Patent Publication Number: US-9889618-B2

Title: Method and system for producing a tyre tread with lugs

Description:
TECHNICAL FIELD 
     The present invention relates to a method and system for producing a tyre tread with lugs. 
     The present invention may be used to advantage for producing farm vehicle tyre treads. It is important to note that the term ‘farm vehicle tyre’ is intended to mean a tyre used on off-road work vehicles, and so refers not only to farm vehicles but also earthmoving machinery or similar. 
     BACKGROUND ART 
     A farm vehicle tyre has a tread with a toroidal base surface that extends about a central axis of rotation; and a number of lugs project radially from the base surface, and extend substantially crosswise to the travelling direction of the tyre. 
     At present, to produce a farm vehicle tyre, a tyre casing is prepared and wound onto a building drum; the tread belts are wound about the casing on the building drum, and a smooth, green-rubber blank tread is wound over the belts; and the casing, wound with the belts and the blank tread, is inserted inside a curing mold negatively reproducing the tread pattern. During the curing process, the rubber of the blank tread is heated and subjected to high pressure to adapt to the shape of the curing mold and so form the tread pattern. 
     The tread pattern includes the lugs, which are exceptionally large and, above all, project upwards considerably from the rest of the tread (even by as much as ten centimeters at the highest point). Forming the lugs in the curing mold therefore involves considerable rubber displacement inside the mold, i.e. a large amount of rubber must migrate from one part of the mold to another. This rubber displacement inside the curing mold, i.e. the large amount of rubber that has to be pressured into forming the lugs, prevents the formation of an evenly thick tread base. In other words, migration of a large amount of rubber inside the curing mold results in the formation of an ‘uneven’ tread base (i.e. differing widely in thickness) which may result in defects, rippling and unsightly flaws. 
     To prevent too thin a rubber thickness at the thinnest parts of the tread base, the average thickness of the tread base must be ‘oversized’ with respect to an optimum thickness that could be used if the tread base were of perfectly even thickness. In other words, to prevent too thin a rubber thickness at the thinnest parts of the tread base, the whole tread base must be made thicker using more rubber. Oversizing the average thickness of the tread base means more material (rubber) is required to produce the tyre (with no advantage whatsoever in terms of nominal tyre performance), thus increasing the manufacturing cost and weight of the tyre. 
     Moreover, because the size of the green-rubber tread cross section is calculated according to tyre size, the known production method described above requires a different green-rubber tread cross section for each tyre radius. In other words, the green-rubber tread cross section of a 420/85 R24 tyre, for example, cannot be used for a 420/85 R28, R30, R34 or R38 tyre. 
     To eliminate these drawbacks, it has been proposed to reduce the thickness of the blank tread wound about the casing (with the belts in between), and to apply green-rubber blocks to the areas of the blank tread where the lugs are to be formed. This reduces the amount of rubber that has to migrate from one part of the curing mold to another, by providing more rubber (in the form of rubber blocks) where it is needed (to form the lugs) right from the outset. Even using rubber blocks, however, the thickness of the tread base still remains significantly uneven, especially at the shoulders of the tyre where the lugs are larger. In other words, even using rubber blocks, the average thickness of the tread base must be ‘oversized’ with respect to an optimum thickness that could be used if the tread base were of perfectly even thickness. 
     Patent Application WO2012001544A1 describes a method of producing a lugged tyre, whereby a green-rubber blank tread is produced; rubber blocks are applied to the areas of the blank tread where the lugs are to be formed; and the blank tread, together with the blocks, is cured in a curing mold. 
     DESCRIPTION OF THE INVENTION 
     It is an object of the present invention to provide a method and system for producing a tyre tread with lugs, which method and system are designed to eliminate the above drawbacks and, in particular, are cheap and easy to implement. 
