The connecting shaft is the device that transmits power from the gear seat to the rolls, or from the rolls of one rolling mill to the rolls of another rolling mill, or directly from the main motor to the rolls. There are two types of universal connecting shafts: universal joint shaft and plum blossom shaft.

All roll diameters have the certain range of variation. From installing new rolls to scrapping them, the axis position of the roll can be changed up and down, while the axis of the herringbone gear in the gear seat is unchanged. The   allows the roll axis to have a large inclination angle (8-10 ° C) to the axis of the herringbone gear, which can smoothly transmit the torsional moment. The permitted inclination angle of the plum coupling is only 1-2 degrees, which is only suitable for rolling mills with a small change in the distance between the centerlines of the work rolls. It is easy to disassemble and easy to change rollers.

The plum bushing and plum bushing should be suitable for the size of the roll journal in order to be disassembled during roll replacement, to withstand large impact loads and to extend service life. The shaft sleeve is generally made of cast iron, and under heavy load, it is made of cast steel. Section steel rolling mills with the roll diameter greater than 500mm have larger weights of the couplings and bushings. In order to prevent the weight of the plum couplings and universal joints from acting on the hinges of the plum couplings or universal joints, usually a coupling bracket is installed in the middle to balance the weight of the coupling shaft and make the coupling rotate smoothly.

Due to the advantages and disadvantages of the plum blossom joint and the universal joint, the joint coupling has recently appeared, that is, the universal joint link is used at the end of the link between the coupling and the gear seat, and the connecting end of the connecting shaft and the roller is connected by a plum coupling. In addition, there are arc gear shafts.