Multilevel load limiting retractor with dual shifting mode

1. A seat belt retractor comprising:

a spool for seat belt protraction and for seat belt retraction;

an energy absorption mechanism for absorbing energy from the spool during seat belt protraction, the energy absorption mechanism having a first part for a relatively high rate of energy absorption and a second part for a relatively low rate of energy absorption; and

a shift mechanism configured to shift between the first part and the second part, the shift mechanism comprising a link movable between a first link position wherein the first part is engaged with the spool and a second link position wherein the second part is engaged with the spool, the link drivable between the first link position and the second link position by energy from seat belt protraction.

2. The seat belt retractor of claim 1 wherein at least one of the first part and the second part comprises a torsion bar.

3. The seat belt retractor of claim 1 wherein the link is drivable between the first link position and the second link position by relative movement between the energy absorption mechanism and the spool.

4. The seat belt retractor of claim 3 wherein the link is drivable between the first link position and the second link position by deformation of the energy absorption mechanism.

5. The seat belt retractor of claim 3 wherein the link comprises a runner linked to move with the spool, wherein the runner is screwed on a threaded member linked to move with the energy absorption mechanism.

6. The seat belt retractor of claim 1 including a coupler for coupling one of the first part and the second part to the spool, the coupler communicating with a control unit for controlling the coupler.

7. The seat belt retractor of claim 6 wherein the coupler has a first coupling position wherein the first part is engaged with the spool and has a second coupling position wherein the second part is engaged with the spool, the coupler moving between the first coupling position and the second coupling position in a direction generally along an axis of rotation of the spool.

8. The seat belt retractor of claim 1 wherein the energy absorption mechanism is arranged to absorb energy from the spool at the relatively high rate following the relatively low rate.

9. The seat belt retractor of claim 8 wherein the energy absorption mechanism is arranged to absorb energy from the spool initially at the relatively high rate, then switches to the relatively low rate, and subsequently switches back to the relatively high rate.

10. A seat belt retractor comprising:

a spool for seat belt protraction and seat belt retraction, the spool rotatable about an axis;

an energy absorption mechanism extending along the axis and supporting the spool, the energy absorption mechanism having a first part for providing a relatively high force for resisting seat belt protraction and a second part for providing a relatively low force for resisting seat belt protraction to the spool; and

a coupler for selectively engaging the first part and the second part to the spool, the coupler having a first coupling position wherein the first part is engaged with the spool and having a second coupling position wherein the second part is engaged with the spool, the coupler arranged to move between the first coupling position and the second coupling position in a direction generally along the axis.

11. The seat belt retractor of claim 10 including an actuator for moving the coupler between the first coupling position and the second coupling position.

12. The seat belt retractor of claim 11 wherein the actuator comprises a gas generator.

13. The seat belt retractor of claim 11 wherein the actuator is arranged to direct an actuation force in a direction transverse to the axis, the actuation force for moving the coupler between the first coupling position and the second coupling position.

14. The seat belt retractor of claim 13 including a member for redirecting the actuation force from the direction transverse to the axis to the direction generally along the axis.

15. The seat belt retractor of claim 14 including a guide structure for redirecting the member.

16. The seat belt retractor of claim 15 wherein the guide structure comprises a ramp.

17. A seat belt retractor comprising:

a spool for seat belt protraction and for seat belt retraction;

an energy absorption mechanism for absorbing energy from the spool during seat belt protraction, the energy absorption mechanism having a first part for a relatively high rate of energy absorption and a second part for a relatively low rate of energy absorption;

a shift mechanism configured to shift between the first part and the second part, the shift mechanism comprising a link movable between a first link position wherein the first part is engaged with the spool and a second link position wherein the second part is engaged with the spool, the link drivable between the first link position and the second link position by energy from seat belt protraction; and

a coupler for selectively coupling one of the first part and the second part to the spool, the coupler in communication with a control unit for controlling the coupler.

18. The seat belt retractor of claim 17 wherein the link is drivable between the first link position and the second link position by relative movement between the energy absorption mechanism and the spool.

19. The seat belt retractor of claim 17 wherein the energy absorption mechanism is arranged to absorb energy from the spool at the relatively high rate following the relatively low rate.

20. The seat belt retractor of claim 19 wherein the energy absorption mechanism is arranged to absorb energy from the spool initially at the relatively high rate, then switches to the relatively low rate through the control unit, and subsequently switches back to the relatively high rate through the shift mechanism.