Process for manufacturing toothed belts of elastomeric material and toothed beltmade by said process

Abstract: In a process for manufacturing toothed belts of any length, a tubular sleeve comprising a layer of cords (1) and at least a layer of elastomeric material (2) is preformed in an autoclave without being vulcanized so as to produce, on the inner surface thereof, a plurality of hollows (5) spaced apart from one another according to a predetermined toothing pitch (P). The sleeve is helically cut to obtain a continuous strip (7) to be subsequently wrapped over two wrapping drums (8, 9) suitably spaced apart from each other depending on the circumferential extension to be given to the belt. The obtained annular element (10), when the wrapping has been completed, is trued up by means of the provided hollows, between a moulding matrix (11) and opposite mould (12) and is submitted to a pressing step and simultaneous cross-linking of the elastomeric material (2) by its being closed according to stages in succession, between the moulding matrix (11) and opposite mould (12) designed to be moved close to each other. Thus the complete formation of the teeth (3) on the inner side of the belt being worked occurs, as well as the firm bonding of the continuous loops (10a) formed by wrapping of the continuous strip (7).

Description: The present invention relates to a process for manufacturing toothed belts of elastomeric material, comprising the following steps: making a semifinished product in the form of a continuous strip; wrapping the semifinished product in the form of a strip over at least two wrapping rollers of parallel axes, to be positioned according to a predetermined distance between centers, said wrapping being carried out according to a wrapping pitch corresponding to the width of the semifinished product in order to achieve an annular element of the desired circumferential extension, of at least the same width as the toothed belt to be achieved and formed with a plurality of loops disposed in mutual side-by-side relation. The invention also relates to a toothed belt of elastomeric material of the type comprising: at least a layer of reinforcing cords of annular configuration; at least a layer of elastomeric material incorporating said reinforcing layer and exhibiting, on an inner surface thereof, a plurality of teeth extending transversely to the circumferential extension of the belt and spaced apart from each other by the same distance, according to a predetermined toothing pitch. It is known that toothed belts of elastomeric material are essentially comprised of a layer of longitudinal reinforcing cords incorporated within at least a layer of elastomeric material in which a plurality of shaped teeth spaced apart from each other by the same distance according to a predetermined toothing pitch is defined, on the inner side of the circumferential belt extension. The manufacture of these belts involves the accomplishment of a tubular sleeve obtained by successively wrapping the layer of reinforcing cords and the layer of elastomeric material in the raw state over a cylindrical drum provided with longitudinal hollows matching the shape of the toothing to be achieved. The sleeve disposed on the drum is then submitted to a moulding and simultaneous cross-linking process within an autoclave. In particular, the sleeve is submitted to a homogenously distributed pressure over a predetermined period of time following which the elastomeric material is urged into the hollows provided in the drum in order to cause the formation of teeth. During this process the overall duration of which is not generally shorter than 15 minutes, the sleeve is submitted to high temperatures, generally in the range of 150° to 170°C in order to cause the complete cross-linking of the elastomeric material and, consequently, the structural stabilization of the obtained toothed sleeve. As far as mass production is concerned, the circumferential extension of the drum on which the sleeve is formed is normally provided to be coincident with the circumferential extension that the finished belts must have. In these case it is sufficient to slit the sleeve according to several suitably spaced circumferentially-parallel lines for achieving a plurality of belts having the desired circumferential extension and width. There are, however, many other cases in which the circumferential extension of the drum, and consequently of the obtained sleeve, is lower than the circumferential extension that the finished belts must have. This occurs for example when it is necessary to produce belts of a given circumferential extension in a relatively limited amount, which does not justify the accomplishment of a drum of sizes selected on purpose for these belts. In other cases the circumferential extension of the belts in demand may be so high that the accomplishment of drums of very big sizes is involved, so that they cannot even be introduced into the equipments available in the factory such as for example vulcanization and moulding autoclaves, grinding and packaging machines. In order to solve these problems it has been currently provided that the sleeve obtained from the moulding and cross-linking process be slit according to a line of helical extension the pitch of which is coincident with the width of the belts to be achieved. In this way a semifinished product in the form of a continuous strip is achieved which lends itself to be cut to size according to the circumferential extension of the desired belt which is thus obtained by jointing end-to-end the opposite ends of the strip length cut to size. By this method it is possible to produce toothed belts of any circumferential extension but it is necessary to accept the unavoidable structural weakening of the belt caused by the end-to-end junction present on the circumferential extension thereof. US Patent No. 4,083,838 discloses a method by which it is possible to obtain belts of thermoplastic material in the desired length without end-to-end junctions of the above described type being required. In accordance with this method, a semifinished product in the form of a continuous strip on which a toothing in its final configuration has been previously produced, is passed over two rollers of parallel axes, mutually positioned according to a distance between centers corresponding to the desired circumferential extension to be given to the obtained belt. The wrapping of the semifinished product is carried out helically according to a pitch corresponding to the width of the semifinished product so as to form a plurality of loops disposed consecutively in side by side relation and mating at the respective opposite edges. A heating means is provided close to one of the rollers on which the wrapping is carried out, said means, assisted by a presser ribbon guided according to a determined wrapping arc about the roller, causing the plasticization of the thermoplastic material forming the semifinished product, so that the loops disposed in side by side relation get firmly bonded to each other. Thus an annular