Ribbon lifting mechanism for selectively placing a printing ribbon and a correction ribbon at a printing position of a typewriter
BACKGROUND OF THE INVENTION 
     The present invention relates to a ribbon lifting mechanism for a typewriter of the type which is so constructed that either of a printing ribbon and correction tape selectively assumes the printing position by the turning of a holder in which both the printing ribbon and correction tape are accommodated. 
     A typical conventional ribbon lifting mechanism of the above-mentioned type is disclosed in U.S. Pat. No. 4,533,267 and it is constituted by the combination of a holder in which a printing ribbon and correction tape are accommodated one above another, a cam member for displacing the holder up and down with the aid of a cam function, a stopper member adapted to assume the restricting position where an extent of upward displacement of the holder is restricted in order that the printing ribbon is located at the printing position during printing operation and further assume the non-restricting position where an extent of upward displacement of the holder is not restricted in order that the correction tape is located at the printing position during corrective printing operation, and an electromagnet for displacing the stopper member from the restricting position to the non-restricting position. 
     In the conventional ribbon lifting mechanism the holder is displaced upwardly under the effect of a cam function caused by rotation of the cam member and the printing ribbon assumes the printing position while the stopper member is located at the restricting position due to the electromagnet which fails to be activated, when normal printing operation is performed. On the other hand, when corrective printing operation is performed, the electromagnet is activated and thereby the stopper member is located at the non-restricting position, resulting in an increased extent of upward displacement of the holder. Thus, the correction tape assumes the printing position. 
     In order that either of the printing ribbon and correction tape selectively assumes the printing position the conventional ribbon lifting mechanism requires special driving elements such as an electromagnet and other elements which serve to displace the stopper member from the restricting position to the non-restricting position. This leads to problems in that the ribbon lifting mechanism is complicated in structure and it is manufactured at an expensive cost. 
     SUMMARY OF THE INVENTION 
     Thus, the present invention has been made with the foregoing problems in mind and its object resides in providing an improved ribbon lifting mechanism of the previously-mentioned type disclosed in U.S. Pat. No. 4,533,267 the teachings of which are incorporated herein by reference which assures that either of the printing ribbon and correction tape selectively assumes the printing position by utilizing rotational operation of a ribbon lifting cam adapted to turn the holder in which both the printing ribbon and correction tape are accommodated. 
     Another object of the present invention is to provide an improved ribbon lifting mechanism of the previously-mentioned type which does not require special driving elements such as an electromagnet and other elements which are employed for the conventional ribbon lifting mechanism in order that either of the printing ribbon and correction tape selectively assumes the printing position. 
     Another object of the present invention is to provide an improved ribbon lifting mechanism of the previously-mentioned type which is simple in structure and can be manufactured at an inexpensive cost. 
     To accomplish the above objects there is proposed according to the invention a ribbon lifting mechanism for a typewriter which includes a holder in which both a printing ribbon and correction tape are accommodated, the holder being turnably supported in a frame, a ribbon lifting cam adapted to be driven by a motor, the ribbon lifting cam being formed with cam sections which serve to turn the holder so as to allow either of the printing ribbon and correction tape to selectively assume the printing position, and a single cam follower operatively connected to the holder, the cam follower being actuated in accordance with the ribbon lifting cam. 
     In a preferred embodiment of the invention the mechanism is provided with a stopper member which serves to locate the printing ribbon at the printing position during normal printing operation while it is engaged to the holder and moreover locate the correction tape at the printing position during corrective printing operation while it is disengaged from the holder. To carry out turning movement of the stopper member the mechanism further includes means for displacing the stopper member and the means is disposed on the ribbon lifting cam. 
     In the present invention based on the aforementioned structure, when a normal printing operation is performed, the ribbon lifting cam is rotated in the normal direction and thereby the holder is turned upwardly with the aid of a cam function of the ribbon lifting cam. At this moment the stopper means displacing means retains the stopper means at the predetermined position while an extent of upward displacement of the stopper means is restricted. Thus, the printing ribbon is located at the printing position. 
     When a corrective printing operation is performed, the ribbon lifting cam is rotated in the reverse direction and thereby the stopper member is disengaged from the holder by means of the stopper member displacing means. Thereafter, the holder is displaced upwardly by rotation of the ribbon lifting cam in the normal direction and the extent of upward displacement increases without any occurrence of restriction caused by means of the stopper member. Thus, the correction tape is located at the printing position. 
     Other objects, features and advantages of the invention will become readily apparent from reading of the following description which has been prepared in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings will be briefly described below. 
     FIGS. 1 to 6 show a ribbon lifting mechanism in accordance with the first embodiment of the invention. 
     FIG. 1 is a vertical sectional side view of the mechanism as seen from the right side. 
