TM10-3930-660-34 If working pressure in the boom lift circuit exceeds 2800 psi (19306 kPa), the relief valve in the main valve opens allowing overflow to the return circuit until the pressure decreases below the relief setting.  The boom lower circuit relief functions the same way but the port relief valve is set at 950 psi (6206 kPa). A  port  relief  in  the  main  valve  protects  the  boom  hoist  circuit  from  hydraulic  shocks  or  spikes  when  the  counterbalance relief  setting  is  exceeded  and  main  valve  spools  are  closed.    When  a  spike  is  sensed  at  the  port  relief  valve,  the  relief valve  is  piloted  open  and  allows  the  excess  pressure  to  be  relieved  to  the  return  line.    As  soon  as  the  pressure  drops below the port relief valve setting, the valve will close.  The port relief valve is set at 3000 psi (20685 kPa). d. Boom Extend System.  The boom extend 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 to the right, pilot pressure oil is allowed to flow into the bottom center pilot port of the main valve.  The pilot pressure oil pushes the center section (extend) spool up, allowing oil to flow through the main valve, out the work port and to the right (head end) port of the boom extend cylinder. Oil  flows  to  the  counterbalance  valves  of  the  extend  cylinder  similar  to  the  frame  tilt  cylinder  and  extends  the  cylinder. From the rod end of the cylinder, oil is routed back to the main valve work port and to the reservoir through the hydraulic filter. With  the  hydraulic  joystick  in  the  center  position,  the  pilot  oil  flow  is  blocked,  as  in  the  boom  hoist  system.    Centering springs keep the extend spool of the main control valve in the center position. As the hydraulic joystick is moved to the left, pilot pressure oil is allowed to flow into the top center pilot port of the main valve.  The pilot oil pressure pushes the center section (extend) spool down, allowing oil to flow through the main valve, out the work port and to the left (rod end) port of the boom extend cylinder. Oil  flows  to  the  counterbalance  valves  of  the  extend  cylinder  similar  to  the  frame  tilt  cylinder  and  retracts  the  cylinder. From the head end of the cylinder, oil is routed back to the main valve work port and to the reservoir through the hydraulic filter. If  working  pressure  in  the  boom  extend  circuit  exceeds  2800  psi  (19306  kPa),  the  relief  valve  in  the  main  valve  opens allowing overflow to the return circuit until the pressure decreases below the relief setting. A port relief set at 3000 psi (20685 kPa) protects the boom extend function from hydraulic shocks or spikes, similar to the boom hoist function. 16-6