Improved method for producing quality hosiery in circular knitting machines and a device for its implementation

Abstract: An improved method for producing quality hosiery in circular knitting machines by which the density of the knitted hose is regulated by measuring the axial advancement of the tensioning device (8) for the produced hose portion by portion and consequently varying the level difference between the sinker knock-down plane and the minimum level reached by the active needles (4).

Description: This invention relates to a method for producing knitted hose of variable knitting density on a circular knitting machine, wherein the knitting density is regulated with precision as the hose production proceeds through consecutive courses. The invention also relates to a circular knitting machine for implementing the method. More particularly, and with reference to the production of quality hosiery articles, the invention relates to a method which enables the length of the stitch loops and thus the extensibility of the article to be adjusted with accuracy so that it adheres with greater comfort and better appearance to the leg of the wearer. As the human leg is typically shaped with a transverse dimension which varies gradually along its length in passing through the ankle, calf, knee and thigh, the extensibility of the knitted hose must be regulated correspondingly. Such an article is made extensible by generally varying the stitch density, ie the length of the stitch loops formed course by course by interaction between the needles and sinkers. To understand the technical problems involved, it is necessary to consider the operation of a circular knitting machine. Figure 1 shows a simplified scheme of a double cylinder circular knitting machine, and reference will be made thereto, it being however understood that the present invention is also advantageously applicable to a single-cylinder circular knitting machine. The upper and lower cylinders are indicated diagrammatically by 1 and 2; the knitted hose is indicatively formed in the zone 3 by the needles 4 which cooperate with the sinkers 5. The needles 4 are arranged on the outer surface of the cylinders 1 and 2 in suitable slide tricks along the generating lines of the cylinders, the sinkers 5 being arranged on the upper end of the cylinder 2. The knitted hose is formed along the outer periphery of the cylinder which supports and guides the needles 4 in their rotary and reciprocating movement in cooperation with the sinkers 5 and the yarn feeds, not shown in the figure. In the scheme of Figure 1, for reasons of simplicity the machine is shown during the production of a portion of plain knitted hose, for which only the needles of the lower cylinder act together with the sinkers. During this manufacture, the needles of the upper cylinder are transferred into the lower cylinder of the machine. If other types of stitch are produced, such as rib stitches, some needles are transferred from the lower cylinder to the upper cylinder. The length of the stitch loops is determined by the difference in level between the plane in which the sinkers 5 retain the yarn F deposited on them, known currently as the knock-over plane and indicated by a dot-and-dash line in Fig. 1, and the plane to which the hooks of the needle 4, after being raised to maximum level to pick up the yarn from the feeds, are lowered to reach minimum level while retaining the yarn in their upper hooks. The loop length is generally varied either by keeping the level of the knock-over plane fixed but, in accordance with the double-direction arrow shown in Fig. 1, positioning at a higher or lower level the knitting cam 6 which lowers the needle to the required level by means of its lower contour engaging the butts 7 of the needles 4, or viceversa by keeping the axial position of the cam 6 fixed and varying the level of the knock-over plane by raising relative to the cylinder 2 the circular ring 9 which supports the sinkers 5. For correct clearance of the knitted hose as it is produced and for the correct formation of the new knitwork courses, the hose must be removed from the zone 3 by making it penetrate into the cylinder 1 and must be kept under tension. This tension must be both constant and substantial, particularly for knitted fabrics of a certain consistency. Generally, tensioning members which engage the hose and move axially inside the circular knitting machine cylinders are used. By way of example, tensioning devices of this type are described in US-A-4516410 and in EP-A-0168871. Figure 1 shows diagrammatically the tensioning device 8 of EP-A-0 168 871. Said device exerts a substantially constant tension and withdraws the gradually produced hose by drawing it upwards from the zone 3 in which the knitwork is formed course by course. It has been found that the length of the produced stitch loops does not correspond unequivocally to the difference in level between the knock-over plane determined by the axial position of the sinkers 5 and the plane representing the minimum level reached by the hooks of the needles 4 by the action of the lowering cam 6. In this respect, after the needles 4 are raised to their maximum level and grasp the yarn from the feeds, the needles in being lowered to minimum level to form the stitches by dragging the yarn with them encounter a certain resistance offered by the yarn itself, which is unwound from an overlying bobbin. This resistance is due to the friction involved in the various direction changes of the yarn between the needle 4 and the feed bobbin and to the unwinding of the yarn from the bobbin itself, which can be of considerable and variable size, such resistance varying considerably. This resistance to the release of the feed yarn sometimes results in considerable elongation of the yarn and even in the withdrawal of yarn from the previously formed loops, so shortening them. Thus the stitch loops formed from such a taut yarn have a length when in their rest state which is less than that desired, once they are released and cleared from the needles. The knitted hose produced in this manner therefore does not have the required density and consequent extensibility. Even if it is desired to take account of the state of tension of the yarn during the loop formation by oversizing to a certain extent the said level difference, ie the length of the active needle stroke, in order to compensate the tension release, this expedient turns out to be unsatisfactory because the yarn tension varies during knitting. To obviate this drawback it has been proposed in GB-A-2193230 to measure the speed at which the yarn is actually transferred by the feed bobbin to the needles, and to correlate said speed with the yarn length which would have had to be knitted in a unit of time along the path between the needles and sinkers on the basis of their predetermined level difference. If any positive or negative deviation from this value is found, the level difference is correspondingly varied so that the formed loops are of the correct length. This expedient however only partly solves the technical problem because the measurement of the speed, ie the length of yarn transferred in a unit of time which itself is related to the yarn length used to form the knitwork courses, is effected on the yarn under tension, ie while still affected by the very uncertainties which cause the variation in the effective length of the stitch loops. Further causes of