Helical Bending of Bars for Strakes

We often receive orders for bending bars the hard way helically. While entering orders for such helical bending of stainless flat bar, I noticed that very often they have the same pitch—57.9 degrees.  Helical bending of steel sections can be used for mixer blades, handrails on staircases, and auger conveyors.

But it turns out that this 57.9 degree pitch is more than just a coincidence: it is by design…the design of helical strakes.  Helical strakes are used to improve the aerodynamic properties of towers, smoke stacks, chimneys, or any other round pole.

Imagine a fluid like wind flowing across a solid smooth long cylinder.  The wind flows on alternating sides of the cylinder create an oscillating force perpendicular to the wind which is called Vortex Induced Vibrations (VIVs). The strakes prevent all of those oscillating forces from working in sync to generate very large oscillations.  So even though strakes increase the overall wind drag, by twisting the vibrations out-of-phase they prevent the much of the damage of VIV’s.

It turns out that the optimal strake pitch is five times the diameter of the pipe (Pitch distance = 5D).   The 5D pitch can be converted to a pitch angle: Angle = arctan( 5D / pi*D) = arctan(5/pi) = 57.9 degrees  (independent of diameter).

Helical bending of hard way stainless bars for strakes.  Note the checking fixture to guarantee that the stakes will fit the cylinder.


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