[0001] The present invention relates to an improvement to self-adjusting thread braking
devices for weft feeders.
[0002] More specifically, the invention relates to a self-adjusting braking device of the
kind described in the prior Italian patent no. 1,251,209 and hereinafter referenced
as conventional device for the sake of brevity.
[0003] Weft feeders are devices which typically comprise: a fixed cylindrical drum, on which
a windmilling hollow arm winds a plurality of turns of thread which constitute a thread
reserve; means for producing the advancement of the reserve turns from the base toward
the free end of the drum; and means for braking at the output the thread that unwinds
from the drum and feeds the loom or other textile machine, passing through a final
thread guide supported in a fixed manner coaxially to the drum.
[0004] The conventional self-adjusting braking device is constituted by a substantially
frustum-shaped braking body which is arranged so that its larger circumference is
in front of the fixed drum of the feeder, with respect to which it is tangent at an
output circumference which is slightly smaller than the maximum circumference of the
cylindrical body of the drum. Said braking body is supported by elastic suspension
means which are shaped like a lamina or an elastic diaphragm and is more typically
suspended at the center of a radial arrangement of traction springs which react on
an annular suspension element, which is rigidly coupled to a movable support, controlled
by an axial movement mechanism which allows to vary the static contact pressure that
the frustum-shaped braking body applies to the drum if the thread is not advancing.
[0005] The thread advances between the drum and the braking body, onto which it discharges
the axial component of its mechanical tension. When the tension increases -- as the
advancement speed of the thread increases -- said axial component tends to move the
braking body axially in contrast with the action of the elastic suspension means,
separating it from the drum and causing a consequent and corresponding decrease in
the braking action, which is thus modulated as a function of said advancement speed.
[0006] In particular, the self-adjusting action of said conventional brake is such as to
decrease the braking of the weft thread as the thread advancement speed increases
and viceversa. The lower the rigidity of the suspension springs of the frustum-shaped
body, which react elastically to said axial movements of the braking body, the greater
the effect.
[0007] In some applications, typically in high-speed looms, the excessive decrease in the
mechanical tension of the weft thread can form a so-called "balloon" of thread ahead
of the frustum-shaped braking body. This has a negative effect on the correct execution
of the weaving action, because it can generate tangles of thread but most of all because
it causes the mechanical tension of the thread to be substantially uncontrollable,
so that the brake at least partially loses the ability to self-adjust.
[0008] Currently, an attempt is made to obviate this drawback by increasing the static pressure
of the braking body, but this entails a corresponding and sometimes undesirable overall
increase in the mechanical tension of the thread during the entire beating cycle of
the weaving loom.
[0009] The essential aim of the present invention is to eliminate this drawback by improving
known self-adjusting braking systems in order to make them adapted to avoid the forming
of the so-called "balloon" of thread without resorting to an increase in the static
pressure of the braking body on the drum of the weft feeder and therefore without
undesirably altering the values of the overall mechanical tension that acts on the
thread during the beating cycle of the loom or the like.
[0010] According to the present invention, this aim and other important objects are achieved
by an improved self-adjusting braking device which has the specific characteristics
stated in the appended claims.
[0011] Substantially, the invention is based on the concept of limiting the axial movements
for the disengagement of the frustum-shaped braking body from the drum of the feeder
through a retention means made of flexible material. Typically, and according to an
embodiment of the invention, the retention means is constituted by a ring made of
soft, natural or synthetic elastomeric material, advantageously having a cellular
structure, which is vulcanized onto a threaded ring which mates with a correspondingly
threaded seat formed in the annular suspension element of the frustum-shaped body,
so that by adjusting the extent to which said ring is screwed into said annular element,
the distance of the elastomeric ring from the outer surface of the frustum-shaped
body is adjusted, consequently adjusting the axial stroke that said frustum-shaped
body can perform before elastically abutting against the retention ring.
[0012] As an alternative, and according to a variation of the invention, the elastomeric
retention ring is supported directly by the annular suspension element of the frustum-shaped
body, so that the distance between the outer surface of the frustum-shaped body and
said elastomeric ring is adjusted by elastic deformation of the suspension spring
of the frustum-shaped body, adjusting the axial position of said supporting ring by
means of the movement mechanism.
[0013] Said variation, which provides considerable simplification, can however be applied
only if the suspension springs of the frustum-shaped braking body have a very low
rigidity, so that the axial movement of said supporting ring does not entail a significant
increase in said static pressure.
[0014] The present invention is described in detail with reference to the accompanying drawings,
provided by way of non-limitative example and wherein:
figure 1 is a partially sectional lateral elevation view of a weft feeder with the
improved self-adjusting braking device according to the present invention;
figure 2 is an enlarged-scale view of a detail of figure 1.
[0015] The weft feeder 10 shown in the figure is of the conventional type which comprises
a fixed cylindrical drum 11, which is adapted to receive a plurality of turns of weft
thread F which constitutes a weft reserve R to be fed to a loom or other textile machine.
A hollow radial arm 12, rigidly coupled to a rotating ring 13 arranged at the base
of the drum, winds the turns of thread onto said drum, and a known system with oscillating
rods 14 transfers the turns from the base to the head of the drum 11 in order to form
the weft reserve R.
