Self-adjusted modulated thread braking devices for a weft feeder

Abstract: A braking device for textile machine weft feeders (14), comprising a frustum-shaped braking body (15), which faces the drum (11) of the weft feeder (10), and an adjustable annular support (16), whereto the frustum-shaped body is rigidly coupled by means of a preloaded elastic suspension (17). According to the invention, and in order to optimize its elastic characteristics, the suspension (17) comprises at least two sets of identical corresponding traction springs which have respective different stiffnesses and preloadings which are inversely proportional to the respective stiffness of the springs.

Description: The present invention relates to self-adjusted modulated thread braking devices for weft feeders for weaving looms and textile machines in general. More specifically, the invention relates to modulated thread braking devices of the type disclosed in Italian Patent no. 1,251,209 or in European Patent Application EP 0 826 619 A1, comprising the features of the preamble of claim 1 with a frustum-shaped braking body which is arranged so as to face the fixed drum of the weft feeder, to which it is tangent at a diameter slightly smaller than the maximum one and against which it is pushed, so as to engage it by elastic contact, by corresponding flexible suspension elements. For this purpose, and according to a conventional arrangement, the braking body is rigidly coupled, at its smaller cross-section, to a rigid ring which is suspended elastically, by means of at least one set of three preloaded helical traction springs, from an annular support which surrounds the braking body and can move with respect to the drum by means of a screw-type adjustment mechanism. The helical suspension springs are subjected to a preloading F in a radial direction in order to keep the braking body correctly centered with respect to the annular support. Through the movement of the annular support towards the feeder drum produced by the adjustment mechanism, each spring is inclined by a generic angle α with respect to the radius of said support and stretches, discharging onto the braking body an axial elastic component Ftgα which can be adjusted by varying, again by means of said mechanism, the axial position of the annular support with respect to the drum of the feeder and therefore the inclination α of the springs. It is evident that depending on the stiffness of the elastic suspension of the braking body, i.e., on the slope of the straight line that plots the elastic characteristic of said suspension, the contact pressure and accordingly the braking action, for an equal adjustment, vary within broad limits. Since the braking action is proportional to the count of the weft thread being processed, it is necessary to have an assortment of uniform units or, briefly, of sets of springs having different stiffnesses in order to adapt the braking action of the frustum-shaped body to the corresponding different thread counts. Typically, in the adjustment range of the screw mechanism, the axial component of the total elastic tension discharged onto the braking body by the springs of the suspension must be able to vary between zero and 5000 g; this range is usually covered by alternately using two or more sets of springs having increasing stiffness and/or by varying the number of springs of each set. Spring replacement, however, is not easy and besides forcing to stop the loom it must be performed by specialized personnel, since it is easy to make mistakes in replacing one or more springs; imbalanced braking actions are accordingly possible. Therefore it is preferred to have a series of braking devices which have different characteristics and are chosen and installed in the weft feeder according to the different operating requirements; the term "braking devices" references the assembly constituted by the frustum-shaped body, the elastic suspension springs and the annular support. This is a drawback, since it requires the provision of multiple braking devices, with a consequent cost increase. Moreover, replacing the devices in any case forces to stop the loom and the selected device is always a more or less acceptable compromise in terms of stiffness of the elastic suspension in order to limit the devices to a reasonable number. The aim of the present invention is to eliminate these drawbacks, and within the scope of this general aim the invention has the important object of improving said braking devices, improving the elastic suspension of the frustum-shaped body so that said suspension has a nonlinear elastic characteristic which can vary according to a curve which in practice allows to cover, over the adjustment range of the screw-type adjustment mechanism, the entire range of values required for the axial elastic tension that acts on the frustum-shaped body. In other words, the aim of the present invention is to provide a braking device which can be used substantially over the entire range of threads being processed without requiring any replacement and/or adjustment. According to the invention, this aim and objects and others which will become apparent from the following detailed description are achieved with an improved braking device as claimed in claim 1. Substantially, the invention is based on the inventive concept of suspending the frustum-shaped braking body from the annular support by means of at least two sets of identical springs and of assigning to each spring set different stiffnesses and preloadings which are inversely proportional to the stiffness of the springs. Typically, the braking device according to the invention comprises a first set of three springs, which are arranged at 120° to each other and have a modest stiffness and a substantial preloading, deriving for example from a preloading elongation δL equal to 30-60% of the rest elongation of each spring, and a second set of three springs, which are interposed between the first ones, have a substantial stiffness, for example 100-200% greater than the stiffness of the springs of said first set, and a correspondingly reduced preloading, produced for example by a preloading elongation δL equal to 1-7% of the rest elongation of each spring. With this arrangement, the overall characteristic of the suspension assumes, in accordance with the stated aim and objects, a curvilinear plot which is characterized by an initial portion, corresponding to the minimum values of the adjustment, which has an almost constant gradient, and by a subsequent portion, corresponding to the higher adjustment values, in which the gradient gradually increases according to a substantially exponential rule. The present invention will become apparent from the following detailed description and with reference to the accompanying drawings, given by way of non-limitative example, wherein: Figure 1 is a lateral elevation view of a weft feeder provided with the improved braking device according to the present invention; Figure 2 is a lateral enlarged-scale view of a detail of Figure 1, illustrating the operating method of the braking device; Figure 3 is a front view of the braking device, taken in the direction of the arrows III-III of Figure 1; Figure 4 is a chart of the elastic characteristic of the suspension of the improved braking device according to the present invention. In Figure 1, the reference numeral 10 generally designates a weft feeder of the conventional type comprising a fixed drum 11 whereon a hollow rotating arm, rigidly coupled to a rotating disk 12 located at the base of the drum, winds a weft reserve in the form of a plurality of turns of thread S. The thread that unwinds from the drum 11 to feed a loom or other textile machine (not shown) passes through a finger 13 which is coaxial to the drum 11 and is subjected, during its unwinding motion, to the action of a modulating braking device, generally designated by the reference numeral 14, having the purpose of keeping said thread under tension and of modulating said tension as a function of the advancement speed of said thread. The braking device 14, interposed between the drum 11 and the finger 13, comprises a frustum-shaped braking body 15 which is arranged so as to face the drum 11, whereto it is tangent at its rounded end part along a circumference which is slightly smaller than the maximum circumference of said drum. By means of an annular support 16 and of an elastic suspension 17, the frustum-shaped braking body 15 is pushed into elastic contact engagement with the drum 11 in order to brake, with a preset elastic force, the thread unwinding from said drum. The annular support 16 is supported by a carriage 18 which can move axially with respect to the drum 11, by means of a screw mechanism 19, in order to vary the static tension that pushes the frustum-shaped braking body 15 against the drum 11. The elastic suspension 17 is formed by a plurality of radial helical springs M which are connected, at one end, to a rigid ring 20 arranged at the smaller cross-section of the frustum-shaped body 15 and, at the other end, to the annular support 16 (Figure 2). Through the movement of the support 16 towards to the drum 11 of the feeder produced by the screw mechanism 19, the springs M are inclined by an angle α (Figure 2) with respect to the radius of the annular support 16. By stretching, each spring discharges onto the frustum-shaped braking body 15 an axial elastic component Ftgα, where F is the radial pretensioning tension of the springs M. According to the present invention, and in order to achieve the stated aim and objects, the elastic suspension 17 comprises at least two sets of identical springs which have different stiffnesses and preloadings which are inversely proportional to the stiffness of the springs of each set. More specifically, and with reference to Figure 3, the improved braking device according to the present invention comprises a first set of three identical springs M1 which are arranged at 120° to each other and have a limited stiffness "c", for example between 0.10 and 0.25, and a substantial preloading which is suitable to ensure correct centering of the frustum-shaped body 15; for example, a preloading value produced from a preloading elongation δL equal to 30-60% of the rest elongation L 0 of the springs. A second set of three springs M2 is associated with the first set of three springs M1; the springs of said second set are interposed between the springs M1 and are likewise mutually spaced by 120°. According to the invention, the springs M2 of the second set of three has a significantly greater stiffness "c", for example between 1 and 4, and instead have a significantly reduced preloading, for example equal to a preloading elongation δL' equal to 1-7% of their rest elongation L' 0 . With this arrangement, the characteristic of the suspension assumes the shape plotted in Figure 4, which shows that it varies according to a curve K in which the initial portion A has an almost constant gradient and a subsequent portion B has a gradient which gradually increases according to a substantially exponential rule. This allows, in accordance with the stated aim and objects, to substantially cover the entire range between 0 and approximately 2700 g of values of the axial tension that acts on the braking body 15 in the corresponding adjustment range 0-64 of the screw adjustment device, expressed as quarter-turns of the knob of the screw mechanism 19. Typically, according to the invention, the springs M1 of the first set are made of spring steel wire having a diameter Φ = 0.4 mm and the springs M2 of the second set are made of spring steel wire having a diameter Φ = 1 mm. Without altering the concept of the invention, the details of execution and the embodiments can of course 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. 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.

