Abstract:
A paper feed mechanism includes a platen rotatably mounted on a frame and a friction feed having a support which rotatably supports a roller, the support being movable between an operable position in which the roller is in biasing engagement with the platen and a non-operable positon in which the roller is not biased toward said platen. A spring biasingly urges the support in its operable position in which the roller is in biasing engagement with the platen. A cam is rotatably mounted on the frame and rotatable between a first cam position which moves the support to its non-operable position and a second cam position which permits the support to be moved to its operable position by the spring, whereby the cam controls the operability of the friction feed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a paper feed mechanism for a printer provided with, for example, both a tractor feed mechanism and a friction feed mechanism as form feed means. 
     Conventionally, in a printer provided with both a tractor feed mechanism and a friction feed mechanism, there is provided a mechanism which switches the elastic contact force of a friction roller which elastically contacts a platen according to the paper feed mode. The arrangement is such that elastic contact is made strongly in the case of the friction feed, and elastic contact is made weakly in the case of the tractor feed. For example, in Japanese Patent application No. SHO60-243323 of the present applicant, a construction is disclosed in which the elastic contact force on the platen of the friction roller is changed over by using a cam having a complicated shape, a cam follower which follows this cam, a plurality of levers, and a spring. 
     In such a conventional mechanism for adjusting the frictional force between the recording paper and the platen according to the tractor feed and the friction feed, respectively, there have been such drawbacks in that the construction is complicated and a long time is required for assembly. In addition, it is difficult to set the frictional force between the recording paper and the platen in respective form feed modes. 
     It is an object of the present invention to make it easy to set the frictional force between the recording paper and the platen for the tractor feed and the friction feed, and further to improve the assembling characteristics by means of a simple construction. 
     SUMMARY OF THE INVENTION 
     In order to achieve the above-mentioned object, a paper feed mechanism according to the present invention comprises a paper guide movable in a direction opposing an outer circumferential surface of a platen, a movable member supported movably below the paper guide, friction rollers which are provided movably on the movable member and are capable of contacting the outer circumferential surface of the platen elastically, a first spring urging the friction roller toward the platen through the movable member, rotatably cam means which engages the movable member when located at a first rotational position so as to move the movable member against the urging force of the first spring and release the engagement with the movable member when located at a second rotational position, and a second spring which has a spring force weaker than that of the first spring and is mounted between the movable member and the paper guide so as to urge the paper guide toward the platen. 
     Also, when a pair of friction rollers are utilized to contact the platen elastically, a rockable roller holder is provided on the movable member, and a pair of friction rollers are provided on either side of the rocking center of the roller holder. The second spring is mounted between the roller holder and the paper guide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-4 show one embodiment according to the present invention wherein: 
     FIG. 1 is a front view showing the operating condition of the tractor feed; 
     FIG. 2 is a front view showing the operating condition of the friction feed; 
     FIG. 3 is a front view of the drive wheel train; and 
     FIG. 4 is a plan view of FIG. 3. 
     FIGS. 5 and 6 show another embodiment according to the present invention wherein: 
     FIG. 5 is a front view showing the operating condition of the tractor feed; and 
     FIG. 6 is a front view showing the operating condition of the friction feed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the drawings. 
     As shown in FIG. 1 and FIG. 2, a platen 1 is fixedly secured on a shaft 2, and both end portions of the shaft 2 are supported rotatably on side plates 3 (shown in FIG. 3), which are opposed to each other at a predetermined spacing. At the lower part of the platen 1, a movably mounted paper guide 4 is disposed opposite to the outer circumferential surface of the platen 1. The paper guide 4 is provided with a hole 4a at one end portion thereof and is supported rockably on the side walls 3 through a shaft not shown. At another end portion of the paper guide 4, a paper retaining plate 4b which can contact the outer circumferential surface of the platen 1 is provided. 
     At the under part of the paper guide 4 a member 6 is movably supported. That is, a baseplate 5 extends parallel to the platen 1, and both end portions of the baseplate 5 are fixedly secured to the side plates 3. One side edge of the baseplate 5 is bent upward and is provided with a receiver 5a at predetermined position thereof. At one end portion of the movable member 6, an engaging portion 6a consisting of a U-shaped groove is formed, and the engaging portion 6a engages with the receiver 5a such that the movable member 6 is thereby supported rockably on the baseplate 5. 
     A contact portion 6b is provided on the upper surface of the movable member 6. The movable member 6 also has a bearing groove 6c which rotatably receives a shaft 8 which is provided with a friction roller 7 formed at another end portion thereof. A first spring 9 is fitted to a projection 5b which extends upward from the baseplate 5. The first spring 9 urges the friction roller 7 toward the platen 1 through the movable member 6. The friction roller 7 can contact the platen 1 elastically upon passing through an opening (not shown) provided on the paper guide 4. 
     A second spring 10 is mounted between the movable member 6 and the paper guide 4. The second spring 10 is set so as to have a spring force weaker than that of the first spring 9 and urges the paper guide 4 toward the platen 1. 
