WO2014060325A1

WO2014060325A1 - Single-cylinder knitting machine and needle cylinder for said machine

Info

Publication number: WO2014060325A1
Application number: PCT/EP2013/071361
Authority: WO
Inventors: Renato Pilotelli; Roberto Pilotelli
Original assignee: Renato Pilotelli; Roberto Pilotelli
Priority date: 2012-10-15
Filing date: 2013-10-11
Publication date: 2014-04-24
Also published as: EP2906743A1; CN105143538B; CN105143538A; EP2906743B1; ITFI20120209A1

Abstract

The single-cylinder circular knitting machine comprises a needle cylinder (1) and a plurality of latch needles (3) sliding in respective tracks of the needle cylinder. An annular channel (9) with an undercut (9D) is provided along one end of the needle cylinder. Between each pair of adjacent tracks respective a substantially radial groove (11) is provided, formed in the needle cylinder and intersecting the annular channel (9). In each groove a respective insert (13) is housed. The inserts define a stitch forming surface. Each insert (13) has an appendix (23) that is inserted in the undercut (9D) of the annular channel (9). At least one blocking element (31) is also provided that holds the inserts (13) in a working position in their respective grooves (11).

Description

"SINGLE-CYLINDER KNITTING MACHINE AND NEEDLE CYLINDER FOR

SAID MACHINE"

Description

Technical Field The present invention relates to the single-cylinder knitting machine, and more in particular to the single-cylinder knitting machines with latch needles. The invention also relates to a needle cylinder for a single-cylinder circular knitting machine.

Background Art Single-cylinder knitting machines are provided with a single circular needle bed formed by needles sliding in tracks provided in a needle cylinder. For stitch forming, the needles cooperate with a ring of sinkers provided with an alternating radial movement synchronized with the downward and upward movement of the needles. Circular knitting machines without sinkers or machines provided with fixed sinkers, for instance on the upper end of the needle cylinder, have been also produced. A knitting machine without sinkers is disclosed in WO-A-2007/074486.

In machines without sinkers or with fixed sinkers the stitch is formed on a stitch-forming plane or surface comprised of components attached to the upper end of the needle cylinder, i.e. to the end in correspondence of which the needle hooks are arranged, which are cyclically extracted and retracted to take the yam and form stitches.

In machines without sinkers or with fixed sinkers tabs are provided projecting between adjacent needles. These tabs are usually produced by machining a cylinder blank. The function of these tabs is to replace the movable sinkers defining the position of the stitch forming plane or surface that, unlike what occurs with movable sinkers, is fixed, i.e. without radial movement. As a result thereof, during stitch formation the yarn always rubs on a fixed position on the tabs, thus causing premature wear of the upper edge of these tabs with a cut that could worsen the features of the produced fabric and facilitate yarn breakage.

To avoid these drawbacks, usually the tabs forming the stitch forming surface are firstly formed by chip removal from the cylinder blank and then hardened through an induction hardening cycle. However, this does not solve the problem in a satisfactory manner.

Furthermore, the hardened tabs are fragile and can break more easily following an impact. Their surface finishing does not allow the yarn to slide in an optimum way. Moreover, the tabs shall be produced with a much reduced thickness to leave sufficient space between them for the movement of the needles and the passage of the yarn during stitch formation. Thin tabs are even more subject to break due to the fragility resulting from the hardening process.

To overcome these disadvantages the production of the tabs with a material other than that of the cylinder has been proposed, producing the tabs as separate elements, adequately shaped and then glued in cuts formed by milling in the edge of the needle cylinder. In this way the inserts can be produced in the more suitable material, with the necessary hardness and optimum surface finishing.

However, in this case again the elements forming the tabs are subject to wear, even if they have longer useful life than the common solutions, wherein the tabs are produced in a single piece from the cylinder, with or without hardening. The replacement or the overhaul of the cylinder and the elements forming the tabs is therefore required. These operations require demounting the needle cylinder by skilled staff. This entails long shut-downs and the need for transporting the machine to a workshop, with consequent costs for production loss, transport and use of skilled staff.

