Patent ID: 8050609

Claim:
A fixing device for fixing a toner image formed on a recording sheet, onto the recording sheet, comprising: a fixing roller adapted to be rotatable about a predetermined axis of rotation; a pressure roller disposed in pressure-contact with the fixing roller so as to be rotatable depending on a rotation of the fixing roller, the pressure roller holding and conveying a recording sheet on which a toner image is formed in cooperation with the fixing roller in a nip region where the pressure roller and the fixing roller abut against each other; a fixing gear coupled to the fixing roller, for driving the fixing roller; and a driving section having a driving gear engageable with the fixing gear, for driving the fixing gear by rotating the driving gear, wherein the fixing roller and the fixing gear are arranged concentrically and coupled to each other by inserting a key formed in one of the fixing roller and the fixing gear into a keyway formed in the other of the fixing roller and the fixing gear, the keyway is designed such that the key is allowed to move along a direction in which the fixing roller is rotated, in a cross section of the keyway perpendicular to the axis of rotation of the fixing roller, a maximum possible angle α of displacement of the key relative to the keyway is equal to or greater than a nip angle θ 1 on the nip region side, wherein θ 1 is an angle made by two line segments which are formed by connecting the axis of rotation of the fixing roller with one and the other of two intersections of the fixing roller and the recording sheet being in the passage through the nip region, a keyway portion defining the keyway, and the key are formed so as to each have a one-side contact region which is allowed to contact each other when the fixing roller rotates in a direction relative to the key, and the other-side contact region which is allowed to contact each other when the fixing roller rotates in the other direction relative to the key, and in the cross section of the keyway perpendicular to the axis of rotation of the fixing roller, a keyway width angle β on the side including the keyway, wherein β is an angle made by a line segment which is formed by connecting the one-side contact region of the keyway portion with the axis of rotation of the fixing roller and a line segment which is formed by connecting the other-side contact region of the keyway portion with the axis of rotation of the fixing roller, and β is equal to or greater than a sum angle (θ 1 +θ 2 ) consisting of the nip angle θ 1 on the nip region side, plus a key width angle θ 2 on the side including the key, wherein θ 2 is an angle made by a line segment which is formed by connecting the one-side contact region of the key with the axis of rotation of the fixing roller and a line segment which is formed by connecting the other-side contact region of the key with the axis of rotation of the fixing roller.