Patent Publication Number: US-5294106-A

Title: Image forming apparatus having detachable sheet cassette

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus having a sheet cassette detachably assembled to a main body portion of the apparatus for feeding each one of a sheets to an image forming section, and more particularly, to the image forming apparatus which facilitates removal of a jamming sheet. 
     In a conventional image forming apparatus of this kind, various proposals have been made in order to provide a mechanism for removing a jamming sheet if sheet jamming occurs in an internal portion of the apparatus. One example of such conventional arrangement is disclosed in a Japanese Patent Publication Hei 2-31465. 
     According to the disclosed image forming apparatus, are provided a sheet cassette provided detachably from a main body portion, a pair of rollers for nippingly feeding a sheet fed from the sheet cassette toward an image forming section, and a pair of guide plates provided spacedly from each other by a predetermined distance for defining therebetween a sheet feed path. 
     One of the rollers is supported on the main body portion, whereas the other roller is supported by the sheet cassette. Further, one of the guide plates is supported on the main body portion and other guide plate is supported on the sheet cassette. 
     If sheet jamming occurs in the above described image forming apparatus, the sheet cassette is detached from the main body portion, so that a space between the pair of rollers and a space between the pair of the guide plates are increased to provide an open space for facilitating removal of the jammed sheet. 
     However, in the conventional image forming apparatus, since the above described other roller and other guide plate are supported on the sheet cassette, distance between the pair of guide plates may be varied dependent on assembling condition of the sheet cassette with respect to the main body portion. If the distance is reduced, the sheet may be easily engaged with the guide plate, which leads to the sheet jamming. Further, sheet nipping force by the pair of rollers may be changed. If the nipping force is reduced, sheet slippage may occur relative to the rollers. Therefore, it becomes impossible to feed the sheet to a desired position, to thereby cause sheet jamming. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to overcome the above described drawbacks, and it is an object of the present invention to provide an image forming apparatus in which probability of sheet jamming may be reduced and jammed sheet, if any, can be easily removed. 
     This and other objects of the present invention will be attained by providing an image forming apparatus for forming an image on a sheet comprising a main body portion provided with an image forming section, a sheet cassette, sheet supplying means, sheet feeding means, sheet guide means and biasing means. The sheet cassette is detachably provided to the main body portion for storing therein a stack of a plurality of cut sheets. The sheet supplying means is adapted for supplying each one of the sheets from the sheet cassette. The sheet feeding means is adapted for feeding the sheet supplied by the sheet supplying means to the image forming section. The sheet feeding means comprises a first sheet feed roller rotatably supported by the main body portion, and a second sheet feed roller supported by the sheet cassette at a position confrontable with the first sheet feed roller when the sheet cassette is fully attached to the main body portion. The sheet guide means includes first and second guide plates provided at the main body portion for defining a sheet feed passage bridging between the sheet feeding means and the image forming section. The first guide plate is pivotally movable in a state of detachment of the sheet cassette from the main body portion for providing an open space at the sheet feed passage. The biasing means is provided in the sheet cassette for urging the second roller toward the first roller when the sheet cassette is attached to the main body portion. 
     In the image forming apparatus, thus constructed, when the sheet cassette is attached to the main body portion, the second roller is urged toward the first roller. With this state, if the sheet is delivered from the sheet cassette by the sheet supplying means, the sheet is fed toward the image forming section while being nipped between the first and second rollers through the sheet feed passage. If sheet jamming occurs, the sheet cassette is detached from the main body portion for moving the first and second rollers away from each other, and for pivotally moving the first guide plate in order to provide the open space at the sheet feed passage, to thereby facilitate removal of the jamming sheet. 
     The image forming apparatus further includes a roller guide portion provided at the main body portion for guiding and positioning the second sheet feed roller at a position confronting with the first sheet feed roller when the sheet cassette is attached to the main body portion. 
