[0001] The present invention relates to a thread braking device for weft feeder, having
reduced intervention times.
[0002] More particularly, the invention relates to a thread braking device for a weft feeder
in shuttle-less looms, particularly gripper looms, projectile looms and air-jet looms.
[0003] Even more precisely, the invention relates to a thread braking device which comprises
a braking means which has a continuous circular shape, typically a frustum-shaped
body, which is supported frontally and coaxially to the drum of the weft feeder, in
order to engage directly, or with interposed flexible elements, the thread that unwinds
from the drum, and wherein the frustum-shaped braking body is subjected to the action
of an electromechanical actuation means which is capable of varying the pressure with
which the body is pressed against the drum of the feeder; the actuation means is supplied
with a modulated excitation current.
[0004] Devices for positive modulated braking of the specified type are known in which the
braking body is subjected to the electrodynamic action produced by the interaction
of the excitation current that circulates in a coil associated with the braking body
and of the magnetic field of a permanent magnet which is fixed with respect to the
braking body.
[0005] Devices of the above-cited type are disclosed in EP-0534263 and EP-0652312 in the
name of this same Applicant, and are referenced concisely hereinafter as known devices.
[0006] Both of these conventional devices, although providing effective modulated braking,
suffer structural and functional drawbacks. From the structural point of view, they
are in fact relatively complicated and bulky and require accurate manufacture of the
moving coils and of the fixed permanent magnet, between which there must be a minimal
air gap in order to achieve a significant electrodynamic action despite excitation
currents of modest intensity.
[0007] On the other hand, the conductor of the moving coils must necessarily be small, and
the coils must be formed by a minimal number of turns in order to avoid increasing
excessively the mass, and therefore the inertia, of the frustum-shaped body that supports
said coil. This is the greatest structural drawback, because in any case it limits
the maximum allowable value of the excitation current; accordingly, the electrodynamic
action that affects the frustum-shaped body, which depends on the ampere-turn ratio,
is in any case limited to modest values, which are sufficient to produce the modulated
thread braking action if the frustum-shaped body acts by direct contact on the drum
of the feeder and are substantially insufficient if said body acts on the thread with
interposed flexible elements, such as coils of flexible laminas or rings of bristles.
[0008] Moreover, Italian patent No. 1,268,111 in the name of this same Applicant discloses
a device for positive modulated thread braking for weft feeders in which the frustum-shaped
body that constitutes the braking means is rigidly coupled to a support which can
move along the axis of the drum of the feeder, is supported and guided by a fixed
support, and in which the movable support is subjected to the action of a reversible
motor which is supported by the fixed support, is supplied with the modulated excitation
current (which is modulated so as to match the variation of the mechanical tension
of the thread during the weaving process) and is connected to the movable support
by interposing a mechanical coupling which is capable of converting the angular movements
of the motor shaft into corresponding translatory motions of the movable support with
respect to the fixed support.
[0009] This device, while constituting a substantial improvement with respect to the two
previously described conventional devices, also suffers drawbacks.
[0010] More particularly, the use of a motor of the reversible type, preferably of the step
type, which is suitable to act on the movable support in order to modulate, proportionally
to the variation of the mechanical tension of the thread during the beat of the loom,
so as to vary the pressure applied by the body to the drum of the braking device,
does not allow to provide immediate intervention, as required by modem looms, which
have a high average speed of the weft thread.
[0011] Moreover, the electrodynamic yield of the step motor used to drive the braking device
is not satisfactory, since it is necessary to use a high driving current in order
to have an adequate movement of the motor shaft.
[0012] The aim of the present invention is to provide a thread braking device for weft feeder
which allows to have extremely short intervention times of the braking device with
respect to conventional devices.
[0013] Within the scope of this aim, an object of the present invention is to provide a
thread braking device for weft feeder in which the reduced intervention times are
associated with a low driving current for actuating the motor shaft of the braking
device.
[0014] Another object of the present invention is to provide a thread braking device for
weft feeder in which the electrodynamic yield of the device is higher than in known
devices for an equal driving current.
[0015] Another object of the present invention is to provide a thread braking device for
weft feeder which allows to transmit to the braking cone of the braking device a movement
which is very precise in relation to the very low thickness that the thread can have.
