Boom Extend System - TM-10-3930-673-340580

TM 10-3930-673-34

16-2. PRINCIPLES OF OPERATION (CONT)

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.

If working pressure in the boom lift circuit exceeds 3050 psi (21029.75 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 3250 psi (22408.75 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 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.

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 3050 psi (21029.75 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 3250 psi (22408.75 kPa) protects the boom extend function from hydraulic shocks or spikes,

similar to the boom hoist function.

e. Attachment System. The attachment function is controlled by the electric joystick and the attachment

control valve. Hydraulic oil flow from the axial piston pump supplies oil pressure and flow for attachment

control valve pilot and function operations. A load sensing line from the attachment control valve to the

piston pump adjusts the pump output. Pilot shuttles in each valve section, allow the pressure of the function

16-4