Abstract:
A printer includes a head and a platen mounted in a chassis. A paper tractor may be mounted on the chassis in either of two rotary positions either to pull or to push the paper, respectively, past the platen. The difference between the two positions is a rotary angle at which the tractor sits relative to the parts of the printing machine.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a printer and, more particularly, to a printer capable of having both a push-tractor type structure and a pull-tractor type structure. 
     Conventional printers were classified into either one of the pull-tractor type and the push-tractor type depending on the relationship between paper and a tractor. 
     Referring now to FIG. 1, a conventional pull-tractor type printer comprises a cylindrical platen 1 for supporting paper 3 from the rearside thereof to absorb the impact generated at the time of printing by a head 4, and a tractor 2 for pulling out the paper 3 from the platen. In this type of structure, however, since the paper is pulled by the tractor 2, the paper 3 cannot be cut at any location between the platen 1 and the tractor 2. For this reason, when a piece of paper 3 is cut out immediately after the printing, an operator must wait until the desired printed portion on the paper has passed through the tractor 2 to cut out said portion at a cutting position 8 behind the tractor 2. As a result, every time the paper is cut, the length of paper equal to the distance between the head 4 and the tractor 2 is inevitably wasted. 
     Referring to FIG. 2, a conventional push-tractor type printer comprises a platen 1 and a tractor 2 for feeding paper 3 to the platen 1. However, this type of structure is not suitable for multiple paper, which is composed of several sheets of paper stacked. As shown in FIG. 3, when multiple paper 10 is used, as the rotational radius of the inner side paper 11 close to the platen 1 is slightly shorter than that of the outer side paper 12 of the paper 10, the amount of feed of the paper 11 becomes smaller than that of the paper 12. This difference in the amount of feed causes a slack 13 of the paper 11. As the multiple paper 10 is continuously fed, the amount of the slack 13 increases and the slack 13 gradually accumulates large enough to jam. As a result, the feed of the paper 10 eventually becomes impossible. Such a conventional pull-tractor type printer is disclosed in the U.S. Pat. No. 4,344,715. 
     One object of the present invention is, therefore, to provide a printer free from the above-mentioned disadvantages of the prior art and capable of achieving both the pull-tractor and push-tractor type structures. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a printer having a head for printing paper and a tractor drive mechanism, both of which are mounted on a chassis, the printer comprising: a cylindrical platen disposed opposite to the head for supporting the paper from the rearside thereof so that the impact caused in the printing operation can be absorbed, the platen being provided between sides of the chassis; tractors for driving the paper across the platen; and support means for supporting the tractors at a first position which allows the tractors to pull out the paper printed by the head from said platen when operated in pull-tractor type fashion and for supporting the tractors at a second position which is inclined from the first position by a predetermined angle to allow the tractors to feed the paper to the platen before printing when operated in push-tractor type fashion; whereby the printer can have both a pull-tractor type structure and a push-tractor type structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a diagram of a conventional pull-tractor type printer; 
     FIGS. 2 and 3 are diagrams of a conventional push-tractor type printer; 
     FIG. 4 is a decomposed perspective view of an embodiment of the invention; 
     FIGS. 5, 6, and 7 are a side view, a perspective view and a partially enlarged side view respectively of the embodiment when used in the pull-tractor type structure; and 
     FIGS. 8, 9, and 10 are a side view, a perspective view and a partially enlarged side view of the embodiment when used in the push-tractor type structure. 
     In the drawings, the same reference numerals represent the same structural elements. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 4 and 5, an embodiment of the present invention comprises a printer body 28 and a tractor assembly 24. The assembly 24 comprises a pair of tractors 2; a support rod 16 for supporting the tractors 2; a drive bar 15 whose rotation causes the tractors 2 to advance paper, a tractor gear 25 coupled onto one end of the bar 15; a chassis 17 for mounting the tractors 2, the rod 16, the bar 15 and the gear 25; support plates 14a and 14b provided on sides 17a and 17b of the chassis 17 via pins 27 respectively; knobs 26a and 26b provided at a space between the side 17a and the plate 14a and at a space between the side 17b and the plate 14b respectively to rotate around the axis of the rod 16; and torsion springs 34a and 34b provided, on both ends of the rod 16 to constantly apply torque to knobs 26a and 26b. 
