TM10-3930-660-34 to the piston pump inlet port and returns to the reservoir through the frame tilt valve. By  moving  the  frame  tilt  valve  lever  forward,  the  cylinder  is  extended  and  tilts  the  machine  to  the  left.    Oil  flows  to  the counterbalance valve for the  head  end  of  the  cylinder,  unseats  the  ball  check,  and  enters  the  cylinder.    Pressure  in  the head  end  begins  to  increase  because  oil  in  the  rod  end  of  the  cylinder  is  trapped;  the  rod  end  counterbalance  valve  is closed and ball is seated.  The pressure in the head end pilots open the counterbalance valve for the rod end.  Oil can now flow from the rod end of the cylinder, allowing the cylinder to extend.  Oil from the rod end of the cylinder continues on to the piston pump inlet port and returns to the reservoir. Moving the frame tilt valve rearward, the cylinder is retracted and tilts the machine to the right.  Oil flows to the rod end of the cylinder and pilots open the head end counterbalance valve.  Oil flows through the tilt cylinder in the opposite direction of the tilt left function.  Oil from the head end is routed to the piston pump inlet port and the reservoir. All counterbalance valves in hydraulic cylinders also serve as a safety device in case of hose burst or power loss.    The counterbalance  valves  require  pressure  to  pilot  them  open  which  will  then  allow  oil  to  flow.    If  pressure  is  removed  the counterbalance valve closes and stops the oil from flowing; the cylinder is “locked up”. c. Boom Hoist System.  The boom hoist function is controlled by the hydraulic joystick and the main valve.  Pilot oil for  the  hydraulic  joystick  is  routed  through  the  shuttle  valve  from  the  piston  pump.    The  pilot  return  oil  is  routed  to  the hydraulic reservoir. As the hydraulic joystick is moved towards the rear, pilot pressure oil is allowed to flow into the left bottom pilot port of the main valve.  The pilot pressure oil pushes the left (boom) spool up, allowing oil to flow through the main valve to the lower work port for the hoist cylinders.  Oil flows to the counterbalance valve in the cylinders, unseats the ball check, and enters the head end of the cylinders.  The oil pressure pushes the pistons up extending the cylinders, thereby raising the boom. Oil  in  the  rod  end  of  the  cylinders  is  routed  through  ball  check  of  the  flow  control,  the  main  valve  and  to  the  reservoir through the hydraulic filter. With the hydraulic joystick in the center position, the pilot oil flow is blocked and each valve in the joystick is connected to the return line to the hydraulic reservoir- The spools of the main control valve are kept in the center position by springs which provides passage for the oil through the valve and back to the reservoir through the hydraulic filter. As the hydraulic joystick is moved forward, pilot pressure oil is allowed to flow into the left top pilot port of the main valve. The pilot pressure oil pushes the left (boom) spool down, allowing oil to flow through the main valve and orifice of the flow control valve to the rod end of the hoist cylinders.  Oil pressure pilots open the counterbalance valve to allow oil in the head end  of  the  hoist  cylinders  to  flow  through  the  main  valve  and  to  the  reservoir  through  the  hydraulic  filter.    The  orifice controls the rate at which the cylinders are retracted.  Boom is lowered at the same rate whether the forks are carrying a load or not. 16-5