Rolling mills — in a four-high mill configuration, which rolls act directly on the strip and which provide stiffness to limit deflection?

Introduction / Context:
Rolling mill configurations are chosen to balance strip thickness control, surface quality, stiffness, and power requirements. The four-high mill is a common arrangement for cold rolling thin strip while maintaining adequate stiffness by using backup rolls to support smaller work rolls.

Given Data / Assumptions:

• Four-high mill with two small-diameter rolls in the middle and two larger rolls outside.
• Objective: reduce strip thickness while controlling roll deflection and spreading.
• Standard definitions: “work (working) rolls” contact the strip; “backup” rolls support work rolls.

Concept / Approach:

The two central small-diameter rolls directly contact and deform the strip (work rolls). Their small diameter lowers rolling force and power but increases the tendency to bend, potentially causing crown/shape defects. To counter this, large-diameter backup rolls are placed above and below the work rolls to stiffen the system and limit elastic flattening.

Step-by-Step Solution:

Verification / Alternative check:

Plant schematics and textbooks depict four-high mills exactly as “work–backup–work–backup”.

Why Other Options Are Wrong:

Options with one or three work rolls misrepresent the symmetric arrangement; “all four working” corresponds to a four-roll mill without backups, not the standard four-high configuration.

Common Pitfalls:

Confusing a “cluster” (Sendzimir/Z-mill) with a four-high; mislabeling backup rolls as idle elements—they are essential for stiffness.

Final Answer:

two are working rolls and two are backing up rolls