     With such arrangement, if the sheet cassette is attached to the main body portion, the second sheet feed roller is subjected to positioning at a position in confrontation with the first sheet roller by the roller guide portion. Simultaneously, the second sheet feed roller is urged toward the first sheet feed roller for providing sheet nipping arrangement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a perspective view showing a laser printer according to one embodiment of the present invention; 
     FIG. 2 is a schematic cross-sectional view showing an internal arrangement of the laser printer; 
     FIG. 3 is a front view showing an insertion slot and ambient portion of the laser printer for installation of a sheet cassette; 
     FIG. 4 is a perspective view showing the sheet cassette; 
     FIG. 5 is a cross-sectional view showing a grip portion of the sheet cassette; 
     FIG. 6 is a side view showing the sheet cassette; 
     FIg. 7 is a cross-sectional view partly showing the sheet cassette; 
     FIG. 8 is a partial cross-sectional view showing a state in which the sheet cassette is attached to the laser printer; 
     FIG. 9 is a partial cross-sectional view showing a state in which the sheet cassette is detached from the laser printer; and 
     FIG. 10 is a view showing a sheet supplying roller and ambient components in a state where the sheet cassette is attached to the laser printer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An image forming apparatus according to one embodiment of the present invention will be described with reference to FIGS. 1 through 10. According to the depicted embodiment, the present invention is applied to a laser printer which performs printing operation based on print data transmitted from an external equipment such as a personal computer and a work station. Throughout the specification, the expressions &#34;front&#34;, &#34;rear&#34;, &#34;above&#34;, &#34;below&#34; and &#34;laterally&#34; are used herein to define the various parts when the printer is disposed in an orientation in which it is intended to be used. 
     A laser printer LP will be briefly described. The laser printer is of an ordinary type and has a main frame 1 in which are provided a laser scanner unit (not shown), a process cartridge 2 (see FIG. 2), a transfer/separation unit having a transfer charger and a eraser needle (not shown), a fixing unit having a heat roller and a pressure roller (not shown), and a sheet feed unit having a supply roller, a resist roller 3, a feed roller and a discharge roller. Further, a manipulation panel 70 having a display portion 70 is provided in the laser printer LP. The display portion 70 is adapted to display an error message such as sheet jamming. 
     The process cartridge 2 will be described with reference to FIG. 2. A peripheral surface of a photosensitive drum 5 is provided with an electrostatic latent image by the laser scanner unit and through an application of a high voltage from a charger 4. The electrostatic latent image becomes a visible image by toners supplied thereto. The developed image is transferred to a sheet P supplied from a sheet feed mechanism 10 in synchronism with the rotation of the photosensitive drum 5. Thereafter, the sheet P is passed through the fixing unit and is discharged onto a discharge tray 6. 
     Next, will be described with reference to FIGS. 2 through 9, the sheet feed mechanism 10 provided in the main frame 1 of the laser printer LP. 
     The sheet feed mechanism 10 generally includes a sheet cassette 11 storing therein a plurality of sheets P, a sheet supplying roller 12 for supplying a sheet P in the sheet cassette 11 toward the process cartridge 2, a pair of first and second feed rollers 13 and 14, and a drive motor (not shown) for drivingly rotating the sheet supplying roller 12 and the first feed roller 13. The sheet supplying roller 12 constitutes sheet supplying means, and the first and second fed rollers 13 and 14 constitute feeding means of the present invention. 
     As shown in FIG. 4, the sheet cassette 11 has a box shape sheet accommodating portion 21 for accommodating therein a stack of the sheets P. A lifter plate 22 is disposed within the accommodating portion 21. The lifter plate 22 has a rear end portion (right side in FIG. 4) pivotally supported to the accommodating portion. The sheets P are stacked on the lifter plate 2. As shown in FIG. 8, a coil spring 24 is interposed between the lifter plate 22 and a bottom plate 23 of the accommodating portion 21. Thus, the lifter plate 22 is normally urged upwardly by the biasing force of the coil spring 24. 
     Each front portion of side walls 25 of the sheet cassette 11 is formed with an opening 26. On the other hand, a front portion of the lifter plate 22 is provided with laterally extending extensions 22a extending through the openings 26. 
     A stop member 27 is disposed at the side wall 25 at a position adjacent the opening 26. The stop member 27 has an upper portion provided with a projection 27a for pivotally moving the stop member about the projection 27a. Further, a notched portion 27b is formed at a lower rear portion of the stop member 27. The notched portion 27b is engageable with the extension portion 22a of the lifter plate 22 when the latter is pushed downwardly toward the bottom plate 23. By the engagement between the notched portion 27b and the extension portion 22a, the lifter plate 22 can maintain a lowermost position as shown in FIG. 9 against the biasing force of the coil spring 24. 
     At a front upper end portion of the stop member 27, a spring portion 27c extending frontwardly is integrally provided. One end of the spring portion 27c is engageable with an engagement wall 25a of the side wall 25. By the spring portion 27c, the stop member 27 is urged toward the extension portion 22a of the lifter plate 22. Because of the biasing force of the spring portion 27c, the notched portion 27b and the extension portion 22a can maintain engagement therebetween. 
