Analysis of lock key fracture of backup roll

　　Problem overview: the driving side of the lower roll of the backup roll was concentrated on the machine for about one week. The lock key was broken and the lock nut was rewired, resulting in roll jumping and affecting the normal production of the rolling mill.
 

　　1、 Roll system structure

　　In the working state of the roll system, the locknut presses the positioning bearing (double row conical bearing). The locknut is divided into inner ring and outer ring. The inner ring and the fixed end face of the roll are connected by threads. In the locked state, the inner ring and the fixed end face of the roll are close to the snap ring, and the end face of the outer ring locks the end face of the inner ring of the positioning bearing. Finally, in order to prevent the relative movement of the inner and outer rings of the locknut, which leads to wire inversion, the end face keyway is set on the end face of the inner and outer ring. After locking, the inner and outer rings are.
 

　　2、 Stress analysis

　　The stress condition of the lock nut can be analyzed through the structure diagram. Taking the structure diagram of the transmission side as an example, the sequence of axial force to the transmission side is: Roll - distance ring - inner ring assembly of the positioning bearing near the roll body side - outer ring of the positioning bearing - Large gland of the outer ring - large gland bolt.

　　When the axial force occurs on the operating side, the sequence is as follows: Roll - snap ring - lock nut inner ring - lock nut outer ring - positioning bearing inner ring assembly close to lock nut side - positioning bearing outer ring - bearing seat.
 

　　3、 Current lock key status

　　At present, the lock key is L-shaped. The main reason is that the bottom of the outer ring keyway of the lock nut is lower than the bottom of the inner ring keyway after installation. In order to ensure that the lock key is firmly fixed without deflection, the lock key now needs to be L-shaped.
 

　　4、 Cause analysis

　　Through the force analysis, it can be judged that when the axial force direction acts on the operating side, it can cause the stress on the locknut. When the inner ring of the locknut and the roll run synchronously at high speed and the rolling direction changes, there will be a relative movement in the circumferential direction between the inner ring and the outer ring of the locknut. Vibration, start-up and stop, and reciprocating rolling will all cause shear force under the action of axial load, resulting in the objective existence of shear force, When the shear force is greater than the shear force of the lock key, fracture occurs.

　　The current locking key structure is shown in the following figure:

　　The fracture position is concentrated at the notch with fracture resistance, and the upper half and fixing bolts are normal.
 

　　5、 Rectification suggestions

　　At present, the damage is concentrated on one rolling mill and mostly on the drive side of the lower roll, so it is recommended to rectify from the following directions:

　　1. The lock key structure is changed from the current l structure to a regular cuboid, and the thickness is increased to the greatest extent to improve the shear resistance;

　　2. Adjust the lock nut to ensure that the bottom of the outer ring keyway is higher than the bottom of the inner ring keyway when the lock nut is locking the inner ring of the bearing, so as to ensure that the lock key has a regular rectangular structure and can be fixed stably and reliably;

　　3. Reduce the axial load borne by the lock nut, that is, reduce the axial force acting on the operating side;