Bottom (lower pinch) roll of a plate rolling machine: deflection, crowning and how it is positioned and driven
The lower pinch roll of a plate rolling machine · why it bends away from the top roll and leaves a barrel-shaped shell, crowning versus counter-deflection, 42CrMo4 roll steel, and the cylinders, drive and readouts that move it
On a double-pinch (four-roll) plate roll the top roll stays at a fixed height and the bottom roll is pushed up from below to grip the plate. Everything the top roll does depends on that grip: if the bottom roll bows away under load, the middle of the plate is pinched less than its ends and the finished shell comes out fatter in the middle. Buying a bottom roll, whether in a new machine or as a replacement, means deciding how its deflection is handled, what steel and hardness it is made of, and how it is moved, driven and measured.
The lower (bottom, pinch) roll of three- and four-roll plate rolling machines, as a component in a new machine or a replacement: its deflection under bending load, crowning and counter-deflection, roll steel and hardening, bearings, hydraulic positioning, drive and position readouts. Machine configurations, overall capacity and pre-bend capacity are covered on the Plate rolling machine page; crowning of press brake beds is covered on the press brake pages.
CNFX editorial research · Sources checked October 12, 2026 · 4 sources
Why the bottom roll bows away from the top roll, and what that does to the shell
Promau's patent US 5,890,386, by Orazio Maria Davi, describes the problem from the bottom roll's side. Rolls supported at their ends deflect in opposite directions under the plate's reaction, with the greatest deflection at mid-length; the gap between the gripping points widens toward the middle; the plate is bent to a larger diameter there; and the shell comes out barrel- or cigar-shaped, with the seam more open at mid-length than at the ends. Besides the working pressure, the patent attributes deflection to the roll's slenderness coefficient, the ratio between its diameter and axial length. [1]
The patent adds that the barrel defect grows with plate thickness and the reel defect with thinness, so a machine whose crown is set for thick plate is not suited to thin plate and vice versa, and operators correct by raising or lowering the grip pressure. [1] Editorial note: this is why the bottom roll's bending-pressure setting matters as much as its crown; ask the maker what crown the bottom and top rolls carry and for which thickness range it was chosen.
Way of handling deflection
What the patent says it does
Drawback the patent gives
Crowned (swelled) rolls: larger diameter in the middle, tapered ends
State the thinnest and thickest plate, the yield strength, the working length and the tolerance on shell diameter along its length, so the maker can say how the bottom roll's deflection will be handled.
02
What a buyer specifies for the bottom roll: steel, bearings, drive, positioning and readout
Bharat Heavy Electricals Limited's specification for a four-roll plate bending machine (3,100 mm rolls, 100 mm plate of 275 MPa yield, 2,000 mm minimum diameter) shows how the bottom roll is pinned down in a purchase document:
The same document asks for a bending-pressure selector for the bottom roll within easy reach of the operator and a flat end of at most 150 mm on 100 mm plate. [2] Editorial calculation: 150 ÷ 100 = 1.5, so the unbent flat left at the plate edge may be up to one and a half plate thicknesses.
Mega Manufacturing's patent US 10,751,779 B2 describes the same arrangement on a four-roll machine: the top roll in a fixed horizontal position, the bottom roll adjustably mounted, positioned by a cooperating pair of hydraulic cylinders and powered by its own hydraulic motor. [3] Editorial note: with one cylinder at each end, the two ends of the bottom roll can be set to different heights, which is what makes the tilting for cones possible, and also what lets an unequal setting produce a tapered shell by mistake; the two readouts in BHEL's list are there to show both ends.
BHEL requirement
Value or wording
Roll material
All rolls to 42CrMo4; vendor to specify hardness and heat treatment [2]
Plate firmly clamped between top and bottom rolls throughout, including edge setting [2]
Compensation
Automatic speed and pressure compensation between top and bottom roll [2]
Bearings
Anti-friction spherical roller bearings on all four rolls [2]
Interlocks
No bottom roll movement while the front bearing housing is tilted, and no tilting while the bottom roll is under pressure [2]
03
Bottom rolls on small three-roll machines: crowned, hardened and grooved
At the light end, Knuth's KRM ST 25/5 asymmetric three-roll machine bends up to 2,550 mm wide, 5 mm thick plate of 450 N/mm² with 180 mm rolls to a minimum diameter of 270 mm. Its rolls are crowned, made of hardened steel so they also suit stainless steel, and the lower and rear rolls carry wire grooves; a 4 kW motor drives the upper and lower rolls through a reduction gearbox. [4]
Editorial calculation: 270 ÷ 180 = 1.5, so on this machine the smallest shell is one and a half times the roll diameter, and the roll's length-to-diameter ratio is 2,550 ÷ 180 ≈ 14. On BHEL's heavy machine the minimum diameter of 2,000 mm can drop to 680 mm only with an optional interchangeable top roll, which shows how roll diameter, not plate capacity, sets the smallest shell. [2]
Editorial note: when replacing a worn bottom roll, ask for the original crown profile and hardness, and check the wire-groove positions if the machine is used to roll wire or bar.
04
What to state for a plate rolling machine bottom roll
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Machine: [three-roll or four-roll; make and model; working length]
Plates: [thickness range; yield strength; minimum shell diameter]
Roll: [diameter; crown profile; steel e.g. 42CrMo4; core and surface hardness]
Support: [bearings; support rollers or counter-deflection device]
Movement: [hydraulic cylinders per end; tilting; position readouts and accuracy]
Drive: [driven or idle; hydraulic motor or gearbox; speed]
Replacement: [original drawing; grooves; journal dimensions]
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
Share this page
Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
Why do rolled shells come out barrel-shaped?
Promau's US 5,890,386 explains that the top and bottom rolls bow apart under load, furthest at mid-length, so the middle of the plate is bent to a larger diameter than the ends. [1]
Does crowning the rolls solve it?
Only for one thickness range: the same patent says crowned rolls leave thin plate reel-shaped, so makers choose a compromise crown and adjust grip pressure. [1]
What steel are plate roll bottom rolls made of?
BHEL's four-roll machine specification requires 42CrMo4 for all rolls, with hardness and heat treatment to be stated by the vendor. [2]
How is the bottom roll moved?
Mega Manufacturing's US 10,751,779 B2 describes a pair of hydraulic cylinders positioning it and a hydraulic motor driving it; BHEL asks for two 0.1 mm readouts of its position. [3][2]
Sources and what still needs a direct check
Sources checked 2026-10-12. Deflection and crowning come from Promau's US patent 5,890,386; purchase requirements for roll steel, drive, positioning and interlocks from a BHEL four-roll machine specification; the hydraulic arrangement from Mega Manufacturing's US 10,751,779 B2; small-machine roll data from Knuth's KRM ST 25/5 datasheet.