toothed element is achieved that lends itself to be cut according to circumferential lines thereby giving rise to a plurality of belts having the desired width and circumferential extension. Presently it is impossible to use the above method for making toothed belts in which elastomeric material is used in place of the thermoplastic material. In fact, cross-linking carried out in the autoclave before the semi-finished product taxes the form of a continuous belt, would make the subsequent melting of the elastomeric material for obtaining the mutual junction between contiguous loops of impossible achievement. It is known from US-A-4,448,621 a method and device for making a toothed belt which comprises the steps of providing first belt sleeve having the desired tooth configuration cutting the first sleeve into a continuous strip, winding the strip to form a second sleeve of desired circumferential length, joining the adjacent strips of the second sleeve in side-a-side fashion, and cutting at least one belt of desired width from the second sleeve. The first sleeve is built in a cylindrical grooved mold body having a plurality of axially extended grooves. First, a layer of fabric is wrapped circumferentially around the grooved mold and then, a load carrying band for the sleeve is formed on the mold by helically winding over the layer fabric a plurality of turns of at least one strand. Further, a layer of rubber is applied over the convolutions of the strands in the form of calender sheet. In a further step, into a vulcanizer known as autoclave, stem is introduced within the mold. The layer of rubber softens below its curing temperature, and there is sufficient volume of rubber to form the top cover of the belt, the complete filling of the grooves of the mold is effective and accurately shaped belt teeth are produced. After the groove have been completely filled, additional heat may be supplied to vulcanize the rubber portion of the belt. After cooling, the partially cured first sleeve is removed from the mold, for the further cited steps of cutting a strip, forming a second sleeve and cutting a belt said second sleeve. In accordance with his own experience, the Applicant has then thought of lowering temperatures and residence times of the manufactured article in the autoclave, in order to avoid cross-linking of the elastomeric material, which operation would have been executed in a subsequent step; however it has been found that the formation of teeth on the inner surface of the sleeve did not take place in a regular manner, so that it was impossible to resort to this procedure. In accordance with the present invention it has been found that in order to obtain toothed belts bade of elastomeric material, of any circumferential extension in the absence of mechanical junctions, it is possible to resort to a process in which the above step in an autoclave is used for fundamentally forming the toothing pitch without carrying out the cross-linking of the elastomeric material, that is forming alternate hollows and projections the configuration of which can only partly correspond to the final configuration of the teeth, in a following step using hollows (or projections) in order to true up the annular element, already formed from the wrapping of a continuous strip over two drums, between two moulds the mutual approaching of which in the presence of heat gives the teeth the ultimate shape and, by cross-linking of the elastomeric material, causes the contiguous loops of the annular element to adhere to each other so as to form a belt of the desired length and devoid of any mechanical junction. In practice, a process is adopted in which a semifinished product in the form of a continuous strip is obtained by helical slit of a sleeve that has been previously submitted in an autoclave to a preforming step which is not necessarily intended for the complete construction of the teeth but only aiming at the formation of projections and hollows spaced out according to the toothing pitch to be given to the belts, without however carrying out the cross-linking of the elastomeric material present in the sleeve. The semifinished product thus obtained in the form of a continuous strip is then helically wrapped about two rollers spaced apart from each other by a distance depending on the desired belt extension, in order to produce an endless ring the adjacent loops of which, as they are in the raw state, enable the cross-linking of the elastomeric material to take place after the ring has been cut to the desired width, which results in the achievement of a junction-free belt. Hollows obtained in the preforming operation are then used to true up the annular element obtained from the strip wrapping, on a matrix employed for moulding and cross-linking the final toothing profile. The present invention in one main aspect thereof relates to a process for manufacturing toothed belts of elastomeric material comprising the following steps: making a sleeve internally provided with hollows; slitting the sleeve according to a cutting line of helical extension so as to obtain a semifinished product in the form of a continuous strip; wrapping said semifinished product in the form of a continuous strip over two wrapping toothed rollers of parallel axes to be positioned according to a predetermined distance between centers, said wrapping being carried out at a pitch corresponding to the width of the semifinished product in order to obtain an annular element of the desired cirumferential extension having at least the same width as the toothed belt to be produced and formed with a plurality of loops disposed in mutual side by side relation, characterized by: a) controlling the temperature and time of the sleeve-making step so as to obtain a plurality of alternate hollows and projections for preforming of the teeth defining the belt pitch while avoiding the cross-linking of the elastomeric material constituting the sleeve; b) truing up the annular element in the raw state, by means of said hollows or projections, between a moulding matrix and the opposite mould; c) moving the matrix and the mould close to each other applying heat for vulcanization thereto, so as to mould the teeth to the final shape and cause the mutual bonding of the side by side loops of the annular element in the raw state, thereby giving rise to a belt of the desired length, in the absence of mechanical junctions. More particularly, the process for manufacturing toothed belts of elastomeric material, is characterized by the fact of comprising the following steps: before said making a sleeve, wrapping a reinforced cord layer and a raw elastomeric layer provided as sheet around a cylindrical preforming drum having a surface with a plurality of substantially longitudinal teeth spaced apart by a toothing pitch, said elastomeric layer hang a quantity of elastomeric material less than a quantity required to form teeth of a cross-linked belt to be formed; introducing the reinforced cord layer, the raw elastomeric material, and the preforming drum into an autoclave; heating