[0016] Another object of the present invention is to provide a thread braking device for
weft feeder which is highly reliable, relatively easy to manufacture and at competitive
costs.
[0017] This and other objects which will become better apparent hereinafter are achieved
by a thread braking device for weft feeder, comprising a braking means which is supported
frontally and coaxially with respect to a drum of a weft feeder in order to directly
or indirectly engage the thread that unwinds from said drum, characterized in that
said braking means is actuated by at least one linear motor which allows the mutual
approach or spacing of said braking means with respect to said drum in order to modulate
the braking of said weft thread.
[0018] Further characteristics and advantages of the present invention will become better
apparent from the following detailed description of preferred but not exclusive embodiments
of the braking device according to the invention, illustrated only by way of non-limitative
example in the accompanying drawings, wherein:
Figure 1 is a schematic partially sectional view of the braking device according to
the present invention;
Figure 2 is a partially sectional view of a second embodiment of the braking device
according to the present invention;
Figure 3 is a partially sectional view of a third embodiment of the braking device
according to the invention;
Figure 4 is a schematic view of the braking device according to the invention and
shown in Figure 1, associated with a weft feeder;
Figure 5 is a partially sectional schematic view of a fourth embodiment of the braking
device according to the present invention; and
Figure 6 is a front view of the braking device according to the invention in its fourth
embodiment.
[0019] With reference to the above figures, 1 designates a conventional weft feeder which
comprises a fixed drum on which a hollow rotating arm, or windmilling arm, winds a
plurality of turns which constitute the weft reserve. The hollow arm is rigidly coupled
to a motor shaft which is likewise hollow, and the thread that arrives from a spool
(not shown) runs in the cavity of the shaft of the arm. At each beat, the loom draws
a certain number of turns from the reserve, and the thread that unwinds from the drum
of the weft feeder 1 is subjected to a modulated braking means which allows to control
the mechanical tension of the thread, keeping it substantially constant as the travel
acceleration that the loom applies to said thread in performing each individual beat
varies.
[0020] The braking means is composed of a braking body 2, typically a frustum-shaped body
which has a continuous circular shape (in a first embodiment of the invention), is
made of high-strength synthetic material, and is supported by a movable support 3,
which is arranged in front of the drum of the feeder 1 and coaxially thereto and with
which the body 2 is pushed into contact in order to engage the thread that unwinds
from the feeder 1.
[0021] The frustum-shaped body 2 is rigidly coupled to the movable support 3 by interposing
springs 4 which provide an elastic suspension which is capable of damping the braking
action but most of all of ensuring the perfect centering of the frustum-shaped body
2 with the drum of the weft feeder 1.
[0022] In the figures, the weft thread is designated by the reference letter T.
[0023] In the embodiment shown in Figure 1, the movable support 3 is connected to a support
5 which is directly connected to the shaft 6 of a motor.
[0024] An important feature of the invention consists in that the motor is a linear motor
M, whose shaft 6 is made of nonmagnetic material and constitutes the rotor of the
linear motor. The shaft 6 is coupled to a supporting element 7 on which two magnetic
rings (for example made of neodymium) are fitted; such rings are magnetized radially
and in opposite directions.
[0025] The two magnetic rings are designated by the reference numerals 10 and 11 and are
fitted on a ring 9 of magnetic material (for example iron), which is in turn fitted
on a spool 8 (made of plastics) rigidly coupled to the shaft 6.
[0026] The stator of the linear motor is constituted by an outer enclosure 20, through which
the shaft 6 passes coaxially, and by at least two pole shoes 12 and 13 which wrap
around two coils 14 and 15.
[0027] Preferably, the linear motor has an additional pole shoe (third pole 16) which greatly
improves the efficiency of the motor.
[0028] The modulation of the braking action of the braking device according to the invention
is performed by acting on a movement of the braking device 30, i.e., of the shaft
6 and therefore of the braking body 2. In practice, the flux generated by the magnets
10 and 11 concatenates with the coils 14 and 15, and by making currents flow through
said coils one obtains a movement of the shaft 6 in one direction or the other according
to the direction of the current.
