Tube bend radius: minimum radius, ovality and wall thinning

Bend radius is the radius of the arc a tube is bent to. On round tube it is normally measured to the centerline and stated as a multiple of the outside diameter: 2D, 3D and so on. The tighter the radius for a given tube, the more the bend flattens into an oval, the thinner its outer wall becomes and the more likely the inside is to wrinkle. Each has a limit, set by the tube’s drawing or specification.

What is bend radius?

Bend radius is the radius of the arc of the bend, but on its own the term is incomplete. The arc can be measured to the inside edge of the tube, its centerline or its outside edge, and the three give different numbers. The tooling maker Bend Tooling says so in its terminology guide:

“When specifying a tube bend, it is necessary to state from which reference point the magnitude of the radius is determined.”

Bend Tooling, Bend Tooling Encyclopedia, entry “bend radius”

The same guide names the convention: the centerline radius (CLR) for round tubing, the inside radius for square and rectangular tubing. If a drawing or quotation does not say which, ask.

What bend radius does a tube bender give?

A tube bender gives the radius built into its tooling. In the FAA’s handbook for aircraft maintenance technicians, the radius blocks of a production bender “are so constructed that the radius of bend varies with the tube diameter”, and the radius “is usually stamped on the block.”

What do 2D and 3D mean?

“D” is the centerline radius divided by the tube’s outside diameter, so it says how tight a bend is relative to the tube. The US Navy’s aviation hose and tube manual (NAVAIR 01-1A-20) uses the same letter in its bend radius table: for 1 inch tube the 3D column reads 3.000 inches and the 4D column 4.000 inches.

The same D is not equally hard on every tube. The second number is the wall factor, the outside diameter divided by the wall thickness. Bend Tooling’s rule of thumb: “The higher the wall factor, the more difficult the bend”, because a thin wall needs more support at the point of bend.

Is there a minimum bend radius chart for steel tube?

There is no single minimum bend radius for steel tube, and no one chart that covers every job. The figure depends on the bending method and on whether the bore is supported. Open sources give these values:

The Fabricator’s figures are centerline radii. The NAVAIR table is for aircraft fluid lines.

A Turkish Ministry of National Education training module states its hand-bending rule as a diameter: at least twice the tube diameter, without saying where it is measured. High-pressure injection lines have their own rule, given in the article on how injector lines are made.

What happens when a bend is too tight?

In a tight bend the outside of the tube stretches and the inside is compressed. All three defects follow from that.

• Flattening (ovality). The Fabricator: the outside surface caves in as it stretches and the cross section loses its roundness. The Navy manual gives the cost: an elliptical section has less area, which reduces the fluid delivered or calls for more pumping power. The FAA handbook adds: “Excessive flattening causes fatigue failure of the tube.”
• Wall thinning. Bend Tooling: “The fixed amount of material contained in the wall must spread out to cover the extrados as its area increases at the point of bend.” The extrados is the outside of the bend; the inside wall thickens. In rotary draw bending the guide calls this thinning unavoidable.
• Wrinkles. The FAA handbook: “Wrinkled bends usually result from trying to bend thin wall tubing without using a tube bender.”

The first two pull against each other: according to the guide, the more flattening is reduced, the more the wall generally thins. The names vary too (ovality, flatness, reduction); what counts is the formula in the specification.

What does a mandrel do?

A mandrel sits inside the tube during bending and supports the wall from within, so the section cannot collapse. The Navy manual: “A mandrel provides internal support to the tube during bending.” The Turkish training module lists fillers that do the same job in hand bending: sand, a chain of balls or a helical spring.

Not every bend needs one. According to Bend Tooling, a thick wall or a generous radius can do without, but “if the wall factor of a tube exceeds 20, a mandrel is needed in most instances.” The second tool is the wiper die, which sits between the bend die and the tube and prevents a wrinkle on the inside radius. After sand or shot, the Navy manual wants all particles washed or blown out.

What should you check on a bent tube?

On a bent tube, check what the radius refers to, then the cross section of each bend, then its inside. The two open documents we found with numbered limits are aircraft maintenance manuals; the measuring suits any tube.

1. Read the drawing. Centerline radius or inside radius? Are there limits for flattening and wall thickness?
2. Measure the section. With a caliper, take the largest and smallest outside diameter at the middle of the bend. NAVAIR defines flatness as their difference divided by the nominal outside diameter; on corrosion-resistant steel and aluminum tube it allows 5 percent at 1000 psi or more, 10 percent below that. The FAA handbook’s test is coarser: the small diameter “must not be less than 75 percent of the original outside diameter”.
3. Look at the inside of the bend. NAVAIR accepts no wrinkles or kinks deeper than 1 percent of the outside diameter at 500 psi or more, 2 percent below that.
4. Ask about wall thickness. Many applications, the guide notes, require a minimum wall thickness in the finished part. It does not say how a buyer checks it; since a caliper cannot measure the wall in the middle of a bend, our inference is that the figure has to come from the maker.
5. Compare with the sample. The Navy manual lays the pattern on the new tube and bends to match; making a tube from a sample is covered in a separate article.

Neither document was written for automotive tube, and their limits are far apart. Our inference from that gap: the limit that applies is the one on the part’s drawing or specification; if there is none, ask before ordering. On an injector line, do not re-bend the tube by hand to make it fit, for the reason given in the injector line length article.