[0016] When requested by the loom, the thread unwinds from the drum 11 and passes through
a fixed thread guide 15, which is coaxial to said drum, and its mechanical tension
is controlled by the modulated and self-adjusting braking action produced by a frustum-shaped
braking body 16, which is arranged so that its larger circumference faces the drum,
against which it is pushed so as to engage it by tangent elastic contact. For this
purpose, the frustum-shaped body 16 is suspended at the center of a radial arrangement
of helical springs 18, typically a set of three springs, which react against an annular
support 19 which is arranged frontally and coaxially to the drum 11 and is supported
by a slider 20 so that it can be adjusted axially. Said slider can move along a guide
21 which is parallel to the drum 11 and is controlled by a mechanism of the screw-and-nut
type, which can be operated by a knob 22, through which it is possible to vary the
static pressure that the frustum-shaped braking body 16 applies to the drum 11.
[0017] In a per se known manner, said braking system reacts automatically to the variations
in the mechanical tension of the thread produced by corresponding variations in the
advancement speed of said thread, by means of corresponding axial movements of the
braking body 16, which moves away from the drum 11, in contrast with the action of
the springs 18, correspondingly reducing the braking action applied to the thread
and therefore the mechanical tension of said thread. However, if the mechanical tension
of the thread decreases excessively, as a consequence of a large axial movement of
the braking body 16, a balloon of thread forms in the region directly before of the
braking body 16.
[0018] According to the present invention, this drawback is overcome by means of an improvement
to said braking system, which consists of the fact that the movement of said frustum-shaped
braking body 16 is limited by a flexible retention element.
[0019] Said retention element is constituted by a ring 23 made of soft elastomeric material,
advantageously having a cellular structure, such as natural or synthetic rubber, adapted
to engage by abutment the outer surface S of the frustum-shaped body 16.
[0020] According to a preferred embodiment of the invention, the elastomeric ring 23 is
vulcanized onto a threaded ring 24, which engages a correspondingly threaded seat
formed in the annular support 19, allowing to adjust, according to the extent to which
it is screwed in, the distance of the elastomeric ring 23 from the outer surface S
of the frustum-shaped body 16 and therefore the maximum axial stroke that said body
can perform in order to abut against the retention ring 23.
[0021] In practice, the position of the retention ring 23 is adjusted by means of the following
procedure: since the ring 24 is fully screwed into the respective threaded seat of
the annular support 19, the static pressure that the frustum-shaped body 16 applies
to the drum 11 when the thread does not advance is set to the minimum value through
the knob 22. The loom is started with this minimum setting of the static pressure,
so that the frustum-shaped body 16 undergoes a sudden and large axial movement, triggering
the forming of said balloon. Starting from this initial configuration, the degree
to which the ring 24 is screwed into the seat of the support 19 is decreased, moving
the retention ring 23 closer to the frustum-shaped body 16 until it almost skims its
surface while still allowing a minimum axial mobility to the latter.
[0022] The position thus assumed by the ring 23 is the optimum configuration of the system
for retaining the frustum-shaped body by abutment, after which the static pressure
can be set by the knob 22 to the correct operating value without generating said balloon
of thread.
[0023] According to a simplifying variation of the invention, the elastomeric ring 23 is
vulcanized directly onto the supporting ring 19 and its axial distance from the frustum-shaped
braking body 16 is adjusted by the knob 22 by means of flexing of the springs 18 and
of the consequent variation in the mutual relative position of said body 16 and said
ring 23. However, this is possible only if the springs 18 are highly elastic, so that
the movement of the annular support 19 and accordingly of the frustum-shaped body
16 and of the retention ring 23 does not entail a significant increase in the static
pressure with which the frustum-shaped body 16 engages the drum 11.
[0024] Of course, without altering the concept of the present invention, the details of
execution and the embodiments may be altered extensively, with respect to what has
been described and illustrated by way of non-limitative example, without thereby abandoning
the scope of the invention.
[0025] Where technical features mentioned in any claim are followed by reference signs,
those reference signs have been included for the sole purpose of increasing the intelligibility
of the claims and accordingly, such reference signs do not have any limiting effect
on the interpretation of each element identified by way of example by such reference
signs.
1. A self-adjusting thread braking device for weft feeders (10), comprising a frustum-shaped
braking body (16) and means (18-19) for elastically suspending said frustum-shaped
body and for pushing it elastically into tangent contact engagement with the drum
(11) of the weft feeder (10), characterized in that it comprises a retention element
(23) made of flexible material, which is associated with the suspension means (19)
of the frustum-shaped braking body (16) and is adapted to elastically engage, by abutment,
the outer surface (S) of said frustum-shaped braking body in order to limit its axial
movements.
2. A device according to claim 1, characterized in that said retention element is constituted
by a ring (23) made of natural or synthetic soft elastomeric material, advantageously
having a cellular structure.
3. A device according to claims 1 and 2, characterized in that said retention ring (23)
is rigidly coupled to a threaded ring (24) which mates with a correspondingly threaded
seat provided in a rigid annular element (19) which constitutes the suspension support
of said frustum-shaped braking body (16); the extent to which said ring (24) is screwed
into said seat of the annular supporting element (19) being used in order to adjust
the distance of the elastomeric retention ring (23) from the outer surface (S) of
the frustum-shaped braking body (16).
4. A device according to claims 1 and 2, characterized in that said elastomeric retention
ring (23) is supported directly by a rigid ring (19) which constitutes the suspension
support of the frustum-shaped braking body (16), which is supported by a radial arrangement
of suspension springs (18) which react on said rigid ring; in that the rigid ring
(19) is controlled by an axial movement mechanism which can be operated by a knob
(22); and in that the adjustment of the mutual relative distance between the outer
surface (S) of the frustum-shaped body (16) and the elastomeric retention ring (23)
occurs by elastic deformation of the suspension springs (18) by adjusting, through
said knob (22), the axial position of said rigid ring (19) that constitutes the suspension
support of the frustum-shaped braking body (16).