Rotation driving force transmitting member, photoreceptor drum, process cartridge, and image forming apparatus

A rotation driving force transmitting member disposed between a driving source which is rotated around an axial line, and a driven body, the rotation driving force transmitting member comprising:

• a concave member provided on one of the driving source and the driven body, the concave member including a concave portion comprising a cross-section which defines a twisted elliptical space;
• and
• a convex member provided on the other of the driving source and the driven body, the convex member including a convex portion that has a twisted elliptical cross-section and which fits into the twisted elliptical space defined by the concave portion,
• a rotation driving force of the driving source is transmitted to the driven body via the convex member and the concave member in a state where the convex member is subjected to a drawing force toward the concave member side, due to rotation of the driving source in a state where the convex portion fits into the concave portion of the concave member;
• wherein a shape of the cross-section of the convex portion of the convex member is an ellipse in which a ratio between a major-axis length and a minor-axis length of the ellipse is 1.1 or more and 1.5 or less, and
• a shape of the cross-section of the concave portion of the concave member is ellipse in which a ratio between a major-axis length and a minor-axis length of the ellipse is 1.1 or more and 1.5 or less;
• and
• wherein a twisted amount of the convex portion of the convex member and a twisted amount of the concave portion of the concave member are 1°/mm or more and 15°/mm or less.