TM10-3930-660-34 CHAPTER 16 HYDRAULIC SYSTEMI AINTENANCE Section I.  DESCRIPTION AND DATA 16-1.  GENERAL. Hydraulic system maintenance procedures not covered in this chapter can be found in TM10-3930-660-20. 16-2.  PRINCIPLES OF OPERATION. a. System  Description.    The  hydraulic  system  of  the  forklift  supplies  a  controlled  flow  of  filtered  hydraulic  oil  for operation  of  the  boom,  carriage,  forks,  and  frame  tilt  functions.    Directions  used  in  the  system  descriptions  are  as  the reader is sitting in the operator’s seat. Principle components of the hydraulic system and how the components are connected are shown on pages 16-2 and 16-3. A  two  section  gear  pump  draws  hydraulic  oil  from  the  hydraulic  reservoir  through  the  suction  strainers  whenever  the engine is operating.  The small section (1.48 cu.  in.) of the gear pump supplies oil for the brake system, frame tilt function , and charge pressure oil for the axial-piston pump. From the small section of the gear pump, oil flows past the relief valve.  If system pressure exceeds 1750 psi (12066 kPa), oil pressure unseats the relief valve and allows excess oil to flow through the hydraulic filter back to the reservoir. Past the relief valve, oil flows through the brake valve, through the frame tilt valve and finally to the axial-piston pump inlet port where the oil is used as charge pressure oil.  The remaining oil flows back to the reservoir. The large section (3.94 cu.  in.) of the gear pump supplies oil to the priority valve for the steering system and to the main valve for boom hoist and extend functions.  From the main valve, oil flows back through the hydraulic filter and is returned to the reservoir. Oil  flows  from  the  large  section  of  the  gear  pump  to  the  priority  valve.    The  steering  system  is  first  priority,  receiving  a portion  of  the  total  oil  flow  first.    When  the  steering  system  is  satisfied,  oil  pressure  increases  and  moves  the  spool  to return the total oil flow to the main valve.  A load sensing line from the steering valve to the spring end of the priority spool monitors the pressure in the steering valve, to insure proper oil flow and pressure is provided to the steering valve.  When the steering cylinder bottoms, pressure in the steering valve increases.  The pilot pressure is relieved through the 2500 psi (17236 kPa) relief valve in the priority valve and all the remaining pump flow is directed to the main valve. 16-1