320C Excavator Hydraulic System – Relief Valve (Main)



Illustration 1 g03424088

Cross section of straight travel control valve and main relief valve

(1) Straight travel control valve

(2) Main control valve

(3) Right travel control valve

(4) Check valve

(5) Check valve

(6) Main relief valve

(7) Delivery line (idler pump)

(8) Delivery line (drive pump)

(9) Idler pump

(10) Drive pump

(11) Internal passage

Main relief valve (6) is located in the left side of the main control valve. The main relief valve limits the maximum operating pressure of the travel hydraulic circuit and the implement hydraulic circuits when the machine is at a load condition.

The oil delivery from drive pump (10) enters main control valve (2) through delivery line (8). The oil delivery from idler pump (9) enters main control valve (2) through delivery line (7). Drive pump oil and idler pump oil flows through check valves (4) and (5) to passage (11). Check valves (4) and (5) ensure that only the higher oil pressure from the drive pump or the idler pump flows through passage (11) to main relief valve (6) .

Closed Position



Illustration 2 g01332464

Main relief valve (closed position)

(11) Passage

(12) Seat

(13) Valve

(14) Spring

(15) Spring chamber

(16) Seat

(17) Poppet

(18) Spring

(19) Passage

(20) Orifice

(21) Return passage

Poppet (17) is positioned to the left against seat (16) by the force of spring (18). Valve (13) is positioned to the left against seat (12) by the force of spring (14) .

System pressure oil in passage (11) flows through orifice (20) into spring chamber (15). The force of the system pressure oil acts on poppet (17). When the force of system pressure oil in passage (11) is less than the force of spring (18), poppet (17) remains against seat (16). The pressure in passage (11) and the pressure in spring chamber (15) are now equal. System pressure oil in spring chamber (15) and the force of spring (14) maintain valve (13) against seat (12). There is no oil flow from passage (11) to return passage (21). When main pump oil pressure in passage (11) is less than the main relief pressure setting, main relief valve (6) remains in the closed position.

Open Position



Illustration 3 g01332615

Main relief valve (open position)

(11) Passage

(12) Seat

(13) Valve

(14) Spring

(15) Spring chamber

(16) Seat

(17) Poppet

(18) Spring

(19) Passage

(20) Orifice

(21) Return passage

(22) Passage

(23) Valve chamber

(24) Locknut

(25) Adjustment screw

System oil pressure in passage (11) and spring chamber (15) nears the main relief valve pressure setting. The force of the system oil pressure in spring chamber (15) becomes greater than the force of spring (18). Poppet (17) away from seat (16). System oil pressure now flows through seat (16) into valve chamber (23). The oil in valve chamber (23) now flows through passage (22) into return passage (21). This low-pressure oil now returns to the hydraulic tank.

At the same time as the oil in spring chamber (15) flows through seat (16), the system pressure oil in passage (11) flows through orifice (20). As the system oil pressure flows through orifice (20) into spring chamber (15) the pressure of the oil in spring chamber (15) decreases. The reduced pressure oil in spring chamber (15) allows the high-pressure oil in passage (11) to force valve (13) away from seat (12). The high-pressure oil in passage (11) now flows into passage (19) and return passage (21) to the hydraulic tank.

The amount of spring force of spring (18) that acts on poppet (17) determines the main relief valve pressure setting. Adjustments to the main relief valve pressure setting are made by changing the spring force of spring (18). The position of adjustment screw (25) determines the spring force of spring (18) .

ReferenceRefer to Testing and Adjusting, “Relief Valve (Main) – Test and Adjust” for adjustment procedures.