COMPONENT OPERATION / Travel Device Counterbalance Valve, Check Valve While Traveling: 1. When pressure oil from the control valve flows to port P1, pressure oil flows unseats the check valve, flows to port BM and the travel motor. 2. On the other hand, return oil from the travel mo- tor is routed to port AM. However, return oil is blocked by the spool in counterbalance valve. 3. When pressure in port P1 increases further, pressure oil from port P1 pushes to open the check valve in spool through oil passage F in spool, and flows to chamber G. Pressure oil from port P1 flows to chamber G through the orifice and orifice A in the spool. 4. Therefore, the spool moves to the lower.
At this time, pressure oil from port P1 flows to port A and is supplied to the parking brake, so that the park- ing brake is released. 5. Return oil from the travel motor flows to port P2 through notch H on the spool. Then, pressure oil is allowed to flow so that the travel motor starts rotating. 6. When the travel lever is returned to neutral, oil in ports P1 and P2 flows to the hydraulic oil tank through the control valve. 7. Oil in chamber G flows to port P1 through the ori- fice and pressure in chamber G is reduced. 8. Therefore, the spool is returned to the original position by spring force and blocks the oil pas- sage, so that the travel motor rotation is stopped. While Descending: 1.
While descending a slope, the travel motor is forcibly rotated by the machine weight so that the motor operates like a pump. 2. If the travel motor draws oil, oil pressure in port P1 and chamber G decrease and the spool moves upward, so that return oil from the travel motor is restricted. Therefore, pressure in port AM increases and brakes the travel motor. 3. Once return oil from the travel motor is restricted, pressure in port P1 increases again and moves the spool downward. 4. As this operation (hydraulic braking operation) is repeated, the machine is prevented from running away. T3-5-6