Conical springs: when a tapered spring solves a packaging problem

What a conical spring is

A conical spring is a helical compression spring wound on a changing diameter, so the coils form a cone rather than a cylinder. The wire diameter is usually constant. The large end sits on a seat, the small end pushes against a plunger or a cap, and the coils can nest inside one another as the spring compresses.

The shape is chosen for one of three reasons: a short solid height, a stable column in a narrow space, or a progressive load curve. Often it is all three at once. The part still behaves as a spring, but the design equations for a cylindrical spring only describe it approximately, and the differences matter at the ends of the travel.

The packaging problem it solves

A cylindrical spring needs free length for travel and coil gap for deflection. Its solid height is roughly the wire diameter multiplied by the number of coils, plus the end contributions. In a shallow housing, that stack can be the reason the product is too tall, or the reason the spring has to be short and highly stressed.

A conical spring can collapse into a nested stack, so the solid height can be a small fraction of the free length. That lets a designer keep the free length and the travel while reducing the installed height. It also lets the spring sit over a shaft or inside a cup without a separate retainer, because the large end locates on the seat and the small end centres on the load.

Solid height and telescoping

When a conical spring compresses, the small coils enter the large coils. The load at which this starts depends on the pitch and the taper. Once the coils touch, the spring becomes much stiffer, and the rate curve turns upward. That is useful if the design needs a soft initial rate and a firm stop, and unhelpful if the design assumes a constant rate.

For illustration, a cylindrical spring with 1 mm wire and ten active coils has a solid height of roughly 11 mm, and its free length is limited by the gaps between those coils. A conical spring with the same wire and a strong taper can let each coil sit inside the one below it, so a five coil spring with a large end of 16 mm and a small end of 6 mm might reach a solid height of 2 to 3 mm. The numbers are illustrative, and the nesting only works if the diameter step between coils is at least two wire diameters.

The rate is not linear

A cylindrical spring with a fixed diameter has a nearly constant rate over its working range. A conical spring does not. The active coil diameter changes as the spring deflects, and the coils that touch stop contributing. The result is a rising rate, sometimes in two stages.

That behaviour has to be reflected in the drawing and the test. A single load at one height does not describe a progressive spring. Specify the load at two or three heights across the working range, plus the solid height and the load at solid. If the application needs a specific rate at a specific point, say so, because the same free length and wire can produce a different curve with a different taper.

Stability and buckling

A tall cylindrical spring can buckle sideways when the free length is large relative to the diameter. A conical spring is more stable because the small end acts like a centring feature and the coils nest rather than bow. That is why conical springs appear in tall, narrow cavities where a cylindrical spring would rub the bore.

Stability is not unconditional. If the small end is not guided, the spring can still lean. If the large end sits on an uneven seat, the first coil takes more load and the spring can tilt. Design a seat that supports the full large coil, and guide the small end with a boss or a recess if the application is sensitive to lateral movement.

Design variables and their limits

The main variables are the large end diameter, the small end diameter, the taper ratio between them, the wire diameter, the number of active coils, the pitch and the end type. Each one changes the rate, the solid height and the stress. The table gives the direction of the change and the limit to watch.

Making a conical spring

Conical springs are usually coiled on a CNC coiler with a variable pitch and a controlled diameter change. The setup is more demanding than a cylindrical spring because the feed, the pitch and the diameter have to change together. A short run may be made on a lathe or a manual coiler, which is slower and less consistent.

After coiling, the spring is usually stress relieved, and the ends may be ground square. Grinding a conical end is harder than grinding a cylindrical one, so the seat face may be less flat. If the application needs a square seat, say so and expect a higher cost. Plating and coating follow the same rules as any spring, with the added risk that a coating can bridge the nested coils and change the solid height.

When not to use one

A conical spring is the wrong choice when the design needs a constant rate, a precise load at a single height, or a long fatigue life at high stress. The small end works harder than the rest of the spring, and a progressive rate is difficult to hold within a tight tolerance. A cylindrical spring with a nested sleeve or a different packaging layout may be the better answer.

It is also a poor choice when the housing cannot support the large end or guide the small end. A conical spring in a loose cavity can tilt and rub, which defeats the stability argument. Check the seat, the guide and the load direction before choosing the shape.