Tube Roll Forming and Welding Rolls
Tube roll forming is a high-precision process that shapes complex and intricate cross sections. It’s a cost-effective way to manufacture parts with tight tolerances and clean contours.
The tooling for tube roll forming is a complex set of metal dies that shape the incoming strip of sheet metal. These forming rolls need to have sliding properties, high hardness, and low elongation.
Welding Rolls
At the end of the tube forming process, welding rolls ensure that the ends of the original flat piece of metal are firmly welded together and pressed in place. This is done through various welding operations, including high-frequency induction (HFI), laser welding, and TIG welding. Depending on the specific welding operation used, different properties are required for these specialized rolls. These include good sliding properties, hardness, and permeability.
During the tube roll forming process, a sheet metal strip is guided between pairs of specially machined steel cylinders called forming rolls arranged in modular sequence until it achieves the desired shape and size. This precise method allows for the production of tubular products with exact dimensions, which are used in many industries ranging from automotive to household products.
Tube forming is a time-consuming and costly process, but the results are often much more cost-effective than alternatives. It also helps manufacturers meet strict quality standards that are essential in high-demand applications.
One important tool in this process is the drive roll, which supports and guides the welding wire as it is being formed. Several factors must be taken into account when selecting a drive roll, including the type of welding process, the wire size and type, the number of grooves on the roll, and the knurling. For a reliable and long-lasting drive roll, an aluminum bronze alloy like QA520H or QA900 is the ideal solution.
Sliding Rolls
Rolling-sliding contact is a type of multi-axial motion that combines rotation (commonly of an axially symmetric object) with translation of the same object with respect to a surface. It is a common form of frictional contact in many machine elements such as wheels/rails, cams/followers and gears which are often only marginally lubricated. Ideally the mating surfaces are separated by a thick layer of lubricant such that no wear occurs. In real materials however, the lubricant film can degrade and the resulting friction/contact stress can induce damage such as micropitting.
Studies have shown that the presence of lubricant can have a cushioning effect on asperity stresses in rolling-sliding contacts and may reduce the contact fatigue lives of roughness asperities. Sliding, by contrast, tends to increase asperity stresses by increasing the number of cyclical loading events which promotes contact fatigue and increases the magnitude of micropitting damage.
In this study a series of tests were performed on the elliptical contact geometry using varying slide-roll ratios and loads to investigate the effect of sliding on micropitting damage. The results show that with all other test parameters held constant, increasing SRR significantly increases micropitting damage which is most severe at negative SRRs, i.e when the roller is sliding faster than the counterface disc. Central elastohydrodynamic (EHL) film thickness measurements were also made and it can be seen from Figure 17 that for the range of SRRs tested here, which covers conditions most relevant to gear contact, the EHL film tube forming rolls thickness does not decrease with increasing SRR. This is consistent with the theory that a reduction in entrainment speed and increased bulk temperature due to increasing SRR can only result in an increase in frictional heat input and not cause a reduction in EHL film thickness.
Forming Rolls
In tube roll forming, a metal strip gets bent into an enclosed shape. This can be a straight line or a series of bends. The bending is accomplished with a set of forming rolls that are precisely machined to bend the metal into the desired profile.
These forming rolls are usually made of aluminum bronze alloy QA520H or QA900 — a material that is well-suited for the application because it offers excellent sliding properties, high hardness, and low elongation. These properties facilitate the forward movement of the forming rolls, reduce friction between the forming rolls and the roll forming line metal strip, and extend the life of the forming rolls through repetitive bending operations.
Forming rolls must also withstand the high angular springback associated with AHSS. This type of steel has higher strength and different microstructures than traditional mild steel or even AISI 1020 carbon steel, so it requires greater bending tolerances. For example, a roll forming line that curves AHSS steel with a ten-degree radius may need to overbend the metal by three to five degrees.
To reduce downtime and labor costs, some tube roll forming lines are designed to reorient themselves. This allows a fabricator to run different profiles on one line, which increases efficiency and productivity. For instance, a duplex line used to make bleacher and seating parts can process different part families with the same equipment. The tooling for each of these different profiles is already loaded and preset on the rafts, so a fabricator can change from one profile to another by simply lifting the raft and replacing it with the raft with the desired new profile.
Finishing Rolls
Many metals are roll formed, including aluminum coil and steel, which is often embossed with wood grain finishes for garage doors. Unlike tubes and pipes, which are made from round shapes, roll forming produces nonround shapes, such as angles and C-sections.
Roll forming is a one-sided process, which means that the male and female rolls only engage with the material at the corners of the shape. This allows for tight, accurate radii when using a welded roll form system. Discrepancies between opposite or adjacent corner radii in a welded tube may occur because the mill tooling does not provide support on the inside of the shape, causing the workpiece to reshape and crown as it is drawn through the process.
Because of this, it is important for forming rolls to have good sliding properties with high load-bearing capacity and low coefficient of friction. For these applications, aluminum bronze alloys like QA520H and QA900 are ideal due to their combination of high hardness and excellent sliding properties.
When a roll-forming machine is used to make a nonround shape, it must use an alternating driven pass and idle side pass pattern to reduce springback. Some manufacturers also use a “form square, weld square” process that applies additional pressure during the last step to stabilize the welding. However, for more complex shapes—such as the steel section bars in farm machinery cabs—it is better to use a rolling mill that provides both bending and forming capabilities.
