A great deal of progress has been made in recent years on how tubes are formed and shaped. There now exist several types of tube bending machines and tools capable of fabricating the most complex pipe and tube designs that designers come up with.

But this doesn’t mean that it has become easy to fabricate tubes: the success of tube fabrication also depends on the designer’s understanding of essential design parameters, engineering material properties, and tooling.

This article presents essential information about simple pipe and tube bending designs. It will serve as a guide to help engineers design pipes and tubes for different applications.

Pipes that have undergone the bending process. Source: frog/Adobe Stock

## What happens during the bending process?

The pipe bending process typically involves using mechanical force to push a pipe against a die, forcing it to conform to the shape of the die. When this process occurs, two things happen:

1. Fibers at the outside wall of the pipe are in tension, causing thinning and elongation of the outside wall
2. Fibers at the inside wall of the pipe are in compression, causing thickening and shortening of the inside wall

High-quality bends usually depend on the type of material used, outside and inside diameter of the tube, wall thickness, and centerline radius of the tube, among others.

## Some key terms and formulas for pipe and tube bending design

Figure 2 shows a section of a pipe that has undergone the bending process. It will be used to illustrate the following key terms.

Sectional view of a bent pipe. Source: Temitayo Oketola

### Centerline and centerline radius (CLR)

The centerline, sometimes referred to as the neutral line, is the imaginary line, or axis, that runs longitudinally along the pipe through the midpoint of its diameter. No compression or stretching occurs along this line during bending.

The centerline radius (CLR) is the distance from the center of curvature to the pipe's centerline or neutral line.

### Outside diameter (OD) and inside diameter (ID) of the pipe

The outside diameter is the longest distance across the outside dimensions of the tube or pipe. In contrast, the inside diameter is the longest distance across the inside dimensions of the tube.

### Wall thickness

This wall thickness describes the distance between the outside and inner diameter of the pipe.

### “D” of bend

Also called the severity of bend, the D of bend is the ratio between the bend's centerline radius and the pipe's outside diameter.

It is recommended that designers work with a high “D” of bend since a smaller “D” of bend usually translates to a higher likelihood of the pipe or tube breaking during the bending process.

But if the functionality of the tube design depends on having a low “D” of bend, here are a few tips designers can take to improve the likelihood of success:

• Choose a material with high ductility as this allows more elongation during the bending process and reduces the possibility of fracture.
• Increase the wall thickness of the pipe as this allows more material to flow during the bending process and reduces the possibility of fracture.

### Length of bend

When bending a pipe or tube, designers should remember that they are only constructing a part of a circle. As such, the length of the bend is simply the circumference of the arc formed from the bending process.

Where:

Bend angle is measured in degrees

So consider a scenario where a designer is looking to bend a pipe using the heat induction bending process. The total length of pipe that must be heated to bend the pipe through 90° to a radius of 70 mm is approximately 110 mm.

## The importance of the material’s ductility

When designing a part for bending, the material’s ductility plays a vital role in the likelihood of accurately fabricating the tube or pipe. Ductility describes a material’s ability to have its shape elongated without losing its strength or breaking.

For bent pipe designs, engineers can express the pipe material’s ductility by calculating the elongation required to achieve the desired angle and centerline radius without fracturing.

So consider a scenario where a designer is looking to bend a tube with an OD of 3 inches on a 6 inches CLR. The material must have an elongation of at least 25%.

As a rule, engineers should choose materials with high ductility as this correlates to easier bending and higher elongation.

## Conclusion

The process of bending tubes and pipes is pretty straightforward. Designers will have no problem fabricating simple bends so long as they adhere to the tips mentioned in this article. For more complex bends, designers are advised to reach out to pipe and tube manufacturers to discuss their projects.