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(11) | EP 1 186 426 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Dot-matrix print head |
| (57) A dot-matrix print head (1) wherein a casing (2) houses a number of sliding print
needles (3) arranged radially about a central axis (7), respective rocking members
(4) associated with the needles (3), and respective electromagnets (5) for controlling
the rocking members and for attracting the respective rocking members in opposition
to elastic contrasting means (6) to move the respective rocking members selectively
into a rest position and a print position; the elastic contrasting means (6) are defined
by a substantially U-shaped monolithic annular element (6) made of elastomeric material
and having two continuous circumferential projections (21, 22) spaced radially to
rest, with respective predetermined loads, on respective first and second contact
portions of the rocking members (4); and the annular element (6) is so shaped and
sized that the ratio between the respective predetermined loads exerted by the projections
(21, 22) on the respective first and second contact portions (23, 24) of the rocking
members (4) ranges between roughly 7:1 and roughly 3:1 when the rocking members are
in the rest position, and between roughly 3:1 and roughly 1:1 when the rocking members
are in the print position. |
[0002] Dot-matrix print heads are known in which the sliding print needles, arranged in a ring, are moved by respective rocking blades activated by respective electromagnets in opposition to elastic contrasting means. In European Patent EP 418268, for example, the elastic contrasting means comprise two elastomeric rings, such as O-rings, compressed between the blades and a wall of the print head casing. One of the O-rings is located directly over the blade pivots to maintain contact between the pivots and electromagnets; the other O-ring is located radially inwards of the pivots and acts as a return spring to oppose the rocking movement of the blades; and respective blade guide pins are inserted between the O-rings. Though fairly effective as elastic contrasting elements, the O-rings in the above patent fail to provide for optimizing movement of and the return force exerted on the blades, thus resulting in malfunctions, vibration and noise. Moreover, using two separate elastic members, which must be positioned correctly and independently inside the print head, complicates assembly.
[0003] It is an object of the present invention to provide a print head designed to eliminate the aforementioned drawbacks of known solutions. More specifically, it is an object of the invention to provide a print head which, in a straightforward, low-cost manner and with a small number of component parts, provides for optimizing movement of the moving parts, thus reducing vibration and noise, and at the same time is highly efficient, reliable, compact, and cheap and easy to produce.
[0004] According to the present invention, there is provided a dot-matrix print head comprising a casing housing a number of sliding print needles arranged radially about a central axis, respective rocking members associated with said needles, and respective electromagnets for controlling said rocking members and for attracting the respective rocking members in opposition to elastic contrasting means to move said respective rocking members selectively into a rest position and a print position; said elastic contrasting means comprising first and second elastic means located circumferentially about said central axis and spaced radially to rest, with respective predetermined loads, on radially-spaced respective first and second contact portions of said rocking members; the print head being characterized in that said first and second elastic means are so shaped and sized that the ratio between said respective predetermined loads exerted by said first and second elastic means on said respective first and second contact portions of the rocking members ranges between roughly 7:1 and roughly 3:1 (is preferably about 5:1) when said rocking members are in the rest position, and ranges between roughly 3:1 and roughly 1:1 (is preferably 2:1) when said rocking members are in the print position.
[0005] The specific ratio between the loads exerted by the first and second elastic means provides, not only for fully efficient, reliable operation of the print head by optimizing movement of the moving parts in a straightforward, low-cost manner, but also, as substantiated by tests, for reducing vibration induced by movement of the moving parts, preventing resonance phenomena, and reducing noise.
[0006] An important aspect of the invention lies in the elastic contrasting means comprising a substantially U-shaped monolithic annular element of elastomeric material; said first and second elastic means being defined by respective continuous circumferential projections of said annular element, which project from a base portion of said annular element towards said rocking members, and are separated radially by a circumferential groove.
[0007] This therefore eliminates the need for two separate elastic members : one component, which is cheap and easy to produce and can be fitted quickly and easily to the print head, ensures contact at all times between the rocking members and respective electromagnets at the respective pivots, and also acts as a return element for restoring the rocking members to the rest position.
