Valve opening-closing timing control apparatus

1. A valve opening-closing timing control apparatus comprising:

a driving-side rotor that rotates about a rotation axis synchronously with a crankshaft of an internal-combustion engine;

a driven-side rotor that is arranged on an inner side of the driving-side rotor coaxially with the rotation axis and rotates integrally with a camshaft for valve opening and closing of the internal-combustion engine; and

a phase adjustment mechanism that sets a relative rotation phase between the driving-side rotor and the driven-side rotor, by using drive force of an electric actuator, wherein

the phase adjustment mechanism includes an output gear disposed on the driven-side rotor coaxially with the rotation axis, an input gear rotating about an eccentric axis in an orientation parallel with the rotation axis, having a number of teeth less than the output gear, being arranged coaxially with the eccentric axis, and being connected to the driving-side rotor via an Oldham coupling, and a cylindrically shaped eccentric member supporting, on an inner side of the input gear, the input gear in such a way as to cause the input gear to rotate about the eccentric axis, and is configured in such a way as to cause the eccentric axis to revolve by means of rotation of the eccentric member driven by drive force of the electric actuator and change a meshing position of the output gear and the input gear,

the Oldham coupling has an engagement arm projecting in a radially outward direction with the rotation axis as a center,

at least either of the driving-side rotor and the input gear has an engagement portion with which the engagement arm is engaged, and is connected to the Oldham coupling,

the engagement arm has a pair of arm flat surface portions perpendicular to a rotational direction of the driving-side rotor,

the engagement portion has a pair of engagement flat surface portions that the pair of arm flat surface portions face in a sliding contact manner,

each of the arm flat surface portions is configured in such a way as to be, within a range in which one of the pair of arm flat surface portions slides with respect to one of the pair of engagement flat surface portions that the arm flat surface portion faces, always in contact with an entirety of an overlapping portion of the engagement flat surface portion with the arm flat surface portion when the arm flat surface portion and the engagement flat surface portion are viewed from a direction that is perpendicular to a sliding direction of the Oldham coupling and in which the arm flat surface portion and the engagement flat surface portion overlap each other,

the pair of arm flat surface portions are both outer side surfaces of the engagement arm,

the engagement portion is a groove being formed in the driving-side rotor and penetrating in a radial direction,

the pair of engagement flat surface portions of the engagement portion are both inner side surfaces of the groove, and

each of the arm flat surface portions has a portion that, in a range in which the arm flat surface portion slides in a radial direction with respect to one of the pair of engagement flat surface portions that the arm flat surface portion faces, always protrudes in the radially outward direction beyond the engagement flat surface portion.

2. The valve opening-closing timing control apparatus according to claim 1 , further comprising

a second engagement arm orthogonal to a projecting direction of the engagement arm,

a second engagement portion engaged with the second engagement arm, the second engagement portion being a protrusion formed on the input gear and projecting in a direction along the rotation axis,

a pair of second arm flat surface portions of the second engagement arm that are both inner side surfaces of an engagement recessed portion formed in the second engagement arm,

a pair of second engagement flat surface portions of the second engagement portion that are both outer side surfaces of the protrusion, wherein

each of the second engagement flat surface portions has a portion that, in a range in which one of the second arm flat surface portions that one of the pair of second engagement flat surface portions faces slides in a direction along the rotation axis, always protrudes in a direction along the rotation axis beyond the second arm flat surface portion.

3. The valve opening-closing timing control apparatus according to claim 2 , further comprising

a front plate fixed to the driving-side rotor on a far side from the camshaft with respect to the Oldham coupling in a direction along the rotation axis, wherein

a plurality of the second engagement portions are disposed on the input gear at positions that face each other with the eccentric axis interposed between the positions among positions in radial directions with the eccentric axis as a center, and

the front plate has a retreat portion formed at a part facing each of the second engagement portions, the retreat portion avoiding the front plate coming into contact with the second engagement portions.