JP 10 131066 A discloses a conventional 7×7 multi-strand steel cord adapted to reinforce heavy duty tyres. In this 7×7 multi-strand steel cord, each strand consists of one core filament and six sheath filaments. This 7×7 multi-strand steel cord construction suffered in some cases from an insufficient degree of rubber penetration. As a result, each strand, and especially the core strand, suffered from core migration, bad fatigue behavior and was subject to corrosion just because there was almost no rubber penetration between the six sheath filaments and the core filament.
Since a few decades the art of steel cords has known a continuous trend towards trying to achieve full rubber penetration between the individual steel filaments of the steel cords, attempts have been made to allow rubber to fully penetrate. EP 0 841430 A discloses a steel cord comprising one core filament with a first wave form and six layer filaments with a second wave form arranged around the core filament wherein the first wave form substantially different from the second wave form. The object of this wavy form is to create micro-gaps between the layer filaments and the core filament to allow rubber to penetrate. Such a layer steel cord with a sufficient rubber penetration can be manufactured using only one twisting step. But in order to achieve full rubber penetration in a multi-strand steel cord, it is not enough only to create micro-gaps between the layer filaments and the core filament(s) in each strand; the micro-gap between the core strand and the adjacent outer strand is also very important.