     According to the present invention, there are provided a method and system for producing a tyre tread with lugs, as claimed in the accompanying Claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A number of non-limiting embodiments of the present invention will be described by way of example with reference to the attached drawings, in which: 
         FIG. 1  shows a view in perspective of a farm vehicle tyre; 
         FIG. 2  shows a front view of the  FIG. 1  farm vehicle tyre; 
         FIG. 3  shows a side view of the  FIG. 1  farm vehicle tyre; 
         FIG. 4  shows a schematic cross section of part of the  FIG. 1  farm vehicle tyre; 
         FIG. 5  shows a schematic of a system, in accordance with the present invention, for producing the  FIG. 1  farm vehicle tyre; 
         FIG. 6  shows a schematic of a building drum of the  FIG. 5  system; 
         FIG. 7  shows a plan view of a portion of a blank tread used in the  FIG. 5  system; 
         FIG. 8  shows a cross section along line VIII-VIII of the  FIG. 7  blank tread; 
         FIG. 9  shows a schematic view in perspective of an extruder for extruding the  FIG. 7  blank tread; 
         FIG. 10  shows a schematic view in perspective of an extruder for extruding green-rubber blocks for application to the  FIG. 7  blank tread; 
         FIG. 11  shows a schematic plan view of a crosswise cut to separate two consecutive  FIG. 10  blocks; 
         FIG. 12  shows a plan view of a portion of an alternative embodiment of a blank tread used in the  FIG. 5  system; 
         FIG. 13  shows a cross section along line XIII-XIII of the  FIG. 12  blank tread; 
         FIG. 14  shows a schematic view in perspective of an extruder for extruding green-rubber blocks for application to the  FIG. 12  blank tread; 
         FIG. 15  shows a schematic plan view of a crosswise cut to shape a  FIG. 14  block; 
         FIG. 16  shows a plan view of a portion of a further embodiment of a blank tread used in the  FIG. 5  system; 
         FIG. 17  shows a cross section along line XVII-XVII of the  FIG. 16  blank tread; 
         FIG. 18  shows a schematic of the  FIG. 6  building drum used in an alternative embodiment; 
         FIG. 19  shows a view in perspective of a different embodiment of a farm vehicle tyre; 
         FIG. 20  shows a front view of the  FIG. 19  farm vehicle tyre. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     Number  1  in  FIGS. 1-4  indicates as a whole a farm vehicle tyre. 
     Tyre  1  comprises a toroidal casing  2 , which extends about a central axis of rotation and supports tread belts  3  wound about casing  2 , and a tread  4  wound about casing  2  and over belts  3 . Tread  4  comprises a tread base  5 , which completely covers casing  2  and is of substantially constant thickness; and a number of lugs  6 , which project upwards (i.e. extend radially outwards) from tread base  5  and are arranged symmetrically about the axis of rotation. Each lug  6  curves from the centreline of tyre  1  to a corresponding shoulder of tyre  1 , has a truncated-cone-shaped cross section (tapering away from tread base  5 ), and has a front wall and a rear wall with respect to a given rolling direction  7  of tyre  1 . Each lug  6  has a lateral portion  8  at a shoulder of tyre  1 ; and a centre portion  9  located in the centre area of tyre  1 . As shown clearly in  FIG. 4 , each lug  6  is thinner at centre portion  9 , and thicker at lateral portion  8 . As shown clearly in  FIG. 2 , centre portion  9  of each lug  6  has a greater curvature than lateral portion  8  (roughly speaking, lateral portion  8  may be assumed to be substantially straight). 
     Number  10  in  FIG. 5  indicates as a whole a production system for producing farm vehicle tyre  1 . Production system  1  comprises a building unit  11  for producing casing  2  of tyre  1 ; a winding unit  12 , which winds first belts  3  and then a green-rubber blank tread  13  about casing  2 ; an application unit  14 , which applies green-rubber blocks  15  to the areas of blank tread  13  where lugs  6  are to be formed; and a curing unit  16  for curing casing  2 , together with belts  3 , blank tread  13  and blocks  15 , in a curing mold  17  negatively reproducing the pattern of tread  4 . In an alternative embodiment, blocks  15  may be made of cured, as opposed to green, rubber, i.e. may be pre-cured. 