the reinforced cord layer and the raw elastomeric layer in the autoclave at a heating temperature and for a time period selected to avoid cross-linking the raw elastomeric layer, thereby preforming said sleeve of elastomeric material had interior and exterior circumferential surfaces and hang a plurality of axially extending partly shaped teeth forming said projections and hollows, said projections heing separated by said hollows; before said step b) of truing up the annular element, applying a plurality of covering layers of non-cured elastomeric material around the outer surface of the annular element to cover adjacent ones of the plurality of loops of the annular element; trueing up the non cross-linked annular element by positioning a lenght of said annular element and the plurality of covering layers between said moulding matrix and opposite mould, said matrix having a plurality of teeth and a plurality of grooves complimentary to the teeth and grooves of the finished belt; pressing the covering layers and the annular element with the matrix and the mould and applying heat to the covering layers and annular element to mold the projections to the final shape of teeth, causing mutual bonding of the side by side loops of the annular element, and cross-linking covering layer and annular element thereby forming said cross-linked toothed belt of desired length. Advantageously, during said pressing step a final mutual truing of the respectively aligned transverse hollows belonging to the individual loops of the semifinished product previously wrapped in the form of a strip, is carried out. During said wrapping step a mutual alignment of the transverse hollows present in the individual loops of said semifinished product in the form of a strip is also preferably executed. This alignment is obtained by engagement of said hollows on longitudinal-alignment teeth provided on said wrapping rollers, mutually spaced apart by the same distance as said toothing pitch. Preferably, for the accomplishment of said cylindrical tubular sleeve the engagement of a coating fabric on said preforming drum is provided before carrying out the wrapping of the textile fiber layer and the raw elastomeric material. In addition, during said pressing step, a covering fabric previously interposed between said moulding matrix and the annular element may also be fastened to the inner surface of the annular element. The application of a covering fabric to the inner surface of the annular element can be exploited in order to execute a presetting of the loops of the semifinished product in the form of a strip before the pressing step. In this case said covering fabric is applied during said wrapping step through interposition of the covering fabric between the wrapping rollers and the annular element being worked. Alternatively, the mutual presetting of the loops formed with said semifinished product can take place through application, after said wrapping step, of an outer coating sheet to the outer surface of the annular element wrapped around said wrapping rollers. The cross-linking step of the elastomeric material is preferably obtained simultaneously with the pressing step, through heating of the moulding matrix and opposite mould. In addition the pressing step is carried out by successive stages to each of which the pressing of a portion of predetermined length of said annular element corresponds. After said cross-linking step, a step involving the trimming of said edges of the obtained toothed belt may also be provided. In the case in which the annular element has an important width, it is possible to execute a circumferential cutting step after said cross-linking step, in order to obtain a plurality of toothed belts of predetermined width. Alternatively, this circumferential cutting step can be executed before the pressing step. In accordance with a further feature of the invention, the preforming step is such accomplished that said hollows exhibit a depth corresponding to at least 1/3 of the height detectable on the individual teeth of the finished belt, and are alternated with projections the transverse preforming outline of which has a surface extension equal to at least 1/2 of the surface extension detectable on the tooth profile of the finished belt. Preferably, in accordance with the process of the invention, after the moulding step at least 10% of said projections should be only partly formed as regards their detectable height and/or their sizes as above described. More preferably at least 30% of the projections is partly formed as above stated and still more preferably at least 50% of the projections is partly formed as above stated. Advantageously the application of centripetal pressure during the forming step takes place concurrently with the administration of heat to the sleeve being worked. In greater detail, the application of centripetal pressure during the preforming step is carried out in an autoclave concurrently with the administration of heat to the sleeve over a period of time included between 1/6 and 1/2 of the time necessary to cause the complete cross-linking of the elastomeric material at the temperature conditions according to which the administration of heat occurs. Alternatively, the administration of heat to the sleeve is carried out by submitting the latter to temperatures of a value included between 1/2 and 4/5 of the value needed for obtaining the complete cross-linking of the elastomeric material over a period of time corresponding to the duration of the administration of heat. In a preferential embodiment, the application of pressure during a preforming step occurs at a value included between 10 bar and 22 bar, over a period of time included between 2 and 15 minutes, while administering heat to the sleeve at a temperature included between 50°C and 205°C at the outside thereof, and included between 100°C and 140°C at the inside thereof, depending on fluid used (that is air, steam or pressurized water). After the preforming step the material has not yet begun the vulcanization process and hence physical parameters of the vulcanized item such as hardness and ultimate tensile strength are not measurable. The invention in another aspect thereof relates to a toothed belt of elastomeric material comprising: at least a reinforced cord layer of annular configuration; at least a layer of elastomeric material incorporating said reinforced cord layer and exhibiting, on an inner surface thereof, a plurality of teeth extending transversely to the circumferential extension of the belt and spaced apart from each other the same distance according to a predetermined toothing pitch (P); said reinforced cord layer, incorporated into said at least a layer of elastomeric material, structured in the form of a continuous strip helically wrapped in several loops disposed in side by side relation and mating at the respective opposite edges, characterized in that it further comprises at least a covering fabric applied to the inner surface thereof provided with said teeth, said covering fabric being structured in the form