the inaccuracy of this measurement are that the resistance offered to the unwinding of the yarn is not constant, the yarn itself has an elongation/tension characteristic which is not a straight line, and the free taut length of the yarn varies periodically from a minimum to a maximum depending on the point at which it is withdrawn from the bobbin. FR-A-1 377 602 discloses a method and a device for controlling the length of a knitwork on a circular knitting machine relative to a predetermined length, whereby the length of the stitch loops is changed when a difference is detected between the measured length of the knitwork and the predetermined length thereof. The purpose is that of obtaining knitwork like stockings all having the same total length on the same knitting machine or on a group of knitting machines. With this method, however, it is not possible to produce knitwork having the desired variable density throughout its length. The purpose of the present invention is to obviate the above drawbacks and to provide a method and a circular knitting machine by which knitted hose having variable knitting density can be produced, in which the density is regulated with precision during knitting to obtain through every portion of the knitted hose the required length of the stitch loops and thus the required and desired extensibility independently of modifications of the state of tension of the yarn. This purpose is achieved by a method as claimed in claim 1 and by a circular knitting machine as claimed in claim 4. In contrast with the suggestion of GB-A-2 193 230, according to which the stitch loop length is changed in response to the speed of the yarn fed under tension, the present invention is based on determining the effective length of the stitch loops once they have been released from the needles. In this respect it has been found that the variation in the length of the loops of the produced knitted hose is strictly related to the movement of the tensioning device 8, which is operated with a constant tensioning force which becomes distributed over the entire circumference of the produced hose. The axial movement of the tensioning device 8 is faster in the case of longer loops in the production of a hose portion and slower in the case of shorter loops, in proportion to the effective increase or decrease in the length of the loops when in their rest state. The present invention allows to produce knitted hose of variable density, ie in which the stitch loops have a variable length which is regulated with precision during knitting along the length of the hose itself, by controlling the actual length of the produced stitch loops and comparing the actual variation in the axial velocity of the tensioning device 8, ie its axial advancement per unit of time, with the variation in the required length of the loops, portion by portion, ie with the predetermined desired variation of this axial velocity. Preferably, the advancement of the tensioning device 8 is determined for a predetermined number of produced courses of knitwork and is then compared with a reference value representing the desired advancement per course. If this comparison of the measured rate of axial advancement of the tensioning device 8 shows that it is less than that required, the level difference between the plane of deposition of the yarn F on the sinkers 5, ie the knock-over plane, and the plane in which the hooks of the knitting needles 4 are at their minimum level after grasping the yarn as determined by the axial position of the knitting cam 6, is then correspondingly increased. This variation can be effected either by raising the plane in which the sinkers 5 lie by axially raising their support 9, or by lowering the cam 6. If instead the measured rate of axial advancement of the tensioning device 8 is shown to be greater than that required, the opposite action is taken by reducing the level difference between the knock-over plane of the sinkers 5 and the minimum level plane of the hooks of the active needles 4. Figure 2 shows by way of non-limiting example a typical embodiment of the invention incorporating means for controlling the length of the stitch loops by varying the level of the position of the knitting cam 6 by level difference varying means described later on. On an axially movable guide rod 9 of the tensioning device 8 there is disposed a series of reference markers 10 which are sensed by a fixed sensor 11, for example a proximity sensor, facing the path of the markers 10. As the rod 9 moves axially, the markers 10 pass by the sensor 11, which senses their passage. The sensor 11 is connected by a connection 12 to a control unit 13 of microprocessor type to which the measured rate of axial advancement of the rod 9 is transmitted and wherein it is compared by comparing means with a predetermined rate of axial advancement referred to the number of knitwork courses produced or to the number of cylinder revolutions, for example every five cylinder revolutions. The commencement of scanning can be advantageously fixed as the commencement of production of each new knitted hose or as the commencement of that portion of hose through which the density is to be regulated. The unit 13 can be advantageously integrated into the control electronics of the circular knitting machine. The unit 13 also contains the series of discrete successive predetermined reference values for the rate of axial advancement for each portion of hose produced, corresponding for example to every five revolutions of the cylinder, and compares the measured values of the rate of axial advancement obtained by the sensor 11 with the predetermined reference values for the same portion. If the comparison shows a deficiency, ie the rod 9 has moved less than it should, the loops must be lengthened, and the cam 6 which is at too high a level is correspondingly lowered. If the comparison shows, an excess, the loops must be shortened and the cam 6 is correspondingly raised. The unit 13 regulates the positioning of the cam 6 on the basis of the comparison between the actual values obtained by the measuring means and the reference values to counteract the differences between these values. For this purpose the unit 13 also contains the control electronics for a stepping motor 14 and operates it via a connection 15. The stepping motor 14 undergoes controlled clockwise or anticlockwise rotations to rotate an output shaft 16 which at one end fixedly carries a rotary cam 17 of variable radius engaging one end point 18 of a lever 19, which is pivoted at 20 and engages with its other end 21 the knitting cam 6 for vertically displacing this cam. An elastic element 22 reacting against a fixed part 23 ensures constant contact between the point 18 and the contour of the rotary cam 17. The clockwise or anticlockwise rotations of the stepping motor 14 and cam 17 thus result in lowering or raising of the knitting cam 6, ie in a variation in the level difference between the knock-over plane of the sinkers 5 and the minimum level of the hooks of the lowered needles, which determines the length of the stitch loops. The method and machine of the invention enable stitch loops of the required length to be obtained through every portion of the produced fitted hose, independently of the state of tension of the yarn during its feed, and independently of the other described causes of disturbance. The resultant hosiery is produced exactly to the required size and shape.