[0008] A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:
Figure 1 shows a schematic longitudinal section of a print head in accordance with the invention;
Figure 2 shows a larger-scale view, with parts removed for clarity, of a detail of the Figure 1 print head.
[0009] Number 1 in the accompanying drawings indicates as a whole a print head comprising a casing 2 housing a number of sliding print needles 3, respective rocking members 4 associated with needles 3, and respective electromagnets 5 for controlling rocking members 4 and for attracting respective rocking members 4 in opposition to elastic contrasting means 6 to move rocking members 4 selectively into a rest position and a print position - shown on the right and left respectively in Figure 1 - in which rocking members 4 in turn move respective needles 3 into a withdrawn rest position and an extracted print position.
[0010] In known manner, needles 3, respective rocking members 4 and respective electromagnets 5 are arranged radially about a central axis 7; rocking members 4 are defined by respective substantially flat blades defined by respective opposite, substantially parallel or slightly converging faces 8, 9; and blades 4 lie substantially, in the rest position, in a plane perpendicular to central axis 7, and rock in respective planes perpendicular to said plane.
[0011] In the non-limiting example shown, print head 1 is a so-called "nonballistic" type, i.e. in which blades 4 are connected integrally, at respective first ends 10, to respective heads of needles 3. It is understood, however, that the invention also applies to "ballistic" print heads, i.e. in which the needles are not connected to the blades, and are moved by the blades striking them.
[0012] Each electromagnet 5 (known and not described in detail for the sake of simplicity) has two opposite ends defining respective side by side, radially spaced, opposite poles 11, 12. Blades 4 comprise respective second ends 13 opposite first ends 10 and located at the radially outer poles 11 of electromagnets 5; and respective intermediate portions 14 having respective transverse edges 15 defining respective pivots of blades 4. Edges 15 are formed on faces 9 of blades 4 facing electromagnets 5, and cooperate with respective substantially flat supporting surfaces 16 defining the radially inner poles 12 of electromagnets 5.
[0013] Blades 4 have respective through holes 17 with axes substantially parallel to central axis 7 and located adjacent to and radially outwards of edges or pivots 15, at poles 12 of electromagnets 5. Holes 17 loosely house respective pins 18 carried by casing 2, e.g. formed in one piece with and projecting perpendicularly from a bottom wall 19 of casing 2; and holes 17 and respective pins 18 define guide means for guiding blades 4 rocking on pivots 15.
[0014] Print head 1 also comprises a central damping device 20, of known type and therefore not described in detail for the sake of simplicity, on which rest first ends 10 of blades 4 in the rest position.
[0015] Elastic contrasting means 6 comprise first and second elastic means 21, 22 located circumferentially about central axis 7 and spaced radially to rest on respective radially spaced first and second contact portions 23, 24 of blades 4. More specifically, the elastic contrasting means comprise a monolithic annular element 6 made of elastomeric material and having a substantially U-shaped radial section; and said first and second elastic means are defined by respective continuous circumferential projections 21, 22 of annular element 6, which project from a base portion 25 of annular element 6 and are separated radially by a circumferential groove 26.
[0016] Annular element 6 is housed in a circumferential seat 27 formed in bottom wall 19 of casing 2, immediately adjacent to and radially inwards with respect to pins 18; base portion 25 rests against a bottom wall of seat 27, possibly with the interposition of an annular spacer 28; projections 21, 22 project from seat 27 towards blades 4, so as to be compressed between faces 8 of blade 4 and the bottom wall of seat 27, and so exert respective predetermined loads on contact portions 23, 24; projection 21, radially outwards with respect to projection 22, rests on contact portions 23, substantially at edges 15; and projection 22 rests on contact portions 24 radially inwards with respect to edges 15.
