A sheet transport device includes a transport unit that transports a sheet to a reading position by a first transport roller, a driving unit that switches a rotating direction, a rotating unit that rotates the first transport roller in one direction, the rotating unit including a one-direction gear that directly meshes with a first planet gear when the driving unit rotates in a first direction and meshes with a second planet gear when the driving unit rotates in a second direction, a switch unit that switches between states in which rotation of the one-direction gear is or is not transmitted to the first transport roller, and a displacement-force applying part that applies a force for displacing the first planet gear to mesh with the one-direction gear when the first planet gear touches the switch unit.

A sheet transport device includes a transport unit that transports a sheet to a reading position by a first transport roller, a driving unit that switches a rotating direction, a rotating unit that rotates the first transport roller in one direction, the rotating unit including a one-direction gear that directly meshes with a first planet gear when the driving unit rotates in a first direction and meshes with a second planet gear when the driving unit rotates in a second direction, a switch unit that switches between states in which rotation of the one-direction gear is or is not transmitted to the first transport roller, and a displacement-force applying part that applies a force for displacing the first planet gear to mesh with the one-direction gear when the first planet gear touches the switch unit.

A rotary element for a rotary transmission has a helical radial projection disposed about a rotary axis thereof, the helical radial projection having leading and trailing edges of different diameter. A helical peripheral surface between the leading and trailing edges has a radial profile which is inclined to a tangent to the envelope and preferably has an elliptic profile in a radial plane. In use with another such rotary element having a helical radial projection of opposite handedness in a rotary transmission, a point (P) of rolling contact of the helical peripheral surface with a helical peripheral surface of the other such rotary element helically traverses the helical peripheral surfaces of both rotary elements to positively transmit rotary drive between them without interdigitation of their respective helical radial projections and the associated sliding friction between them as arising in a gear transmission.

A gear structure includes two gears meshing with each other to transmit power. In one of the two gears, some of a plurality of teeth are partially missing in a tooth width direction. The other of the two gears has engaging portions that engage with the missing tooth portions at inter-teeth portions corresponding to the partially missing teeth among a plurality of inter-teeth portions.

A bevel gear pair with constant meshing characteristics with a constructed tooth pair is provided. The bevel gear pair with a constructed tooth pair includes a bevel gear I with a constructed tooth pair and a bevel gear II with a constructed tooth pair based on conjugate curves. In the present disclosure, a normal tooth profile curve of the bevel gear I with a constructed tooth pair and a normal tooth profile curve of the bevel gear II with a constructed tooth pair are continuous combined curves with the same curve shape, which facilitates machining by the same cutter. A common normal at an inflection point or a tangent point of the continuous combined curve passes through a pitch point of the bevel gear pair with a constructed tooth pair, and a position of the inflection point or the tangent point can be adjusted according to an actual demand.