Conical springs

The main advantage of conical springs is their ability to compress into themselves, significantly reducing the axial space required at full compression.
They require more radial space at the large-diameter end, but because the coils nest inside one another, the compressed height can be extremely low.
Another advantage is their non-linear load characteristic. During compression, the larger coils deflect first. As compression increases, progressively smaller and stiffer coils engage, increasing the spring force. This behaviour provides effective shock absorption.
Conical springs are therefore widely used in compact equipment such as battery contacts and push-button mechanisms.