     FIG. 2 is a vertical sectional side view of the mechanism as seen from the left side, particularly showing the state where the stopper member is located at the operative position. 
     FIG. 3(A) is an enlarged side view of the engagement member as seen from the left side. 
     FIG. 3(B) is an enlarged plan view of the engagement member. 
     FIG. 3(C) is a perspective view of the engagement member in FIGS. 3(A) and (B) shown in the disassembled state. 
     FIG. 4(A) is a side view of the ribbon lifting cam as seen from the first cam follower side. 
     FIG. 4(B) is the same side view of the ribbon lifting cam as FIG. 4(A), particularly showing the operational relation between the ribbon lifting cam and the first cam follower. 
     FIG. 5(A) is a side view of the ribbon lifting cam as seen from the second cam follower side. 
     FIG. 5(B) is the same side view of the ribbon lifting cam as FIG. 5(A), particularly showing the operational relation between the ribbon lifting cam and the second cam follower, and 
     FIG. 6 is a vertical sectional side view of the mechanism similar to FIG. 2, particularly showing the state where the stopper member is located at the inoperative position. 
     FIGS. 7 to 10 show a ribbon lifting mechanism in accordance with the second embodiment of the invention. 
     FIG. 7 is a vertical sectional side view of the mechanism as seen from the left side, particularly showing the state where the stopper member is located at the operative position. 
     FIG. 8 is a vertical sectional side view of the mechanism similar to FIG. 7, particularly showing the state where the stopper member is located at the inoperative position. 
     FIG. 9 is an enlarged fragmental view of the mechanism as seen in the direction as identified by an arrow mark X in FIG. 7, particularly showing the ribbon lifting cam and associated components. 
     FIG. 10 is an enlarged fragmental view of the mechanism as seen in the direction as identified by an arrow mark Y in FIG. 7, particularly showing the ribbon lifting cam and associated components. 
     FIG. 11 is a perspective view of a prior art typewriter which may house the mechanisms shown in FIGS. 1-10. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be described in a greater detail hereunder with reference to the accompanying drawings which illustrate preferred embodiments thereof. First, description will be made as to the first embodiment of the invention with reference to FIGS. 1 to 6. 
     As shown in FIG. 11, the typewriter 1 includes a frame 2 on which a platen 11 is supported rotatably. Further, the typewriter 1 includes a carriage 3 which is movable in parallel with the rotational shaft 11a of the platen 11 and the carriage 3 is provided with a support shaft 12 which serves to turnably support the holder 13 thereon. As is apparent from the drawing, a tension spring 14 serving as spring means is spanned between the holder 13 and the carriage 3 so that the holder 13 is normally turned in the anticlockwise direction as seen in FIG. 1 under the effect of the resilient force of the tension spring 14. A ribbon cartridge 15 with printing ribbon 50 accommcdated therein is mounted on the upper surface of the holder 13 and a part of printing ribbon 50 which is exposed from the cartridge 15 is located opposite to the fore part of the platen 11. Further, the typewriter 1 includes a correction tape 60 in the space as defined between the holder 13 and an opposing pair of downwardly projected side walls 13a which are made integral with the former and the correction tape 60 is spanned between a pair of spools 4 so that it is wound and unwound thereabout. Specifically, the correction tape 60 is located just below the exposed part of printing ribbon 50 and it extends in parallel with the latter. 
     A ribbon lifting cam 17 is designed in the form of a thick disc of which both side faces are formed with cam grooves respectively which have a contour different from one another. The ribbon lifting cam 17 is rotatably supported by means of the support shaft 16 and it is adapted to rotate in the direction as identified by an arrow mark L in FIGS. 1, 2, 4 and 5 or in the opposite direction to the foregoing one by driving a step motor 5 attached to the carriage 3 which serves as reversible driving motor. 
     As shown in FIG. 4(A), the ribbon lifting cam 17 is formed with a groove-shaped cam on the one side face thereof. Specifically, the groove-shaped cam comprises a first cam section 18 of which radius is designed to gradually increase as seen in the direction opposite to that as identified by an arrow mark L, the first cam section 18 serving to lift printing ribbon 50, a second cam section 19 of which radius is designed to increase as an extension from the first cam section 18, the second cam section 19 serving to lift correction tape 60, and a relieving section 20 of which radius is left unchanged and which is located opposite to the second cam section 19 to form the inside cam wall. As is apparent from the drawing, a boundary point 18a between the first cam section 18 and the second cam section 19 and a starting point 20a of the relieving section 20 are located opposite to one another and the former is located outwardly of the latter. Further, the groove-shaped cam includes an inclined section 21 as an extension from the relieving section 20 having the arched configuration of which depth is designed to decrease gradually and of which width is left unchanged and a shallow groove section 22 having the outer wall face of which radius is left unchanged. The end point of the outer wall face of the shallow groove section 22 is located in the proximity of the boundary point 18a. 