     At a front lower end portion of the stop member 27, a projection 27d is integrally provided. The projection 27d projects laterally of the sheet cassette. The projection 27d laterally extends through a cut-out portion of the side wall 25 for engagement with a protruded portion 65 (see FIG. 3, and described later) when the sheet cassette 11 is assembled in the main frame 1 of the laser printer LP in order to disengage the notched portion 27b of the stop member 27 from the extension portion 22a of the lifter plate 22. 
     A pair of sheet guides 30, 31 are provided movably in the lateral or widthwise direction of the sheet P in the sheet accommodating portion 21. A co-actuation mechanism (not shown) is provided between the two sheet guides 30, 31 for automatically moving one of the sheet guides 30, 31 toward the remaining sheet guide 31 or 30 if the remaining sheet guide is moved toward the one of the sheet guides, and for automatically moving one of the sheet guides 30, 31 in a direction away from the remaining sheet guide 31 or 30, if the remaining sheet guide is moved away from the one of the sheet guides. 
     The sheet guides 30, 31 have guide surfaces 32 extending in parallel with a supplying direction of the sheet P. Each side edges of the stacked sheets P are abuttable on the guide surfaces 32 for regulating positions of the stacked sheets P. The guide surfaces 32 also serve to guide the travel of the sheet P which is delivered from the sheet cassette by means of the supply roller 12. 
     As shown in FIGS. 6 and 7, the sheet guide 30 has a box-like portion 33 at a front portion thereof and at a side confronting the side wall 25 of the sheet accommodating portion 21. Within the box-like portion 33 and at a front portion thereof, a recessed portion 34 is formed. The recessed portion 34 has an upper narrow part and a lower wide part sectioned by a stepped portion 34a whose height is approximately the half of the height of the box-like portion. A vertically movable sub-guide 35 is accommodated within the recessed portion 34. 
     The sub-guide 35 has a sub-guide surface 35a. A cut out portion 32a is formed at the guide surface 32. The cut out portion 32a is located in a range from an upper edge of the guide surface 32 to a half height position thereof, and the sub-guide surface 35a is positioned within the cut out portion 32a and is flush with the guide surface 32. 
     At a lower portion of the sub-guide 35, a spring portion 35b having S-shape is disposed. One end of the spring portion 35b is engaged with a bottom surface of the box-like portion 33. By the spring portion 35b, the sub-guide 35 is urged upwardly. The sub-guide 35 is provided with a pawl portion 35c engageable with the stepped portion 34a. Because of the biasing force of the spring portion 34b, the pawl portion 35c of the sub-guide 35 maintains engagement with the stepped portion 34a of the recessed portion 34. With this state, a top portion of the sub-guide 35 is protruded from the upper edge of the box-like member 33. The sub-guide surface 35a has a part corresponding to the protruded part. The protruded part of the sub-guide surface 35a is adapted for guiding the sheet P instead of the guide surface 32 when the stacked sheets P are partly lifted by the lifter plate to a position exceeding the guide surface 32 of the sheet guide 30 as shown in FIG. 10. 
     Incidentally, the other sheet guide 31 has the identical vertically movable sub-guide 35. Upper portions of these sub-guides 35 are normally protruded from the upper edge of the sheet guides 31. 
     At the right side sheet guide 30, a through hole 36 (FIG. 7) extending in a vertical direction is formed at a rear portion of the box-like portion 33. Within the through hole 36, a locking member 37 is provided. The locking member 37 includes two side plates 37a having right-angled triangular shape and an operating portion 37b connecting together the two side plates 37a. 
     The operating portion 37b of the locking member 37 has upper half portion protruded toward the guide surface 32 and a lower half portion positioned inward of the box-like member, so that a stepped portion is provided at a boundary between the upper and lower half portions. A part of the guide surface 32 is cut away at a position corresponding to the upper half portion, so that the operating portion 37b is fitted with the cut away portion. Incidentally, the upper half portion of the operating portion 37 is flush with the guide surface 32 when the operating portion is fitted with the cut away portion. 
     The side plate 37a of the locking member 37 is provided with a laterally extending projection 37c. Further a recessed portion 36a is defined at a wall of the box like portion 33. The projection 37c of the locking member 37 is insertable into the recessed portion 36a. Thus, the locking member 37 is pivotally movable about the projection 37c. 