     As shown in  FIG. 6 , casing  2  produced on building unit  11  is mounted onto a building drum  18  powered to successively receive belts  3 , blank tread  13  and blocks  15 . In other words, once casing  2  is mounted on building drum  18 , belts  3  are wound about casing  2 , blank tread  13  is wound about casing  2  and over belts  3 , and blocks  15  are applied to blank tread  13  wound about casing  2 . Each block  15  has an outer wall  19  at a shoulder of blank tread  13 ; and an inner wall  20  opposite outer wall  19  and located in the centre area of blank tread  13 . 
     In the  FIGS. 6, 7 and 8  embodiment, blank tread  13  is thinner at the sides and thicker at the centre, so as to define a raised centre portion  21  against which inner walls  20  of blocks  15  rest. Inside curing mold  17 , blocks  15  therefore eventually form lateral portions  8  of lugs  6 , and raised centre portion  21  eventually forms centre portions  9  of lugs  6  (that is, the centre portion of the pattern of tread  4 ). In other words, inside curing mold  17 , the rubber needed to form lateral portions  8  of lugs  6  comes mostly from blocks  15 , while the rubber needed to form centre portions  9  of lugs  6  (which define the centre portion of the pattern of tread  4 ) comes mostly from raised centre portion  21  of blank tread  13 . 
     In a preferred embodiment shown in  FIG. 8 , blank tread  13  is of constant thickness outwards of raised centre portion  21 ; and raised centre portion  21  of blank tread  13  has a trapezoidal cross section. (Alternatively, raised centre portion  21  of blank tread  13  may have a rectangular cross section). 
     In a preferred embodiment shown in  FIG. 9 , blank tread  13  has a constant cross section along its whole length, and is extruded by an extruder  22 . 
     In a preferred embodiment shown in  FIG. 7 , each block  15  is positioned at an angle with respect to raised centre portion  21 , i.e. with respect to a circumferential plane of blank tread  13  (following, as stated, the areas where lugs  6  are to be formed) so that block  15  forms an acute angle α with a circumferential plane. 
     To distribute the green rubber more evenly inside curing mold  17 , the inner wall  20  of each block  15  contacting raised centre portion  21  is shaped on a slant to form an acute angle β with the longitudinal axis of block  15 . Preferably (but not necessarily), acute angle β equals acute angle α, so that inner wall  20  is parallel to raised centre portion  21 . 
     Preferably, to distribute the green rubber more evenly inside curing mold  17 , the inner wall  20  of each block  15  contacting raised centre portion  21  is shaped on a slant to also form an acute angle δ with the tangential plane of blank tread  13 . Preferably (but not necessarily), acute angle δ is supplementary to the angle formed by the lateral walls of raised centre portion  21  with the tangential plane of blank tread  13 . In other words, inner wall  20  of each block  15  slopes doubly: by acute angle β with respect to the longitudinal axis of block  15 , and by acute angle δ with respect to the tangential plane of blank tread  13 . 
     Preferably, to distribute the green rubber more evenly inside curing mold  17 , the outer wall  19  of each block  15  is also shaped on a slant to form an acute angle γ with the longitudinal axis of block  15 . Preferably (but not necessarily), acute angle γ equals acute angle α, so that outer wall  19  is parallel to raised centre portion  21  and inner wall  20 . In other words, preferably (but not necessarily), the two opposite walls  19  and  20  of each block  15  are parallel to each other and slope with respect to the longitudinal axis of block  15 . 