     As shown in FIGS. 1 and 2, the holder 13 is provided with an engagement member 25 on the one side wall 13a which is fixedly secured thereto by means of a pair of set screws 24. The engagement member 25 includes a engagement piece 26 which is bent outwardly therefrom, as shown in FIG. 3(B). A support piece 29 of which the fore end part is designed in the form of an U-shaped portion 29a is fixedly attached to the engagement member 25 in the middle area of the latter by means of a pair of pins 27 and 28. The U-shaped portion 29a is so designed that a pin-shaped cam follower 30 is inserted therethrough in such a manner that it moves toward the support shaft 16. A plate spring 31 is immovably held between the engagement member 25 and the holder 13 by means of the set screws 24 and a bifurcated portion 32 is formed at the free end of the plate spring 31 so that the cam follower 30 is operatively engaged thereto. As long as the cam follower 30 is operatively engaged to the bifurcated portion 32, it is forcibly urged to come in abutment against the bottom of the aforesaid sections 18 to 22 in the ribbon lifting cam 17 under the effect of the resilient force of the plate spring 31 without fail. Since the holder 13 is urged to turn about the support shaft 12 in the anticlockwise direction as seen in FIG. 1 under the effect of the resilient force of the tension spring 14, both the engagement member 25 and the plate spring 31 fixedly secured to the holder 13 are caused to turn in the same direction. As a result, the cam follower 30 is normally brought in contact with the outer wall face of the sections 18 to 22 in the ribbon lifting cam 17. 
     The carriage 3 is fixedly provided with a turn shaft 36 at the position located downwardly of the support shaft 16 of the ribbon lifting cam 17. As will be best seen in FIG. 2, a stopper member 34 is operatively supported to turn about the shaft 36 and the free end of the stopper member 34 is formed with a bent piece 34a adapted to come in engagement to the upper end of the engagement piece 26 of the engagement member 25. A second pin-shaped cam follower 35 is fixed to the stopper member 34 at the intermediate position between the turning point and the free end of the latter. It should be noted that the stopper member 34 is constructed of a plate spring and thereby the second cam follower 35 is normally urged to come in contact with the bottom of a stopper cam 37. 
     On the other hand, as shown in FIGS. 2 and 5(A), the ribbon lifting cam 17 is formed with the groove-shaped stopper cam 37 on the other side face thereof. Specifically, the stopper cam 37 essentially comprises a third cam section 38 for allowing the stopper member 34 to assume the operative position where a printing operation is carried out (as shown in FIG. 2) and a fourth cam section 39 for allowing the stopper member 34 to assume the inoperative position where a corrective printing operation is carried out (as shown in FIG. 6). As is apparent from the drawing, the fourth cam section 39 has a circular configuration of which diameter is determined larger than that of the third cam section 38. While the stopper member 34 assumes the operative position as shown in FIG. 2, the engagement piece 26 of the engagement member 25 is brought in contact with the bent piece 34a of the stopper member 34 during turning movement of the holder 13. On the other hand, while the stopper member 34 assumes the inoperative position as shown in FIG. 6, the engagement piece 26 of the engagement member 25 is disengaged from the bent piece 34a of the stopper member 34 during turning movement of the holder 13. 
     As shown in FIG. 5(A), the stopper cam 37 is further formed with a first introduction section 40 through which the second cam follower 35 is introduced from the third cam section 38 into the fourth cam section 39 during rotation of the ribbon lifting cam 17 in the reverse direction and a second introduction section 41 through which the second cam follower 35 is introduced from the fourth cam section 39 into the third cam section 38. 
     Further, the stopper cam 37 includes a first stepped section 40a at the boundary position where the first introduction section 40 is transferred to the fourth cam section 39. The side of the first stepped section 40a connected from the fourth cam section 39 is designed lower than the first introduction section side of the latter. A part of the second introduction section 41 at which the latter intersects the fourth cam section 39 at the middle part of the circular track of the latter is designed in the form of a stepped section and the part of the fourth cam section 39 located leftwardly of the second introduction section 41 as seen in FIG. 5(A) is designed higher than the part of the same located rightwardly of the second introduction section 41 as seen in the same drawing. Further, the stopper cam 37 includes a second stepped section 38a at the boundary position where the second introduction section 41 is transferred to the third cam section 38. The side of the second stepped section 38a facing to the third cam 38 is designed lower than the second introduction section 41. The inside wall 40b of the first introduction section 40 constitutes a stepped section which extends at a right angle relative to the third cam section 38 and the part of the third cam section 38 located upwardly of the inside wall 40b as seen in FIG. 5(A) is determined lower than the part of the same downwardly of the inside wall 40b as seen in the same drawing. 