     As shown in FIG. 7, an engagement projection 37d is provided at a lower end of the operating portion 37b of the locking member 37. The engagement projection 37d has one lateral side face, which confronts the right side wall 25, extending vertically, and another lateral side face, which faces a central portion of the sheet accommodating portion 21, extending obliquely. (In FIG. 7, a left side face of the projection 37d extends vertically, and a right side face thereof extends obliquely.) On the other hand, at position on the bottom plate of the sheet accommodating portion 21 and at a position confronting the engagement projection 37d, engagement grooves 23a are formed. The engagement grooves 23a extends in the sheet supplying direction, and spaced with each other corresponding to several widths of sheets. Each of the engagement grooves 23a has a one lateral side, which confronts the right side wall 25, extending vertically, and another lateral side face, which faces the central portion of the sheet accommodating portion 21, extending obliquely. 
     With the arrangement of the engagement projection 37d and the engagement grooves 23a, the sheet guide 30 is easily movable toward the opposite sheet guide 31, but cannot be movable in a direction away from the sheet guide 31. 
     The operating portion 37b of the locking member 37 is integrally provided with a spring portion 37e having one end engaged with an inner surface of the box-like portion 33 for urging the operating portion 37b toward the guide surface 32. 
     At a front center portion of the sheet accommodating portion 21, a separation member 38 (FIGS. 4, 8 and 9) is provided for supplying only an uppermost sheet P of the stacked sheets in the accommodating portion 21 in co-operation with the sheet supply roller 12. As shown in FIGS. 8 and 9, the separation member 38 is provided movably in a direction toward the supplying roller 12. Further, a coil spring 39 is provided between the separation member 38 and an inner wall 11a of the sheet cassette 11. Therefore, the separation member 38 is normally urged toward the supplying roller 12. Incidentally, a rubber plate 38a is attached to a separation member surface confronting the sheet supplying roller 12. 
     A grip portion 41 is provided at a front portion of the sheet accommodating portion 21 of the sheet cassette 11. Within the grip portion 41, the above described second feed roller 14 is disposed. The second feed roller 14 includes a shaft 14a and two rollers 14b mounted on the shaft 14a and spaced away from each other by a predetermined distance. Both ends of the shaft 14a extends laterally outwardly of the grip portion 41 through openings 43 formed at lateral sides of recesses 42 (FIG. 4) positioned at lateral sides of the grip portion 41 and at confronting sides of the sheet accommodating portion 21. The shaft 14a is movable within the openings 43. The grip portion 41 has a rear wall 44 at which slots 45 are formed for allowing the parts of the rollers 14b to be protruded therethrough. A leaf spring 46 is disposed in the grip portion 41, and the leaf spring 46 has one end contactable with the shaft 14a and another end fixed to the inner wall of the grip portion 41 for urging the rollers 14b toward the slots 45. The leaf spring 46 constitutes biasing means of the present invention. 
     A pair of locking plates 47 are disposed at a position adjacent to the both lateral side portions of the grip portion 41. That is, the recesses 42 at the lateral side portions have sides confronting the sheet accommodating portion 21, and slots 42a (FIG. 4) are formed at the confronted sides. The locking plates 47 extend through the slots 42a out of the grip portion 41. The locking plates 47 have base portions 47a supported by supporting pins 48 extending laterally inwardly from side walls in the grip portion 41, so that the locking plates 47 are pivotable about the supporting pins 48. 
     Each of the base portions 47a of the locking plate 47 is connected to each operation member 49 for rotating the locking plates 47 in a clockwise direction in FIGS. 8 and 9. Parts of the operation members 49 extend outwardly of the grip portion 41. Further, a coil spring 50 is interposed in the recessed portion 42 of the grip portion 41 and between a bottom wall 41a of the grip portion 41 and a lower end of the locking plate 47 for normally urging the locking plate 47 in a counterclockwise direction in these Figures. Furthermore, the exposed portion of the locking plate 47 is formed with an engaging groove 47b for engagement with a positioning pin 64 described later. 
     As best shown in FIG. 8, a sheet passage 52 is defined between the sheet accommodating portion 21 and the grip portion 41 for delivering a sheet fed from another sheet feed mechanism connected to a lower end portion of the main frame 1 toward the feed rollers 12 and 13. 
