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A seat belt retractor has a winding shaft to wind a seat belt. A motive spring pretensions the winding shaft in the winding direction of the seat belt. An electric motor provides a torque that can be transmitted to the motive spring to change the spring force. A switch coupling transmits the torque

A seat belt retractor has a winding shaft to wind a seat belt. A motive spring pretensions the winding shaft in the winding direction of the seat belt. An electric motor provides a torque that can be transmitted to the motive spring to change the spring force. A switch coupling transmits the torque of the electric motor drive to the winding shaft to retract the seat belt. To transmit the torque, a drive wheel, which is rotatably mounted on a shaft non-rotatably and coaxially connected to the winding shaft and which is non-rotatably connected to an outer spring end of the motive spring by a rigid rotational connection. A first active surface of the coupling is movably mounted against a restoring force to a component that is securely connected to the rigid rotational connection and rotatable around the shaft. A second active surface of the coupling is non-rotatably connected to the shaft.

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I claim: 1. A seat belt retractor comprising: a winding shaft to wind a seat belt; a motive spring that pretensions the winding shaft in a seat belt winding direction; a spring housing in which the motive spring is arranged on a side of the spring housing; an electric motor, from which a torque can

I claim: 1. A seat belt retractor comprising: a winding shaft to wind a seat belt; a motive spring that pretensions the winding shaft in a seat belt winding direction; a spring housing in which the motive spring is arranged on a side of the spring housing; an electric motor, from which a torque can be transmitted to the motive spring to change a spring force; a shaft extension that is non-rotatably and coaxially connected to the winding shaft; a drive means that is non-rotatably connected to an outer spring end of the motive spring by a rigid rotational connection connected to the spring housing, the drive means being rotatably mounted on the shaft extension; and a switch coupling located radially inside the rigid rotational connection, via which the torque of the electric motor can be transmitted to the winding shaft to retract the seat belt, the switch coupling having a first active surface, a second active surface, a coupling part and a second coupling part, the first active surface being located on the coupling part mounted in a movable manner to the spring housing on an opposite side of the spring housing relative to the motive spring and movably mounted against a restoring force to the coupling part which is securely connected through the spring housing to the rigid rotational connection and rotatable around the shaft extension and the second active surface of the switch coupling is non-rotatably connected to the shaft extension by the second coupling part; encircling the coupling part to transmit torque of the electric motor to the winding shaft the switch coupling has the first active surface brought into an engaged position with the second active surface causing the rotation of the shaft extension to rotate the winding shaft wherein the rigid rotational connection provides transmission of force or torque from the drive means to the motive spring. 2. The seat belt retractor according to claim 1, wherein the first active surface of the switch coupling can be moved into an engaged position with the second active surface by an electromagnetic signal or by inertial or a centrifugal force. 3. The seat belt retractor according to claim 2, wherein the drive means is a gear comprising a drive wheel that transmits the torque. 4. The seat belt retractor according claim 3, wherein the electric motor, the motive spring and the switch coupling are a unit that can be assembled on the shaft extension. 5. The seat belt retractor according to claim 3, wherein the drive wheel of the gear is a gear wheel. 6. The seat belt retractor according to claim 3, wherein the gear further comprises a screw gear, comprising a front screw gear, or a worm gear. 7. The seat belt retractor according to claim 3, wherein the gear is a worm gear in combination with the drive wheel. 8. The seat belt retractor according to claim 3, wherein the gear, the switch coupling and the motive spring are located on a side of the seat belt retractor on which a vehicle sensor and a belt webbing sensitive sensor are also located. 9. The seat belt retractor according to claim 8, wherein the seat belt retractor has another side where a pyrotechnic driven tightening means is located, the pyrotechnic driven tightening means arranged to drive the winding shaft in the winding direction to tighten the seat belt. 10. The seat belt retractor according to claim 9, wherein the electric motor and the pyrotechnic driven tightening means are secured to a seat belt retractor frame. 11. The seat belt retractor according to claim 10, wherein a unit consisting of the gear, the switch coupling and the motive spring is also secured to the seat belt retractor frame. 12. The seat belt retractor according to claim 8, wherein the electric motor can be activated by a sensor signal indicating a dangerous driving situation. 