Troubleshooting the rotation fault of the PC200-5 excavator

1. Fault phenomenon

One of the Komatsu PC200-5 excavators in our company experienced the following abnormal conditions during the construction operation: The left rotation was normal, meaning that the motor could immediately stop when the rotation control lever returned to the middle position. The right rotation is abnormal. That is, when the control lever is turned back to the center position, the motor must continue to rotate by a large Angle before it can stop.

2. Possible causes of the fault and inspection

In light of the above situation and in combination with the working principle of the rotary system, after analysis, it is believed that the main causes of this fault are as follows (the hydraulic principle of the rotary system of this machine is shown in the attached figure) :

(1) Main control valve

Due to the wear of the rotary main control valve 3, some impurities will be squeezed between the valve core and the valve hole, causing the valve core to get stuck and thus unable to return to the center position in time.

② The control spring on the left-hand side of the rotary main control valve failed, causing the valve core to fail to return to the center position in time (when checking, the left and right springs can be swapped).

③ The pilot control oil on the right-hand side of the rotary main control valve failed to flow back to the oil tank in time and completely, thus forming hydraulic resistance and causing the valve core to fail to return to the center position accurately and promptly. In this regard, we believe that it is due to the fact that the rotary control valve 7 cannot be accurately in place or the slow return valve 6 is blocked, which causes the oil return to be not smooth. (When checking, the pilot control oil circuits for the left and right rotation directions can be swapped.)

(2) Rotary hydraulic motor

According to the hydraulic principle of the rotary system, if the rotary motor operates normally, when rotating to the right, the pressure oil enters the rotary motor 4 through the B1R oil circuit and then returns to the oil tank through the A1 R oil circuit. When the right rotation is completed, the main control valve of the rotation returns to the center position. At this point, the A1 R oil circuit and the B1R oil circuit have been closed, thus forming hydraulic resistance, and the rotation motor stops immediately. According to the fault phenomenon of this machine, it indicates that the A1 R oil circuit has formed an open circuit due to leakage. The reasons for this state are as follows: The valve core of safety valve A1 is stuck or the set pressure is too low. When the right rotation ends, the hydraulic oil in the A1 R oil circuit is unloaded and flows back to the oil tank through valve A1 (during inspection, it can be swapped with valve B1). There is internal leakage in the rotary motor (as the left rotation of this machine is normal, it indicates that there is no internal leakage in the motor of this machine).

Although we have thoroughly investigated and eliminated all the causes of the above-mentioned faults one by one, the fault phenomenon still exists. Therefore, we suspect that the check valve A is in the open state (under normal circumstances, Valve A is not open). When it was taken apart for inspection, there were indeed some small metal thin pieces stuck inside, thus creating an open circuit. After being removed, the fault phenomenon disappeared immediately.