Claims: A self-adjusting modulated thread braking device (14) for weft feeders (10), comprising a frustum-shaped braking body (15) having a longitudinal axis, which faces the drum (11) of the weft feeder (10), and an adjustable axially movable annular support (16), whereto said frustum-shaped body is rigidly coupled by means of a preloaded elastic suspension (17), characterized in that said suspension (17) comprises at least two sets of identical corresponding radially arranged traction springs (M1-M2) which have respective different stiffnesses and preloadings (δL) which are inversely proportional to the respective stiffness of the springs. A braking device (14) according to claim 1, characterized in that said suspension (17) comprises a first set of three springs (M1), arranged at 120° to each other, which have a modest stiffness (c) and a substantial preloading, and a second set of three springs (M2), which are interposed between the first springs and likewise spaced and have a substantial stiffness (c'2) and a correspondingly reduced preloading. A braking device (14) according to claim 2, characterized in that the first set of three springs (M1) has a stiffness (c) whose value is between 0.10 and 0.25 and a preloading elongation (δL) which is 30-60% greater than the rest elongation (L 0 ) of the springs, and in that the second set of three springs (M2) has a stiffness (c) whose value is between 1 and 4 and a preloading elongation (δL) which is 1-7% greater than the rest elongation (L' 0 ) of the springs. A braking device (14) according to claim 2, characterized in that the springs (M1) of the first set are made of spring steel wire having a diameter Φ of 0.4 mm and the springs (M2) of the second set are made of spring steel wire having a diameter Φ of 1 mm. A braking device (14) according to claim 1, characterized in that said at least two sets of springs (M1-M2) are provided in order to form an elastic suspension of the frustum-shaped braking body (15) which has a curvilinear characteristic (K), with an initial portion (A) which has a substantially constant gradient and a subsequent portion (B) in which the gradient increases gradually according to a substantially exponential rule.

Inventors: PEDRINI GIOVANNI, ZENONI PIETRO, PEDRINI, GIOVANNI, ZENONI, PIETRO
Applicants: LGL ELECTRONICS SPA, L.G.L. ELECTRONICS S.P.A.
Application year: 1998
Main classification: B65H51
IPC codes: D03D47, B65H51