Seat belt pretensioner with brake

Apparatus for helping to protect an occupant of a vehicle seat, comprising:

• a seat belt;
• a seat belt retractor including a spool supported for rotation about a spool axis in a seat belt withdrawal direction and an opposite seat belt retraction direction;
• a pretensioner energizable to rotate said spool in the seat belt retraction direction to apply a predetermined amount of tension to said seat belt;
• a first mechanism to maintain the predetermined amount of tension on said seat belt after said pretensioner applies the predetermined amount of tension to the seat belt;
• and
• a second mechanism to block rotation of said spool in said seat belt withdrawal direction, said second mechanism being actuatable if the tension on said seat belt exceeds said predetermined amount.

Apparatus for helping to protect an occupant of a vehicle seat, comprising:

• a seat belt;
• a seat belt retractor including a spool supported for rotation about a spool axis in a seat belt withdrawal direction and an opposite seat belt retraction direction;
• an electric motor energizable to rotate said spool in the seat belt retraction direction and de-energizable, said electric motor being energizable to rotate said spool in the seat belt retraction direction to apply a predetermined amount of tension to said seat belt;
• and
• a brake for preventing rotation of a shaft of said motor, said brake comprising a magnetorheological fluid.