of a continuous strip having the same width as the belt. Still in a further aspect the invention relates to a belt comprising an annular element had a plurality of teeth spaced apart by grooves and incorporating longitudinal extending reinforcing cords, said belt being manufactured in at least two steps, respectively a first step wherein a strip of noncross-linked elastomeric material incorporating longitudinally extending reinforced cords and having teeth partially shaped is wrapped up helically according to a plurality of loops disposed in mutual side by side relation to obtain said annular element and a further step wherein by means of a matrix and an opposing mold member moved toward each other the interposed annular element is molded and cured to had the final shape of the teeth, characterized by the fact of comprising a fabric having the same width of the said plurality of loops disposed in mutual side by side relation, said fabric being applied to the plurality of teeth and grooves of said plurality of loops by mechanical pressure, the final thickness of the fabric originating a thickness of the belt, measured from the plane of the longitudinal cords and the centers of the grooves, corresponding to a predetermined value for obtaining a correct meshing with a toothed pulley. Preferably the belt is characterized in that it further comprises at least a coating fabric applied to the inner surface thereof provided with said teeth, said coating fabric being structured in the form of a continuous strip wrapped in several side by side loops mating at the respective opposite edges. Further features and advantages will become more apparent from the detailed description of a preferred embodiment of a process for manufacturing toothed belts of elastomeric material and a belt obtained by said process, in accordance with the present invention, given hereinafter by way of non-limiting example with reference to the accompanying drawings, in which: Fig. 1 shows, by way of example only, a diagram indicating the variations in time of the feed pressures in an autoclave for the execution of a preforming step in accordance with the process in question; Fig. 1a is a diagram similar to the one shown in Fig. 1 and referred to pressure variations in the autoclave according to the moulding and cross-linking processes of the known art; Figs. 2a and 2b are two diagrams obtained through the so-called "Monsanto tests" carried out respectively on a coupon of elastomeric material submitted to the preforming cycle in an autoclave and on a coupon of raw elastomeric material, of the same type, not submitted to said cycle; Fig. 3 is a diagrammatic perspective view of the execution of a wrapping step of a semifinished product in the form of a continuous strip over a pair of mutually spaced apart wrapping rollers; Fig. 4 is a partly sectional side view of a portion of an annular element disposed between a matrix and an opposite mould to be submitted to a final-moulding and cross-linking step; Fig. 5 diagrammatically shows the superimposition of the outlines of a completely formed tooth (in dotted line) and a tooth not completely formed (in solid line), respectively. The process of the present invention aims at manufacturing driving belts of elastomeric material of the type essentially comprising at least a layer of reinforcing cords 1 made of textile fibers, fiberglass (Fig. 4) or other materials such as aramide fibers or the like, as currently used in belts, incorporated within at least a layer of elastomeric material 2 shaped such as to define, on the inner surface of the circumferential belt extension, a plurality of shaped teeth 3 spaced apart from each other a predetermined toothing pitch "P". To this end, a cylindrical tubular sleeve is first made in a manner known per se by wrapping the reinforcing cord layer 1 and the elastomeric material layer 2 in a raw state and in the form of a sheet of constant thickness about a cylindrical preforming drum provided, on the inner surface thereof, with a plurality of substantially longitudinal teeth spaced apart from each other a distance corresponding to the toothing pitch "P". These operations are preferably preceded by the engagement on the preforming drum of a tubular stocking of textile material designed to form an inner coating fabric 4 for the sleeve and hence the finished belts. The inner coating fabric 4 not only strengthens the structure of the individual teeth 3, but also makes the subsequent removal of the sleeve from the drum easier. The tubular sleeve together with the preforming drum on which it has been formed, is then introduced into an autoclave of the type normally used for moulding and cross-linking of sleeves in the production processes of the known art. Inside this autoclave not shown as known per se, in an original manner the sleeve undergoes a preforming step aiming at creating, on the inner surface of the sleeve, a plurality of shaped hollows 5 spaced apart from each other a given toothing pitch "P" and alternated with surface projections 3a, each of which will form one of said teeth 3. The sleeve preforming in the autoclave is carried out by applying a homogeneously distributed centripetal compression to the sleeve in a manner known per se but at all events also described below, so that the elastomeric material 2 urged through the textile reinforcing layer 1 and against the longitudinal teeth of the drum will at least partly fill the hollows defined in the drum between two adjacent teeth. In order to increase the malleability of the elastomeric material 2, simultaneously with the centripetal compression, heat is administered to the sleeve being worked. However the administration of heat and the treating time in the autoclave must be adjusted so that cross-linking does not occur in the elastomeric material. For the purpose, the residence time of the sleeve in the autoclave and/or the treatment temperatures therein are conveniently reduced as compared to those of the known art where the working cycles aim at achieving the complete formation of teeth 3 and the simultaneous complete cross-linking of the elastomeric material. For an easier understanding, the graph in Fig. 1 shows, by way of example, the variations in time of the treatment pressures in the autoclave with reference to the preforming step carried out in accordance with the invention on a sleeve of chloroprene-based elastomeric material having the following composition: - Chloroprene 52.35% - Magnesium oxide + zinc oxide 3.95% - Silica + activators 11.8% - Blacks 28.8% - Antifatigue agents 1.55% - Antiozone 1.55% It is pointed out that chloroprene is one example of the materials used; other materials could be polybutadiene rubber-based materials, nitrile, hydrogenated and chlorosulfonated rubbers. For comparing purposes, the graph in Fig. 1a shows the development of pressures that can be found in the moulding and cross-linking step of the known art on a sleeve of elastomeric material identical to the one specified above. Referring to the conventional cycle in an autoclave as shown in Fig. 1a, the