There is thus a need for improving single-cylinder machines with fixed sinkers or without sinkers as regards the resistance of the stitch forming surfaces. Summary of the Invention

According to an embodiment, a single-cylinder knitting machine is provided, comprising a needle cylinder and a plurality of latch needles sliding in respective tracks of the needle cylinder. According to a different aspect, the invention also relates to a cylinder for circular knitting machines.

According to the invention, an annular channel with an undercut is advantageously provided along one end of the needle cylinder. Between adjacent tracks where the needles slide a substantially radial respective groove is provided, formed in the needle cylinder and intersecting the annular channel. In each groove a respective insert can be housed. The various inserts, form with their respective upper edges a stitch forming surface or plane, i.e. a sinker surface. Each insert can have an appendix that is inserted and blocked in the undercut of the annular channel. A blocking element holds the inserts in a working position in their respective grooves. In the working position the appendices of the inserts are advantageously pushed in the undercut of the annular channel by the blocking element, so as to maintain the inserts fixed during the knitting process, preventing the inserts from exiting the grooves. By removing or deactivating the blocking element, one or more inserts can be removed and replaced, for instance when worn or damaged.

This operation can be done by a machine operator, without the need for transporting the machine to a workshop, thus making the use and maintenance of the circular knitting machine much easier.

In practical embodiments the blocking element pushes the inserts in a substantially radial direction. Depending upon the position of the undercut, this thrust may be radially inwards or radially outwards. The thrust exerted by the blocking element is preferably a resilient thrust, so as to balance any dimensional tolerance and facilitate removal and replacement of the inserts.

In advantageous embodiments the substantially radial grooves, wherein the inserts are housed, have a depth smaller than the depth of the annular channel, so that along one flank of the channel a continuous surface forms, defining the undercut, with which the inserts cooperate.

In advantageous embodiments the undercut is delimited by a surface that is inclined towards the bottom of the annular channel, for instance a surface shaped like a solid of revolution, typically for instance a conical surface.

Advantageously, the appendices of the inserts can project from a lower edge of the insert, which edge is opposite with respect to the upper edge defining the stitch forming surface. The appendices can advantageously have an inclined edge, forming with the lower edge an angle lower than 90°, for instance comprised between 80° and 20° , and preferably between 70° and 30° .

Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. Brief description of the drawings

The invention will be easier to understand by means of the description below and the attached drawing, which shows a non-limiting, practical embodiment of the invention. More in particular, in the drawing: figure 1 is a cross-sectional view along line I-I in figure 2, of the upper part of the needle cylinder in an embodiment; figure 2 shows a plan view according to line II-II in figure 1; figure 3 is a cross-sectional view similar to that of figure 1 with some components removed and with the insert 13 separate from the needle cylinder; figure 4 is a view according to line IV-IV of figure 3 ; figures 5 A to 5 L show a stitch forming sequence with a cylinder according to figures 1 to 3; figure 6 shows a cross-sectional view similar to the section of figure 1 in a second embodiment; figure 7 shows a view similar to that of figure 2 in the embodiment of figure

6; figure 8 shows a cross-sectional view similar to that of figure 1 in a third embodiment; and figure 9 shows an enlarged cross-sectional view of the upper end of the needle cylinder in a further embodiment.

Detailed description of embodiments

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. Reference throughout the specification to "one embodiment" or "an embodiment" or "some embodiments" means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase "in one embodiment" or "in an embodiment" or "in some embodiments" in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

Figure 1 shows a cross-section of the upper part of a needle cylinder in a single-cylinder circular machine according to a nearly radial surface according to I-I in figure 2, which in turn shows an enlargement of a portion of the needle cylinder in a front view according to Π-ΙΙ of figure 1. In the cross-section shown in figure 1, reference number 1 indicates the needle cylinder and reference number 3 indicates one of the latch needles, the needle cylinder 1 is equipped with. In practice, as well known to those skilled in the art, needles 3 are arranged in a number corresponding to the fineness of the machine around the whole circular extension of the needle cylinder 1.