13. The seat belt retractor according to claim 1, wherein the drive means is a gear that transmits the torque. 14. The seat belt retractor according claim 1, wherein the electric motor, the motive spring and the switch coupling are a unit that can be assembled on the shaft extension. 15. The seat belt retractor according to claim 1, wherein the switch coupling is located between the driving means and the motive spring. 16. The seat belt retractor according to claim 1, wherein the switch coupling is engaged at a minimum rotational speed or a minimum acceleration in during rotation of the winding shaft in the belt winding direction and released during rotation of the winding shaft in a belt extraction direction. 17. A seat belt retractor comprising: a winding shaft to wind a seat belt; a motive spring that pretensions the winding shaft in a seat belt winding direction; a spring housing in which the motive spring is arranged on a side of the spring housing; a shaft extension that is non-rotatably and coaxially connected to the winding shaft; a drive means that is non-rotatably connected to an outer spring end of the motive spring by a rigid rotational connection connected to the spring housing, the drive means being rotatably mounted on the shaft extension; an electric motor, from which a torque can be transmitted to the motive spring to change a spring force, wherein the electric motor can be activated by a sensor signal indicating a dangerous driving situation; a switch coupling located radially inside the rigid rotational connection via which the torque of the electric motor can be transmitted to the winding shaft to retract the seat belt, the switch coupling having a first active surface, a second active surface, a coupling part and a second coupling part, the first active surface being located on the coupling part mounted in a movable manner to the spring housing on an opposite side of the spring housing relative to the motive spring and movably mounted against a restoring force to the coupling part which is securely connected through the spring housing to the rigid rotational connection and rotatable around the shaft extension and the second active surface of the switch coupling is non-rotatably connected to the shaft extension by the second coupling part; to transmit torque of the electric motor to the winding shaft the switch coupling has the first active surface brought into an engaged position with the second active surface causing the rotation of the shaft extension to rotate the winding shaft wherein the rigid rotational connection provides transmission of force or torque from the drive means to the motive spring, wherein the first active surface of the coupling can be moved into an engaged position with the second active surface by an electromagnetic signal or by inertial or a centrifugal force; and a pyrotechnic driven tightening means arranged to drive the winding shaft in the winding direction to tighten the seat belt. 18. A seat belt retractor comprising: a winding shaft to wind a seat belt; a motive spring that pretensions the winding shaft in a seat belt winding direction; a spring housing in which the motive spring is arranged on a side of the spring housing; a shaft extension that is non-rotatably and coaxially connected to the winding shaft; a drive means that is non-rotatably connected to an outer spring end of the motive spring by a rigid rotational connection connected to the spring housing, the drive means being rotatably mounted on the shaft extension; an electric motor, from which a torque can be transmitted to the motive spring to change a spring force, wherein the electric motor can be activated by a sensor signal indicating a dangerous driving situation; a switch coupling located radially inside the rigid rotational connection, via which the torque of the electric motor can be transmitted to the winding shaft to retract the seat belt, the switch coupling having a first active surface, a second active surface, a coupling part and a second coupling part, the first active surface being located on the coupling part mounted in a movable manner to the spring housing on an opposite side of the spring housing relative to the motive spring and movably mounted against a restoring force to the coupling part which is securely connected through the spring housing to the rigid rotational connection and rotatable around the shaft extension and the second active surface of the switch coupling is non-rotatably connected to the shaft extension by the second coupling part; to transmit torque of the electric motor to the winding shaft the switch coupling has the first active surface brought into an engaged position with the second active surface causing the rotation of the shaft extension to rotate the winding shaft wherein the rigid rotational connection provides transmission of force or torque from the drive means to the motive spring, wherein the first active surface of the coupling can be moved into an engaged position with the second active surface by an electromagnetic signal or by inertial or a centrifugal force; a pyrotechnic driven tightening means arranged to drive the winding shaft in the winding direction to tighten the seat belt; and the coupling and the motive spring are located on a side of the seat belt retractor on which a vehicle sensor and a belt webbing sensitive sensor are also located, and the seat belt retractor has another side where the pyrotechnic driven tightening means is located.