     Preferably, to distribute the green rubber more evenly inside curing mold  17 , outer wall  19  of each block  15  is also shaped on a slant to form an acute angle ε with the tangential plane of blank tread  13 . Preferably (but not necessarily), acute angle ε equals angle δ, so outer wall  19  is parallel to inner wall  20 . In other words, outer wall  19  of each block  15  slopes doubly: by acute angle γ with respect to the longitudinal axis of block  15 , and by acute angle ε with respect to the tangential plane of blank tread  13 . 
     In a preferred embodiment shown in  FIG. 10 , each block  15  has a constant trapezoidal cross section along its whole length, and is extruded by an extruder  23 . More specifically, extruder  23  extrudes a continuous rubber strip with the same trapezoidal cross section as blocks  15 ; and blocks  15  are cut off the continuous rubber strip from extruder  23  by a crosswise cut in a plane inclined at an angle equal to acute angle β (normally equal to acute angle γ), and at a further angle equal to acute angle δ) normally equal to acute angle ε). 
     In the  FIG. 7-10  embodiment, each block  15  has a constant trapezoidal cross section along its whole length. In the  FIG. 12-14  embodiment, each block  15  has a trapezoidal cross section varying lengthwise, so as to be thinner at inner wall  20  and thicker at outer wall  19 . Preferably, the thickness of each block  15  increases evenly (linearly) from inner wall  20  to outer wall  19 . Alternatively, the thickness of each block  15  may increase discontinuously (in steps) from inner wall  20  to outer wall  19 . 
     Varying the cross section of each block  15  lengthwise provides for further improving rubber displacement inside curing mold  17 , by providing more rubber at the shoulders of tyre  1  (i.e. at lateral portions  8 , where lugs  6  are thicker) and less rubber in the centre area of tyre  1  (i.e. at centre portions  9 , where lugs  6  are thinner). On the other hand, forming blocks  15  with a lengthwise-varying cross section is more complicated (and therefore takes longer and is more expensive). 
     As shown in  FIG. 14 , blocks  15  may be formed by extruding, on extruder  23 , a continuous rubber strip with a cross section identical to the longitudinal section of blocks  15 ; and by cutting blocks  15  off the continuous rubber strip by a crosswise cut in an inclined plane (as shown clearly in  FIG. 14 , the blocks are cut off the continuous rubber strip so they face alternately upwards and downwards, by continually inverting the angle of the cutting plane). As shown in  FIG. 15 , once cut off the continuous rubber strip, each block  15  is cut at opposite ends to form inner wall  20  and outer wall  19  sloping at corresponding (possibly different) angles β and γ, and at corresponding (possibly different) angles δ and ε. 
     In a further embodiment shown in  FIGS. 16 and 17 , when (and only when) blocks  15  vary lengthwise in cross section (i.e. are thinner at inner wall  20  and thicker at outer wall  19 ), a blank tread  13  of constant thickness (i.e. with no raised centre portion  21 ) may be used. 
     In one possible embodiment, to improve grip of blocks  15  to blank tread  13  (and so prevent accidental movement of blocks  15  when inserting casing  2  inside curing mold  17 ), cement is applied between a bottom surface of each block  15  and a top surface of blank tread  13 . Alternatively or in addition to cement, to improve grip of blocks  15  to blank tread  13 , the bottom surface of each block  15  and/or the top surface of blank tread  13  may be pre-heated (e.g. using infrared-ray lamps for non-contact heating). 
     In one possible embodiment, blank tread  13  is made from a softer first rubber compound, and blocks  15  are made from a different second rubber compound harder than the first. This embodiment makes it possible to improve both performance and working life of tyre  1 , by making lugs  6  (substantially formed from the rubber of blocks  15 ) harder (and therefore more wear resistant), while tread base  5  (substantially formed from the rubber of blank tread  13 ) is softer and so able to better distribute mechanical stress produced by the rolling movement of the tyre. 
