How to Choose
the Right Plate Roll
Basic Facts of Selecting a Plate or Sheet Rolling Machine
written by David Donnell, President Carell Corporation
With recent advances in technology, Plate Roll builders now offer more choices of geometry, options and automation. This makes choosing the most appropriate machine for the application more critical than ever before for the buyer.
Carell Corporation has compiled some basic educational information on plate rolls and their use with ultimate goal of making it easier to more accurately select and size a Plate or Sheet Rolling Machine for a given application. For even more in-depth information and engineered application solutions, please contact a Carell Corporation Application Specialist.
Plate or Sheet Bending Rolls fall into 2 general categories, single or double pinch types. However they may have different geometries or operational features. Matching the most appropriate machine style to the application is of primary importance to the user in order to maximize the return on investment (ROI).
General Common Roll Styles are as follows:
3 Roll Initial Pinch
(also known as 3 Roll Single Initial Pinch or I.P. Rolls or Slip Rolls)
3 Roll Double Pinch
(also known as 3 Roll Double Initial Pinch)
4 Roll Double Pinch
(also known as 4 Roll Double Initial Pinch or simply 4 Rolls)
3 Roll Variable Translating Geometry
3 Roll Pyramid or True Pyramid Rolls
2 Roll Machines
Vertical Rolls are also available in a variety of the above styles (See below for details on each style and geometry)
Machine Capacity is of equal or greater importance to style as steel mills are producing alloys with increasingly higher yields and tensile strengths in order to meet today’s applications.
Plate roll manufacturers commonly rate machines according to baseline material yields between 36 to 38,000 psi. When choosing a roll you must refer to your mill delivery certificates, to your proposed supplier or to the internet to specifically verify the average yields of the plate you need to roll. In our current manufacturing environment, the term “A36 Steel” has little correlation to the actual yield since the A36 generalization encompasses many specific sub-denominations. Some of these may have yields strengths of 70,000psi or more. The old common term “Mild Steel” will generally represent steels with yields of 45,000 to 55,000 psi.
Machine capacity must be commensurate to your material yield. Most roll manufacturers can provide tables or graphs that allow you to match machine capacity to the material yield.
Pre-Bending vs. Rolling Ratings will often be presented in numerous formats and tied to a minimum shell diameter at a given thickness for each Plate Roll model. The Pre-Bending operation is performed at the leading and trailing edge of the sheet (in most cases, the eventual seam). Sheets cannot physically be bent completely to the leading edge and will thus leave what is referred to as an ‘Unbent Flat’. As a rule of thumb, the minimum you should expect for an unbent flat length is 1.5 times the material thickness. This may approach 2.5 to 3.5 times the material thickness for thicker plate. It is this pre-bending operation which attempts to reduce the unbent flat to the minimum possible length which requires the most power from the machine. This operation is also very critical as care must taken to ensure that the sheet does not disengage from between the rolls. Because of this “safety factor” claims of unbent flats equal to 1.1 times the material thickness should be thoroughly investigated as it is rare that this can be achieved.
Pre-bending ratings with the shortest possible unbent flats are necessarily lower than the overall rolling capacity rating for any given machine. You must be mindful when reviewing machine ratings since the maximum total rolling capacity is generally expressed with the intrinsic requirement of multiple rolling passes, longer un-bent flats and larger minimum shell diameters, even if not entirely clear from reading a simple roll specification. Un-bent flat lengths for any given sheet are a function of thickness, width, cylinder diameter, machine geometry, material yield, top roll size, operator proficiency and other factors.
Rolling ratings anticipate that the plate is in motion, translating through the machine. This is called “dynamic” as opposed to the “static” pre-bending operation when the sheet is held between the rollers and one or more rolls move upward to “curl” the leading edge. Dynamic rolling mode will always allow a greater thickness to be rolled than can be bent during static pre-bending.