Each needle 3 slides in a respective track 4 substantially parallel to the axis A-A of the cylinder. In some embodiments each track where the needles 3 slide can be formed by milling of the cylindrical surface of the needle cylinder 1. In the illustrated embodiment, each track is defined by two adjacent and consecutive bars 5, extending parallel to the axis A-A of the needle cylinder 1 and fixed in corresponding grooves provided in the needle cylinder 1. Some or all the bars 5 can be provided with feet 5A for the purposes described below. The needles 3 can be provided with butts 3T cooperating with control cams arranged in a cam mantle, not shown and arranged around the needle cylinder 1 , as well known to those skilled in the art. The needles 3 can be held in the tracks by means of annular springs 7.

As shown in particular in figure 3, where the needle cylinder 1 is shown in a cross-section similar to that of figure 1 but without the needles and the other components mounted on it, the needle cylinder 1 has an upper end or edge 1 A along which an annular channel 9 is provided, extending along the entire circular extension of the needle cylinder 1. Upper end means the end at which are arranged and from which exit the hooks of the needles 3 during the knitting process (see sequence of figures 5A to 5L). In some embodiments the channel 9 has a bottom surface 9A, for instance flat and laying on a plane orthogonal to the axis A-A of the needle cylinder 1. 9B and 9C indicate two flanks, respectively the radially inner and the radially outer flank of the channel 9. In some embodiments, the flank 9B has a substantially cylindrical extension, while the flank 9C has advantageously an axial-symmetrically shaped surface portion, in particular a surface of revolution around the axis A-A. In the example illustrated in figures 1 and 3, the surface portion in question has a substantially conical shape flared downwards, i.e. towards the bottom 9A of the channel 9, and is coaxial with the needle cylinder 1. In general, the surface defining the flank 9C is shaped so as to define an undercut 9D inside the channel 9. Moreover, substantially radial grooves 11 are provided along the upper end

1A of the needle cylinder 1. The radial grooves 11 intersect the annular channel 9. Advantageously, a radial groove 11 can be provided in correspondence of each bar 5 delimiting a corresponding track where the needles 3 slide. Substantially, for each needle 3 of the cylindrical needle bed defined by the needle cylinder 1, two substantially radial grooves 11 are therefore provided, arranged at the side of the track where the needle slides.

Inside each groove 11 an insert 13 can be inserted, shown separate from the cylinder in the upper part of figure 3 and mounted in the groove 11 in figure 1.

As shown in particular in the front view of figure 4, each insert 13 can have a substantially laminar shape with variable thickness. More in particular, an upper portion 13A of the insert 13 has a thickness SI lower than the thickness S2 of a lower portion 13B of the same insert 13. When the insert 13 is mounted in the respective radial groove 11, the lower, thicker part 13B is integrally housed in the radial groove 11 while the thinner part 13A is partially housed in the radial groove 11 and partially or completely projecting there from. In other embodiments the thickness of the insert 13 is constant along the whole height, or can vary continuously from a maximum to a minimum thickness. The configuration of figure 4 is particularly advantageous, as simple to be produced and particularly robust and effective. The thicker part is an optimal guide in the groove 11 and increases the mechanical resistance of the insert. The thinner part leaves more space for the yarn in the stitch forming step, when the needle arranged between two adjacent inserts 13 goes down below the stitch forming surface.

In advantageous embodiments, each insert 13 has an upper edge 15 (figure 3) and a lower edge 17, in as well as two side edges or flanks 19 and 21. "Upper" and "lower" refer to the position that these inserts normally take when mounted on the needle cylinder 1 and this latter is positioned in the most frequent vertical position, with the edge 1A facing upwards. It is also possible that the needle cylinder 1 is positioned in a different manner.

