The transmission mechanism of the bridge crane is a multi mass torsion system. However, both theoretical calculation and test results show that in order to estimate the maximum load of the mechanism strength calculation, the general transmission mechanism is simplified as a dual-mass single degree of freedom system can meet the requirements of engineering accuracy. In order to understand the problem is simple, you can also omit the damping.

The transmission mechanism of the bridge crane is simplified as the motion differential equation of the two-mass single degree of freedom System. With reference to relative coordinates, the degree of freedom of the dual-mass system becomes a. The form of the solution of the equation depends on the form of excitation and the change of the resistance torque. The resistance torque of bridge crane can be regarded as invariable. The form of change over time depends on the nature of the mechanism's original motivation.

If the drive is an AC winding type electric machine, the general starting torque is linearly reduced with the increase of the mechanism speed; if the motor is driven, then the start-up of the mechanism relies on the access clutch. The driving torque increases with time, and the increase time depends on the speed of the clutch. In order to simplify the technology of the dynamic load of the crane mechanism, it can be seen as a step, that is, the mechanism is subjected to a certain dynamic torque in the start-up moment and then it remains unchanged.

Due to the high natural frequency of torsional vibrations, the torque of torsional vibrations is very short, usually one-tenth or one-tenth of the start-up time.

Therefore, it is assumed that the error of the bridge crane in this short time is not the same as that of the AC winding type motor. When the mechanism starts with a friction clutch, it increases in a straight line. If the speed of the motor is solved according to the change, the accurate result can be obtained.