[0029] The braking device according to the invention can further be provided with means
for measuring the movement of the motor, i.e., the movement of the shaft 6 that directly
actuates the braking device. The means for sensing the movement of the motor are conveniently
constituted by a sensor device 24 which faces a magnet 25 which is rigidly coupled
to the end of the shaft 6 of the motor that lies opposite the end that is connected
to the braking body 2.
[0030] The sensor device 24 is sensitive to the magnetic field and measures the distance
between it and the magnet 25.
[0031] Such distance is directly proportional to the movement that the shaft 6 of the motor
performs in order to move the braking body 2 into contact with the drum of the weft
feeder 1.
[0032] In this manner it is possible to know in real time the exact position of the shaft
6 and therefore the braking force applied to the braking device (braking body 2).
[0033] The actuation of the linear motor can be controlled by a signal which arrives from
the loom, or more specifically from a sensor, which directly measures the actual tension
of the weft thread T, and by means of a microprocessor regulates the braking action
of the braking device according to the parameters outside the loom or according to
the tension of the weft thread T.
[0034] In practice, the command issued by the microprocessor consists of a current signal
which excites the coils 14 and 15. The use of a linear motor allows to have high transmission
efficiency for the motion applied by the shaft 6 to the braking body 2, since the
connection between the two elements is substantially direct.
[0035] Moreover, for equal excitation currents of the coils 14 and 15, and with coil excitation
sources used in a conventional braking device, the electrodynamic yield, i.e., the
force that can be generated by the motor, is much higher in the case of a linear motor
and therefore the ratio of dissipated power to performance is highly in favor of the
linear motor.
[0036] Figure 2 is a view of a second embodiment of the braking device according to the
invention, in which the linear motor is used as shown in Figure 1, with the variation
that the braking body, designated by the reference numeral 32 in this case, is formed
by a ring which has, on its internal circumference, a ring of bristles 33 which are
meant to apply pressure, pushing the weft thread T against the drum of the weft feeder
1.
[0037] Figure 3 illustrates a third embodiment of the braking device according to the invention,
in which the linear motor, designated by the reference letter M in this figure and
in the preceding figures for the sake of simplicity, is applied in order to actuate
a braking body 42 which is constituted by a multiple-lamina brake.
[0038] Figure 4 is instead a general view of the weft feeder with which the linear motor
is associated in order to provide a braking device according to the present invention.
[0039] Figure 5 is a view of another embodiment of the braking device according to the invention,
in which a plurality of linear motors, particularly three linear motors, are used
as shown in detail in Figure 6. The three linear motors, each designated by the reference
letter M, are arranged so that the braking cone is perfectly centered on the drum
of the weft feeder 1 in order to achieve constant braking of the weft thread T once
a certain braking force has been preset.
[0040] This allows to transmit the movement to the braking cone with very high precision
and therefore to adapt the braking device also to very low thicknesses of the weft
thread T.
[0041] By means of the three linear motors M it is possible to preset initially a certain
force on the motor axis so as to make the braking cone mate exactly with the drum
of the weft feeder 1. After performing this operation, the microprocessor (not shown)
reads on the feedback sensor the resulting movement and sets the zero point of each
motor M, determining for the three points the zero plane for which the braking cone
is centered exactly on the rounded end portion of the weft feeder 1.
[0042] The linear motors M are then actuated by a signal which originates from the weaving
loom or from a tension meter arranged on said weft T, in order to maintain the programmed
tension value.
[0043] It is further important to use linear motors having a low inertia and with a moving
coil or magnet, because the times and responses must be on the order of a few hundred
microseconds, up to a few milliseconds.
[0044] The movable braking part must be very light.
[0045] Figure 5 illustrates the connection between the linear motor M and the movable support
3 which is connected to the braking body 2.
[0046] This connection is provided by means of a ball joint 40. The three linear motors
M are all engaged in the same ball joint 40 and allow to form a plane on which the
base of the braking cone (braking body 2) is arranged, so as to achieve exact self-centering
of the braking cone 2 with respect to the drum of the weft feeder 1.
[0047] As shown in Figures 2 and 3, instead of the braking cone 2 it is possible to have
a brake of the bristle type or a multi-lamina brake.
[0048] The use of a linear motor as described, according to the present invention, therefore
allows to have high movement speeds and therefore very short intervention times of
the braking body on the weft thread T.
