Round and square tubing bending process
When a metal tube is bent, the cross section material fibers experience tension on the outside of the bend area and compression on the inside of the bend area. Wall thickness on the outside cross section of the bend will thin-out due to tensile stresses, because the outer wall tends to be pulled in toward the centerline or neutral axis of the part.
If the ultimate tensile stress of the material is surpassed, then the tube will rip along the outer bend surface. The wall thickness on the inside of the tube bend will thicken due to compressive stresses. If the ultimate compressive strength of the material is reached, the tube will experience localized buckling resulting in rippling of the inside bend surface.
The Differences Between Pipe Bending And Tube Bending
It’s worth a mention firstly that there are two common terms used in industry, tube bending and pipe bending. Tube is measured by the outside diameter whereas pipe is measured by the inside diameter. Pipe sizing refers to the nominal inside diameter, sometimes called nominal bore, rather than the actual inside diameter.
Tube bending, or pipe bending, dates back over 100 years. There are dozens of different kinds of machines dependent on the desired shape, which can range from simple hand-operated tube benders through to a fully computerised numerically controlled machines.
When considering pipe bending or tube bending, there are many factors that need to be taken into account, all of which have a significant bearing on the end results. It’s always good practice to consult a specialist pipe bending sub-contractor who will offer advice on what material to use and which method will produce the best results. For example, compression pipe bending and draw pipe bending feasibility is limited to the tooling the specialist bending sub-contractor has in their stock, and even if they do have suitable tooling, the material grade of a pipe might render the production unfeasible. For instance, a draw pipe bending machine that can bend a 4” (101.6mm) pipe to a 3D (3 x Diameter) radius in mild steel would have difficulty bending the same size pipe to the same parameters in stainless steel due to the additional strength (torque) required to bend.
Diffs between bending round and square tubing
In tube bending you can bend round or square tubing. Round tubing has less material in the most highly stressed regions. The plane of highest tension or compression is tangent to the cross section so it is less prone to distortion. In square tubing the entire tube wall is parallel to this plane and so is exposed to the tensile and compressive stress at the outside and inside of the bend. This makes the square tube more susceptible to distortion.
Round tube allows metal to flow evenly in all directions during the bend which makes it easier to control distortion. Square tube material on the inside of the bend surface has a tendency to expand vertically and flow horizontally along the tube face. These forces behave in a certain way to draw in the vertical edges of the tube distorting the cross section into a concave shape. It is physically tougher to control distortion in a square tube than a round tube. A good rule of thumb when designing curved tubing is that tubes become harder to bend as the outside diameter increases and the wall thickness decreases.