Each rolling mill stand has an electric motor coupled to a main reduction gearbox containing single or double helical gears. The main gearbox output shaft (pinion shaft or roll) is flexibly coupled to a pinion stand that drives the rolls. Each pinion stand has a gearbox, also with helical gears, that splits the power between upper and lower mill rolls.
The power required to roll steel varies, depending on the type of steel, plus its thickness and width. Most of the time, the gearbox operates at moderate load, usually less than 15% of the motor rating. But, when a bar enters the mill, the drive is subjected to a sudden, large momentary torque caused by squeezing the bar between upper and lower rolls. This peak torque sometimes exceeds four times the rated motor torque.