HYD AR-4 BANK 4745474 - Caterpillar

Accumulator
Accumulator (2) provides the cushion (shock absorber) for the. The accumulator has a sealed piston that moves up and down inside its bore. The chamber above the piston has a charge of dry nitrogen gas. The charge of dry nitrogen gas is put into the accumulator through valve (1). Oil from the pump and the leveling valve comes through outlet (3) into the chamber below the piston. The oil pushes the piston up and causes compression of the dry nitrogen gas.The accumulator keeps a constant supply of oil available to the load cylinder. It also prevents (absorbs) sudden shocks caused by the load cylinder. This gives the tractor a smoother ride.
Right Side Of Hitch
(1) Nitrogen charging valve. (2) Accumulator. (3) Outlet to leveling valve.Cushion-Hitch Hydraulic System
Lockdown Operation
When the engine is started, oil from hydraulic tank (14) goes to hydraulic pump (5) and servo valve (10). When the output of the pump is lower than the setting of the servo valve, the hydraulic pump is at maximum output (displacement). The setting of servo valve (10) is approximately 16 000 kPa (2300 psi).The oil then goes to accumulator bleed valve (9) on the 627E Push-Pull machine. Check valve (4) and spool (8) move to the left. The oil goes through check valve (4) to selector valve (3). The 621E, 623E and 627E machines do not have the accumulator bleed valve (9).When the engine is started, oil from the steering valve supply enters valve (3) at inlet (17). The oil moves piston (2) to the left against the spring force. Whenever the engine is stopped, pressure of the oil on piston (2) is stopped and the spring force pushes the piston back to the right. If valve spool (1) is in CUSHION-HITCH position, the valve spool will automatically move to LOCKDOWN position.As pump oil goes to selector valve (3), pump oil also goes to leveling valve (6) and divides. When the engine is first started, leaf spring (12) has leveling stem (11) in the RAISE (up) position. The oil goes around leveling stem (11) and directional spool (7) to the rod end of load cylinder (15). The weight of the scraper retracts the rod as far as it will go (LOCKDOWN position). The head end of load cylinder (15) is open to hydraulic tank (14) because of the position of the directional spool.The oil from leveling stem (11) also goes to accumulator (13). The oil charges the accumulator to approximately 16 000 kPa (2300 psi). At this time, servo valve (10) causes the swashplate of hydraulic pump (5) to move to its minimum angle. The pump is now at minimum displacement. Pressure in the system is at the maximum.
Cushion-Hitch Hydraulic System (Engine Running, LOCKDOWN Position)
(1) Valve spool. (2) Piston. (3) Selector valve. (4) Check valve. (5) Hydraulic pump. (6) Leveling valve. (7) Directional spool. (8) Spool. (9) Accumulator bleed valve (627E Push-Pull only). (10) Servo valve. (11) Leveling stem. (12). Leaf spring. (13) Accumulator. (14) Hydraulic tank. (15) Load cylinder. (16)