     A cassette insertion slot 55 is formed in the main frame 1 for assembly of the sheet cassette 11 thus constructed. Lower portions of pair of side plates 56 which support the sheet supplying roller 12 and the first feed roller 13 are visible through the insertion slot 55, these side plates 56 being visible at the upper lateral end portions of the slot 55. 
     The pair of side plates 56 rotatably support the above described resist rollers 3, the first feed roller 13 and the sheet supplying roller 12 these being arranged in order in a downward direction. Further, a guide plate 57 bridging between the sheet supplying roller 12 and the resist rollers 3 is fixedly secured between the side plates 56 for providing a sheet feed passage 58. At a position below the guide plate 57, a perforated slot 57a is formed for allowing a part of the first feed roller 13 to be protruded into the sheet feed passage 58 through the slot 57a. 
     A guide plate 59 is fixedly secured between the side plates 56 at a position between the sheet supplying roller 12 and the first feed roller 13 and in confronting relation to the guide plate 57 by a predetermined distance in order to guide travel of the sheet P delivered by the sheet supplying roller 12 toward a portion between the feed rollers 13 and 14 in co-operation with the guide plate 57. Further, a guide plate 60 is fixedly secured to the side plates 56 at a position between the first feed roller 13 and the resist roller 3 in confronting relation to the guide plate 57 by a predetermined distance for defining a sheet passage 58 in co-operation with the guide plate 57. 
     The guide plate 60 has a cut away portion at a position immediately above the second feed roller 14. Further, at a position adjacent the lower end of the guide plate 60, a pin 61 extends between the side plates 56 for pivotally supporting a movable guide plate 62. The movable guide plate 62 serves to define a part of the sheet feed passage 58 instead of the guide plate 60. At both lateral edge portions of the movable guide plate 62, projections 62a extend toward the guide plate 57. Because of own weight of the movable guide plate 62, tip ends of the projections 62a abut the guide plate 57. Thus, a predetermined space is defined between the guide plate 57 and the movable guide plate 62 because of the projections 62a. Incidentally, since the projections 62a are positioned at lateral edges of the movable guide plate 62, the projections do not interfere with the sheet P fed along the sheet feed passage 58. The above described guide plate 57 and the movable guide plate 62 constitute the guide portion of the present invention. 
     The movable guide plate 62 has a portion confronting the first feed roller 13. At the confronting portion of the movable guide plate 62, an opening 62b is formed, so that a part of the second feed roller 14 protrudes through the opening 62b and brings into contact with the first feed roller 13, when the sheet cassette 11 is inserted into the insertion slot 55. 
     As shown in FIGS. 8 and 9, guide portions 63 are provided at the side walls 56 at a position leftwardly relative to a supporting position of the fist feed roller 13 for guiding the shaft 14a of the second feed roller 14 along the guide portions 63, to thereby permit the second feed roller 14 to be accurately confronted with the first feed roller 13. In accordance with the inserting operation of the sheet cassette 11 through the insertion slot 55, the end portions of the shaft 13a laterally outwardly extending from the recessed portions 42 of the grip portion 41 are entered into the guide portion 63, so that the second feed roller 14 can undergo positioning to have an accurate confronting position relative to the first feed roller 13. The guide portion 63 constitute a roller guide portion as claimed in claim 2. 
     At a lower portion of the guide portion 63, positioning pins 64 extend from the side walls 56 in alignment with each other. When the sheet cassette 11 is inserted through the insertion slot 55, the engagement grooves 47b of the locking plate 47 are brought into engagement with the positioning pins 64 as best shown in FIG. 8, to thereby fix the sheet cassette 11 at a given position. 
     At both lateral sides and at lower portion of the insertion slot 55, a pair of projected portions 65 are provided. When the sheet cassette 11 is inserted through the insertion slot 55, each projection 27d of the stop member 27 is brought into engagement with the projected portions 65, so that the extension portion 22a of the lifter plate 22 is disengaged from the notched portion 27d of the stop member 27. Accordingly, the lifter plate 22 is pivotally moved toward the sheet supplying roller 12 by the biasing force of the coil spring 24. 
     As shown in FIG. 3, the sheet supplying roller 12 is positioned at a center portion of its support shaft. The sheet supplying roller 12 has a irregular shape in which a part of a cylindrical configuration is cut away. First pair of collars 66 are disposed on the support shaft at positions adjacent the both ends of the sheet supplying roller 12. Further, another collars are mounted on the support shaft, each being positioned between the side wall 56 and the first collar 66. These four collars 66 are provided rotatably about the support shaft, whereas displacement of these collars in an axial direction of the support shaft is prevented. Further, a radius of the collars 66 is slightly smaller than that of the arcuate portion of the sheet supplying roller 12. These collars 66 are adapted for preventing the sheet P from being fed diagonally in case of the sheet delivery by the sheet supplying roller 12. 
