Inflatable curtain with passive pretensioner

1. An apparatus for helping to protect an occupant of a vehicle that has a side structure and a roof, the apparatus comprising:

an inflatable curtain that is inflatable away from the vehicle roof to a position adjacent the side structure of the vehicle; and

a pretensioner for connecting the curtain to the vehicle, the pretensioner comprising a rotational element that is rotatable relative to the vehicle, the inflatable curtain being connected to the rotational element;

the inflatable curtain upon deployment applying a force to the rotational element that causes the rotational element to rotate, the rotational element being configured such that angular momentum causes the rotational element to continue to rotate and thereby tension the inflatable curtain.

2. The apparatus recited in claim 1 , wherein the inflatable curtain is connected to the rotational element at a location on the rotational element that is spaced from an axis of rotation of the rotational element.

3. The apparatus recited in claim 1 , wherein the inflatable curtain applies the force to the rotational element until the curtain reaches a predetermined deployed condition, at which point further rotation of the rotational element due to angular momentum applies tension to the inflatable curtain.

4. The apparatus recited in claim 1 , wherein the rotational element comprises a hub and a tensioning arm that extends radially from the hub, the inflatable curtain being connected to the tensioning arm.

5. The apparatus recited in claim 1 , further comprising a tether that connects the inflatable curtain to the pretensioner, the pretensioner tensioning the inflatable curtain via the tether.

6. The apparatus recited in claim 1 , wherein the pretensioner when mounted to the vehicle is positioned at least partially below a vehicle belt line.

7. The apparatus recited in claim 1 , wherein the inflatable curtain is connected to the pretensioner along a lower edge of the curtain.

8. The apparatus recited in claim 7 , wherein the inflatable curtain is connected to the pretensioner adjacent a corner of the curtain.

9. The apparatus recited in claim 1 , wherein the pretensioner further comprises a locking mechanism for blocking rotation of the rotational element in a direction opposite the direction of rotation imparted by the deploying curtain.

10. The apparatus recited in claim 9 , wherein the locking mechanism comprises one of a rack mechanism and a ratcheting mechanism.

11. The apparatus recited in claim 9 , wherein the locking mechanism is located internally within the rotational element.

12. The apparatus recited in claim 1 , wherein the rotational element comprises an inertial mass that when rotated by the deploying curtain has an angular momentum sufficient to apply a tensioning force to the inflatable curtain.

13. The apparatus recited in claim 12 , further comprising a tether that connects the inflatable curtain to the rotational element, the rotational element tensioning the tether to apply the tensioning force to the inflatable curtain.

14. A pretensioner for applying a pretensioning force to an inflatable curtain, the pretensioner comprising:

a rotational element connectable to the inflatable curtain, the rotational element being configured such that deployment of the inflatable curtain causes the rotational element to rotate in a first direction, the rotational element being configured to continue rotating in the first direction due to angular momentum after the force applied by the inflating curtain is relieved, the continued rotation of the rotational element applying a tensioning force on the inflatable curtain.

15. The pretensioner recited in claim 14 , further comprising a locking mechanism for blocking rotation of the rotational element in a second direction opposite the first direction in order to maintain the tension applied to the inflatable curtain by the rotational element.

16. The apparatus recited in claim 15 , wherein the locking mechanism comprises one of a rack mechanism and a ratcheting mechanism.

17. The apparatus recited in claim 15 , wherein the locking mechanism is located internally within the rotational element.

18. The apparatus recited in claim 14 , wherein the rotational element comprises a hub and a tensioning arm that extends radially from the hub, the inflatable curtain being connected to the tensioning arm.

19. A method for pretensioning an inflatable curtain that is inflatable away from the vehicle roof to a position adjacent the side structure of the vehicle, the method comprising:

fixing a pretensioner to the vehicle, the pretensioner comprising a rotational element that is rotatable relative to the vehicle

connecting the inflatable curtain to the rotational element so that deployment of the inflatable curtain applies a force to the rotational element that causes the rotational element to rotate about the axis; and

configuring the rotational element such that angular momentum causes the rotational element to continue to rotate and thereby tension the inflatable curtain.

20. The method recited in claim 19 , wherein:

connecting the inflatable curtain comprises connecting the inflatable curtain to the rotational element via a tether; and

configuring the rotational element comprises:

configuring the tether to apply tension to the rotational element when the inflatable curtain deploys, the force applied to the rotational element causing the rotational element to rotate; and

configuring the tether and rotational element such that the rotational element continues to rotate after the inflatable curtain has at least substantially deployed such that the rotational element applies tension to the inflatable curtain via the tether.

21. The method recited in claim 20 , further comprising the step of configuring the rotational element with a locking mechanism for blocking rotation of the rotational element in a direction opposite the direction of rotation imparted by the deploying curtain.