[0001] The present invention relates to a needle printing head and more specifically to
an improvement of the actuation armatures of the printing needles.
[0002] It is know that among the various kinds of impact matrix printing heads the permanent
magnet printing heads have recently been introduces on the market.
[0003] These printing heads offer the advantage of requiring a lower electrical feedint
power and allow more compact embodiments and higher performances.
[0004] However more sophisticated manufacturing technologies and a more complex product
design are needed to produce them.
[0005] In said printing heads a plurality of magnetic circuits (each of which comprises
a magnetic core with a permanent magnet and a demagnetizing coil) are radially arranged
round a centre.
[0006] For each one of said circuits, a leaf spring which is radially arranged round the
same centre, constitutes a movable armature that closes the magnetic circuit.
[0007] The leaf spring is cantilever fixed in the outer area, with respect to the centre,
and it extends radially inwards, almost right to the centre, with an actuator arm.
[0008] A printing needle is fixed to the end of said actuator arm, substantially perpendicular
to the leaf spring plane.
[0009] Normally the magnetic circuit is magnetized owing to the effect of the permanent
magnet so that the leaf spring is normally actracted by the magnetic circuit in a
bent position and therefore the spring is elastically biased.
[0010] When the demagnetizing coil is energized the magnetic field generated by the coil
neutralizes the field generated by the permanent magnet thus causing the leaf spring
to be relased.
[0011] Due to this release a printing stroke is applied to the needle in the direction of
the same and the needle performs a print on a print media.
[0012] The leaf springs, or armatures, are a critical element of said printing heads because,
in time, them under go relevant and repeated stress which leads to fatigue failure.
[0013] Furthermore in order to be able to work at high frequencies they must have a suitable
stiffening and, at the same time, a minimal inertia.
[0014] In pratice, it is appropriate that said armatures have an adequate elasticity near
the cog so as to bend in the plane that is perpendiĀcular to the leaf plane, whereas
in correspondence with the actuator arm they must be very stiff so as to avoid deformation
to occur in the plane that is perpendicular to the leaf plane.
[0015] Generally this result is obtained by fixing on one of the faces of the actuator arm,
through resistance welding or brazing, a stiffening rib arranged perpendicularly to
the leaf spring plane.
[0016] An embodiment of this kind is described, for example, in the U.S. Patent N
o 4.295.250.
[0017] However this solution is not quite satisfactory because the alternate stress applied
to the armature causes, in time, the detachment of the stiffening rib and, as a result
of this, the printing head failure. This drawback is overcome by the needle printing
head, which is the object of the present invention where the stiffening of the armature
is obtained by means of a stiffening rib extending radially inwards to constitute
the actuator arm. This stiffening rib is fixed by means of a cogging groove at the
free end of the leaf spring so as to obtain a clamp active on both spring faces.
[0018] This and other features will appear more clearly from the following description of
a preferred embodiment of the invention and from the enclosed drawings where:
Fig. 1 shows, in a partial sectional view, a preferred ambodiment of the printing
head according to the present invention.
Fig. 2 and 3 show, in top and sectional view respectively, an actuation armature of
the printing head shown in Fig. 1.
Fig. 4 show in front view a stiffening rib for the armature.
Fig. 5, shows in front view, the armature depitched in Fig. 2 assembled with the stiffening
rib shown in Fig. 4.
Fig. 6 shows in perspective view the armature depitched in Fig. 5.
[0019] Fig. 1 shows, in partial sectional view, a preferred embodiment of a printing head
according to the present invention.
[0020] The printing head comprises a base circular plate 1 made of magnetic material which
is axially magnetized, a toroidal ring 3 made of magnetic material, a plurality of
cylindrical magnetic cores which are radially arranged round the central axis of the
printing head. A coil is inserted on each one of these magnetic cores.
[0021] One of these cores and the corresponding coil are shown in Fig. 1. where they are
indicated by reference numbers 4 and 5 respectively. The printing head comprises also
a thin circular ring 6 made of magnetic material and a further circular ring 7 made
of a material as elastic and magnetic as steel, which is provided with a plurality
of elastic leaves that extend inwards towards the central axis of the printing head.
Their number is equal to that of the cylindrical magnetic cores and each one of these
leaves is radially aligned with a corresponding cylindrical core.
[0022] Fig. 1 shows one of these elastic leaves which is indicated by reference number 8.
[0023] Furthermore the printing heas comprises a circular ring 8 that retains ring 7 and
a closing LID (10) which is provided with a nose on the end of which a ruby needle
guide (12) is mounted.
[0024] The various elements 1,2,3,6,7,9,10 are so rigidly packed as to constitute a single
assembly which is held together by means of screw set in suitable seats, two of which,
13 and 14, are shown with dashed lines in Fig. 1.
[0025] Each of the elastic leaves, as 8, is provided with a post 15 made of magnetic material
which is axially aligned with a corresponding cylindric core (as 4).
[0026] The various cores, as 4, board 1, and rings 2,3,6,7 form a plurality of magnetic
circuits, each of which is closed by a movable armature formed by an elastic leaf
and by a post.
