Electric power steering device

An electric power steering device, comprising:

• a steering shaft;
• a gear fitted on an outer circumference of the steering shaft;
• a steering assistance motor generating a torque to be transmitted to the steering shaft via the gear;
• a first rotation restraining element, sandwiched between and always contacting both the outer circumference of the steering shaft and an inner circumference of the gear, so as to be radially deformed;
• a second rotation restraining element fitted to the steering shaft and the gear via clearances so as to restrain relative rotation between the steering shaft and the gear;
• wherein;
• a frictional restraining torque for restraining the relative rotation between the steering shaft and the gear is generated by exerting a radial force corresponding to the radial deformation of the first rotation restraining element on the steering shaft and the gear;
• the restraining torque is less than a breaking torque of the second rotation restraining element;
• and
• when the second rotation restraining element is broken, the relative rotation between the steering shaft and the gear is allowed so as to enable manual steering operation for fail-safe driving.