When mounted in their grooves 11 , the inserts 13 form, with their respective upper edges 15, a stitch forming or sinker surface. In some embodiments, the upper edges 15 of each insert 13 may have a first portion 15A inclined from the top downwards and from the outside towards the inside of the needle cylinder 1, and a second portion 15B radially more external with respect to the first portion 15 A, with a substantially horizontal extension, i.e. orthogonal to the axis A-A of the needle cylinder 1 when the insert 13 is mounted in the respective groove 11. In some embodiments, the inserts 13 may be provided with a wear-resistant coating or treatment. In particular, according to particularly advantageous embodiments, the wear-resistant coating or treatment may be limited to the portion 15B.

In advantageous embodiments each insert 13 has an appendix 23 projecting from the lower edge 17 opposite to the edge 15. In mounted arrangement (figure 1) the insert 13 engages the respective appendix 23 in the undercut 9D formed by the flank 9C of the annular channel 9. To this end, the appendix 23 may have advantageously an inclined edge 23A, with an angle corresponding to the opening angle of the conical surface 9C delimiting the undercut 9D of the annular channel 9.

In advantageous embodiments, a blocking element 31 is provided for holding each insert 13 inside the annular channel 9 with the respective appendix 23 engaged in the undercut 9D. The blocking element 31 may comprise a ring 33 associated with the needle cylinder 1. In some embodiments the ring 33 may be housed in an annular seat 35 with which the needle cylinder 1 is provided and which is arranged in a position radially more internal and substantially concentric with respect to the annular channel 9.

The ring 33 may advantageously have a pushing surface 37 cooperating with the inserts 13 so as to push the inserts 13 radially outwards so as to push the appendices 23 of the inserts 13 in the undercut 9D, bringing the inclined edges 23 A against the conical surface of the flank 9C of the annular channel 9. In some embodiments the pushing surface 37 has a substantially conical shape diverging upwards.

The conical surface 37 cooperates with the radially more internal side edge or flank 19 of each insert 13. The radially more internal side edge 19 has an inclined portion, with an inclination angle preferably corresponding to the inclination angle of the conical pushing surface 37 of the ring 33. The radial dimension of each insert 13 is such that the radially more internal side edge 19 of each insert 13 may project from the respective groove 11 radially towards the inside of the needle cylinder 1 when the inclined edge 23A of the appendix 23 is abutting against the inclined surface 9C defining the undercut 9D of the annular channel 9, as shown in figure 1. In this way the conical pushing surface 37 of the ring 33 is into contact with the radially more internal side edges 19 of all the inserts 13 and the ring 33 may push radially outwards each insert 13 to hold it fixed with the edge 23 A of the appendix 23 to the conical surface 9C of the undercut 9D of the annular channel 9.

In some embodiments the ring 33 may be rigidly fixed to the needle cylinder 1 through screws in the seat 35. In the illustrated embodiment, the ring 33 of the blocking element 31 is mounted resiliently. To this end a ring of screws 39 is provided, engaging in threaded holes 41 provided in the bottom of the seat 35 of the needle cylinder 1. Each screw 39 is inserted in a seat 33A of the ring 33 and a compression spring 43, for instance a helical spring, is interposed between the bottom of the hole 33A and the head of each screw 39. With this arrangement the ring is resiliently pressed downwards according to arrow F by the force of the springs 43 and generates a resilient force on the inclined surface formed by the radially internal side edge 19 of each insert 13 through the conical surface 37.

With this arrangement, by loosing the screws 39 it is possible to remove one or more worn or damaged inserts 13 that must be replaced. This operation may be performed easily by the machine user, without the need for removing the cylinder and bringing it to a workshop.

