Electric power steering system

An electric power steering system comprising:

• a worm that has a first end portion and a second end portion;
• a worm wheel that engages with the worm at an engagement region;
• a speed reducer that includes the worm and the worm wheel and that reduces a speed of rotation of an electric motor;
• a housing that houses the speed reducer;
• a first bearing that includes an outer ring fixed to the housing and an inner ring loosely fitted to the first end portion of the worm, the first bearing supporting the first end portion of the worm so that the first end portion is rotatable and movable in an axial direction;
• a second bearing that includes an outer ring and an inner ring press-fitted to the second end portion of the worm, the second bearing being supported by the housing so that the second bearing is able to be displaced in a direction in which an inter-center distance between the worm and the worm wheel increases and in a direction in which the inter-center distance decreases, and the second bearing supporting the second end portion of the worm so that the second end portion is rotatable;
• a first elastic member and a second elastic member that are disposed on respective sides of the inner ring of the first bearing in the axial direction, and that elastically support the worm in the axial direction with respect to the inner ring of the first bearing;
• and
• an urging member that urges an outer circumference of the outer ring of the second bearing in the direction in which the inter-center distance decreases,
• wherein a sum of first frictional resistance in the axial direction between the inner ring of the first bearing and the worm and second frictional resistance in the axial direction between the urging member and the outer circumference of the outer ring of the second bearing is less than engagement frictional resistance in the axial direction of the worm.