Failure Analysis of Backup Roll’s Locating Bearing of 1250 Cold-rolling Mill

At present, the linear speed of rolling mill remains at 600m/min-1000m/min, owing to the upgraded AGC system as well as the rapid development in cold-rolling mill industry, especially for the high-precision evolution of four-roll cold rolling strip mill. The fast growth in speed of rolling mills must lead to higher standard on its bearings, particularly in the bearing capacity, limit speed, stability and flexibility. Therefore, further studies are necessary to be conducted on the design, process, installation and lubrication of bearings for roll system of the rolling mill.

Elementary analysis is given taking the backup roll's locating bearing of 1380 four-roll cold-rolling mill :

一、Performance and main parameters of the machine set

二、Issues

351080 locating bearing in backup roll system rises in temperature frequently, and the rolling element even gets adhesive to the retainer, locking the bearings, which further break mating parts such as jam nut, distance ring and change the diameter of roll. What’s more, the burnt main bearing may not be easily disassembled due to the adhesion, and the gas cutting may also damage the roll when tear down the damaged bearings.

In this case, LYEF technicians took an overall investigation by examining all parameters of bearings, making a real-time trace of the bearing assembly, learning the working condition as well as the installation, teardown and maintenance. After analysis, they found :  

1. Broken bearings are mostly located in the driving side of the rolling mill.

2. Bearings are largely broken down in a short time, not more than 1 month.

3. Bearings only have one row broken.

4. There is obvious relative sliding between inner race and the roll.

5. The larger end face of bearing inner race, the head face of jam nut as well as the head face of distancing ring wear out badly.

6. The bearing rolling elements and capped edge wear out badly.

See the figure :

Based on the comprehensive analysis, LYEF technical department drew a conclusion on three reasons for the heating and damage of bearings :

1. Insufficient lubrication: the inlet and outlet of oil on the bearing shell are set improper, as a result, the bearing cannot be fully lubricated, thus got heated to broken. 

2. During installation, the inner race of bearings is not pre-tightened, which causes the change of bearing play and the relative sliding of inner race, generating heat; 

3. Low bearing capacity in axial direction and the insufficient lubrication result in the wear of the rolling element and the capped edge and the heat generation.

LYEF then made effective solution to promoting bearing products and their installation and lubrication, etc. The issue got solved at last.