Sheet transport device including a sensor that detects positions of recording sheet being transported through transport route, and switching nail, and image forming apparatus

A sheet transport device comprising:

• a first transport route that guides a sheet;
• a roller that transports the sheet guided through the first transport route;
• a second transport route that guides the sheet transported in an opposite direction by reverse rotation of the roller;
• a switching nail supported by a first shaft parallel to a rotary shaft of the roller, to be set, by being made to reciprocatively rotate about the first shaft, to one of a first transport position for conducting the sheet guided through the first transport route to the roller, and a second transport position for conducting the sheet transported in the opposite direction by the reverse rotation of the roller, to the second transport route;
• a lever protruding from the first shaft, in a direction orthogonal to the first shaft;
• a sheet detection piece rotatably supported by a second shaft parallel to the first shaft, and protruding to the first transport route, the sheet detection piece being configured to retreat from the first transport route by rotating about the second shaft, upon being contacted by the sheet transported through the first transport route;
• a cam connected to the sheet detection piece via the second shaft, to rotate about the second shaft together with the sheet detection piece, when the sheet detection piece retreats from the first transport route;
• an intermediate rotary member rotatably supported by a third shaft parallel to the first shaft, to rotate, when the switching nail is switched to the second transport position and the first shaft and the lever rotate, about the third shaft by being contacted by the rotating lever, thereby contacting the cam and causing the cam to rotate;
• a rotary member rotatably supported by a fourth shaft parallel to the first shaft, to rotate, when the sheet detection piece retreats from the first transport route and the cam rotates, and when the switching nail is switched to the second transport position and the cam rotates, about the fourth shaft by being contacted by the rotating cam, and be displaced between a first posture of being spaced from the cam, and a second posture of being in contact with the cam;
• a sensor that detects that the rotary member is in the second posture of being in contact with the cam;
• a driver that switches the switching nail to one of the first transport position and the second transport position, by causing the switching nail to reciprocatively rotate about the first shaft;
• and
• a controller including a processor, and configured, when the processor executes a control program, to:
• switch the switching nail to one of the first transport position and the second transport position, by controlling the driver;
• decide that the sheet is transported through first transport route, when the sheet detection piece retreats by being contacted by the sheet transported through the first transport route, with the switching nail being set to the first transport position, and the sensor detects that the rotary member is in the second posture;
• and
• decide that the switching nail has been switched from the first transport position to the second transport position, when the sensor detects that the rotary member is in the second posture, after the sheet has passed the sheet detection piece.