. The pilot oil at the left end of lift valve spool (22) goes back to lift stem (7) and on to hydraulic  tank  (18). The  movement  of  lift  valve  spool  (22)  stops  the flow  of  oil  through  lift  and  tilt  control  valve  (26). Implement  pump  pressure  increases  and  opens  load check  valve  (25).  The  oil  from  the  implement  pump section (27) now goes to the rod end of lift cylinders (21)   and   causes   the   cylinder   pistons   and   rods   to retract.  The  lift  arms  cause  the  bucket  to  lower.  The movement of the cylinder pistons and rods pushes the oil out of the head end of the lift cylinders. This oil comes  into  lift  and  tilt  control  valve  (26)  and  goes around  lift  valve  spool  (22)  on  its  way  back  to  oil filters  (29)  and  hydraulic  tank  (18). When  the  lift  control  lever  is  released,  the  lever and  lower  stem  (6)  go  back  to  HOLD  position.  The flow of pilot oil through lift and tilt pilot valve (5) is stopped. The springs on lift valve spool (22) move the valve  spool  back  to  HOLD  position.  The  pilot  oil  at the right end of the lift valve spool goes back to lower stem  (6)  and  on  to  hydraulic  tank  (18).  The  imple- ment oil in lift cylinders (21) is held by lift valve spool (22)  and  movement  of  the  cylinder  pistons  and  rods stops. The bucket is held in the lowered position until the  lift  control  lever  is  moved  again. FLOAT Operation When  the  lift  control  lever  is  moved  to  FLOAT detent   position,   lower   stem   (6)   moves   to   FLOAT position. The flow of pilot oil to lift valve spool (22) is the same as in LOWER position. The only difference is the vent passage in lift and pilot valve (5) is opened to  hydraulic  tank  (18)  by  the  movement  of  lower stem  (6).  This  permits  make-up  valve  (24)  to  open and  lets  implement  oil  go  directly  to  hydraulic  tank (18). The head end and rod end of lift cylinders (21) are also open to the hydraulic tank. Outside forces on the bucket will control the movement of the cylinder pistons  and  rods. The lift control lever must be manually moved out of  the  FLOAT  detent  position.  There  is  no  kickout for  the  FLOAT  position. LOWER Operation With Engine Off The bucket can be lowered with the engine off. The pilot  oil  supply  comes  from  the  head  end  of  the  lift cylinders.  This  oil  goes  through  check  valve  (14)  to selector  and  pressure  control  valve  (10)  which  de- creases the pressure of the oil to approximately 1030 kPa  (150  psi).  The  oil  then  goes  to  lift  and  tilt  pilot valve  (5).  When  the  lift  control  lever  is  moved  to LOWER  and  on  to  FLOAT  detent  position,  lower stem (6) moves to FLOAT position. Pilot oil goes to lift  valve  spool  (22)  and  moves  it  to  LOWER  posi- tion. The head  end of lift cylinders (21) is now  open to hydraulic tank (18). Make-up valve (24) is vented so it  opens  and  the  rod  end  of  lift  cylinders  (21)  is open  to  hydraulic  tank  (18)  also.  The  weight  of  the bucket and lift arms causes the bucket to lower to the ground. NOTE:  When  the  engine  is  off  and  the  lift  arms  are raised,  the  bucket  can  also  be  dumped. ATTACHMENT   (THIRD)   CIRCUIT CLOSE  or  LOWER  Operation When   the   engine   is   running   with   the   control levers  in  HOLD  position,  pilot  pump  section  (19) takes  oil  from  hydraulic  tank  (18)  and  sends  it  first to  pilot  section  relief  valve  (17)  which  controls  the maximum   pressure   in   the   pilot   system.   The   pilot oil  flow  then  divides.  Some  oil  goes  through  line (16) to the hand metering unit of the steering system. Some oil goes through check valve (15) and selector and  pressure  control  valve  (10)  to  lift  and  tilt  pilot valve  (5).  The  pilot  oil  then  goes  on  to  attachment (third)  pilot  valve  (33).  The  valve  stems  in  the  pilot valves  stop  the  flow  of  oil  in  HOLD  position.  The oil  from  pilot  pump  section  (19)  goes  through  pilot section  relief  valve  (17)  through  oil  filters  (29)  and back  into  hydraulic  tank  (18). At  the  same  time,  steering  pump  section  (20) sends  oil  through  line  (23)  to  the  steering  control valve  of  the  steering  system.  Also,  implement  pump section (27) sends oil to main relief valve (4) which controls   the   maximum   pressure   in   the   implement oil  system.  The  oil  then  goes  through  attachment (three   spool)   control   valve   (38)   because   all   three valve  spools  are  in  HOLD  position.  The  oil  then goes  through  oil  filters  (29)  and  back  into  hydraulic tank ( 18). If oil filters (29) get full of foreign material (debris),  filter  bypass  valve  (28)  will  open  and  let the  return  oil  go  directly  into  the  hydraulic  tank. 3-113