Through mathematical modeling and deducing the analytical relationship between the chain length deviation and the tilt angle of the pendulum during operation,as well as geometric drawing and field test,it is confirmed that the main reason leading to the failure of picking up a car is the length deviation of the left and right chain from the exit to the entrance,which leads to the deflection of the support rods at the bottom of the pendulum at loading position.When the deviation exceeds half of the width of the skid carrying bowl,the bearing rod will not be inserted into the bowls,resulting in failure of a car picking-up.A variety of technical measures to quickly adjust the chain length deviation,avoid the loading failure and the technical scheme for predictive maintenance of chain wear are put forward:sensors are used to monitor the height difference of the weight boxes and the synchronization of the left and right chain at the exit and entrance,the chain length deviation is calculated and compared with the threshold value in real time.