     Next, operation of the laser printer LP, thus constructed, will be described. 
     Prior to the printing operation, in case where the sheets P must be supplemented or the sheets must be replaced by another kind of sheets having another size, an operator grips the grip portion 41 and the operating member 49 so as to rotate the operating member 49 in a clockwise direction in FIG. 8. Consequently, the locking plate 47 is angularly rotated in the clockwise direction, so that the engagement groove 47a of the locking plate 47 is disengaged from the positioning pin 64 protruded from the side plate 56. With this state, the operator can pull out the sheet cassette 11. 
     For supplementing sheets P in the sheet cassette 11 which has been pulled out, the front end portion of the lifter plate 22 lifted by the coil spring 24 is manually depressed downwardly, so that the extension portion 22a of the lifter plate 22 is brought into engagement with notched portion 27b of the stop member 27. Since the stop member 27 is urged toward the extension portion 22a by the spring portion 27c, the engagement between the extension portion 22a and the notched portion 27b can be maintained, to thereby maintain the lower depressing position of the lifter plate 22 as shown in FIG. 9. With this state, the operator supplements sheets P. 
     If sheets having size different from that of the sheets P already stored in the pulled out sheet cassette 11 are to be used and accommodated in the cassette 11, the already stored sheets P are removed, and maintains downward position of the lifter plate 22. Then, the sheet guide 30 is moved dependent on a size of the newly stored sheets. 
     In this case, if the new sheets to be accommodated have size smaller than that of the already accommodated sheets, the new sheets are firstly placed on the center portion of the sheet accommodating portion 21 of the sheet cassette 11. Then, the sheet guide 30 are laterally depressed toward the center until the guide surface 32 abuts side edges of the newly stacked sheets. In this instance, the slanting surface of the locking projection 37d of the locking member 37 disposed at the sheet guide 30 is slidingly moved with respect to the slanting surface of the engagement groove 23a. Therefore, the locking member 37 is angularly moved in the counterclockwise direction in FIG. 7. Thus, the locking projection 37d is brought into locking engagement with a neighboring engagement groove 23a. Since the movement of the sheet guide 30 toward the opposite sheet guide 31 can be attained easily by the locking projection 37d and the engagement groove 23a, the position of the sheet guide 30 can be changed easily without manipulation to the locking member 37 for the purpose of releasing engagement between the locking projection 37d and the engagement groove 23a. 
     On the other hand, if the newly accommodating sheets have size greater than that of the already accommodated sheets, the operator depresses the operating portion 37d of the locking member 37 of the sheet guide 30 toward the side wall 25 of the sheet cassette 11. By this operation, the locking member 37 is angularly rotated about the projection 37c in a counterclockwise direction, so that the locking projection 37d is moved away from the engagement groove 23a of the sheet accommodating portion 21, to thereby provide disengagement of the locking projection 37d from the engagement groove 23a. With this state, the operator depresses the sheet guide 30 toward the side wall 25, so that a distance between the sheet guides 30 and 31 is increased. 
     The operating portion 37b is continued to be depressed so as to further move the sheet guide 30 toward the side wall 25 until the distance between the sheet guides 30 and 31 becomes greater than a width of the newly accommodating sheets. If the distance between the sheet guides 30 and 31 becomes greater than the sheet width, the movement of the sheet guide 30 is stopped, and the new sheets are placed on the lifter plate 22. Then, as described above, the sheet guide 30 is depressed toward the center of the sheet cassette 11 until the guide surface 32 abuts the side edges of the sheets. 
     The sheet cassette 11 in which the sheets P are accommodated in the sheet accommodating portion 21 is inserted through the insertion slot 55 of the laser printer LP in such a manner that a tail end of the cassette 11 is first inserted thereinto. During this installation, the sub-guides 35 upwardly protruded from the upper edges of the sheet guides 30 and 31 may occasionally abut collars 66 dependent on the position of the sheet guides 30, 31. However, since the sub-guides 35 are vertically movable, the sub-guides 35 are moved downwardly into the sheet guides 30 and 31 when they abut the collars 66. Therefore, the operator can insert the sheet cassette 11 through the insertion slot 55 without any special attention to the position of the sheet guides 30, 31. 
