Improvement in backup bearing life in cold rolling mill

摘要

Tandem Mill II in Cold Rolling Mill Complex of Bokaro Steel Plant is equipped with Morgoil system A & B for the lubrication of babbit bush bearings of backup rolls. Back up roll imparts the necessary force on the work rolls for the deformation of strips. Torque is imparted to the work rolls and back-up rolls rotates through friction between the work and back up rolls. Bearings after assembling with sleeves are covered with bearing housings and works in hydrodynamic lubrication regime. Pockets are provided in the bearings for stability in load carrying capacity with unbalanced load on rolls taking into consideration. All together 20 babbit bush fluid film bearings are in positions in the stands for rolling operations to take place. Frequent failures of these bearings were witnessed and thus resulted in downtime of the mill and loss in production. Viscosity of this hydro dynamically operated bearing play a vital role in load bearing capacity of the lubricant. The threshold limit of loading of such bearings have an direct bearing on the frictional forces acting on the torque generated for rotational turbulence .The frictional force will give rise to the temperature of the oil in circulation . The load bearing capacity of Servo steel 680-grade oil was measured and found to be suitable for this application. Based on test results servo steel 680 grade oil was put into service in backup bearing lubrication system. Temperature was monitored with online monitoring device. The return line temperature of the lubricant was also monitored with the help of Infra Red non contact type thermometer for eight uniformly spaced locations in a set of roll chokes during rolling. The maximum temperature recorded for the top sets of back up roll bearings was confined to 44 degree centigrade. The temperature of bottom bearing was confined to 35 degree centigrade, as the bottom rolls are water cooled with rolling oil emulsion. As a result, the backup bearing failure has reduced drastically.