Claims: A method of producing knitted hose of variable knitting density on a circular knitting machine, said hose having stitch loops of a variable length which is regulated during knitting along the length of the hose itself by varying the level difference between the knock-over plane of the sinkers (5) and the plane representing the minimum level reached by the hooks of the needles (4) under the effect of a knitting cam (6) which lowers the needles (4) while the hose being knitted is subjected to constant tension, characterised in that said hose is engaged by means of an axially displaceable hose engaging tensioning device (8) and in that the length of the stitch loops of said hose is controlled portion by portion as a function of the rate of axial advancement of the tensioning device (8) such that said rate of axial advancement is compared with a predetermined rate of axial advancement established for the same portion whereby said level difference is regulated to counteract the difference between the actually measured rate of axial advancement of the tensioning device (8) and the respective predetermined value for said rate of axial advancement. A method as claimed in claim 1, characterised in that the predetermined values for said rate of axial advancement and the actually measured values for said rate of axial advancement are related to the number of revolutions made by the needle cylinder (2) of the circular knitting machine or to the number of knitwork courses knitted, starting from the commencement of every knitted hose or the commencement of a portion of the hose for which the density is to be regulated. A method as claimed in claim 1 or 2, characterised in that each time the measured axial advancement of the tensioning device (8) is less than the predetermined axial advancement the level difference between the knock-over plane of the sinkers (5) and the plane representing the minimum level reached by the hooks of the knitting needles (4) is consequently and proportionally increased, and each time the measured axial advancement of the tensioning device (8) is greater than the predetermined advancement said level difference is consequently and proportionally decreased. A circular knitting machine for implementing the method claimed in claim 1, comprising sinkers (5) defining a knock-over plane, a knitting cam (6) for lowering the knitting needles (4), means (14,16-21) for varying the level difference between the knock-over plane of the sinkers (5) and the plane representing the minimum level reached by the hooks of the knitting needles (4) lowered by said cam (6), and tensioning means providing a constant tension onto the hose being knitted, characterised in that said tensioning means comprise an axially displaceable tensioning device (8) which moves axially inside the needle cylinder (1) of the circular knitting machine associated with means for controlling the length of the stitch loops of the hose portion by portion by regulation of said level difference, said controlling means comprising means (10,11) for measuring the rate of axial advancement of said tensioning device (8) during knitting of a given hose portion and means (13) for comparing said measured rate of axial advancement with a predetermined value for the rate of axial advancement contained in said comparing means for the same portion, whereby said means (14,16-21) for varying said level difference are operated by said controlling means in a way such as to counteract a difference between the measured rate of axial advancement and the respective predetermined value for the rate of axial advancement of said tensioning device (8) detected by said comparing means. A circular knitting machine as claimed in claim 4, characterised in that said means for measuring the rate of axial advancement of said tensioning device (8) comprise a series of reference markers (10) provided on a guide rod (9) of said tensioning device (8) and a fixed sensor (11) facing the path of said markers (10), said sensor (11) being connected to a microprocessor unit (13) containing said comparing means and controlling said level difference varying means (14,16-21). A circular knitting machine as claimed in claims 4 and 5, characterised in that the microprocessor unit (13) controls a stepping motor (14) operating a lever system (19,20) for vertically displacing said knitting cam (6). A circular knitting machine as claimed in claim 6, characterised in that said stepping motor (14) has a rotary cam (17) fixed on its output shaft (16), said rotary cam (17) operatively engaging one end (18) of a lever (19) whose other end (21) engages said knitting cam (6) for raising or lowering said knitting cam (6).

Inventors: NEGRI ETTORE, NEGRI, ETTORE
Applicants: MATEC SRL, MATEC S.R.L.
Application year: 1990
Main classification: D04B9
IPC codes: D04B9, D04B15