centripetal compression on the sleeve is obtained for example by admitting water overheated to 164°C and at a pressure of 18 bar as shown by curve "N", into an elastomeric plenum chamber conventionally provided in the autoclave and externally surrounding the sleeve fitted on the drum. These conditions are the same over the whole operating cycle of the autoclave which is of approximately 16 minutes. Still with reference to Fig. 1a, the curve marked by "K" shows the variation in time of the pressure of steam admitted into the drum carrying the sleeve, according to known procedures. In a starting step lasting about tre minutes, as shown by the initial length K' of curve K, the steam pressure in the drum keeps a value of about 2.2 bar at a temperature of 134°C, in order to cause a plasticization of the elastomeric material. In a subsequent cross-linking step, shown by length "K''" of curve K, the steam pressure within the drum rises up to about 6 bar, at a temperature of 164°C and keeps these conditions over the remaining 13 minutes of the operating cycle, during which the cross-linking of the elastomeric material comes to an end. As said, according to the known art, in the above conditions the final formation of teeth 3 at the inside of the sleeve occurs, as well as the cross-linking of the elastomeric material 2. Unlike the above statements, in the preforming step carried out in the example shown in Fig. 1 the operating cycle in the autoclave is reduced to about 6 minutes during which steam admitted into the drum keeps a pressure of 1.2 bar at a temperature of 120°C, as shown by curve "Z". The overheated water introduced into the plenum chamber surrounding the sleeve keeps a pressure of about 18 bar and a temperature of 164°C, as shown by curve "X". It is noted that the important reduction in time of the operating cycle in the autoclave together with the light reduction in the treatment temperatures and steam pressure within the preforming drum causes the preforming step to occur without any risk of giving rise to cross-linking phenomena in the elastomeric material 2. The important reduction in time of the operating cycle in the autoclave, due to possible temperature differences in the various areas of the autoclave or for other reasons, leads to the formation of projections that can at least partly correspond to the final shape of teeth 3, as intentionally emphasized in Figs. 3 and 4. In particular, taking into account the fact that the sleeve is disposed in the autoclave according to a vertical axis, teeth 3 can show a gradual reduction in height along the longitudinal extension of the sleeve towards the lower part thereof, but they will at all events exhibit the respective base walls 5a (Fig. 4) mutually livelled adjacent the cord layer 1. Advantageously in the process of the invention, for reasons better clarified in the following, the presence of possible projections which are different from one another and from the final form of the teeth 3 does not give rise to any qualitative decay of the finished product. In fact it has been found that in order to achieve a final product having satisfactory characteristics it is sufficient that the depth of hollows 5 denoted "h" in Fig. 4, be at least equal to 1/3 of the overall height "H" of the tooth 3 in the finished product. It is also important to point out that preferably the surface extension of the transverse profile of each projection 3a defined between two contiguous hollows 5 must at least correspond to 1/2 of the surface extension of the transverse profile of each individual tooth 3. It has also been found that the above parameters are complied with if at the inside of the autoclave the administration of heat to the sleeve is carried out over a period of time included between 1/4 and 1/2 of the time needed for causing the complete cross-linking of the elastomeric material at the temperature conditions according to which the heat administration occurs. In other words, it is pointed out that even if the temperature and pressure conditions are kept substantially unchanged as compared to the autoclave cycle of the known art, the duration of the operating cycle for achieving the preforming cycle is included between 1/6 and 1/2 of the duration of the operating cycle in an autoclave when the conventional manner is followed. Alternatively, the administration of heat to the sleeve can be carried out by submitting the latter to temperatures ranging from 1/2 to 4/5 of the value required for achieving the complete cross-linking of the elastomeric material, over a period of time corresponding to the duration of the administration of heat. In other words, the duration of the preforming step can even be as long as the duration of the known operating cycle, in which case in order to prevent cross-linking from starting in the elastomeric material, the operation temperatures within the preforming drum and in the autoclave plenum chamber should conveniently be reduced to a value in the range of 2/3 to 4/5 of the temperatures detectable in a conventional autoclave cycle. Obviously, depending on the different requirements, within the above specified limits it will be possible to select any time and temperature combination adapted to produce sufficiently deep hollows 5 without causing the elastomeric material cross-linking. In a preferential solution it is at all event provided that the centripetal compression in the preforming step should be included between 15 bar and 21 bar, the residence time in the autoclave between 3 and 10 minutes and the temperature in the preforming drum, that is within the sleeve, between 100°C and 140°C, the temperature at the outside of said sleeve ranging from 50°C to 205°C. The absence of cross-linking in the elastomeric material submitted to the preforming step can be checked by examining the rheometric features of a sample of this material, comparing them with the rheometric features detectable on a coupon of identical material under the same test conditions that has not been submitted to the preforming cycle in the autoclave. Just as an indication, diagrams shown in Figs. 2a and 2b show the results achieved by carrying out a test, currently known to those skilled in the art as "Monsanto test", on a sample of elastomeric material previously submitted to a preforming cycle and a sample of the same raw material not submitted to said cycle, respectively. For the sake of completeness, it is pointed out that the "Monsanto test" substantially consists in putting a sample under examination of 5 cm diameter and 3 mm thickness, in a chamber heated to a controlled temperature, in this case corresponding to 185°C, and detecting the variation in time of torque "C" (to be measured in Newton x metre) necessary to cause the angular oscillation (according to an angle of 1° to 3° in absolute value) of a disk rotatably housed within said chamber. The increase of the torque value in time, detectable from graphs in Figs. 2a and 2b, is the cross-linking index taking place in the elastomeric material by effect of its standing at high temperature and will