Reversing Rolling Mill

A reversing rolling mill reduces a heavy slab, ingot, or plate by passing it through a single roll gap many times. After each pass the rolls stop and turn the other way, the plate runs back through, and the gap is closed a little more for the next reduction. A 200 mm slab can reach a 12 mm plate in roughly 15 to 19 passes on one stand. The same stand does all of the work, so it must be built heavily and its main drive must reverse under load on every cycle. Plate mills, aluminium hot and cold mills, and slabbing or blooming mills all use this layout because it rolls a wide range of thicknesses on one stand without the capital cost of a long continuous train.

How reversing rolling works

The operator or the control model follows a pass schedule. Each entry on the schedule sets a target draft, the reduction in thickness for that pass, along with a roll speed and a roll gap. The plate enters, the roll set reduces it, and the plate leaves on the far side. The screwdown and gap control then closes the gap, the reversing main drive turns the rolls the other way, and the plate is rolled back. Early passes take large drafts to break down the cast structure. Later passes take small drafts to hit the final thickness, flatness, and surface. On wide plate the plate turning device rotates the piece 90 degrees so it can be cross rolled to width before finishing along its length.

Mill stand and housing

The mill stand is a closed-top steel housing built from two housing posts and two housing caps held by pre-stressed tie rods. The housing reacts the full roll separating force, which on a plate mill runs from 30 to 90 MN, with little spread so the gap stays accurate. The posts sit on sole plates anchored to a deep foundation. Replaceable liner plates and window guides line the window so the chocks slide cleanly and the wear stays in bolt-in parts.

Roll set and chocks

The four-high stack uses two work rolls in contact with the plate, backed by two larger back-up rolls that limit work-roll deflection across a wide barrel. Each roll neck sits in a work-roll chock or back-up-roll chock. Work-roll chocks run on four-row tapered roller bearings. Back-up rolls carry the heaviest load on oil-film bearings, where a hydrodynamic film on a babbitt bushing supports the neck. A neck seal and an oil seal keep cooling water out of the bearing.

Screwdown and gap control

The screwdown sets the gap in two stages. Two screwdown screws running in bronze nuts give coarse position, driven through a screwdown gearbox from the screwdown motor. Fast, accurate control comes from the hydraulic gap control capsule, a short-stroke cylinder fed by a servo valve and read by a position sensor. The controller takes roll force from the load cells and corrects the gap for housing stretch, so the plate leaves at the scheduled thickness.

The reversing main drive

The reversing main drive is the defining subsystem. A main drive motor accelerates the rolls to rolling speed, holds peak torque through the bite, decelerates, stops, and reverses, all within a few seconds, then repeats on the return pass. Peak torque in a heavy draft can reach several mega-newton-metres. The motor is built from a stator assembly, a rotor assembly, a copper winding set, and a motor housing, with a commutator on a DC machine. A flywheel stores energy so the supply sees a steadier draw despite the swing in load. Torque reaches the rolls through a gear coupling, the pinion stand that splits drive to both work rolls, and two universal spindles that allow for the changing roll gap.

Manipulators, roller tables, and plate handling

Between passes the plate is moved and repositioned by the manipulators and roller tables. Driven table rollers turned by roller-table motors carry the plate to and from the stand and reverse with the rolling direction. Hydraulic side guides center the plate and edge it square so the next pass tracks straight. The work-roll table guides the plate into the bite.

Cooling, descaling, hydraulics, and controls

On a hot mill the cooling and descaling system runs roll cooling sprays through a spray header and strips scale with a high-pressure descaling header fed by the descaling pump. The hydraulic and lube unit supplies the gap control and bending circuits from a hydraulic pump and accumulator, while a lube pump and oil cooler serve the oil-film bearings. The control system sits in the control panel and reads encoders, pyrometers, pressure sensors, and load cells to run the pass schedule and command each reversal. Variants range from cold aluminium mills with very tight gap control to large slabbing and blooming mills built as two-high reversing stands for primary breakdown.