[0049] Average weft speeds of up to 1800 m/min are in fact reached in modem shuttle-less
looms. With a reserve drum having a diameter of 140 mm, for example, one obtains approximately
4000 rpm. Since the instantaneous speed can be as much as 50% higher than the average
speed, main motor speeds of 6000 rpm may also occur. This means that the weft thread
T travels along one turn in approximately 10 msec, and since a loom with a height
of 1800 mm requires at least 4 turns of thread to form a beat, the brake must modulate
the braking action at least twice per beat and therefore intervene in a few milliseconds
if one wishes it to intervene in real time. It is thus evident that intervention speed
is an essential fact.
[0050] In practice it has been observed that the weft thread braking device fully achieves
the intended aim and objects, since it allows to intervene with extremely short times
on the braking of the weft thread, thus meeting the requirements of extremely fast
intervention of modern looms.
[0051] The device thus conceived is susceptible of numerous modifications and variations,
all of which are within the scope of the inventive concept; all the details may furthermore
be replaced with other technically equivalent elements.
[0052] For example, the shaft 6 of the motor M can be moved to the end of its stroke so
as to allow the brake to open completely if it is necessary to replace it or if it
is necessary to insert the thread through said braking means.
[0053] In practice, the materials used, so long as they are compatible with the specific
use, as well as the dimensions, may be any according to requirements and to the state
of the art.
[0054] The disclosures in Italian Patent Application No. TO99A000459 from which this application
claims priority are incorporated herein by reference.
[0055] 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 thread braking device for weft feeder, comprising a braking means which is supported
frontally and coaxially with respect to a drum of a weft feeder in order to directly
or indirectly engage the thread that unwinds from said drum, characterized in that
said braking means is actuated by at least one linear motor which allows the mutual
approach or spacing of said braking means with respect to said drum in order to modulate
the braking of said weft thread.
2. The braking device according to claim 1, characterized in that the rotor of said linear
motor is constituted by a shaft which is connected to said braking means, the movement
of said shaft allowing to modulate the braking action of said braking means on the
weft thread.
3. The device according to claim 2, characterized in that said linear motor comprises
at least two poles.
4. The device according to claim 2, characterized in that said linear motor comprises
three poles.
5. The device according to one or more of the preceding claims, characterized in that
said linear motor comprises an outer enclosure suitable to accommodate said shaft
which is supported by a supporting element which is arranged coaxially to said shaft,
a ring of magnetic material being fitted on said supporting element, two magnetic
rings being fitted on said ring of magnetic material, said magnetic rings being magnetized
radially and in opposite directions.
6. The device according to claim 4, characterized in that the outer enclosure of said
motor constitutes the stator of the motor, two coils being accommodated between said
outer enclosure and said poles of the linear motor.
7. The device according to one or more of the preceding claims, characterized in that
said shaft has, at the end that lies opposite the end for connection to said braking
means, a magnet which faces at least one sensor for detecting the movement of said
shaft of the linear motor.
8. The actuation device according to one or more of the preceding claims, characterized
in that said coils are current-driven according to the braking force to be obtained
on said weft thread.
9. The actuation device according to claim 7, characterized in that the current for driving
said coils is supplied by a microprocessor which is connected to a sensor for detecting
the tension of the weft thread, said microprocessor being further connected to said
sensor for detecting the movement of said shaft of the linear motor in order to produce
the intended movement of said shaft according to the intended braking force, which
is determined by comparing the braking force applied by said shaft with the tension
of said weft thread.
10. The device according to claim 1, characterized in that said braking means is a frustum-shaped
body which is connected, by virtue of flexible means, to a movable support which is
in turn connected to a support which is keyed on said shaft of said at least one linear
motor.
11. The device according to claim 1, characterized in that said braking means comprises
a brake with bristles which is connected to said shaft of said at least one linear
motor.
12. The device according to claim 1, characterized in that the braking means comprises
a multi-lamina brake which is connected to the shaft of said at least one linear motor.
13. The device according to claim 1, characterized in that it comprises three linear motors
which are connected to said braking means by a supporting element, said three linear
motors being suitable to allow the centering of said braking means with respect to
the drum of said weft feeder.