ME12002HYDRAULICSYSTEM

SectionDriveCounterbalancing System(straighttravel)

Straight travel spool (305) allows the machine to continue to be driven straight while performing other operations, for example, swinging the boom while driving. See Figure 3-150.

If driving and not performing any other operations, hydraulic fluid from front pump (27) flows through straight travel spool (305), travel left spool (301L), and into the left travel motor. Hydraulic fluid from rear pump (28) flows through travel right spool (301R) and into the right travel motor. See Figure 3-150.

Unless there are travel resistance differences between the two drive motors, the excavator continues to drive straight because the left and right drive circuits are isolated from each other.

PerformingOtherFunctionsWhileDriving

If operating the excavator using both valves:

1.When driving, only hydraulic fluid from rear pump (28) flows into both travel motors. See Figure 3-150.

1Plug

2Main pressure limiting valve (MR)

3Spool

PLPort pump 1

PRPort pump 2

2.When driving and performing other functions, only hydraulic fluid from front pump (27) flows for all other functions. Flow required for all other functions while driving reduces because these functions operate at a reduced speed to maintain the safety and stability of the machine. See Figure 3-150.

Any remaining hydraulic fluid from front pump (27) flows into both travel motors. See Figure 3150.

If the machine does not have the straight travel function, hydraulic fluid flow supplied into each drive motor from one pump differs from that of the other pump. Because of the difference between flow suppliled, the machine cannot travel straight.

Figure 3-151 Performing Other Functions While Driving

If raising the boom while driving forward, the boom and travel pilot control valves supply pilot pressure into pilot path (4), pilot path (3) and pilot path (2). See Figure 3-151.

Pilot pressure then operates boom 1 (303), travel left (301L) and travel right (301R) spools. While the pilot pressure is shifting boom 1 spool (303), path (11) shuts-off. Pilot path (18) and pilot path (19) pressurize for operating straight travel spool (305). See Figure 3-151.

Pilot path (25) passes through all spools. If any spool other than boom 1 operates, path (17) and path (16) shut-off pilot path (25). Pilot path (18) and pilot path (19) pressurize and operate straight travel spool (305). See Figure 3-151.

When all other spools are in the neutral position, pilot path (18) leads to the tank through pilot path (25) and drain line (32). Pilot path (18) pressure decreases, deactivating straight travel spool (305). See Figure 3-151.