374F Excavator Hydraulic System Displacement Change Valve Caterpillar

Small Displacement Change Operation (HIGH SPEED MODE)

Illustration 1	g02266814
Small displacement change operation (1) Piston chamber (2) Piston (3) Swashplate (4) Barrel (5) Port (supply oil or return oil) (6) Port (supply oil or return oil) (7) Passage (8) Displacement change valve (9) Port (pilot system oil pressure) (10) Passage (11) Shuttle valve

When the travel speed control switch on the keypad is pressed and the rabbit appears on the display, the machine is in HIGH SPEED MODE. In this condition, an input signal from the travel speed control switch is sent to the machine ECM. The machine ECM energizes the travel speed solenoid in the pilot manifold. Pilot pressure flows to port (9) of the travel motor. Pilot oil shifts displacement change valve (8). System pressure from port (5) or port (6) flows through shuttle valves (11) and passage (10) to displacement change valve (8). System pressure flows through displacement change valve (8) and passage (7) to piston chamber (1). System pressure in piston chamber (1) shifts piston (2) to the right. Piston (2) forces swashplate (3) to the minimum angle position. As a result, the angle of swashplate (3) is decreased and the motor displacement is decreased. Barrel (4) must rotate faster to displace the same amount of oil during HIGH SPEED MODE. The travel speed increases.

The pressure sensor for the pump delivery also provides an input signal to the machine ECM. When the travel load is light and when the pump delivery pressure is below a certain level, no signal is sent to the machine ECM. The machine ECM energizes the travel speed solenoid. The travel motor continues to be in HIGH SPEED MODE.

Large Displacement Change Operation (LOW SPEED MODE)

Illustration 2	g02266833
Large displacement change operation (1) Piston chamber (2) Piston (3) Swashplate (4) Barrel (5) Port (supply oil or return oil) (6) Port (supply oil or return oil) (7) Passage (8) Displacement change valve (9) Port (pilot system oil pressure) (10) Passage (11) Shuttle valve (12) Passage

When the travel speed control switch on the keypad is pressed and the tortoise appears on the display, the machine is in LOW SPEED MODE. In this condition, an input signal from the travel speed control switch is sent to the machine ECM. The travel speed solenoid in the pilot manifold is de-energized. Pilot oil in port (9) flows to the return hydraulic system. Displacement change valve (8) is not shifted. Oil in piston chamber (1) flows through passage (7) and displacement change valve (8) to the return hydraulic system. System pressure from port (5) or port (6) is blocked from flowing to passage (7) by displacement change valve (8). Swashplate (3) forces piston (2) back against the motor housing. As a result, the angle of swashplate (3) increases and the motor displacement increases. Barrel (4) rotates slower to displace the same amount of oil during LOW SPEED MODE. The travel speed decreases.

When the angle of swashplate (3) in the travel motor increases, the displacement of the travel motor increases.

The angle of swashplate (3) in the travel motor will increase and the travel speed will decrease during the following two conditions.

1. An increase in pump pressure occurs.

   1. When the machine is in HIGH SPEED MODE and the pump delivery pressure increases above a certain level, the machine ECM de-energizes the travel speed solenoid. When the travel speed solenoid is de-energized, pilot system oil stops flowing into pilot port (9) of displacement change valve (8). Main pump oil flow from port (5) or port (6) and shuttle valve (11) is blocked by displacement change valve (8).

   2. The force of swashplate (3) moves piston (2) to the left. The oil in piston chamber (1) now flows through passages (7) and (12) to the hydraulic tank. The angle of swashplate (3) is increased. The displacement of the travel motor increases and the travel speed decreases.

2. The travel speed control switch is pressed in order to obtain LOW SPEED MODE.

   1. When the travel speed control switch on the keypad is pressed and the tortoise appears on the display, the machine is in LOW SPEED MODE. In this condition, an input signal from the travel speed control switch is sent to the machine ECM. The machine ECM de-energizes the travel speed solenoid. The angle of swashplate (3) increases and the displacement of the travel motor increases . The travel speed decreases.