     A cam means 11 is disposed above the contact portion 6b. The cam means 11 is provided with a first cam surface 11a having a circular arc surface which engages with the contact portion 6b of the movable member 6 to thereby push down the movable member 6 against the urging force of the first spring 9 and a second cam surface 11b having a plane surface which releases the engagement with the movable member 6, thereby allowing the movable member 6 to be pushed up by the urging force of the first spring 9. Thus it will be seen that when the cam means 11 is located at a first rotational position, the first cam surface 11a engages the contact portion 6b, and when the cam means 11 is located at a second rotational position, the second cam surface 11b is separated with a fixed space from the contact portion 6b. The cam means 11 has a rod shape and is supported rotatably by the side plates 3 at both ends thereof. 
     A printing head 12 is disposed on a carriage (not shown) opposing the platen 1, and a tractor mechanism 13 is disposed on the opposite side of the platen. The tractor mechanism 13 is fitted in a relatively non-rotatable manner and is movable in an axial direction with respect to a shaft 14 which is supported rotatably on the side plates 3 at both end portions thereof. P denotes a recording paper. 
     FIG. 3 shows the platen shaft 2, the shaft 14 along with the rotational drive mechanism of the tractor mechanism 13, and the rotational mechanism of the cam means 11. That is, the end portions of the shaft 2 of the platen 1, the shaft 14 of the tractor mechanism 13 and the cam means 11 are projected outwardly of the side plate 3. 
     First, a wheel train which drives the shafts 2 and 14 will be described A gear 17 which is fixedly secured to the shaft 2 and a gear 16 mesh successively with a drive gear 15 which receives a rotational driving force from a drive motor (not shown), thus forming a first gear wheel train A for rotatably driving the platen 1. 
     In order to rotatably drive the tractor mechanism 13, a gear 19 which is fixedly secured to the shaft 14 and a gear 18 mesh successively with the gear 16, thus forming a second wheel train B. 
     In the case of the tractor feed, the gear 16 and the gear 18 mesh with each other, and the rotational driving force is transmitted both to the first wheel train A and the second wheel train B, so that the platen 1 and the tractor mechanism 13 are driven to rotate at the same time. The rotating speed of the platen 1 is set to be somewhat faster than the rotating speed of the tractor mechanism 13. 
     In the case of the friction feed, it is arranged so that only the platen 1 is driven to rotate by interrupting the transmission of the torque to the second wheel train B such that the tractor mechanism 13 does not rotate. Interruption of transmission of the torque to the second wheel train B is effected by a conical cam 18a (FIG. 4) which is provided integrally and concentrically with respect to the gear 18 and an operation lever 21 provided rockably on a shaft 20 provided on the side plate 3. The gear 18 is rotatably provided on a shaft 29 (FIG. 4) secured to the side plate 3. A retainer ring 31 is provided on the end of the shaft 29, and a spring 30 is wound around the shaft 29 between the retainer ring 31 and the gear 18. 
     At the end portion of the operation lever 21, an engaging portion 21a having a U-shape and a side surface thereof forming a tapered surface 21c is formed. The engaging portion 21a of the operation lever 21 enters from the side of the conical cam 18a by means of the rocking of the lever 21 as shown with chain lines in FIG. 3 such that the gear 18 is moved axially, thereby to separate the mesh between gear 18 and gear 16. A protrusion 22 which regulates the retreat position of the operation lever 21 is provided on the side plate 3. When the operation lever 21 is positioned at the retreat position as shown by solid lines in FIG. 3, the conical cam 18a is biased leftwardly in FIG. 4 by the force of the spring 30. 
     In the case of friction feed, the operation lever 21 is moved to the position shown by the broken lines in FIG. 3, and the engaging portion 21a of the operation lever 21 enters from the side of the conical cam 18a. Accordingly, the conical cam 18a is moved rightwardly in FIG. 4 against the bias of spring 30, and the gear 18 is disengaged from the gear 16. 
     In order to rotate the cam means 11 interlocking with the rocking of the operation lever 21 when the friction feed and the tractor feed are changed over from each other, a sector gear 21b is formed concentrically with the shaft 20, which becomes the rocking center at the central part of the operation lever 21. A pinion 23 is fixedly secured to the cam means 11, and the sector gear 21b and the pinion 23 mesh with each other. As a result, the cam means 11 is rotated interlocking with the rocking of the operation lever 21. 
     With such a construction, in the case of tractor feed with the operation lever 21 at the retreat position as shown in solid lines in FIG. 3, the engaging portion 21c of the operation lever 21 does not engage with the conical cam 18a, and the gear 16 and the gear 18 are in a meshed state. Accordingly, the torque from the drive gear 15 is transmitted to the first wheel train A and the second wheel train B, and rotates the shafts 2 and 14 so as to rotatingly drive the platen 1 and the tractor mechanism 13, thus effecting paper feed by the tractor mechanism 13. In such a state, the cam means 11 is located at the first rotational position as shown in FIG. 1, and the first cam surface 11a contacts the contact portion 6b and pushes down the movable member 6 against the urging force of the first spring 9. As a result, the friction roller 7 does not contact the platen 1 elastically, and the strong spring force of the first spring 9 is not exerted on the platen 1. Even in such a state, however, the recording paper P is pressed against the platen 1 through the paper guide 4 by the weak spring force of the second spring 10 so that an appropriate frictional force is applied between the recording paper P and the platen 1. Also, since the platen 1 is set so as to rotate somewhat faster than the tractor mechanism 13, the recording paper P is applied with an appropriate tension when the recording paper P is fed by the rotation of the tractor mechanism 13. 