In the embodiment illustrated in figures 1 to 4, the inserts 13 cooperate with respective movable elements 51 housed in horizontal slide tracks 53 provided in a crown 55. The crown 55 is mounted coaxially to the needle cylinder 1 and fixed to this latter for instance through the feet 5A of the bars 5 delimiting the sliding tracks of the needles 3. The movable elements 51 are controlled so as to move with a reciprocating motion according to the double arrow £51 (figure 1) by cams (not shown) cooperating with the throat 51A with which the movable elements 51 are provided.

Each movable element 51 has furthermore a finger 5 IB oriented radially inwards and superposed onto a respective insert 13.

The stitch formation with a circular knitting machine designed as illustrated in figures 1 to 4 occurs with the sequence illustrated in figures 5A to 5L. As it is clear from these figures, the inserts 13 defining the stitch forming surface perform at least partially the function usually performed by the movable sinkers, with which the single-cylinder knitting machines are provided. The movable elements 51 hold the stitch near the stitch forming surface defined by the set of the upper edges 15 of the inserts 13 when the needles are lifted, so that the stitch is not drawn upwards through the lifting of the needles and is pushed towards the center, maintaining it into contact with the needles.

In the sequence of figures 5A to 5L, each figure shows the stitch forming members and an enlargement thereof. In figure 5A the needle 3 has brought the thread or yarn F below the stitch forming surface, here indicated with P and formed by the upper edges 15 of the inserts 13. In figure 5B the needle 3 starts the upward movement, while the movable element 51 has moved radially inwards so that in the subsequent step (figure 5C) the finger 5 IB of the movable element 51 holds the yarn preventing it from being drawn upwards by the upward movement of the needle towards the position of figure 5D. The upward movement of the needle continues (figures 5E, 5F) until the stitch is discharged from the opened latch 3L of the needle 3, and, held by the finger 5 IB of the movable element 51, remains adjacent to the stitch forming or sinker plane or surface P.

Figures 5G, 5H and 51 show the movable element 51 translating radially outwards, the needle 3 with the yarn F moving downwards for forming the subsequent stitch, and the latch 3L of the needle 3 closing again until allowing the passage of the just formed stitch M above the hook 3U of the needle 3, as shown in figures 5 J, 5 and 5L.

Figures 6 and 7 show a modified embodiment of the blocking systems for blocking the inserts 13 in the respective radial grooves 11. Equal or equivalent parts to those of the embodiments illustrated with reference to figures 1 to 4 are indicated with the same reference numbers and will not be described again.

In this embodiment the appendix 23 projecting from the lower edge 17 of the insert 13 has an inclined edge or flank 23 A oriented radially inwards, and not outwards as in figures 1 to 4. Correspondingly, in addition to the bottom 9 A, the annular channel 9 has a substantially cylindrical, radially outer flank 9C and a radially inner flank 9B forming an undercut 9D. In some embodiments the undercut 9D is formed by an axial-symmetrical surface with respect to the axis A-A of the needle cylinder 1 , similarly to what is provided in the embodiment of figures 1 to 4 for the surface 9C. In particular, the surface 9B may be conical. The inclination of the conical surface 9B of the radially inner flank of the annular channel 9 substantially corresponds to the inclination of the edge or flank 23A of the appendices 23 of the single inserts 13, that in this embodiment is arranged on the radially inner side instead of being arranged on the radially outer side of the appendix 23.

The inserts 13 are mounted in the respective grooves 11 with the appendices 23 engaging the channel 9 and the undercut 9D of this channel. This position is ensured by a thrust oriented radially inwards rather than outwards, as in figures 1 to 4. The radial inwards push may be generated by a blocking element surrounding the needle cylinder 1. In this embodiment the blocking element comprises at least one annular spring 61 engaging in an annular throat 63 provided in the needle cylinder 1 at the height of the grooves 11. Each insert 13 may have a throat 22 along the radially outer edge 21. When the insert 13 is mounted in the respective groove 11, the throat 22 is arranged nearly in correspondence of the annular throat 63. The dimension and the shape of each insert 13 are such that, when the inclined edge 23 A of the insert 13 is in abutting contact with the conical surface 9B defining the undercut 9D, the profile of the throat 22 projects radially with respect to the bottom of the annular groove 63, ensuring in this way an effective resilient thrust of the annular spring 61 holding the single inserts 13 in place.