[0027] When there is no stress, the elastic leaves 8 extend in a plane which is perpendicular
to the axis of the printing head and a small gap, in the order of 0,3 mm, is present
between the lower faces of the post as 15 and the top of cores as 4.
[0028] However the magnetic field developed by the permanent magnet 3 normally causes the
elastic leaves as 8 to be bent and attracted towards the cores as 4, so that the posts
are in contact with cores as 4.
[0029] The selective energization of the various coils as 5 permits the various magnetic
circuits to be selectively demagnetized and the various armatures consituted by leaves
as 8 to be selectively release too.
[0030] At the free end each leaf as 8 is provided with an actuation arm which is constituted
by a stiffening rib 16 which extends radially towards the printing head axis and is
perpendicular to the leaf plane.
[0031] At the end of each stiffening rib a printing needle as 17 is fixed which inserts
its Vip in the needle guide ruby 12.
[0032] When a strip as 8 is released, the corresponding needle 17 protrudes with its tip
from the needle guide ruby 12 thus permitting a dot to be printed on a printing support.
[0033] Fig. 2 and 3 show in plan and sectional view, and in greater detail, the shape and
structure of one the various armatures. As already mentioned the armature consists
of a leaf 8 made of elastic and magnetic material that extends radially from an external
ring 7 towards the centre of ring 0.
[0034] In correspondence with the connection to ring 7, leaf 8 is suitably radiused to the
ring through radiuses 18, 19.
[0035] The free end of the leaf is suitably rounded.
[0036] Near its free end leaf 8 has a circular opening into which a magnetic post 15 is
inserted and fixed.
[0037] To this end the magnetic post is provided with a stop collar 18 having the same diameter
as the circular opening and a height equal to the thikness of the leaf, which is inserted
in the circular opening of the leaf.
[0038] The diameter of the post is much greater than that of the stop collar, so as to offer
the leaf, when these two elements are assembled, a stop ledge.
[0039] The two elements can be fixed to each other by calking of the stop collar.
[0040] Besides the stop collar and on the same side of stop collar 18, post 15 has a cylindrical
appendix whose diameter is sligthly lesser than that of stop collar 18.
[0041] In said appendix a diametral grooves 20 having a suitable width is provided, with
orientation towards axis 0 of the printing head. It is designed to receive stiffening
rib 16 which acts as an actuation arm.
[0042] Fig. 4 shows in detail, in front view, siffening rib 16.
[0043] A stiffening rib consists of a small plate made of a material as resistant as steel,
having a suitable thickness (for instance 0,5 mm) and a generically elongated and
rectangular shape.
[0044] Stiffening rib 16 has a straight edge 21 which has been designed to rest on one of
the faces of strip 8 and a terminal portion 22 for insertion into groove 20 of Appendix
19.
[0045] In addition this siffening rib has a groove 23 which separates straight edge 21 from
a retaining tooth 24.
[0046] Groove 23 is intended to receive the distal end of strip 8 which is cogged therein.
[0047] At the end of rib 16, which is opposite to the terminal portion 22, a printing needle
17 is fixed through brazing or laser welding.
[0048] Fig. 5 shows in front view stiffening rib 16 Which is set on the armature formed
by strip 8 and post 15.
[0049] In this position rib 16 is fixed and rigidly held in place through either brazing
or laser welding.
[0050] Fig. 6 shows in perpective view the armature completed with the printing needle actuation
arm which is constituted by stiffening rib 16 and evidences the various areas along
which either brazing or welding of the composing elements can be carried out.
[0051] The use of a stiffening rib of the describe form ensures a strong structural bond
resistant to alternate stress.
[0052] In fact, with reference to Fig. 5 it can be noticed that whenever the armature is
released and as a result of this needle is projected upwards, the welding areas undergo
stress which is caused by a clockwise movement that is determined by inertial mass
of both the needle and the rib. This inertia tends to make said rib 16 to rotate clockwise
round cogging and to detach terminal portion 22 from the leaf 8.
[0053] This stress is easily absorbed by a wide welding area which pratically extends along
the whole terminal portion 22 embedded in Appendix 19 which, in its turn, is an integral
part of post 15.
[0054] Conversely, when the armature is recalled in the biases position, the welding areas
undergo the stress that is caused by a counter clockwise movement which tends to make
the rib rotate round its distal point B leaning on the leaf.
[0055] Missing tooth 24 such rotation would be obstacolated mainly by the limited welding
area, near point A, which undergoes tensile stress with the possibility of rapid fatigue
failure.
[0056] Vice versa the presence of tooth 24 determines the following two effects: on the
one hand it allows the realization of a wider and more winding welding area in correspondence
with the cogging, on the other hand it allows the stress to be transmitted directly
to rib 16 through tooth 24.
[0057] The weld material that might be present between tooth 24 and the end of strip 8 undergoes
a compression stress rather than a tension stress.
[0058] Therefore the maximum stress it allows is much higher and also much less affected
by fatigue.