     Since the second cam follower 35 is normally brought in abutment against the bottom of the cam groove of the stopper cam 37 by means of the stopper member 34 which is a plate spring, it is caused to move from the higher area to the lower area of the track of the cam groove but it does not move in the opposite direction to the foregoing one in any way. As will be described later, the second cam follower 35 is retained within the third cam section 38, as long as no corrective printing operation is performed. Once the second cam follower 35 enters the fourth cam section 39 beyond the first stepped section 40a via the first introduction section 40, it can not return to the third cam section 38 due to existence of the height difference as mentioned above, unless it moves via the second introduction section 41 and the second stepped section 38a. 
     While the normal operative state is maintained, the ribbon lifting cam 17 and the first cam follower 30 assume the operative position as shown in FIGS. 1 and 4, that is, their home position. Further, while the normal operative state is maintained, the stopper cam 37 and the second cam follower 35 assume the operative position as shown in FIGS. 2 and 5, that is, their home position. Thus, when both the first cam follower 30 and the second cam follower 35 assume their home position, the holder 13 is located at the inoperative position (as represented by real lines in FIG. 1) and the stopper member 34 is located at the operative position as mentioned above. 
     When a printing operation is initiated by a single stroke, the ribbon lifting cam 17 is caused to rotate from the above-mentioned position by a predetermined angle of θ1 in the direction as identified by an arrow mark L in FIGS. 1 and 4(B). When the ribbon lifting cam is rotated by an angle of θ1 in that way, the first cam follower 30 carries out relative movement on the track T1 while coming in contact with the first cam section 18, as shown in FIG. 4(B). As a result, the first cam follower 30 is gradually displaced away from the support shaft 16 and the holder 13 is caused to turn in the anticlockwise direction by the resilient force of the tension spring 14 as seen in FIG. 1. On the other hand, the second cam follower 35 carries out relative movement on the track T4 along the third cam section 38, as shown in FIG. 5(B). During the relative movement, the second cam follower 35 is kept in the operative position. As a result, the engagement member 25 is raised up and its engagement piece 26 comes in engagement with the bent piece 34a of the stopper member 34, as represented by phantom lines in FIG. 2. This causes the printing ribbon 50 on the holder 13 to be raised up from the inoperative position as represented by real lines in FIG. 2 to the printing position as represented by phantom lines in the same drawing whereby a printing operation is ready to start. After completion of a single printing operation the step motor 5 is rotated in the reverse direction and thereby the ribbon lifting cam 17 is caused to rotate in the direction opposite to that as identified by an arrow mark L in FIGS. 1, 4(A) and 4(B). As a result, the first cam follower 30 resumes its home position and the holder 13 is turned back in the clockwise direction as seen in FIG. 1 against the resilient force of the tension spring 14 until the printing ribbon 50 resumes the inoperative position. 
     When continuous printing operation is to be performed, the ribbon lifting cam 17 is rotated further by a predetermined angle of θ3 in the direction as identified by an arrow mark L in FIGS. 1, 4(A) and 4(B) from the operative state where the printing ribbon 50 assumes the printing position for single printing operation (the operative state where it is rotated by a predetermined angle of θ1). At this moment the engagement piece 26 of the engagement member 25 is operatively engaged to the bent piece 34a of the stopper member 34, resulting in the first cam follower 30 from being inhibited from parting away from the support shaft 16 beyond a predetermined distance. Thus, the first cam follower 30 is introduced into the relieving section 20 from the end part of the first cam section 18 to carry out relative movement on the track T3, whereas the second cam follower 35 carries out relative movement on the track T4 in the third cam section 38 in the same manner as in the case of a single printing operation. 