     If the sheet cassette 11 is further inserted through the insertion slot 55 from a state shown in FIG. 9, the projection 27d of the stop member 27 of the sheet cassette 11 is brought into engagement with the projection 65 positioned within the insertion slot 55. If the inserting movement is continued, the projection 65 urges the stop member 27 against the biasing force of the spring portion 27c, and the stop member 27 is angularly rotated about the projected portion 27a in a clockwise direction in FIG. 4. Thus, the notched portion 27b of the stop member 27 is disengaged from the extension portion 22a of the lifter plate 22. Accordingly, the lifter plate 22 can be angularly moved upwardly in the clockwise direction in FIG. 4 because of the biasing force of the coil spring 24 about the rear portion of the lifter plate 22, so that the uppermost sheet P of the sheet stack accommodated in the sheet accommodating portion 21 is brought into contact with the outer peripheral surfaces of the collars positioned adjacent to the sheet supplying roller 12. 
     If the sheet P is in contact with the collars 66 by the angular rotational movement of the lifter plate 22, a portion of the sheet P in the vicinity of the collars 66 is positioned above the upper edges of the sheet guides 30, 31 as shown in FIG. 10. With this state, if the sheet cassette 11 is further inserted through the insertion slot 55, the upper portion of the stacked sheets P may be misaligned from each other. However, according to the illustrated embodiment, the sheet cassette 11 has the sheet guides 30, 31 provided with the sub-guides 35 whose guide surfaces 35a can still guide the side edges of the sheets P. Thus, it is possible to avoid misalignment of the upper portion of the sheets. 
     If the sheet cassette 11 is further inserted through the insertion slot 55 from the state shown in FIG. 9, the locking plate 47 of the sheet cassette 11 is brought into contact with the positioning pin 64. By this contact, the locking plate 47 is angularly rotated in a clockwise direction in FIG. 9. If the sheet cassette 11 is further inserted, the engagement groove 47b of the locking plate 47 is brought into engagement with the positioning pin 64. By this engagement, the installation of the sheet cassette 11 into the main frame 1 of the laser printer LP is completed. Simultaneously, the both ends of the shaft 14a of the second feed roller 14 is moved into the guide portion 63, so that the second feed roller 14 is brought to a position in confrontation with the first feed roller 13 through the opening 62b of the movable guide plate 62. Moreover, the second feed roller 14 is brought into contact with the first feed roller 13 projected through the slot 57a of the guide plate 57. (see FIG. 8). 
     In the cassette installation state shown in FIG. 8, the second feed roller 14 is in contact with the first feed roller 13, and the shaft 14a of the second feed roller 14 is urged toward the first feed roller 13 by the leaf spring 46. Therefore, the sheet cassette 11 is urged in a direction to detach the cassette out of the insertion slot 55 due to the reaction force from the first feed roller 13. However, since the engagement groove 47b of the locking plate 47 is engaged with the positioning pin 64, the sheet cassette 11 can undergo positioning at the engagement position therebetween. 
     Will be described operation of the laser printer LP assembled with the sheet cassette 11 in a manner described above. 
     As shown in FIG. 8, in case where the sheet cassette 11 is assembled in the laser printer LP, the irregularly configured portion of the sheet supplying roller 12 is facing with the uppermost sheet P of the sheet stack stored in the sheet cassette 11. If print command signal together with print data are outputted from a host computer (not shown) connected to the laser printer LP, the laser printer LP starts printing operation in accordance with the outputted print data. 
     In this printing operation, the sheet supplying roller 12 is rotated in a clockwise direction in FIG. 8. Therefore, the cylindrical portion of the sheet supplying roller 12 is in contact with the uppermost sheet P for feeding the sheet P toward the first and second feed rollers 13 and 14. During this feeding, even if two or three sheets are simultaneously fed by the sheet supplying roller 12, feeding of the sheets other than the uppermost sheet is restrained by the rubber plate 38a attached to the separation member 38, so that only the uppermost sheet can be fed. Further, during this feeding operation, the portion of the sheet P at a position adjacent the sheet supplying roller 12 is positioned above the upper edges of the sheet guides 30, 31. Therefore, the sheet may not be subjected to accurate guiding by the guide surface 32 of the sheet guides 30, 31 in the sheet feeding direction. Thus, there is a probability that the sheet may be diagonally fed with respect to the sheet feeding direction. However, in the illustrated embodiment, since the sub-guides 35 extend from the upper edges of the sheet guides 30, 31, and since the guide surfaces 35a of the sub-guides 35 serve to guide the sheet at a position adjacent the sheet supplying roller 12 in the sheet feeding direction, diagonal feeding of the sheet P can be obviated. 