be stabilized when the cross-linking has been completed. As can be seen when comparing Figs. 2a and 2b, graphs therein reproduced are pratically identical, which means that the elastomeric material sample submitted to the cycle corresponding to the preforming step is still in a raw state, in that it exhibits the same behaviour as the raw material sample that has not yet been submitted to any treatment in the autoclave. In principle, the elastomeric material 2 of the preformed sleeve is preferably provided to exhibit, after a test in a Mooney viscosimeter carried out in accordance with ASTM 1646-80 standards, a viscosity increase lower than 10%. It is also noted that in the described example the hardness of the elastomeric material after the preforming step is not measurable because the material is not vulcanized, whereas the hardness of the same material on the finished belt has a value ranging from 60 to 90 Shore A. In accordance with the process of the invention the preformed sleeve is subsequently submitted to a cutting step following a helical-extension cutting line, the pitch of which is preferably the same as, or a multiple of the pitch according to which the wrapping of the reinforcing cord layer 1 on the preforming drum has been previously carried out, in order to prevent as much as possible the cords from being cut on their length. Preferably the cutting is carried out on a rubber drum on which the sleeve withdrawn from the preforming drum has been fitted. Following this helical cutting step a semifinished product is obtained in the form of a continuous strip 7 (Fig. 3) of predetermined width and coincident with the pitch of the helical-cutting line, in which hollows 5 produced during the preforming step will extend in the transverse direction. As clearly diagrammatically shown in Fig. 3, the semifinished product in the form of a continuous strip 7 is then wrapped over at least two wrapping rollers 8, 9 of parallel axes, to be mutually positioned according to a predetermined distance between centres depending on the circumferential extension to be given to the finished belts. In greater detail, the wrapping of the semifinished product 7 obtained by preferably driving in rotation rollers 8, 9 is carried out helically according to a pitch corresponding to the width of the semifinished product itself, so as to produce an annular element 10 exhibiting at least the same width as one of the belts to be obtained. Advantageously, during the wrapping of the semifinished product 7 the mutual alignment of the transverse hollows 5 present on the individual loops 10a formed from the semifinished product, is achieved too. This alignment results from the engagement of hollows 5 on alignment teeth 8a, 9a extending longitudinally on each of the wrapping rollers 8, 9 and mutually spaced apart a distance corresponding to the toothing pitch "P". The alignment teeth 8a, 9a have a height corresponding to, or greater than the depth "h" of hollows 5. In this way the alignment teeth 8a, 9a can act on the base portion 5a of each individual hollow 5, so that the individual loops 10a formed from the semifinished product 7 wrapped on rollers 8, 9 are perfectly levelled. Advantageously, the engagement of the transverse hollows 5 on the alignment teeth 8a, 9a also enables the perfect alignment of the hollows and the individual loops 10a to be achieved in the wrapping step without any tensioning being required to be produced on the semifinished product 7. Thus, any risk of deforming the elastomeric material 2 still in the raw state is eliminated. The annular element 10 obtained from the wrapping step is subsequently trued up by means of hollows 5 and submitted to a pressing step between a moulding matrix 11 and an opposite mould 12 to be moved close to each other. As shown in Fig. 4, the mould 12 acts by a substantially flat contact surface 12a thereof, on an outer surface 10c of the annular element 10, whereas the moulding matrix 11 acts on an inner surface 10b of the annular element by a shaped surface 11a thereof. This shaped surface 11a exhibits ridges 13 matching the shape of the transverse hollows 4 spaced out by recesses matching the shape of the outline of the individual teeth 3 to be obtained. Following the mutual approaching of matrix 11 and mould 12, the final truing up of the respectively aligned hollows 5 belonging to the different loops 10a of the annular element 10 is advantageously obtained. Simultaneously, the elastomeric material 2 located at the projections 3a is urged into the recesses 14 of matrix 11 thereby surely determining the exact formation of teeth 3. Concurrently with the pressing step, a cross-linking of the elastomeric material 2 is carried out through heating of matrix 11 and opposite mould 12, in a manner known per se and conventional, so as to cause the final mutual bonding of loops 10a and structural stabilization of the belt being worked. As can be guessed from Fig. 4, the pressing step and the related cross-linking step are carried out according to stages in succession, to each of which the execution of said steps on a portion of predetermined length of the annular element 10 corresponds, which length in turn corresponds to the longitudinal extension of matrix 11. In particular, a longitudinal stepping movement is provided to be transmitted to the annular element 10 fitted on rollers of parallel axes, in synchronism with the alternate movement imparted to the matrix 11 and/or opposite mould 12 for obtaining the pressing and simultaneous cross-linking on the individual portions of the annular element. The truing up of hollows 5 ensures the elimination of possible misalignments present between the hollows and/or projections 3a after the wrapping step. Still in accordance with the present invention, during the pressing step a covering fabric 15 previously interposed between the moulding matrix 11 and the annular element itself is preferably provided to be applied during the pressing step to the inner surface 10b of the annular element 10. This covering fabric has the function of increasing the mechanical strength of teeth 3 and the wear resistance of the obtained belt. Provision may be made for the pressing step and other steps of the process related thereto to be carried out directly on the annular element 10 still fitted on the wrapping rollers 8, 9 when the wrapping step is over. In this case the covering fabric 15 should be preferably disposed around the wrapping rollers 8, 9 before or after the execution of the wrapping step of the semifinished product in the form of a strip 7. The covering fabric 15 will be thus disposed intermediate the wrapping rollers 8, 9 and the annular element 10 and will be finally fastened to the annular element as a result of the pressing and cross-linking step. The preliminary application of the covering fabric 15 during the wrapping step can also be exploited for the purpose of achieving a mutual presetting of the loops 10a forming the annular element 10, should the annular element be disengaged from the wrapping rollers 8, 9 and be transferred to