The Yishan TY220 bulldozer found during on-site operations that the bulldozer does not turn when pulling the steering rod on either side, but if the left and right steering rods are pulled at the same time, the bulldozer can still move forward in a straight line. The preliminary judgment is that the steering system is malfunctioning.

The steering system of this type of bulldozer adopts a fully mechanical control mechanism. Check that the pull rods and springs are all connected intact, and the gap between the rods is also appropriate. It is determined that the fault lies in the steering hydraulic circuit.

The steering pump is connected with the transfer case to convert the mechanical energy into hydraulic energy, even if the oil is sucked from the rear axle box and filtered by the coarse filter and the fine filter and enters the diverter valve, the diverter valve distributes the liquid flow in a ratio of 3:1 To the steering valve and brake circuit. The working pressure of the steering valve is 1.25 MPa, which is guaranteed by the sequence valve. The oil overflowing from the sequence valve enters the transmission circuit and merges with the oil in the transmission circuit, and then enters the torque converter. Under normal circumstances, when the steering handle is not operated, the pressure oil flowing from the diverter valve to the left or right steering valve is enclosed in the steering valve and does not supply oil to the steering clutch. At this time, the steering clutch is in an engaged state. There is a small hole on the left and right steering valves, and a small amount of oil is continuously added to the left and right steering clutches through this small hole, so that the working chamber of the clutch is always filled with oil. Once the steering valve is manipulated, the clutch is immediately disengaged, improving sensitivity. Operate the two steering valves separately to separate the left or right clutch and realize left or right steering; if the two steering valves are simultaneously operated to separate the left and right clutches at the same time, the bulldozer will stop.

When troubleshooting, the oil pressure measured at the fine filter is 2.3 MPa, indicating that there is no problem with the steering pump, and it is sufficient to provide 1.25 MPa of pressure oil to the steering hydraulic circuit. In addition, since the brake and steering hydraulic oil circuits of this machine are all supplied with hydraulic pressure by the steering pump, the braking effect is very good, which also shows that the steering pump is intact.

Then check the combination of the diverter valve, sequence valve and steering valve, that is, remove the upper cover of the rear axle box, disassemble each valve, and check the spool of each valve. At this time, it was found that the spool of the sequence valve was stuck by debris. When the spool was taken out, it was found that the piston in the spool was also stuck by debris. It can be seen that the sequence valve is always in an overflow state at this time. At this time, the oil pressure of the steering circuit is too low. When the steering valve is operated, the oil pressure of the steering circuit cannot overcome the elastic force of the steering clutch spring. Therefore, the steering clutch cannot be separated, so the bulldozer cannot steer. After cleaning the sequence valve and spool, a retest machine is installed, the bulldozer turns flexibly, and the fault is eliminated.