     Next, when a corrective printing operation is to be carried out, the ribbon lifting cam 17 is rotated by a predetermined angle of θ4 in the direction opposite to that as identified by an arrow mark L in FIGS. 1, 4(A) and 4(B) from the operative state where the first cam follower 30 assumes its home position. This causes the second cam follower 35 to be introduced into the fourth cam section 39 from the home position in the third cam section 38 by carrying out relative movement on the track T5 in the first introduction section 40. As a result, the stopper member 34 is turned in the clockwise direction as seen in FIG. 2 to reach the inoperative position. This means that the first cam follower 30 assumes the operative state where it can move freely in the radial direction relative to the support shaft 16 which is a center of rotation of the ribbon lifting cam 17, because the engagement piece 26 of the engagement member 25 is disengaged from the bent piece 34a of the stopper member 34. Due to the fact that the stopper member 34 made of a plate spring normally urges the second cam follower 35 toward the cam face and the fourth cam section 39 has a cam groove of which depth is designed more than that of the first introduction section 40 while the first stepped section 40a is interposed therebetween, the second cam follower 35 can not return to the third cam section 38 but it carries out relative movement on the track T6 along the fourth cam section 39, after the first cam follower 30 resumes its home position. When the ribbon lifting cam 17 is rotated by a predetermined angle of θ4+θ2 in the direction as identified by an arrow mark L in FIGS. 1, 4(A) and 4(B) from the above-mentioned state, the first cam follower 30 which is forcibly urged outwardly under the effect of resilient force of the tension spring 14 carries out further relative movement on the track T1 in the first cam section 18 and thereafter it does so on the track T2 in the second cam section 19. During relative movement as mentioned above the second cam follower 35 carries out relative movement on the track T6 along the fourth cam section 39. As a result, the holder 13 is caused to turn about the support shaft 12 from the operative state as shown in FIG. 2 whereby the correction tape 60 assumes the printing position as shown in FIG. 6. After completion of a corrective printing operation the ribbon lifting cam 17 is rotated in the direction as identified by an arrow mark L in FIGS. 1, 4(A) and 4(B), whereas the first cam follower 30 returns to its home position from the second cam section 19 via the first cam section 18. As a result, the holder 13 is caused to turn about the support shaft 12 in the anticlockwise direction from the operative state as shown in FIG. 6 whereby the correction tape 60 resumes the inoperative position. In the meanwhile, the second cam follower 35 is displaced to its home position in the third cam section 38 from the fourth cam section 39 via the second introduction section 41 beyond the second stepped section 38a, resulting in the stopper member 34 assuming the operative position as mentioned above. 
     Next, operation of the ribbon lifting mechanism constructed in accordance with the first embodiment of the invention will be described below. 
     FIG. 2 shows the operative state where the second cam follower 35 is located at its home position in the third cam section 38 of the stopper cam 37 and thereby the stopper member 34 assumes the operative position. While the above-mentioned state is maintained, the first cam follower 30 is located also at its home position in the first cam section 18 of the ribbon lifting cam 17 as shown in FIG. 1. As long as the first cam follower 30 is held at its home position, the holder 13 maintains the substantially horizontal posture against the resilient force of the tension spring 14 and thereby both the printing ribbon 50 and the correction tape 60 assume their lower inoperative position. 
     When a predetermined letter key 6 is depressed by an operator while the above-mentioned state is maintained, the step motor 5 is rotated in the normal direction by a predetermined angle whereby the ribbon lifting cam 17 is caused to rotate in the direction as identified by an arrow mark L in FIGS. 1, 4(A) and 4(B) by a predetermined angle of θ1. At this moment the second cam follower 35 moves on the track T4 along the third cam section 38 of the stopper cam 37 and thereby the stopper member 34 is kept in the operative position. On the other hand, the first cam follower 30 moves on the track T1 in conformance with the contour of the first cam section 18. Thus, the first cam follower 30 is displaced away from the support shaft 16 and both the holder 13 and the engagement member 25 are turned about the support shaft 12 in the clockwise direction as seen in FIG. 2 by an angle corresponding to the extent of displacement as mentioned above with the aid of the resilient force of the tension spring 14, resulting in the engagement piece 26 of the engagement member 25 coming in engagement with the bent piece 34a of the stopper member 34, as shown by phantom lines in the drawing. As long as the engagement piece 26 is kept in engagement with the bent piece 34a, the printing ribbon 50 on the holder 13 is located opposite to the fore part of the platen 11 whereby a printing operation is ready to start. When a printing hammer 7 is depressed while the above-mentioned operative state is maintained, a required letter is printed on a sheet of printing paper 8 on the platen 11. 
     If a next letter key 6 is not depressed until a predetermined period of time elapses, the step motor 5 is rotated in the reverse direction and thereby the ribbon lifting cam 17 is rotated by a predetermined angle of θ1 in the direction opposite to that in FIGS. 1, 4(A) and 4(B). Namely, both the first cam follower 30 and the second cam follower 35 resume their home position. This means that the holder 13 is turned back to the position as shown in FIGS. 1 and 2 against the resilient force of the tension spring 14 so that the printing ribbon 50 assumes the inoperative position. Thus, a single printing operation is completed. 
     Next, when a next letter key 6 is depressed by an operator subsequent to a printing operation with the use of the preceding letter key 6 before a predetermined period of time elapses, the step motor 5 rotates the ribbon lifting cam 17 by a predetermined angle of θ3 at every time when the depressing operation is performed successively. This causes the second cam follower 35 to carry out relative movement on the track T4. During relative movement as mentioned above the stopper member 34 is held in the operative position and the engagement member 25 is operatively engaged with the stopper member 34. On the other hand, the first cam follower 30 carries out relative movement from the first cam section 18 on the track T3 which extends back to the position located in the proximity of the end of the first cam section 18 via the relieving section 20, the inclined section 21 and the shallow groove section 22. 