another work station designed to carry out the pressing and cross-linking steps. Alternatively, the mutual presetting of loops 10a can be obtained through the application of at least a coating sheet of plastic material, not shown, to the outer surface 10c of the annular element 10, still fitted on the wrapping rollers 8, 9. This coating sheet may be optionally eliminated during the following rasping step executed in known and conventional manner on the belt back. Should the annular element 10 exhibit substantially the same width as that of each individual belt, after the pressing and cross-linking step a trimming step may be executed on the side edges of the annular element so as to give the same a prefectly parallel extension and define the exact belt width. This trimming operation is also necessary because, in order to be sure to obtain a tooth of the desired form, the elastomeric material of the annular element is always in excess and thus there is the formation of loose threads at the edges of the mould after its being closed with the moulding matrix (Fig. 4). Alternatively, should the width of the annular element 10 be a multiple of the width of each individual belt, before or after the pressing and cross-linking operations a final-cutting step can be executed on the annular element following circumferential lines spaced apart from each other the same distance as the width of the belts to be produced. Belts manufactured following the process in question, as a result of the helical cut carried out on the sleeve and the subsequent helical wrapping for the accomplishment of the annular element 10, will exhibit the respective textile reinforcing layers 1 and coating fabrics 4 each structured in the form of a continuous strip forming loops disposed in side by side relation and mating at the respective opposite edges. In addition, belts made in accordance with the invention may be provided with the covering fabric 15 disposed on the respective inner surface thereof exhibiting the teeth 3 and structured in the form of a continuous strip of the same width as the belt. Another peculiar feature of belts made in accordance with the invention resides in the high ultimate tensile strenght they offer. Actually, it has been found that the ultimate tensile strength of the belts of the invention is greatly higher than 50% of the ultimate tensile strength found on belts made of the same materials and obtained from a circumferential cutting of a sleeve directly moulded and vulcanized in the autoclave following the known art. More particularly, the tensile strength of the belts of the invention can easily exceed 85% of the tensile strength found on junction-free belts of the above type. The slight reduction in the tensile strength of the belts of the invention when compared to the belts obtained from the circumferential cutting of a moulded and cross-linked sleeve, is only due to the helical arrangement of the textile fibers forming the layer 1, owing to the helical wrapping carried out on rollers 8 and 9. At all events the strength values obtained by the invention represent an important improvement over conventional belts obtained by butting the opposite ends of a strip length previously cut to size. In these belts, in fact, the ultimate tensile strength does not generally exceed 35% of the ultimate tensile strength detectable on junction-free belts. Therefore the present invention achieves important advantages as compared to the known art. It will be recognized, among other things, that based on knowledges in this field, the irregularity of hollows 5 and surface projections 3a obtained in the preforming step should be interpreted as a factor adversely affecting the satisfactory manufacture of a finished product. On the contrary, surprisingly the invention succeeds in advantageously exploiting the presence of said hollows, even if of irregular form, for achieving the perfect alignment of the individual loops 10a and surface projections 3a in the following wrapping and pressing steps, making thereby possible the accomplishment of a final product offering excellent qualitative features. In particular, circular, trapezoidal and parabolic-sided teeth, and in general teeth of any form, can be obtained. In addition it is possible on demand to convert a projection of the annular element for example of circular form into one in which the tooth sides are for example of parabolic or hyperbolic form by merely disposing the same type of annular element between moulds of the desired configuration. Obviously modifications and variations may be made to the invention as conceived, all of them falling within the scope of the inventive idea characterizing it; for example it is possible to achieve belts having a single fabric on the toothing, of the same width as that of the belt itself. According to an example, the mutual presetting of the loops 10a of the annular element 10 can be obtained by applying a plurality of pincers pressing the two opposite surfaces of one portion of the annular element 10. The plurality of pincers is used to transport the annular element 10 to the press and the pincers may be used to maintain a portion of the annular element 10 between the matrix 11 and the mould 12. Further as already cited, according to the most preferred method of the invention it is foreseen the use in the autoclave of a quantity of elastomeric material less than that required to completely shape the teeth of the belt. For example, the quantity of elastomer used for the sleeve introduced in the autoclave for the preforming cycle may be that needed to shape a belt whose thickness between the back of the belt and the top of the teeth is 2/3 of the thickness desired for the finished belt. In the final pressing step, a plurality of layers of elastomeric material, reciprocally overlapped and not yet cured, are applied around the outermost surface of the annular element 10 covering all the adiacent turns of the strip 7. Preferably, the plurality of layers forming a single foil is unrolled from a drum placed at the right side of figure 4. First of all, the first edge of the said plurality of layers is unrolled from the corresponding drum and applied to the first portion of the annular element 10 to be pressed between the matrix 11 and the mould 12. Successively, after the pressing and curing of the first portion of the belt, the mould 12 and the matrix 11 are distanced and a further piece of the said plurality of layers is unrolled from the drum and placed under the press. The number of layers selected must determine, after the pressing step, the desired thickness of the finished belt between its back and the top of the teeth. Preferably the elastomeric material of the sleeve in the preforming cycle and that of the said plurality of layers is the same. Advantageously, during the pressing step, the plurality of layers connects physically all the adiacent turns of the strip 7 determining a single annular element. Further, the said plurality of layers fills in any voids or discontinuities that could exist between adiacent turns of the strip 