308C CR Excavator Hydraulic System – Control Valve (Straight Travel)

Straight travel (tracking) can be maintained even though there is a swing operation or implement operation during travel.

Straight Travel Operation

Straight travel control valve (1) maintains straight travel even though there is a swing operation or implement operation during travel.

Straight travel control valve (1) also improves the control of operations such as pipe laying or placement of timbers. When the machine travels without swing operation or implement operation, oil from right pump (8) flows through line (7), straight travel control valve (1) and right travel control valve (4) to the right travel motor. Oil from left pump (9) flows through line (6) and left travel control valve (3) to the left travel motor. Because both travel circuits are separated, the machine continues to travel straight, unless a difference in travel resistance occurs between the right and left tracks.

The straight travel system ensures the straight travel of the machine when other circuits are operated during travel. The right pump and the left pump supply oil to the travel motors. The right and the left pump also supply oil for a swing or implement operation. During travel, if an implement operation or a swing operation occurs the oil supply to each travel motor will differ. This would cause the right travel motor and the left travel motor to rotate at different speeds. The different speeds will cause the machine to turn.

The following actions occur when pilot oil pressure activates straight travel control valve (1) .

• Left pump (9) supplies oil to the left travel circuit and to the right travel circuit in order to drive both motors in parallel.

• The swing circuit and implement circuits get oil from right pump (8). When the machine is travelling, the swing circuit and implement circuits do not require a large amount of flow. The swing circuit and implement circuits are operated at speeds that are low enough for stable machine operation. The remainder of the oil is shared by the right travel circuit and the left travel circuit.

Implement, Swing And Travel Operation

When the travel control is activated with either the implement control or the swing control, pilot pressure in pilot passage (1) increases. The increased oil pressure (1) shifts stem (2) to the right against the force of spring (11). Pump oil now flows in the following manner.

• Oil from left pump (14) flows from port (7) to center bypass passage (8) and the oil separates into two flow paths. One path flows through center bypass passage (8) and into the left travel control valve. The other path flows through center bypass passage (10) and into the right travel control valve. It is possible to keep straight travel, because only left pump (14) supplies oil to both travel motors.

• Oil from right pump (13) flows from port (9) to parallel feeder passage (12). The right pump oil separates into three paths. One path flows through parallel feeder passage (12) and into the swing control valve and the stick I control valve. The second path flows through parallel feeder passage (4). Oil flows from parallel feeder passage (4) into the boom I control valve and the bucket control valve. The third path flows through orifice (3) and check valve (6) to center bypass passage (10). In center bypass passage (10), the right pump oil combines with the left pump oil. This will reduce shock load when you activate the straight travel valve.