A difficult to detect fault

A fault occurred in the JS2102 offset press of our factory. The phenomenon was that the automatic lift limit switch frequently issued a paper lift signal, and the cone motor was also frequently started. However, the main paper stack had a very small increase and sometimes it did not move.

Press the open or fall button by hand, the motor can normally raise or lower. Use a current meter to measure the three-phase operating current of the motor. This failure was analyzed to eliminate the possible problems of the motor, mechanical transmission mechanism, AC contactor, thermal relay and insurance, etc., and the idea of ​​finding problems in the automatic lifting control circuit was determined. The auto-lift control circuit has a total of 2 electrical control elements: one is a limit switch mounted on the platen foot mechanism, and the other is an electronic delay which is determined by the limit switch and installed on the machine wall board to determine the length of the motor working time. Time relay. Check the limit switch on the platen foot first. After the power is turned off, when it is removed, the user can press the head to hear the sound of the internal contacts, and use a multimeter to block the test, which proves that the switch is obviously damaged. Switch to a new limit switch test machine, fault remains the same. Is it because the time delay of the electronic delay relay is too short? The delay is adjusted long test machine, the failure has not been ruled out. After further analysis, it is considered that the problem should also be on the automatic lift control circuit. The automatic lift limit switch is operated by the top screw of the presser foot. Therefore, it is necessary to find out the cause of the action of the presser foot mechanism. The test machine, when the presser foot is rotated and pressed down, the trial hand assists it with a little force to make the screw contact the eye position switch more reliably. At this time, the motor returns to normal, and the rise of the paper pile immediately becomes effective. Once you do not use your hands to help, the fault reappears. This fully proves that, besides the damage of the limit switch, the fault also has the cause of insufficient force when the presser foot is pressed down. Temporarily adopt emergency measures: Fix a 20cm long bicycle inner tube on both sides of the presser foot mechanism, and let the middle part press on the presser foot. The elasticity of the eraser increases the force of the presser foot when it is pressed down. After retrying the machine, the fault is finally eliminated.

Later, it was found that the gap between the universal long axis on the presser foot mechanism and its universal drive sleeve was excessively worn, which caused the presser foot to move out of position when it was pressed down. This is really a fault that cannot be easily detected.