     Next, in the case of friction feed, the operation lever 21 is driven rockably clockwise to the position shown by chain lines in FIG. 3 such that the engaging portion 21c enters from the side of the conical cam 18a and axially moves the gear 18. Thus, the gear 18 is no longer in mesh with the gear 16, and the torque from the drive gear 15 is transmitted to the first wheel train A, but is not transmitted to the second wheel train B. Accordingly, the platen 1 is driven to rotate by the rotation of the shaft 2, but the tractor mechanism 13 is not driven to rotate because the shaft 14 does not rotate. 
     By means of the rocking of the operation lever 21, the pinion 23 is rotated counterclockwise through the sector gear 21b such that the cam means 11 is positioned at the second rotational position as shown in FIG. 2, and the second cam surface 11b thereof faces the contact portion 6b at a predetermined space. As a result the movable member 6 has applied thereto the strong spring force or the first spring 9 and presses the recording paper strongly to the platen 1 elastically by means of the friction roller 7. Furthermore, since the movable member 6 is located at the upward position the second spring 10 also presses the recording paper against the platen 1 elastically through the paper guide 4 to provide a stronger urging force than the case mentioned above. 
     In another embodiment, shown in FIGS. 4 and 5, a pair of friction rollers are used. In this second embodiment, the same numerals are assigned to those parts that have the same construction. 
     In the embodiment of FIGS. 4 and 5, a movable member 26 located under the paper guide 4 has an engaging portion 26a consisting of a U-shaped groove formed at one end portion thereof, and the movable member 26 is supported rockably by a baseplate 5 by the fact that the engaging portion 26a engages with the receiver 5a. A contact portion 26b is provided on the upper surface of the movable member 26, and a bearing portion 26c is provided at another end of the movable member 26. The first spring 9 contacts the lower surface of the movable member 26 elastically, and exerts its urging force in a direction tending to raise the bearing portion 26c. 
     A center shaft 25c of a roller holder 25 is fitted rotatably in the bearing portion 26c to thereby receive the roller holder 25 rockably. On both sides of the center of rotation of the roller holder 25, bearing grooves 25a and 25b receive rotatably shafts 28a and 28b provided with friction rollers 27a and 27b. The previously mentioned second spring 10 is mounted between the roller holder 25 and the paper guide 4. The cam means 11 is positioned above the contact portion 26b and has the same construction as that previously described. 
     With such a construction, when the tractor feed is selected, the cam means 11 is located at the first rotational position as shown in FIG. 4, the movable member 26 has moved downwardly against the urging force of the first spring 9, and the roller holder 25 also moves downwardly. As a result, the friction rollers 27a and 27b do not contact the platen 1 elastically, and the strong urging force of the first spring 9 is not exerted on the platen 1. However, the weak urging force of the second spring presses the recording paper against the platen 1 through the paper guide 4 in a manner as previously described in the first embodiment. 
     Next, when the friction feed is selected, the cam means 11 is disposed at the second rotational position and releases the engagement with the movable member 26 as shown in FIG. 5. Therefore, the movable member 26 receives the strong urging force of the first spring 9, pushes up the roller holder 25 and elastically presses the recording paper strongly against the platen 1 by means of the friction rollers 27a and 27b. Since the roller holder 25 is rockably mounted, the friction rollers 27a and 27b contact the platen elastically with a uniform force. Also, with the roller holder 25 being at an upward position, the second spring 10 pushes the recording paper against the platen 1 elastically through the paper guide 4 with a stronger urging force than that just described. 
     Although the movable members 6 and 26 are described as being rockable, they are not necessarily limited to such an arrangement but may be arranged so as to be movable vertically. 
     As described above, in a paper feed mechanism according to the present invention, the pressing force of the paper guide against the platen is changed in accordance with the changeover of the tractor feed to and from the friction feed. Therefore, it is possible to set the frictional force between the recording paper and the platen at an appropriate value in accordance with respective modes of form feed. Also, it is only required to mount the movable member, the roller holder, the friction roller and so forth consecutively in assembly, so assembly may be performed very easily. Furthermore, since there are only a few parts having simple shapes such as the movable member and the roller holder, etc., the construction is simple and the cost may be reduced. 
     Although the present invention has been described through specific terms, it should be noted here that the described embodiment is not necessarily exclusive and that various changes and modifications may be imparted thereto without departing from the scope of the invention which is limited solely by the appended claims.