Gantry crane with large span is inclined to move at a skew with bad balance, so a deviation rectification system is needed to monitor and correct the movement of gantry crane.

Optical-electricity encoders shall be respectively installed at two sides of crane moving mechanism to check their relative travel limit, as moving deviation is among 1～3‰ span, automatic deviation rectification will be done by computer; as it is more than 3‰ span, stop the crane, deviation rectification shall be conducted by driver to reach 1‰ span, then start crane moving mechanism for normal work. Electro-magnetic induction block shall be installed every 10-20m between trolley and ground for zero calibration.

Besides, limit switch will be set up at connection part of flexible leg top and main girder flexible hinge, when deviation reaches 3‰ of crane span, limit switch works, moving system is in emergency power off, the driver rectify the deviation manually by operating moving mechanism of flexible leg, crane moving can restart after eliminating the deviation. Deviation in 1‰ span can not be rectified, emergency cut-off actually means delayed brake, a large shock will bring danger without it.