Cold Mill Shaft Solution

Challenge

Several mechanical failures including a yielded pinion shaft and fractured coupling keys prompted the mill personnel to evaluate the actual torque being transmitted throughout the drive train. Torque measurements showed a large variation in TAF (Torque Amplification Factor) even across sequential strips rolled. Torsional vibration analysis of the drive train was completed to better understand the system dynamics assuming that a sudden instantaneous torque would produce the highest TAFs. To better simulate the torque being applied at the rolls in the model, a sinusoidal curve was fit to the actual torque measurements. The analysis showed that the high torques resulted in stresses that exceeded the endurance limit of the key material – these stresses induced cracks in the coupling keys after repeated cycles.

The yielded shaft was most likely the result of an instantaneous torque overload,  due to the fact that yielding occurred rather than crack propagation. In an effort to minimize the peak torques transmitted throughout the drive train, damping style couplings were evaluated. Replacement of existing gear couplings with damping style couplings reduced the TAF up to 43%.

A reduction of this magnitude will lessen the likelihood of fatigue failures. The adage of keeping the TAF to a maximum of 2.5 is no longer suitable with increased rolling torques.  Performing a torsional analysis can provide users with a valuable and proactive maintenance tool for identifying the cause of equipment failures and the solutions to eliminate the sources.  Actual torque measurements should be conducted with transient torque meters to ensure that analyses are completed correctly and the return on investment for any drive train modification is fully realized.

Solution

In many cases, rolling stands are subject to frequent mechanical failures and motor problems causing unplanned downtime and expensive repairs. By installing strain gage telemetry systems on gear spindles, actual torque measurements can be collected. TAF values can then be measured and evaluated for peak torques and their effect on the power transmission components. The largest reduction in TAF’s can usually be achieved by installing damping style couplings or modifying spacer type couplings from a tubular to a solid design. Damping style couplings and spacer stiffness changes can result in a reduction of peak torques and increase reliability of the drive train.