     The sheet P fed by the sheet supplying roller 12 passes between the guide plates 57 and 59 and reaches the first and the second feed rollers 13 and 14. A predetermined nipping pressure is established between these rollers 13 and 14 by the leaf spring 46. Therefore, these rollers 13 and 14 nip the sheet P therebetween, and the sheet P is directed toward the resist roller 3 by way of the sheet passage 58. 
     The sheet P passing through the sheet feed passage 58 by the feed rollers 13 and 14 passes between the guide plate 57 and the movable guide plate 62 and reaches the resist roller 3. The resist roller 3 is not rotated when the sheet P firstly reaches the roller 3. Thereafter, the resist roller 3 starts rotation in synchronism with the photosensitive drum 5. Upon rotation of the resist roller 3, the sheet P is fed toward the photosensitive drum 5, and the toner image formed on the photosensitive drum 5 is transferred onto the sheet P by the charger 4. The toner image is then fixed by the fixing unit (not shown), and the image carrying sheet is discharged onto the discharge tray 6. 
     If the sheet P cannot be fed during the printing operation, that is, if sheet jamming occurs, the jamming is detected by a sensor (not shown), and the printing operation is immediately stopped, and at the same time, the sheet jamming is displayed at the display portion 70a of the manipulation panel 70. Therefore, the operator can acknowledge the sheet jamming state, and manipulates the operating member 49 of the sheet cassette 11 for detaching the sheet cassette 11 from the laser printer LP. Consequently, the second feed roller 14 is moved away from the first feed roller 13. By angularly moving the movable guide plate 62 in the clockwise direction in FIG. 9, open space can be provided at the sheet feed passage 58 for facilitating removal of the jammed sheet. 
     As described above, the laser printer LP according to the depicted embodiment is provided with the guide plates 57 and 62, which constitute the sheet feed passage 58, and the first feed roller 13 at the main body portion, and the second sheet feed roller 14 at the sheet cassette 11. Therefore, a predetermined fixed distance can be provided between the guide plate 57 and the movable guide plate 62. Accordingly, it is possible to reduce possibility of sheet jamming at the sheet feed passage 58. Further, even if sheet jamming occurs at the sheet feed passage 58, the first and second feed rollers 13 and 14 are moved away from each other by detaching the sheet cassette 11 from the laser printer LP, and the sheet feed passage 58 can be opened by pivotally moving the movable guide plate 62. As a result, jamming sheet can be easily removed. Moreover, in spite of the movable arrangement of the second feed roller 14 relative to the first feed roller 13, stable nipping pressure can be established therebetween because of the accurate positioning of the second feed roller 14 relative to the first feed roller 13 by the engagement between the pin 64 and the engagement groove 47b and between the guide portion 63 and the shaft 14a of the second feed roller 14. Accordingly, it becomes possible to restrain slippage of the sheet relative to the surfaces of the rollers 13 and 14, to thereby further avoid sheet jamming. 
     In the sheet cassette 11 according to the illustrated embodiment, the sheet guide 30 can be moved toward the opposite sheet guide 31 without any manipulation to the locking member 37. Therefore, the sheet guide 30 can be easily displaced to a desired position in accordance with the size of the sheet P. Further, since the sheet guide 30 cannot be moved away from the opposite sheet guide 31 unless the locking member 37 is manipulated. Therefore, accidental displacement of the sheet guide 30 is avoidable. 
     Further, in the sheet cassette 11 according to the present embodiment, the sub-guides 35 are provided which protrude from the upper edges of the sheet guides 30 and 31, and the sub-guides can be sunk into the sheet guides 30, 31. Therefore, it is unnecessary to draw special attention to the position of the sheet guide 30 when the sheet cassette 11 is attached to the laser printer LP through the insertion slot 55, and diagonal feeding of the sheet P can be obviated by the supplemental sheet edge guide by the sub-guides 35. 
     As is apparent from the foregoing descriptions, according to the image forming apparatus of the present invention, probability of occurrence in sheet jamming can be reduced, and the jammed sheet, if any, can be easily removed from the sheet feed passage. 
     While the invention has been described in detail and with reference to specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention.