Forming Angle Formula of Spiral Steel Pipe

The spiral steel pipe forming angle is the acute angle between the center line of the strip steel (or coil) and the axis of the formed pipe. In SSAW pipe production, it is calculated with the forming angle formula cos α = B / (π × D), where B is the strip width and D is the mean diameter of the pipe. The angle determines which strip width is needed for a given pipe diameter, so it directly affects raw material use, forming quality, and production efficiency. The formula is given below, followed by a calculation example.

Spiral Steel Pipe Forming Angle Formula

The forming angle formula for spiral steel pipe (SSAW) is:

cos α = B / (π × D)




Where:
α = forming angle, the acute angle between the center line of the strip steel or coil and the axis of the formed spiral steel pipe
B = width of the strip or coil
D = mean diameter of the steel pipe (outer diameter − wall thickness)

Once B and D are known, calculate the ratio B / (π × D), then take the inverse cosine to get α.

Calculation Example

Given a pipe with outer diameter 1,016 mm and wall thickness 16 mm, made from strip with a width of 1,500 mm:

Mean diameter D = 1,016 − 16 = 1,000 mm
π × D = 3.1416 × 1,000 = 3,141.6 mm
cos α = 1,500 / 3,141.6 = 0.4775
α = arccos(0.4775) ≈ 61.5°

The spiral pipe forming angle in this case is about 61.5°. The formula can also be rearranged to find the strip width for a chosen angle: B = π × D × cos α.

Why is the Forming Angle Important?

As the formula shows, the forming angle and the strip width are linked for any given pipe diameter. The choice of forming angle therefore affects:

Raw material: The same pipe diameter can be made from different strip widths by changing the angle, so the angle influences which coil widths can be used and how well they are utilized.
Forming and product quality: The angle sets the orientation of the helical weld seam, and with it the quality of the finished pipe.

Production efficiency: The angle also determines the length of helical weld seam per meter of pipe, which affects welding time.

Forming Angle and Spiral Weld Characteristics

The SSAW pipe forming angle also sets the geometry of the helical weld. Because the strip edges run parallel to the strip center line, the weld seam lies at the forming angle α to the pipe axis. Two weld characteristics follow from this.

Weld seam length. The length of helical weld per meter of pipe is:

L = 1 / cos α = π × D / B

In the example above (α ≈ 61.5°), this gives about 2.1 m of weld per meter of pipe. A wider strip for the same diameter means a smaller angle and a shorter seam.

Stress across the weld. A straight seam lies along the pipe axis, so the full hoop stress acts across it. A helical seam is inclined to the hoop direction, so the stress acting on it is lower. Under internal pressure, taking the axial stress as half the hoop stress:
σ = σh × √(¼ × sin²α + cos²α)
where σh is the hoop stress. At α ≈ 61.5°, σ is about 65% of σh.
A larger forming angle therefore means a narrower strip, a longer weld per meter, and a lower stress across the weld.