320C and 322C Excavators Attachment Hydraulic System – Control Valve (Boom Lowering)

Note: Refer to Systems Operation, RENR 3816, “320C Excavator Hydraulic System”, “Main Control Valve – Individual Valve Operation” for an explanation of the operation of the boom control valve.




Illustration 1 g00723961

(1) Drain to tank

(2) Pilot line for boom lower

(3) Return to tank

(4) Boom cylinder

(5) Valve

(6) Relief valve

(7) Check valve

(8) Valve for manual boom lowering

(9) Directional valve

(10) Check valve

(11) Supply line to head end

(12) Boom down valve

(13) Boom up valve

(14) Boom II control valve

(15) Boom I control valve

(16) Supply line to rod end

(17) Valve group

(18) Boom regeneration valve

(19) Relief valve

(20) Pilot supply line

(21) Pilot line for boom raise

(22) Pilot line for stick in

(23) Pilot pump

(24) Hydraulic activation control valve

(25) Oil supply line for control

(26) Driven pump

(27) Drive pump

(28) Engine

(29) Oil supply line for control

(30) Relief valve

(31) Boom lowering control valve

The boom lowering control valve has two functions:

  • A check valve that is mounted on the boom cylinder prevents the boom from falling if a hydraulic supply line or a supply tube to the boom cylinder fails.

  • A manual valve allows the operator to manually lower the boom when the engine is shut down.

Boom Raise Operation

When the boom pilot control (13) is moved to the BOOM RAISE position, valve (13) is activated. Pilot supply oil flows from hydraulic activation control valve (24) and line (20) through line (21). The spool in boom control valve (14) is shifted. The spool in boom control valve (15) is shifted. Pump oil flows from lines (25) and (29) through boom control valves to line (11). Main pump oil from line (11) flows into the boom lowering control valves (31). The main pump oil passes through check valves (10) and into the head end of the boom cylinders (4) .

The oil in the rod end of the boom cylinders flows through line (16) and back to the hydraulic tank.

Boom cylinders are operated simultaneously. When the joystick is moved to the NEUTRAL position, boom cylinders are held in the raised position. Check valves (10) and (7) prevent the oil in the head end of the boom cylinders from flowing out of the boom cylinders and returning to tank.

Boom Lower Operation

When the boom pilot control is moved to the BOOM LOWER position, valve (12) is activated. Pilot supply oil flows from hydraulic activation control valve (24) and line (20). Pilot oil pressure from line (2) shifts the spool in boom control valve (15). Also, pilot oil pressure from line (2) shifts the spool in boom regeneration valve (18). Main pump oil flows from line (29) through boom control valve (15) and to line (16). Pump oil from line (16) flows into the rod end of boom cylinders (4) .

The oil in the head end of the boom cylinders flows into boom lowering control valves. Directional valve (9) is shifted when the boom lower operation is initiated. If directional valve (9) is not shifted, oil from the head end cannot return to the tank and the boom cannot lower. If directional valve (9) is shifted, oil can leave the head end and flow into valve (5). Valve (5) divides the oil to directional valve (9). If the boom is lowered quickly, the higher pressure oil shifts the piston in valve (5). More oil returns to the tank through control valve (15). If the boom is lowered slowly, the lower pressure oil flow does not shift the piston in valve (5) very much. Oil returns to the tank more slowly through the restrictions in valve (5) and in directional valve (9) .

Relief Operation of the Cylinder Head End

When the boom is pushed downward by an outside force the oil pressure in the head end of the boom cylinder increases. The oil pressure is sensed at relief valve (6) and at relief valve (30). When the relief valves on both cylinders open the oil flows back to the hydraulic tank.

Manual Boom Lower Operation

This function is used to manually lower the boom when the engine is shut down and when no pilot oil pressure is supplied to the pilot circuit for boom lowering.

Oil flows from the head end of the cylinder through the manual valves for boom lowering (8). The oil flows through line (3) to the hydraulic tank.