Export the spring at its working length, not just the free state

Every CAD workflow knows this old problem: the spring modeled in its free state is never the spring that appears in the assembly. In the real machine, the spring lives compressed between two seats, stretched between two hooks, or loaded at a working position. The free-state 3D model clashes through the neighbouring parts, and the designer ends up remodeling the spring by hand just to close the drawing.

That manual remodel usually follows a familiar recipe: recompute the pitch for the loaded length, redraw the helix, hope nobody changes the load afterwards. Besides being slow, the result almost always carries a silent error: assuming every coil closes by the same amount.

Why deflection is not uniform

In a constant-pitch cylindrical spring, all active coils share the same stiffness and really do close equally. In two very common cases, that stops being true.

• Variable pitch: the close-wound zone binds first. Once it lands, only the open zone keeps moving. The deflected pitch distribution is different from the free one.
• Conical, barrel and hourglass springs: each coil's stiffness depends on the cube of its diameter. The larger coils are far softer and bottom out first, while the small ones have barely moved. A uniformly shrunken helix is simply wrong.

What changes when geometry follows the physics

In our studio, the same model that computes the load-deflection curve drives the exported geometry. Ask for the spring at a working length and every coil closes exactly as much as the physics says: the tight zone of a variable-pitch spring lands first, the large coils of a conical spring pack before the small ones.

The result is a STEP, IGES, DXF or centerline CSV file that drops into the assembly at the right length, with the right shape. No remodeling, no macros, no constant-pitch approximation.

How to use it

• In the studio, scrub the simulator to the working position you want and export: the file comes out at that length.
• In the designer, type the working length in the CAD export menu, or leave it empty for the free state.
• If you prefer raw coordinates, the centerline CSV lists the wire's X, Y, Z points in millimetres, ready for a curve-through-points feature in any CAD package.

When this really matters

Clearance checks in tight assemblies, where the compressed spring defines the true envelope. Interference studies against the shaft or the bore. Product renders and documentation that show the mechanism in its working position. 3D-printed prototypes in the installed condition. In all of these, correct deflected geometry saves a full round of rework.

Exporting at length is one click. The engineering time it hands back is the part that counts.