Fixing device and image forming apparatus using the same

1. A fixing device for fixing a toner onto a recording medium on which an unfixed toner image is formed, said fixing device comprising:

a heating roller;

an endless belt to rotate around a pressure applying member, the pressure applying member having a recess portion on a surface thereof opposed to the heating roller when the pressure applying member is not pressed toward the heating roller, the pressure applying member comprising an elastic layer that presses the endless belt against the heating roller;

a nip through which the recording medium is passed; and

a low-friction member disposed between the endless belt and the pressure applying member, and provided along a moving direction of the recording medium,

wherein a ratio of a radius of curvature of the recess portion in a transferring direction to a radius of curvature of an outer diameter of the heating roller is set to 1.1 or more,

wherein a radius of curvature of the pressure applying member is larger than a radius of curvature of the heating roller, and

wherein a pressure distribution on the nip in the transferring direction has a protrusion shape with a peak at a nip centerline.

2. The fixing device according to claim 1 , wherein when viewed from a sectional section, a width of the pressure applying member with respect to the transferring direction is increased gradually from a side opposing the heating roller toward a surface on an opposite side to a surface opposing the heating roller.

3. A fixing device according to claim 1 , wherein the pressure applying member includes at least two protrusion portions on a surface thereof opposing the heating roller,

wherein top ends of the at least two protrusion portions are provided between both end surfaces of the pressure applying member in the transferring direction, and

wherein the top ends of the at least two protrusion portions have a curved shape.

4. A fixing device according to claim 3 , wherein the at least two protrusion portions are smoothly connected to the recess portion.

5. The fixing device according to claim 1 , further comprising:

a lubricating oil supply device that supplies a lubricating oil on an inner surface of the endless belt.

6. An image forming apparatus, comprising:

a fixing device for fixing a toner onto a recording medium on which an unfixed toner image is formed, comprising:

a heating roller;

an endless belt to rotate around a pressure applying member, the pressure applying member having a recess portion on a surface thereof opposed to the heating roller when the pressure applying member is not pressed toward the heating roller, the pressure applying member comprising an elastic layer that presses the endless belt against the heating roller; and

a nip through which the recording medium is passed; and

a low-friction member disposed between the endless belt and the pressure applying member, and provided along a moving direction of the recording medium,

wherein a ratio of a radius of curvature of the recess portion in a transferring direction to a radius of curvature of an outer diameter of the heating roller is set to 1.1 or more,

wherein a radius of curvature of the pressure applying member is larger than a radius of curvature of the heating roller, and

wherein a pressure distribution on the nip in the transferring direction has a protrusion shape with a peak at a nip centerline.

7. An image forming apparatus according to claim 6 , wherein the pressure applying member has at least two protrusion portions on a surface thereof opposing the heating roller,

wherein top ends of the at least two protrusion portions are provided between both end surfaces of the pressure applying member in the transferring direction, and

wherein the top ends of the at least two protrusion portions have a curved shape.