  We show that the optimal packing of hard spheres in an infinitely long
cylinder yields structures characterised by a screw symmetry. Each packing can
be assembled by stacking a basic unit cell ad infinitum along the length of the
cylinder with each subsequent unit cell rotated by the same twist angle with
respect to the previous one. In this paper we quantitatively describe the
nature of this screw operation for all such packings in the range 1 <= D/d <=
2.715 and also briefly discuss their helicity.