In this case again one or more inserts 13 may be replaced by removing the single inserts against the resilient force of the annular spring 61.

The inserts 13 are shaped with their upper edges 15, 15 A, 15B so as to perform at least a part of the functions of a normal movable sinker, and cooperate with movable elements 51 for forming stitch according to what has been already described with reference to figures 5A-5L.

Figure 8 shows a further embodiment. In this embodiment the inserts 13 are shaped like fixed sinkers that do not need radial movable elements for stitch formation. In figure 8, equal or equivalent parts to those of the embodiment of figures 1 to 4 are indicated with the same reference numbers and will not be described again. The methods for mounting and the opportunity of replacing the single inserts 13 are the same as in the embodiment of figures 1 to 4.

In this embodiment, a radially inwards oriented hook 16 projects from the upper edge 15 of each insert 13 and cooperates with the respective latch needle 3 and more in particular with the leading edge of the needle, i.e. with the edge oriented radially towards the outside of the needle cylinder 1, to hold the stitch during the upward movement of the needle, thus making the use of the movable elements 51 unnecessary. Figure 9 illustrates an enlarged section of the upper part of the needle cylinder 1 with the respective inserts 13 in an embodiment similar to that of figures 1 to 4, wherein the appendix 23 has an inclined edge 23A, whose inclination is different from the inclination angle of the flank 9C forming the undercut 9D of the channel 9. More in particular, the difference in inclination is such that the contact between the edge 23A and the surface of the flank 9C of the channel 9 is near the end of the appendix 23, i.e. nearer to the bottom 9A of the channel. In this way the insert 13 is blocked in an optimal way by abutting against the surface of the flared flank 9C of the channel 9 without the need for accurate machining and particularly stringent tolerances. The same solution, with inclination angles different between flank of the channel and edge of the appendix 23 may be used also in other embodiments.

The embodiments described above and illustrated in the drawings have been explained in detail as embodiments of the invention. It will be clearly apparent to those skilled in the art that modifications, variants, additions and omissions are possible, without however departing from the principles, the scope of the concept and the teachings of the present invention as defined in the attached claims. The scope of the invention shall be therefore determined exclusively based upon the widest interpretation of the attached claims, wherein these modifications, variants, additions and omissions are included within this scope . The terms "comprising" "to comprise" and the like do not exclude the presence of further elements or steps in addition to those specifically listed in a claim. The term "a" or "an" before an element, means or feature of a claim does not exclude the presence of a plurality of these elements, means or features. If a claim of a device claims a plurality of "means", some or all these "means" can be actuated by a single component, member or structure. The enunciation of given elements, features or means in distinct depending claims does not exclude the possibility of combining said elements, features or means together. When a method claim lists a sequence of steps, the sequence with which these steps are listed is not binding and can be changed, if the particular sequence is not indicated as binding. Any reference numerals in the appended claims are provided to facilitate reading of the claims with reference to the description and to the drawing, and do not limit the scope of protection represented by the claims.

Claims

1. A needle cylinder for a single-cylinder knitting machine, comprising a plurality of tracks where latch needles slide, wherein:

- an annular channel with an undercut is provided along one end of the needle cylinder;

between each pair of adjacent tracks a respective substantially radial groove is provided, formed in the needle cylinder and intersecting the annular channel, in each said groove a respective insert being housed, the inserts housed in the grooves defining a stitch forming surface with respective upper edges;

each insert has an appendix that is inserted in the undercut of the annular channel;

at least one blocking element holds the inserts in a working position in their respective grooves, in the working position the appendices of the inserts being pushed in the undercut of the annular channel by said at least one blocking element, so as to maintain the inserts fixed during the knitting process, preventing the inserts from exiting the grooves.

