Sheet feeding device and image forming apparatus

1. A sheet feeding device comprising:

a casing;

a sheet feeding tray attached to the casing in a manner that the sheet feeding tray can be slid into and out from the casing and including a stacking plate on which sheets are to be stacked;

a conveying unit configured to convey the sheets stacked on the stacking plate in a direction orthogonal to both a sliding direction of the sheet feeding tray and a stacking direction of the sheets;

an optional extension unit configured to be removably attached to an upstream side of the stacking plate in a conveying direction, and including an extension plate configured to extend the stacking plate to allow a long sheet to be stacked; and

a first locking unit configured to be capable of being held in a first lock position where the sheet feeding tray cannot be slid when the optional extension unit is attached while, when the optional extension unit is removed, held in a first unlock position where the sheet feeding tray can be slid.

2. The sheet feeding device according to claim 1 , further comprising:

a slide operation unit with which a sliding operation of the sheet feeding tray is to be performed; and

a second locking unit configured to move, in conjunction with the sliding operation performed using the slide operation unit, from a second lock position where the sheet feeding tray cannot be slid to a second unlock position where the sheet feeding tray can be slid,

wherein the first locking unit is configured to lock the second locking unit to prevent the second locking unit from moving from the second lock position to the second unlock position when the first locking unit is in the first lock position.

3. The sheet feeding device according to claim 2 , wherein the first locking unit is configured to be moved from the first unlock position to the first lock position by being pushed by the optional extension unit.

4. The sheet feeding device according to claim 2 , wherein the first locking unit includes:

a lock piece configured to be movable between the first lock position where the sheet feeding tray cannot be slid and the first unlock position where the sheet feeding tray can be slid; and

an urging unit configured to urge the lock piece to the first unlock position.

5. The sheet feeding device according to claim 2 , further comprising a lower-to-limit operation unit with which an operation for lowering the stacking plate to a lower-limit position is to be performed.

6. The sheet feeding device according to claim 5 , further comprising a control unit,

wherein the optional extension unit includes an openable cover that can be opened and closed and configured to cover the extension plate from above, and

the control unit is configured to lower the stacking plate to the lower-limit position when the lower-to-limit operation unit is operated, and lift the stacking plate to an upper-limit position when the openable cover is changed from an open state to a closed state.

7. The sheet feeding device according to claim 2 , further comprising a control unit,

wherein the optional extension unit includes an openable cover that can be opened and closed and configured to cover the extension plate from above, and

the control unit is configured to lower the stacking plate to the lower-limit position when the openable cover is opened, and lift the stacking plate to an upper-limit position when the openable cover is closed.

8. The sheet feeding device according to claim 2 , wherein the first locking unit is provided at both sides of the sheet feeding tray in the conveying direction.

9. The sheet feeding device according to claim 2 ,

wherein a plurality of the sheet feeding trays is housed in the casing in a vertically-stacked arrangement, and

the optional extension unit is removably attached to each of the sheet feeding trays.

10. The sheet feeding device according to claim 2 , further comprising:

an air blowing unit configured to blow air to the sheets, thereby lifting up downstream ends of the sheets in the conveying direction toward the conveying unit; and

a height increasing unit configured to cause the sheets to be stacked such that upstream portions of the sheets in the conveying direction are positioned at a higher level in the stacking direction than downstream portions of the sheets in the conveying direction.

11. The sheet feeding device according to claim 1 , wherein the first locking unit is configured to be moved from the first unlock position to the first lock position by being pushed by the optional extension unit.

12. The sheet feeding device according to claim 1 , wherein the first locking unit includes:

a lock piece configured to be movable between the first lock position where the sheet feeding tray cannot be slid and the first unlock position where the sheet feeding tray can be slid; and

an urging unit configured to urge the lock piece to the first unlock position.

13. The sheet feeding device according to claim 1 , further comprising a lower-to-limit operation unit with which an operation for lowering the stacking plate to a lower-limit position is to be performed.

14. The sheet feeding device according to claim 13 , further comprising a control unit,

wherein the optional extension unit includes an openable cover that can be opened and closed and configured to cover the extension plate from above, and

the control unit is configured to lower the stacking plate to the lower-limit position when the lower-to-limit operation unit is operated, and lift the stacking plate to an upper-limit position when the openable cover is changed from an open state to a closed state.

15. The sheet feeding device according to claim 1 , further comprising a control unit,

wherein the optional extension unit includes an openable cover that can be opened and closed and configured to cover the extension plate from above, and

the control unit is configured to lower the stacking plate to the lower-limit position when the openable cover is opened, and lift the stacking plate to an upper-limit position when the openable cover is closed.

16. The sheet feeding device according to claim 1 , wherein the first locking unit is provided at both sides of the sheet feeding tray in the conveying direction.

17. The sheet feeding device according to claim 1 ,

wherein a plurality of the sheet feeding trays is housed in the casing in a vertically-stacked arrangement, and

the optional extension unit is removably attached to each of the sheet feeding trays.

18. The sheet feeding device according to claim 1 , further comprising:

an air blowing unit configured to blow air to the sheets, thereby lifting up downstream ends of the sheets in the conveying direction toward the conveying unit; and

a height increasing unit configured to cause the sheets to be stacked such that upstream portions of the sheets in the conveying direction are positioned at a higher level in the stacking direction than downstream portions of the sheets in the conveying direction.

19. The sheet feeding device according to claim 1 , wherein

the optional extension unit includes an extension-unit base fixed to the casing, and

an end fence is selectably and removably fixable to the extension-unit base and at least one of the extension plate and the stacking plate.

20. An image forming apparatus comprising the sheet feeding device according to claim 1 .