In-situ-rubberized layered cord that can be used in a tire belt

1. A metal cord having two layers (Ci, Ce) of 3+N construction, rubberized in situ, comprising

an inner layer (Ci) formed from three core wires of diameter d 1 wound together in a helix with a pitch p 1 and an outer layer (Ce) of N wires, N varying from 6 to 12, of diameter d 2 , which are wound together in a helix with a pitch p 2 around the inner layer (Ci), wherein said cord is obtained by sheathing the inner layer formed by the three core wires with a diene rubber composition called a “filling rubber” which, for any length of cord of at least 2 cm, is present in the central channel formed by the three core wires and in each of the gaps lying between the three core wires and the N wires of the outer layer (Ce); and

the content of filling rubber in the cord is between 5 and 30 mg per g of cord, and

wherein said cord has the following characteristics (d 1 , d 2 , p 1 and p 2 are expressed in mm):

0.20<d 1 <0.50;

0.20<d 2 <0.50;

p 1 /p 2 ≦1;

5<p 1 <30;

10<p 2 <30,

wherein d 1 and d 2 are wire diameters and p 1 and p 2 are winding pitches,

wherein substantially no filling rubber is present on a periphery of the cord,

wherein the filling rubber is uniformly distributed inside and around the inner layer,

wherein, in an air permeability test according to ASTM D2692-98, the cord has an average air flow rate of less than 2 cm 3 /min, and

wherein the filling rubber is uniformly distributed inside and around the inner layer by twist balancing.

2. The cord according to claim 1 , wherein the diene elastomer of the filling rubber is chosen from the group formed by polybutadienes, natural rubber, synthetic polyisoprenes, butadiene copolymers, isoprene copolymers and blends of these elastomers.

3. The cord according to claim 2 , wherein the diene elastomer is natural rubber.

4. The cord according to claim 1 , wherein the following relationships are satisfied (d 1 and d 2 being in mm):

0.23≦ d 1 ≦0.40;

0.23≦ d 2≦0.40.

5. The cord according to claim 1 , wherein the following relationship is satisfied:

0.5≦ p 1/ p 2 ≦1.

6. The cord according to claim 1 , wherein p 1 =p 2 .

7. The cord according to claim 6 , wherein p 2 lies in the range from 12 to 25 mm.

8. The cord according to claim 1 , wherein the wires of the outer layer (Ce) are wound in a helix with the same pitch and in the same direction of twist as the wires of the inner layer (Ci).

9. The cord according to claim 1 , wherein the outer layer is a saturated layer.

10. The cord according to claim 1 , wherein the outer layer (Ce) comprises 8, 9 or 10 wires.

11. The cord according to claim 10 , wherein the wires of the outer layer (Ce) satisfy the following relationships:

for N= 8: 0.7≦( d 1 /d 2 )≦1;

for N= 9: 0.9≦( d 1 /d 2 )≦1.2;

for N= 10: 1.0≦( d 1 /d 2 )≦1.3.

12. The cord according to claim 1 , wherein d 1 =d 2 .

13. The cord according to claim 12 , wherein the outer layer comprises 9 wires.

14. The cord according to claim 1 , wherein the content of filling rubber is between 5 and 25 mg per g of cord.

15. The cord according to claim 14 , wherein the content of filling rubber lies in the range from 10 to 20 mg per g of cord.

16. The cord according to claim 1 , wherein, in the air permeability test according to ASTM D2692-98, the cord has an average air flow rate of less than 0.2 cm 3 /min.

17. A multistrand cord, at least one of the strands of which is a cord according to claim 1 .

18. A tire comprising a cord according to claim 1 .

19. A tire according to claim 18 , said tire being an industrial vehicle tire.

20. A tire according to claim 18 , the cord being present in the belt of the tire.