Typewriting machines – Platen or platen-moving mechanism – Bar platen
Patent
1997-03-12
1999-03-09
Burr, Edgar
Typewriting machines
Platen or platen-moving mechanism
Bar platen
400658, B41J 1108
Patent
active
058790918
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an impact printer comprising a printhead that travels along a guide for printing a line of print of determined length on a document, contrast means for supporting the document against the head and contrasting the impulse exerted by the head during printing of the line, spacing means borne by the printhead and cooperating with the contrast means for maintaining a constant distance between the head and the document along the line of print, and elastic pressure means for applying pressure between the spacing means and the contrast means.
RELATED TECHNOLOGICAL ART
A known printer of characteristics similar to those described above is represented in diagram form in FIG. 1, wherein it is labelled 10. A frame 11 of the printer 10 supports two springs 12 that act on the ends 18 of a platen 13, ideally rigid type and on which a document 14 rests. A needle printhead 16, of known type, moves in a fixed rectilinear trajectory along the document 14 for printing characters on the document 14 through the interposition of an ink ribbon not shown in the drawings, and bears a spacing roller 17, which presses the document 14 against the platen 13 during movement of the head 16. In this way, the springs 12 become deformed and thus allow the platen 13 to move with respect to the head 16 and adapt itself to thickness S of the document 14, so that the roller 17 keeps the document 14 at a constant distance D from the head 16, regardless of thickness S of the document.
A pressure P that the roller 17 exerts on the document 14 varies considerably during movement of the head 16. In fact the roller 17, when in proximity of one of the two ends 18 of the platen 13, is contrasted solely by a force corresponding to the elastic reaction of one of the springs 12, whereas, as the head 16 approaches the centre of the platen 13, as indicated by the dashed line in FIG. 1, this force tends to increase and become theoretically, assuming null flexibility of the platen 13, twice that at the ends of the platen 13 and corresponding to the yielding of both springs 12.
The pattern of P is illustrated in graphic form in FIG. 2 by a curve 19 which, for a given thickness S of the document 14, represents the variation of the pressure generated between the roller 17 and the platen 13, along an entire length L of the platen 13; the value assumed by the pressure P in proximity of the two ends 18 of the platen 13 is indicated with P.sub.est, while the value assumed at the centre L/2 is indicated with P.sub.max.
Variation of the pressure of the spacing roller on the document during head movement gives rise to numerous drawbacks, notably: if the latter comprises carbon copy sheets or tracing paper, by passing over it during movement of the head or during feeding of the document when a line feed is effected; type, a needle printer for example. In this case, in fact, the elements of the printhead that touch the document exchange energy with the bar, and printing density may be greater at the centre of the line, where the platen contrasts the head with a pressure twice that exerted on the ends, where printing may appear faint. To the detriment of printing density, in fact, the platen tends to dynamically absorb a greater portion of the energy of the head impact elements at its ends, where it contrasts the head with a pressure markedly lower (approximately half) than at the centre of the bar.
Moreover, if a sufficient printing density is also desired in proximity of the ends of the platen and not solely at the centre, the force exerted by the springs on the ends must be increased, but this requirement conflicts with that of reducing the force in order to avoid the roller causing marks on the document at the centre of the bar.
SUMMARY OF THE INVENTION
One object of the present invention is to eliminate the drawbacks described in the foregoing, and this object is achieved by the printer of the invention, which is characterized by the fact that the pressure between the spacing means and the contrast means
REFERENCES:
patent: 1836906 (1931-12-01), Degener
patent: 2881893 (1959-04-01), Metzner
patent: 4215946 (1980-08-01), Nakabo et al.
patent: 4227819 (1980-10-01), Manriquez
patent: 4318452 (1982-03-01), Reitner
patent: 4575267 (1986-03-01), Brull
patent: 4632577 (1986-12-01), Brull et al.
patent: 4940994 (1990-07-01), Habelt et al.
Statutory Invention Registration, No. H338,Louie J. Lipp, Oct. 1987.
De Ambrogio Attilio
Oberto Massimo
Scarton Gianrico
Burr Edgar
Grohusky Leslie
Ing. C. Olivetti & C. S.p.A.
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