Troubleshooting for TY220 Bulldozer Deviation

The reasons for the occurrence of the deviation fault

(1) The disengagement spring of the right-turn clutch loses its elasticity due to annealing, fatigue or breakage.

(2) The friction plates of the right-hand steering clutch are severely worn (with a wear limit of 3.7mm), or the powder metallurgy material on the friction plates has fallen off;

(3) The sealing ring of the right-hand steering clutch piston is damaged, resulting in insufficient clutch engagement pressure.

(4) The travel of the right brake pedal is adjusted too little (the normal travel is 140 to 150mm), resulting in a too small gap between the brake band and the brake hub.

(5) There are foreign objects between the brake band and the brake hub, causing friction between the brake lining and the outer hub.

Among the above reasons, (1) to (3) cause the right steering clutch to slip, and (4) to (5) cause the brake to exert braking effect even when not in operation.

2. Troubleshooting of deviation faults

Eliminate the deviation fault by using the elimination method, starting from the causes that are easiest to identify. First, check if there are any foreign objects between the brake bands. If there are none, cause (5) has been ruled out. Then, the brake pedal travel and brake band clearance were checked. Both the pedal travel and clearance were normal, and cause (4) was ruled out. Then, the right steering clutch was lifted out for disassembly and inspection. It was found that the friction plates, the friction plate separation spring and the piston oil seal were all in good condition. Thus, all possible causes were ruled out. So, what exactly causes a bulldozer to veer off course?

The suspicious points were mainly concentrated on the newly installed steering valve. After disassembly and inspection, it was found that there was a piece of drill cuttings left during processing in the small piston cavity at the right end of the right steering valve stem (see attached figure). It was precisely because of the negligence of the processing personnel that the mechanical failure occurred.

The steering control valve is shown in the attached figure. The integral valve body 1 is equipped with two valve stems, the right-turning valve stem 2 and the left-turning valve stem 4. Each valve stem has two positions, namely "closed" and "closed". When not in operation, the valve stem is stopped in the closed position by the return spring 5. The position of the valve stem shown in the illustration is the closed position. The pressure oil from the main relief valve enters the D chamber of the steering valve, and the D chamber is in a closed state.

When the right steering control lever is operated, the linkage mechanism drives the valve stem 2 and the compression spring 5 to move to the right. The pressure oil in chamber D flows through chamber A to the right steering clutch, causing the right steering clutch to disengage, thereby achieving the right turn of the bulldozer. When the right steering control lever is released, the right steering valve stem 2 returns to its original position under the elastic force of spring 5, cutting off chamber A and Chamber D, and thus connecting chamber A with the drain chamber B. The oil originally filled in the right steering circuit flows back to the rear axle box through Chamber B from Chamber A.

To ensure that the steering clutch can disengage quickly when operating the steering, the steering clutch circuit should be filled with oil to make the operation sensitive. However, in reality, when the machine is not steered for a long time, the oil in chamber B will leak along the gap. Therefore, during the design and processing, a small piston 3 was added to the right end of the right steering valve stem 2. The right end of the piston is supported by a small spring. Oil holes are machined on the piston. When the oil in Chamber B leaks, the pressure drops, and the oil in the pressure chamber D will be replenished into chamber B through the small holes, thus ensuring that the steering clutch is always filled with oil.

When the left end of the small piston 3 is blocked by small drill cuttings, the oil passage to chamber B is blocked. Due to oil leakage, the pressure in chamber B drops, that is, the pressure at the right end of the right-turning valve stem 4 decreases. The left end of the valve stem 4 communicates with the pressure chamber D through a small hole. The pressure at the left end of the small piston is very high, which will force the small piston to move to the right. After pressing against the spring, it will drive the right-turning valve stem 2 to move to the right. Thus, chamber A is first separated from Chamber B, and then chamber D is connected to Chamber A. The pressure oil flows to the right steering clutch, achieving the disengagement of the right steering clutch. Therefore, even if the right steering is not operated, the bulldozer will still turn right.

After identifying the cause of the fault, the small drill cuttings blocking the oil hole were removed. After the installation and test run, the deviation fault was eliminated. Deviation faults caused by this reason are extremely rare.