308C CR Excavator Hydraulic System – Displacement Change Valve

Small Displacement Change Operation



Illustration 1 g00875075

Travel motor (partial view)

(1) Swashplate

(2) Piston

(4) Passage

(5) Displacement change valve

(6) Pilot port (automatic speed change)

(7) Shuttle valve

(8) Passage

(9) Port

(10) Port

(11) Spring

(12) Piston

(13) Spring



Illustration 2 g00875076

Small displacement change operation

(1) Swashplate

(2) Piston

(4) Passage

(5) Displacement change valve

(6) Pilot port (automatic speed change)

(7) Shuttle valve

(10) Port

(14) Plug

(15) Line

(16) Spring

(17) Cover

(18) Spool

(19) Passage

(20) Spool Chamber

(21) Passage

(22) Passage

(23) Passage

(24) Pivot

(25) Piston chamber

(26) Passage

(27) Passage

When the travel speed control switch is placed in the HIGH position, the travel speed solenoid valve is energized. If the pump delivery pressure is below a certain level, pilot oil flows from line (15) to pilot port (6) of displacement change valve (5). The oil then flows into spool chamber (20). Spool (18) shifts to the right against the force of spring (16) and pressure at passage (27). Spool (18) shifts against cover (17). Pump oil enters motor port (10) and flows through passage (8), shuttle valve (7), passage (22), passage (19), passage (4) and the oil flows into piston chamber (25). The force of piston (2) overcomes force (F) which is produced by motor piston (12) and spring (13). Swashplate (1) rotates in order to decrease the angle. Swashplate (1) is held at the minimum angle position. Decreasing the angle decreases the displacement.

Large Displacement Change Operation



Illustration 3 g00757236

Large displacement change operation

(1) Swashplate

(2) Piston

(4) Passage

(5) Displacement change valve

(6) Pilot port

(7) Shuttle valve

(10) Port

(14) Plug

(15) Line

(16) Spring

(17) Cover

(18) Spool

(21) Passage

(22) Passage

(23) Passage

(24) Pivot

(25) Piston chamber

(26) Passage

(27) Passage

(28) Passage

(29) Passage

When the travel speed control switch is placed in the LOW position, the travel speed solenoid valve is not energized. Pilot oil does not flow to port (6) of displacement change valve (5). Spool (18) is shifted to the left by spring (16) and the pressure at passage (27). Spool (18) shifts to the left against plug (14). Passage (23) closes and passage (29) opens. Force (F), which is produced by piston (12) and spring (13), rotates swashplate (1) .

Swashplate (1) rotates in order to increase the angle with pivot (24) as the axis. The oil in piston chamber (25) goes through passage (4), passage (29) and passage (28). The oil then flows back to the hydraulic tank. Swashplate (1) is held at the maximum angle position. Increasing the angle increases the displacement.

Automatic Change From Small Displacement To Large Displacement



Illustration 4 g00757236

Large displacement change operation

(1) Swashplate

(2) Piston

(4) Passage

(5) Displacement change valve

(6) Pilot port

(7) Shuttle valve

(10) Port

(14) Plug

(15) Line

(16) Spring

(17) Cover

(18) Spool

(21) Passage

(22) Passage

(23) Passage

(24) Pivot

(25) Piston chamber

(26) Passage

(27) Passage

(28) Passage

(29) Passage

When the load pressure in passage (28) increases to approximately 24500 kPa (3550 psi), the combined forces of this pressure and spring (16) overcomes the pilot pressure from port (6). Spool (18) is shifted to the left. Pump oil in passage (4) flows through passage (28) to the motor case drain line. Swashplate (1) rotates to the maximum angle for a large displacement.

When the load pressure in passage (27) decreases to approximately 18600 kPa (2700 psi), swashplate (1) rotates to the minimum angle for a small displacement.

When the travel speed control switch is placed in the HIGH position and pilot oil is supplied to port (6), the machine will automatically change the travel speed to HIGH position or LOW position. This travel speed position is determined by the load that is placed on the machine.