Automation in the workplace is becoming more common as pressure increases to reduce labor costs. Most all CNC and NC controlled machines are 4 roll types. Automated controls are recommended for high volume or significantly repetitive cylinder or shell production or to roll complex shapes that are not easily reproduced by using manual machine controls. Multiple bends, variable radius bends, ovals and the like are some examples. Over automation on a roll when the production volume is not significant can be a costly mistake.
Three Roll Initial Pinch or Single Initial Pinch Plate Rolls are generally for lighter capacity applications and may be electro-mechanical or hydraulic. They work by "pinching" the flat sheet between two counter-opposed rolls while the third offset roll or ‘bending roll’ moves upward to contact and then bend the sheet. When rotation of the rollers is activated, the sheet exits at a given radius. With the sheet cut to the developed length and the bending roll properly positioned; the part is rolled into a cylindrical form, where it can then be welded at the seam to produce a cylinder. The upper roll is in a fixed position; the lower pinch roll can move up/down to pinch the material. The third roll (the forming roll) is also adjustable. To remove a rolled cylinder, it must be extracted from off of the top roll. Machines are generally equipped with some type of release mechanism on the top roll to allow extraction of the cylinder. Typical methods are either a forward tilting or releasing top roll or a removable end yoke. In most applications, these machines require removal and re-insertion of the sheet in order to pre-bend both ends. They are cost effective but in contrast may be more labor intensive in a production setting. There were many large mechanical initial pinch machines built during the 1930’s, 40’s & 50’s and can be found on the used market. All have cast frames as modern alloys and welding techniques had yet to be invented.
Double Pinch Plate Rolls are available from light to extremely heavy capacities and may have three or four rolls. The terminology can be confusing as these units may sometimes be referred to as ‘double pinch pyramid plate rolls’ or ‘double initial pinch plate rolls’. Both three and four roll styles have fixed position top rolls and two offset rollers or ‘lateral rolls’, one on each side. The 4 Roll styles have an additional roller directly underneath the top roll, which constantly pinches the plate during rolling. Double Pinch Rolls can pre-bend both ends of the plate without removal as is required with single pinch rolls. Three Roll machines generally require pre-bending the leading end, running the sheet through the machine to pre-bend the trailing end then switching roll rotation direction to roll the cylinder body. Four Roll Plate Rolls have a slight advantage in cycle time since they permit pre-bending of the leading edge, rolling the cylinder body and finishing off of the trailing edge all while rolling in the same direction. High volume products can be produced faster when using a four roll machine. Smaller machines can be mechanical but most modern machines are hydraulic and include drop end yokes for easy extraction of the work-piece.
Automatic NC or CNC Controls and Four Roll Plate Rolls: Automatic cycle systems are generally used only on four roll plate rolls because there is minimal chance for slippage since the sheet is constantly pinched between the top roll and the 4th roll or “pinching roll”. Automatic controls use an encoder to track movement of the plate through the machine. If the plate slips, the bending roll movements will be out of synch with the material’s translation through the machine.
Variable Geometry Three Roll Plate Rolls are not new but are gaining in popularity around the world. They are built for medium, heavy to extremely thick plate applications. The top roll moves up/down and the lower two rolls each move horizontally. This lower roll movement allows increasing the offset distance from the top roll and commensurately increasing mechanical advantage in bending. A machine of this type works well over a wide range of material thickness. By adjusting the geometry they can be used similar to single pinch, double pinch and pyramid style machines and require minimal sheet movement during the pre-bend operations. In the past these machines were commonly found in shipyards but now are being placed in general job shop and manufacturing applications.
True Pyramid Machines are not generally manufactured by modern roll builders. They may be found on the used market and are more common in developing countries. They have 3 rolls, both lower rolls fixed in position with the top roll moving up/down. In general they leave very long un-bent flats and have only one or two rolls motorized in rotation. Some special application machines are still produced on a custom build basis and are mostly found in shipyards. They will have the lower two rolls spaced closely together and be of small diameter with midpoint support trunion rollers.