     In the  FIG. 6  embodiment, building unit  11  produces a new casing  2  for producing a new tyre  1  (in fact, belts  3  are first wound about the ‘virgin’ casing  2 ). In an alternative embodiment shown schematically in  FIG. 18 , building unit  11  starts with a worn tyre  1  for retreading. First of all, the worn tread is removed from tyre  1  to expose an intermediate surface of casing  2  underneath (at any rate, a surface over the existing belts  3 ), and the intermediate surface of casing  2  is skived. Once the skiving operation is completed, casing  2  is wound with a green-rubber cushion  25 ; blank tread  13  is then wound about casing  2 , on top of green-rubber cushion  25 ; and blocks  15  are applied as described above to blank tread  13 . Finally, casing  2 , together with green-rubber cushion  25 , blank tread  13  and blocks  15 , is cured in a toroidal curing mold  17  to produce a retreaded tyre  1 . 
     In the above embodiments, blocks  15  are applied to blank tread  13  after it is wound about casing  2 . In an alternative embodiment, blocks  15  may be applied to the flat blank tread  13 , which is then wound, complete with blocks  15 , about casing  2 . In this embodiment, the flat blank tread  13 , complete with blocks  15 , may even be cured to form a pre-cured tread (PCT)  4 , which is then wound about casing  2  as part of a ‘cold’ tyre building or retreading process. 
     In the  FIG. 1-4  embodiment, the tread  4  pattern only comprises lugs  6  projecting upwards (i.e. extending radially outwards) from tread base  5 . In the  FIGS. 19 and 20  embodiment, the tread  4  pattern, in addition to lugs  6 , also comprises a central rib  26 , which projects upwards (i.e. extends radially outwards) from tread base  5 , is annular in shape (i.e. is in the form of a ring seamlessly encircling the whole of tread  4 ), and is located centrally along the centreline of tread  4 . In this embodiment, centre portions  9  of lugs  6  blend (i.e. merge) seamlessly with central rib  26 . The  FIGS. 19 and 20  tread  4  with central rib  26  is preferably produced from blank tread  13  with raised centre portion  21 , which minimizes the amount of rubber migration, inside curing mold  17 , required to form central rib  26  of tread  4 . Raised centre portion  21  of blank tread  13  is normally designed to form both central rib  26  of tread  4  and centre portions  9  of lugs  6 . Alternatively, raised centre portion  21  of blank tread  13  is designed to only form central rib  26  of tread  4 , while blocks  15  form the whole of lugs  6  (i.e. both lateral portion  8  and centre portion  9  of each lug  6 ). 
     The above method of producing tread  4  of farm vehicle tyre  1  has numerous advantages. 
     First and foremost, the method described provides for minimizing rubber migration inside curing mold  17 , and so achieving a highly even tread base  5 , i.e. of more or less constant thickness over its whole area. As such, no ‘oversizing’ of the mean thickness of tread base  5  is necessary, thus enabling a significant reduction in the cost and total weight of tyre  1  for a given performance. It is important to note that, for casing  2 , complete with blank tread  13  and blocks  15 , to fit smoothly inside curing mold  17 , there are strict geometric limitations as to the size of blank tread  13 , and especially the size and shape of blocks  15 . By virtue of raised centre portion  21  of blank tread  13  and/or the variation in the cross section of blocks  15 , it is possible to achieve more or less perfect rubber distribution (i.e. to provide the rubber exactly where it is needed in the tread  4  pattern), as well as smooth insertion of casing  2 , complete with blank tread  13  and blocks  15 , inside curing mold  17 . 
     Moreover, the method described allows the same size green-rubber tread cross section to be used for producing same-type tyres of different radii (i.e. permits ‘size rim scaling’) by maintaining a constant thickness of blank tread  13 . That is, the different amount of rubber required to form a different number of lugs  6  is obtained by varying the number of blocks  15 , with no change whatsoever in the thickness of blank tread  13 . For example, the same size green-rubber tread cross section may be used to produce tyres 420/85 of radius R24, R28, R30, R34 and R38.