     If a next letter key 6 is not depressed by an operator within a predetermined period of time after completion of successive printing operations, the step motor 5 is rotated in the reverse direction and thereby the ribbon lifting cam 17 is rotated in the direction opposite to that as identified by an arrow mark L in the drawings in response to the function of a reference position detecting sensor 52 comprising an encoder 54 and other elements which are not shown, until the first cam follower 30 reaches its home position as shown in FIGS. 4(A) and 4(B). Thus, the printing ribbon 50 is lowered to the inoperative position in the same manner as in the case where a single printing operation is completed. 
     On the other hand, if incorrect printing is efrected, a back space key 6a is depressed so that the carriage 3 is returned to the incorrect printing position. Thereafter, the step motor 5 is rotated by a predetermined angle in the reverse direction by depressing a correction key 6b whereby the ribbon lifting cam 17 is rotated by a predetermined angle of θ4 in the direction opposite to that as identified by an arrow mark L in FIGS. 1, 4(A) and 4(B). This causes the second cam follower 35 to carry out relative movement on the track T5 and thereby it is introduced into the fourth cam section 39 from the third cam section 38 via the first introduction section 40. Since the fourth cam section 39 is designed to have a cam diameter larger than that of the third cam section 38, the stopper member 34 is caused to turn about the pivotal shaft 36 in the clockwise direction as seen in FIG. 2 until the inoperative position is reached. As long as the stopper member 34 is held at the inoperative position, its bent piece 34a is displaced away from the engagement piece 26 of the engagement member 25 without any occurrence of engagement therebetween. 
     While the above-mentioned operative state is maintained, the stopper member 34 is kept in the inoperative position, because the second cam follower 35 carries out relative movement on the track T6 of the fourth cam section 39 even when the ribbon lifting cam 17 is rotated in the direction as identified by an arrow mark L in FIGS. 1, 4(A) and 4(B). Thus, the first cam follower 30 which has moved along the first cam section 18 is ready to move to the second cam section 19 under the effect of the resilient force of the tension spring 14 without any occurrence of such a malfunction that it is inhibited from moving to the relieving section 20 from the end part of the first cam section 18. Thereafter, when a letter key 6 is depressed to correct the incorrectly printed letter, the ribbon lifting cam 17 is rotated by a predetermined angle of θ4+θ2 whereby the first cam follower 30 is caused to move on the tracks T1 and T2. As a result, the holder 13 is turned about the pivotal shaft 12 to the position as shown in FIG. 6 with the aid of the resilient force of the tension spring 14 and the correction tape 60 is located at the printing position. Then, the incorrectly printed letter can be erased by operating a printing hammer 7. 
     On completion of the erasing operation the step motor 5 is rotated in the reverse direction and thereby the ribbon lifting cam 17 is rotated by a predetermined angle of θ2 in the direction opposite to that as identified by an arrow mark L in FIGS. 1, 4(A) and 4(B), resulting in the first cam follower 30 returning to its home position. Thus, the holder 13 is turned in the anticlockwise direction as seen in FIG. 6 against resilient force of the tension spring 14 owing to the cam function of the second cam section 19 and the first cam section 18 whereby the correction tape 60 assume the inoperative position as shown in FIG. 1. On the other hand, when the second cam follower 35 is returned by a predetermined angle of θ2 in the direction opposite to that as identified by an arrow mark L in FIG. 5(B), it is displaced to the third cam section 38 from the fourth cam section 39 via the second introduction section 41. As a result, the stopper member 34 is caused to turn about the pivotal shaft 36 in the anticlockwise direction from the position as shown in FIG. 6 and thereby it is returned to the position as shown in FIG. 2, that is, the operative position. 
     Next, description will be made below as to the ribbon lifting mechanism in accordance with the second embodiment of the invention with reference to FIGS. 7 to 10. 
     As will be apparent from the drawings, the platen 11, the holder 13, the tension spring 14, the ribbon cartridge 15, support shaft 16, the engagement member 25 and the first cam follower 30 are designed and constructed in the same manner as those in the first embodiment and therefore their description will not be repeated. 
     The ribbon lifting cam 17a is formed with a first cam section 18, a second cam section 19, a relieving section 20, an inclined section 21 and a shallow groove section 22 on the one side surface thereof in the same manner as in the first embodiment and a first cam follower 30 is located opposite to the above-mentioned sections. 