2. Cylinder according to claim 1, wherein said at least one blocking element pushes said inserts in a substantially radial direction.

3. Cylinder according to claim 1 or 2, wherein said at least one blocking element resiliently pushes the appendices of the inserts in the undercut of the annular channel.

4. Cylinder according to claim 1 or 2 or 3, wherein the grooves have a depth smaller than the depth of the annular channel.

5. Cylinder according to one or more of the previous claims, wherein the undercut is delimited by a surface inclined towards the bottom of the annular channel and having an axial-symmetrical, preferably conical, shape, coaxial with the needle cylinder.

6. Cylinder according to one or more of the previous claims, wherein the appendices of the inserts project from a lower edge of the insert, which edge is opposite with respect to the upper edge defining the stitch forming surface and said appendices have an inclined edge, forming, with the lower edge, an angle of less than 90°.

7. Cylinder according to claims 5 and 6, wherein the inclined edges of the appendices of the inserts have an inclination corresponding to the inclination of the axial-symmetrical surface of the undercut.

8. Cylinder according to claims 5 and 6, wherein the inclined edges of the appendices of the inserts have an inclination not corresponding to the inclination of the axial-symmetrical surface of the undercut, so as to increase the wedge effect.

9. Cylinder according to one or more of the previous claims, wherein the undercut is provided along a radially outer flank of the annular channel and said at least one blocking element generates on the inserts a radially outwards oriented thrust.

10. Cylinder according to one or more of the claims 1 to 8, wherein the undercut is provided along a radially inner flank of the annular channel and said at least one blocking element generates on said inserts a radially inwards oriented thrust.

11. Cylinder according to one or more of the previous claims, wherein the inserts have a side edge which is inclined with respect to the axis of the needle cylinder and projecting radially inwards from the respective groove, and wherein said at least one blocking element acts on said inclined side edge to force the inserts towards an insert blocking position, wherein said inserts are held with the respective appendices in the undercut.

12. Cylinder according to one or more of the previous claims, wherein said at least one blocking element comprises a ring housed in a seat of the needle cylinder, radially inside with respect to the annular channel, the ring comprising a pushing surface cooperating with the inserts to transmit them a radial push towards the position of holding the appendices of the inserts in said undercut.

13. Cylinder according to claim 12, wherein the ring is resiliently biased in an axial direction towards the needle cylinder.

14. Cylinder according to claim 12 or 13, wherein the pushing surface has a substantially conical shape.

15. Cylinder according to claims 12 and 14, wherein the pushing surface cooperates with the inclined side edges, projecting radially inwards from the respective insert groove.

16. Cylinder according to one or more of claims 1 to 10, wherein: each of said inserts has a throat provided on a side edge facing radially outwards when the insert is inserted in the respective groove; the needle cylinder has an annular throat extending along the grooves in which said inserts are housed, the throats of the inserts being aligned with the annular throat when the inserts are housed in the respective grooves; and said at least one blocking element comprise at least one annular resilient element inserted in the annular throat to hold the inserts in the respective grooves, pushing them radially inwards holding the appendices of the inserts in said undercut.

17. Cylinder according to one or more of the previous claims, wherein the inserts are made of a metal foil.

18. Cylinder according to one or more of the previous claims, wherein each of said inserts has a thickness which is variable along the height of the inserts, with a lower part with greater thicker than an upper part.

19. Cylinder according to claim 18, wherein the lower thicker part of each insert is guided in the respective groove, while the upper thinner part projects at least partially from the respective groove, the upper edge that defines the stitch forming surface being provided in the thinner part.

20. A single-cylinder circular knitting machine, comprising a needle cylinder according to one or more of the previous claims and a plurality of latch needles, slidingly housed in the needle tracks.