[0017] Annular element 6 is so shaped and sized that projections 21, 22 exert on contact portions 23, 24 respective predetermined loads having a specific ratio. More specifically, the ratio between the predetermined loads exerted by projection 21 on contact portions 23 and by projection 22 on contact portions 24 ranges between roughly 7:1 and roughly 3:1 when blades 4 are in the rest position, and between roughly 3:1 and roughly 1:1 when blades 4 are in the print position.
[0018] Said ratio is preferably about 5:1 when blades 4 are in the rest position, and about 2:1 when blades 4 are in the print position. Purely by way of example, if, when blades 4 are in the rest position, the load exerted by projection 21 on contact portions 23 (at edges 15) is 250 g, the load exerted by projection 22 on contact portions 24 is about 50 g. When blades 4 are in the print position, the load exerted by projection 21 on contact portions 23 remains about 250 g, while the load exerted by projection 22 on contact portions 24 increases to about 125 g. Obviously, the above absolute values of the loads exerted by projections 21, 22 are purely indicative; the important aspect of the invention being that the ratio of said loads remains within the ranges indicated.
[0019] In actual use, the load exerted by radially outer projection 21 on contact portions 23, and therefore on pivots 15, holds blades 4 against electromagnets 5 and ensures constant contact between edges 15 and respective supporting surfaces 16 of electromagnets 5; while the load exerted by radially inner projection 22 on contact portions 24 acts as a return spring for restoring blades 4 from the print to the rest position and holding first ends 10 of blades 4 against damping device 20.
[0020] Print head 1 operates in exactly the same way as known print heads of the same type, so that no detailed description is required. Quite simply, when a selected electromagnet 5 is activated, respective blade 4 is rocked on respective pivot 15 from the rest to the print position and so moves relative needle 3 into the extracted print position. Blade 4 rocks in opposition to projection 22, which is further compressed elastically by blade 4; and, once printing is completed and blade 4 is no longer attracted by electromagnet 5, projection 22 restores blade 4 to the rest position resting on damping device 20.
1. A dot-matrix print head (1) comprising a casing (2) housing a number of sliding print
needles (3) arranged radially about a central axis (7), respective rocking members
(4) associated with said needles (3), and respective electromagnets (5) for controlling
said rocking members and for attracting the respective rocking members in opposition
to elastic contrasting means (6) to move said respective rocking members selectively
into a rest position and a print position; said elastic contrasting means (6) comprising
first and second elastic means (21, 22) located circumferentially about said central
axis (7) and spaced radially to rest, with respective predetermined loads, on radially-spaced
respective first and second contact portions of said rocking members (4); the print
head being characterized in that said first and second elastic means (21, 22) are so shaped and sized that the ratio
between said respective predetermined loads exerted by said first and second elastic
means (21, 22) on said respective first and second contact portions (23, 24) of the
rocking members (4) ranges between roughly 7:1 and roughly 3:1 when said rocking members
are in the rest position, and between roughly 3:1 and roughly 1:1 when said rocking
members are in the print position.
2. A print head as claimed in Claim 1, characterized in that said ratio between the loads exerted by said first and second elastic means (21,
22) on said respective first and second contact portions (23, 24) of the rocking members
(4) is about 5:1 when said rocking members are in the rest position, and about 2:1
when said rocking members are in the print position.
3. A print head as claimed in Claim 1 or 2, characterized in that said elastic contrasting means comprise a substantially U-shaped monolithic annular
element (6) of elastomeric material; said first and second elastic means being defined
by respective continuous circumferential projections (21, 22) of said annular element,
which project from a base portion (25) of said annular element towards said rocking
members (4), and are separated radially by a circumferential groove (26).
4. A print head as claimed in Claim 3, characterized in that said first elastic means (21) rest on said first contact portions (23) of the rocking
members (4), substantially at respective pivots (15) of said rocking members (4);
said second elastic means (22) resting on said second contact portions (24) of the
rocking members (4), radially inwards with respect to said pivots (15).
5. A print head as claimed in Claim 4, characterized by also comprising guide means (18) for guiding said rocking members (4) and located
radially outwards of said pivots (15); said annular element (6) being located radially
inwards with respect to said guide means (18).