     The ribbon lifting cam 17a is rotatably mounted on the support shaft 16 and a stopper member 43 is turnably supported also on the support shaft 16 with the aid of its U-shaped portion 43a by the side of the ribbon lifting cam 17a. Further, as shown in FIGS. 9 and 10, the stopper member 43 is adapted to slide on the support shaft 16 in the axial direction. Further, it is normally urged toward the ribbon lifting cam 17a under the effect of the resilient force of a compression spring 44 which is mounted on the support shaft 16. The stopper member 43 is formed with a bent piece 43b at the one extreme end thereof which is adapted to come in engagement with an engagement piece 26 of the engagement member 25. The stopper member 43 includes an extension 43c on the opposite side to the engagement piece 26 and a tension spring 45 adapted to normally turn the stopper member 43 in the anticlockwise direction as seen in FIG. 7 is attached to the extension 43c. 
     Further, a limiting member 46 for limiting an extent of turning movement of the stopper member 43 in both the clockwise and anticlockwise directions as seen in FIG. 7 is fixedly secured to the carriage 3. The limiting member 46 includes an inclined piece 47 for displacing the stopper member 43 in the direction as identified by an arrow mark N in FIGS. 9 and 10 against the resilient force of the spring 44 by way of engagement with the extension 43c of the stopper member 43 and an engagement piece 48 for inhibiting further turning movement of the stopper member 43 in the direction as identified by an arrow mark M from the position as shown in FIG. 7 at the fore end thereof. 
     The ribbon lifting cam 17a is integrally formed with a projection 49 on the opposite side surface to that on which the first cam section 18 and other cam sections are provided and the projection 49 is projected toward the stopper member 43. As shown in FIGS. 8, 9 and 10, the projection 49 is located away from the support shaft 16 by a predetermined distance and has the arched configuration. As will be best seen in FIG. 10, the projection 49 has an inclined face 49a of which height linearly increases from the one end to the other one of the arched configuration. The height of the inclined face 49a is so determined that it abuts against the U-shaped portion 43a of the stopper member 43 at the position located midway of the inclined face 49a. Orientation of the inclined face 49a is determined such that the one end of the U-shaped portion 43a of the stopper member 43 comes in abutment against the inclined face 49a when the ribbon lifting cam 17a is rotated in the direction as identified by an arrow mark M by a predetermined angle. 
     Next, operation of the ribbon lifting mechanism in accordance with the second embodiment of the invention will be described below. 
     FIG. 7 shows the inoperative state of the ribbon lifting mechanism. As will be apparent from the drawing, the stopper member 43 is brought in engagement with the engagement piece 48 of the limiting member 46 under the effect of the resilient force of the tension spring 14. While the above-mentioned state is maintained, the bent piece 43b of the stopper member 43 assumes the position where it is operatively engaged with the engagement piece 26 of the engagement member 25 when the latter is raised up, that is, the operative position where the printing ribbon 50 is ready to be in use for a printing operation. Further, while the above-mentioned state is maintained, the first cam follower 30 assumes its home position as shown in FIG. 4(A) and the holder 13 is maintained in the horizontal posture against the resilient force of the tension spring 14 whereby both the printing ribbon 50 and the correction tape 60 are located in their lower inoperative position. 
     When a predetermined letter key 6 is depressed to perform a printing operation, the step motor 5 is rotated in the normal direction by a predetermined angle in the same manner as in the first embodiment and thereby the ribbon lifting cam 17a is rotated in the direction as identified by an arrow mark M in FIG. 7 by a predetermined angle of θ1. During the rotating operation as mentioned above the first cam follower 30 is caused to move along the contour of the first cam section 18 so that both the holder 13 and the engagement member 25 are turned about the pivotal shaft 12 in the clockwise direction as seen in FIG. 7 under the effect of the resilient force of the tension spring 14. At this moment the U-shaped portion 43a of the stopper member 43 abuts against the inclined face 49a of the projection 49 on the ribbon lifting cam 17a and then it carries out relative movement in such a direction that the height of the inclined face 49a increases. As long as the stopper member 43 comes in contact with the inclined face 49a, it moves in the direction as identified by an arrow mark N in FIG. 10 while it is held at the position as shown in FIG. 7, that is, the operative position. Thereafter, as the ribbon lifting cam 17a is rotated in the direction as identified by an arrow mark M in the drawings by a predetermined angle which is determined for a printing operation, the engagement piece 26 of the engagement member 25 is brought in engagement with the bent piece 43b of the stopper member 43 and thereby the printing ribbon 50 on the holder 13 is raised up to the printing position as represented by phantom lines in FIG. 7 which is located opposite to the fore part of the platen 11. Thus, printing operation is ready to start. 