Two Roll Machines are designed for thin or light gauge material rolled to reasonably small diameters. They use a large diameter urethane coated pinch roller which moves up with extreme pressure against a small diameter steel top roller. A mandrel or drum, very close in OD to the desired ID of the part is fitted over the top roll. Two roll sheet rolls are extremely fast and will roll round cylinders or shells even if the blank has large cutouts, voids or holes. Since they require a mandrel for each different part diameter and material thickness they are not as versatile as some other machines. For dedicated high speed production of light gauge cylinders they can be an optimal solution.
Vertical Format Plate Rolls can be of nearly any geometry, double pinch, single pinch or pyramid. Vertical rolls are most often used for rolling large diameters and can be advantageous when used in the field for storage tank production. Carell offers automated Vertical Tank Production Systems which are fed from coil stock and can include vertical sheet cutoff and automatic vertical seam welding among other options. These Systems can be used in-house or in the field and increase production output while reducing labor input.
Optional Equipment and Accessories are offered for most Plate Rolling Machines. The most important items to consider are hardened roll surfaces and cone rolling devices. With today’s harder materials and laser/plasma cutting techniques it is imperative to have a hardened outer surface on the rollers. Look for hardness ratings in the range of 52 to 55 Rockwell C Scale and also for the indication that the hardening process used is “induction hardening”. Hardness in the 52 to 55 RCS range will have a reasonable hardness penetration depth to provide long lasting protection against roll surface wear. Hardness exceeding 58 RCS will have a shallow penetration and a more brittle outer surface which can result in cracking or crazing.
Cone rolling devices permit the operator to roll a conical shape and are standard on some machines. Some machine brands require a machine to be pre-disposed at the time of purchase if cones will be rolled in the future. This important point should not be overlooked.
Lateral as well as Central Overhead Supports are optional on nearly all Double Pinch Plate Rolling Machines. Some rolls may require pre-disposition for the eventual addition of hydraulic powered supports. Overhead supports prevent lighter materials from collapsing when rolled to large diameters and must be sized according to plate thickness and shell diameter. An overhead support is not a crane and it is designed to sustain nominally only 40 to 60% of the total shell or cylinder weight. A side support can also assist in this function to prevent light materials from re-curving toward the floor if the radius is very large.
Some plate rolls are designed with Extended Roll Shafts that protrude through the machine frame at the yoke end. Section or Pipe Dies can be fitted on these ‘stub’ or extended shafts. Some machines are designed with collars inside the machine frame that can be used for profile rolling but when rolling a complete 360 degree hoop it will be impossible to remove from the machine. It is also not possible to coil with inboard collars. It is not practical to anticipate rolling angle iron on a plate roll. Angle will always twist when being rolled. Plate rolls do not have outboard adjustable lateral profile guides to prevent this inherent twist. You should consider using a section rolling machine or double pinch angle roll instead for these applications. In general, section dies on plate rolls are good for bending flat bar the hard way, rods or small pipe.
Roll Translation or Roll Movement Up/Down is accomplished either by using inclined linear guides or via swing arms. Direct rectilinear guides direct the thrust of the roll directly toward the top roll and have the fewest moving parts. Rectilinear guides are found on small as well as extremely high capacity rolls. Swing arm designs are not common on large machines and have more moving parts, pins, and bushings.
Roll Balancing can be achieved in a variety of methods. Maintaining the bending roll or rolls parallel to the top roll is necessary in order to produce a good true cylinder. Volumetric type and electronic type balancing are the most trouble free for general job shop use. They have up to 80% fewer moving parts than swing arm balancing systems and thus significantly lower maintenance costs in the long run.
General Note: Roll Bending Machines are very dangerous if used carelessly. It is the responsibility of the owner to ensure the installation and proper use of point of operation safety guards or devices.