Driving force transmission unit and image forming apparatus including same

1. A driving force transmission unit, comprising:

a first coupling to transmit a rotary driving force, the first coupling including a first contact portion and a hole, an inner circumferential surface of the hole having a rounded rectangular shape in cross section including a circular arc portion;

a second coupling including a second contact portion disposed at a position facing the first contact portion to contact the first contact portion along a direction of rotation of the second coupling; and

a first rotary shaft connected to the first coupling, the first rotary shaft including a shaft portion, an outer circumferential surface of the shaft portion having a rounded rectangular shape in cross section including a circular arc portion inserted into the hole of the first coupling,

the first contact portion being disposed between a first imaginary line extending from a center of rotation of the first rotary shaft passing through one end of the circular arc portion of the first rotary shaft and a second imaginary line extending from the center of rotation of the first rotary shaft passing through the other end of the circular arc portion in a direction of rotation of the first rotary shaft, and within a first area including the circular arc portion of the hole as viewed from the second coupling side,

the first coupling includes a first projection having the first contact portion;

the second coupling includes a second projection having the second contact portion and disposed adjacent to the first projection, and

a radius of an outer circumference of the first projection, relative to the center of rotation, is different from a radius of an outer circumference of the second projection, relative to the center of rotation.

2. The driving force transmission unit according to claim 1 ,

wherein the first contact portion is on one end of the first projection in a direction of rotation of the first coupling, and the entire first projection is disposed within the first area.

3. The driving force transmission unit according to claim 1 , wherein the first coupling includes, as the first contact portion, a plurality of first contact portions in a direction of rotation of the first coupling, and the second coupling includes, as the second contact portion, a plurality of second contact portions in the direction of rotation of the second coupling, and

wherein the plurality of first contact portions is disposed in the first area.

4. The driving force transmission unit according to claim 1 , wherein the first coupling is a drive coupling and the first contact portion is deviated from an imaginary center plane including a rotation center axis of the first rotary shaft.

5. The driving force transmission unit according to claim 4 , wherein the first contact portion is deviated away from a center of the first coupling.

6. The driving force transmission unit according to claim 4 , further comprising a second rotary shaft connected to the second coupling,

wherein an amount of deviation of the first contact portion from the imaginary center plane including the rotation center axis of the first rotary shaft is greater than at least one of an amount of liner axial incongruity between the rotation center axis of the first rotary shaft and a rotation center axis of the second rotary shaft and an amount of deviation between the rotation center axis of the first rotary shaft and the rotation center axis of the second rotary shaft at a connecting portion at which the first coupling and the second coupling are connected, attributed to angular axial incongruity between the rotation center axis of the first rotary shaft and the rotation center axis of the second rotary shaft.

7. The driving force transmission unit according to claim 6 , wherein the direction of deviation of the first contact portion is not parallel with a direction of the parallel shift.

8. The driving force transmission unit according to claim 1 , further comprising a second rotary shaft connected to the second coupling,

wherein the second coupling is a driven coupling, and the second contact portion is deviated from an imaginary center plane including a rotation center axis of the second rotary shaft.

9. The driving force transmission unit according to claim 8 , wherein the second contact portion is deviated away from a center of the second coupling.

10. The driving force transmission unit according to claim 8 , wherein an amount of deviation of the second contact portion from the imaginary center plane including the rotation center axis of the second rotary shaft is greater than at least one of an amount of liner axial incongruity between a rotation center axis of the first rotary shaft and the rotation center axis of the second rotary shaft and an amount of deviation between the rotation center axis of the first rotary shaft and the rotation center axis of the second rotary shaft at a connecting portion at which the first coupling and the second coupling are connected, attributed to angular axial incongruity between the rotation center axis of the first rotary shaft and the rotation center axis of the second rotary shaft.

11. The driving force transmission unit according to claim 10 , wherein the direction of deviation of the second contact portion is not parallel with a direction of the parallel shift.

12. An image forming apparatus, comprising:

a rotary member; and

the driving force transmission unit according to claim 1 to rotatably drive the rotary member.