     If a next letter key 6 is not depressed by an operator before a predetermined period of time elapses, the step motor 5 is rotated in the reverse direction in the same manner as in the first embodiment and thereby the ribbon lifting cam 17a is rotated in the direction opposite to that as identified by an arrow mark M in FIG. 7. This causes the holder 13 to be turned about the pivotal shaft 12 in the anticlockwise direction with the aid of a cam function whereby the printing ribbon 50 is returned to the lower inoperative position as shown by real lines in FIG. 7. At this moment the position where the stopper member 43 is engaged with the projection 49 is displaced from the higher area of the inclined face 49a to the lower one and thereafter the stopper member 43 is disengaged from the projection 49. During displacement of the stopper member 43 in that way it moves in the direction opposite to that as identified by an arrow mark N in FIG. 10 while it slides down along the inclined face 49a under the effect of the resilient force of the compression spring 44. 
     On the other hand, when a next letter key 6 is depressed subsequent to a printing operation of the preceding letter key 6 before a predetermined period of time elapses, the ribbon lifting cam 17 is successively rotated by a predetermined angle in the direction as identified by an arrow mark M in FIG. 7. Thus, a printing operation can be performed continuously. It should be noted that during a continuous printing operation the stopper member 34 moves only in the direction as identified by an arrow mark N in FIGS. 9 and 10 as well as in the opposite direction to the foregoing one without any occurrence of displacement from the operative position as shown in FIG. 7 and the holder 13 is held at the operative state where the printing ribbon 50 is located at the printing position. 
     If an incorrect printing operation is performed, the carriage 3 is caused to resume the incorrect printing position by depressing operation of the back space key 6a. Then, a correction key 6b is depressed and thereby the ribbon lifting cam 17 is rotated by means of the step motor 5 in the normal direction until the projection 49 goes over the stopper member 43, that is, in the direction as identified by an arrow mark M in FIG. 7. Immediately after completion of rotation in the normal direction the ribbon lifting cam 17a is rotated by a predetermined angle in the reverse direction, that is, in the direction opposite to that as identified by an arrow mark M in the same drawing. As shown in FIG. 8, this allows the projection 49 to come in abutment against the U-shaped portion 43a of the stopper member 43. Thus, the stopper member 43 is turned in the clockwise as seen in the drawing in dependence on rotation of the ribbon lifting cam 17a in the direction opposite to that as identified by an arrow mark M, resulting in the bent piece 43b of the stopper member 43 being disengaged from the engagement piece 26 of the engagement member 25. Once disengagement is achieved in that way, the holder 13 is turned about the pivotal shaft 12 in the clockwise direction as seen in the drawing under the effect of the resilient force of the tension spring 14. At this moment the first cam follower 30 is located at the relieving section 20 and when the stopper member 43 is disengaged from the engagement member 25, the holder 13 is caused to turn until the first cam follower 30 is displaced from the relieving section 20 to the second cam section 19. As a result, the correction tape 60 is displaced to the printing position as shown in FIG. 8 and the incorrectly printed letter can be erased by actuating the printing head 7a. 
     After completion of the erasing operation as mentioned above the ribbon lifting cam 17a is rotated further in the direction to that as identified by an arrow mark M in FIG. 7. This causes the extension 43c of the stopper member 43 to come in abutment against the inclined face 47 of the limiting member 46 and thereafter it slides upwardly along the inclined face 47 as the ribbon lifting cam 17a is rotated further. Thus, as is apparent from FIGS. 9 and 10, the whole stopper member 43 is displaced in the direction as identified by an arrow mark N in the drawings by means of the inclined face 47 of the limiting member 46. The projection 49 is later disengaged from the U-shaped portion 43a of the stopper member 43, resulting in the stopper member 43 resuming the operative position as shown in FIG. 7 under the effect of the resilient force of the. tension spring 45. 
     On the other hand, since the first cam follower 30 is displaced to the first cam section 18 from the second cam section 19 as the ribbon lifting cam 17a is rotated, the holder 13 is turned backwardly in the anticlockwise direction as seen in FIG. 8 against the resilient force of the tension spring 14 until its lower inoperative position as shown in FIG. 7 is reached. 
     As will be readily understood from the above description, the ribbon lifting mechanism of the invention is so constructed that either of the printing ribbon 50 and the correction tape 60 selectively assumes the printing position by mcchanically rotating the ribbon lifting cam 17 or 17a adapted to turn the holder 13 without any necessity for special actuating means such as electromagnet or the like which are employed for the conventional ribbon lifting mechanism. Thus, advantageous features of the ribbon lifting mechanism of the invention are that it is simple in structure and can be fabricated at an inexpensive cost. 
     While the present invention has been described above only with respect to two preferred embodiments, it should of course be understood that it should not be limited only to them but various changes or modifications may be made in any acceptable manner without departure from the spirit and scope of the invention as defined by the appended claims.