     The printer body 28, on the other hand, comprises a chassis 29, first projections 19a and 19b and second projections 18a and 18b formed on sides 30a and 30b of the chassis 29, a print head 4, and a platen 1. 
     Referring to FIGS. 6 and 7, the support plate 14a (14b) includes notches 22a and 23a (22b and 23b) formed adjacent to each other, and a curved portion 21a (21b) having a contour engageable to the contour of the projection 18a (18b). 
     Referring to FIGS. 5 through 7, description will now be given to the case where the embodiment is applied to a pull-tractor type structure. The projections 19a and 19b are first engaged with the notches 22a and 22b. Then, an external force is applied to the knobs 26a and 26b in the direction of an arrow A (shown in FIG. 7) by an operator&#39;s finger to rotate the knobs 26a and 26b around the rod 16 by a desired angle. This permits the lower ends of the knobs 26a and 26b to move in the direction of an arrow C. As a result, the curved portions 21a and 21b of the plates 14a and 14b becomes engaged with the projections 18a and 18b. After the engagement of the portions 21a and 21b with the projections 18a and 18b, if the force applied to the knobs is released, the knobs 26a and 26b  will rotate around the rod 16 in the direction of an arrow B due to the elasticity of the springs 34a and 34b. This revolution allows the projections 18a and 18b  to be held between the portion 21a of the plate 14a and a curved portion 31a of the knob 26a and between the portion 21b of the plate 14b and a curved portion 31b of the knob 26b. In this manner, the assembly 24 and the body 28 are fixed to each other with the plates 14a  and 14b and the knobs 26a and 26b of the assembly 24 as well as with the projections 18a, 18b, 19a and 19b of the body 28. The gear 25 is driven with a motor pinion gear 33 via a gear 32 provided on the side 30a of the body 28. The gear 33 is attached to a motor (not shown) and rotated by the motor. The rotation of the gear 25 drives the tractor 2 to pull out the paper 3 which has been printed with the head 4. 
     Referring to FIGS. 8 through 10, description will next be given to the case where the embodiment is applied to a push-tractor type structure. The notches 23a and 23b of the support plates 14a and 14b are engaged with the projections 19a and 19b of the sides 30a and 30b. Then an external force is applied to the knobs 26a and 26b in the direction of an arrow A (shown in FIG. 10) by an operator&#39;s finger. In the manner similar to that described for the pull-tractor type structure, the projections 18a and 18b are held between the curved portion 21a and the curved portion 31a and between the portion 21b and the portion 31b. In this state, the assembly 24 is supported but inclined by an angle θ from the position of the assembly 24 in the pull-tractor type structure. In the push-tractor type structure, the gear 25 is directly driven by the gear 33. The tractors 2 driven by the rotation of the gear 25 feed the paper 3 to be printed near the head 4. The reduction ratio of the gear 25 against the gear 33 is expressed by the following equation, where the number of teeth of the gear 33 is n 1 , that of the gear 32 is n 2  and that of the gear 25 is n 3  : ##EQU1## for the pull-tractor type structure, and ##EQU2## for the push-tractor type structure. The reduction ratio is equal in both structures. Therefore, the rotation speed of the gear 25 is the same in both pull-tractor type and push-tractor type structures so that the paper can be fed in both structures at the same speed without taking trouble to change the revolution-per-minute speed of the motor. 
     As described in the foregoing description, the invention has an advantage in that if multiple paper is employed, the printer can be switched to the pull-tractor type structure while if the paper should be cut near the platen immediately after printing, it can be switched to the push-tractor type structure.