7 and determine uniformity of thickness for the whole lenght and width of the finished belt. Still advantageously, the plurality of layers forms a resistant structure opposing to any reciprocal sliding between adjacent turns of the strip 7 when the finished belt is associated with two pulleys whose axis are not aligned in the same direction. Preferably, the method comprises the step of forming an inner coating fabric 4 of reduced thickness for the sleeve introduced in the autoclave and a covering fabric 15 placed, before the pressing step, for the whole width on the inner surface of the belt. The total thickness of the fabric 4 and the covering 15 must determine on the finished belt, after the pressing step, a thickness, between the cords 1 and the center of the groove, as required for the appropriate meshing of the belt with the pulleys of the transmission. More specifically, the thickness of the finished belt, measured from the plane of the longitudinal cords 1 to the center of the groove between adjacent teeth of the belt, must correspond to the value of the known parameter "a" called PDL (Pitch Difference Lenght). For example, said parameter "a" has a value of 0,685 mm for a belt having the following geometrical parameters: pitch = 8 mm height between the land area and the top of the tooth = 3,2 mm total thickness = 5,2 mm pressure angle = 16° root of the tooth = 5,50 mm In the same example, one of the pulley of the transmission has the following geometrical parameters: Teeth = 30 -Diameter of the pitch line = 76,39 mm -Diameter of the outer circumference of the pulley = 76,39 - 2*a = = 76,39 - 2*0,685 = 75,2 The fabric 4 and the covering 15 may have the following characteristics: Covering 15. Material = for example nylon. Weight = between 180 and 300 gr/mq and more preferably between 220 and 250 gr/mq. Thickness = between 0,75 and 0,85 mm Elasticity = between 7% and 15 %. The elasticity is measured applying a tensile stress of 2 kg to a sample of fabric having the lenght of one meter and a width of 10 centimeter. Fabric 4. Thickness of about 0,25 and 0,30 mm. During the pressing step, the covering 15 is unrolled from a drum and applied to the toothing of the belt substantially as already explained for the plurality of layers applied to the back of the annular element 10. Advantageously, during the pressing step the covering 15 avoids any leakage of elastomeric material to the outside allowing for the finished belt the appropriate meshing with the surface of the pulley of the transmission. Furthermore, the covering 15 allows during the pressing step a controlled and uniform flux of elastomeric material between the recesses of the matrix 11.

Claims: A process for manufacturing toothed belts of elastomeric material comprising the following steps: making a sleeve internally provided with hollows; slitting the sleeve according to a cutting line of helical extension so as to obtain a semifinished product in the form of a continuous strip (7); wrapping said semifinished product in the form of a continuous strip (7) over too wrapping toothed rollers (8, 9) of parallel axes to be positioned according to a predetermined distance between centres, said wrapping being carried out at a pitch corresponding to the width of the semifinished product in order to obtain an annular element (10) of the desired cirumferential extension having at least the same width as the toothed belt to be produced and formed with a plurality of loops (10a) disposed in mutual side by side relation, characterized by: a) controlling the temperature and time of the sleeve-making step so as to obtain a plurality of hollows (5) alternated with projections for preforming of the teeth defining the belt pitch (P) while avoiding the cross-linking of the elastomeric material constituting the sleeve; b) truing up the annular element in the raw state, by means of said hollows, between a moulding matrix (11) and an opposite mould (12); c) moving the matrix and the mould close to each other applying heat for vulcanization thereto, so as to mould the teeth to the final shape and cause the mutual bonding of the side by side loops of the annular element in the raw state, thereby giving rise to a belt of the desired length, in the absence of mechanical junctions. A process for manufacturing toothed belts of elastomeric material, according to claim 1 characterized by the fact of comprising the following steps: before said making a sleeve, wrapping a reinforced cord layer (1) and a raw elastomeric layer (2) provided as sheet around a cylindrical preforming drum had a surface with a plurality of substantially longitudinal teeth spaced apart by a toothing pitch, said elastomeric ayer having a quantity of elastomeric material less than a quantity required to form teeth of a cross-linked belt to be formed; introducing the reinforced cord layer, the raw elastomeric material, and the preforming drum into an autoclave; heating the reinforced cord layer and the raw elastomeric layer in the autoclave at a heating temperature and for a time period selected to avoid cross-linking the raw elastomeric layer, preforming said sleeve of elastomeric material having interior and exterior circumferential surfaces and having a plurality of axially extending partly shaped teeth forming said projections and hollows, said projections heing separated by said hollows; before said step b) of truing up the annular element, applying a plurality of covering layers of non-cured elastomeric material around the outer surface of the annular element to cover adjacent ones of the plurality of loops of the annular element; trueing up the non cross-linked annular element by positioning a lenght of said annular element and the plurality of covering layers between said moulding matrix and opposite mould, said matrix had a plurality of teeth and a plurality of grooves complimentary to the teeth and grooves of the finished belt; pressing the covering layers and the annular element with the matrix and the mould and applying heat to the covering layers and annular element to mold the projections to the final shape of teeth, causing mutual bonding of the side by side loops of the annular element, and cross-linking covering layer and annular element forming said cross-linked toothed belt of desired length. A process according to claim 1 or 2, characterized in that during said pressing step a final mutual truing of the respectively-aligned transverse hollows (5) belonging to the individual loops (10a) of the semifinished product previously wrapped in the form of a strip (7), is carried out. A process according to claim 1 or 2, characterized in that during said wrapping step a mutual alignment of the transverse hollows (5) present in the individual loops (10a) of said semifinished product in the form of a strip (7) is executed. A process according to claim 4, characterized in that said alignment of hollows (5) is obtained by engagement of said hollows on longitudinal-alignment teeth (8a, 9a) provided on said wrapping rollers (8, 9), mutually spaced apart from each other by the same distance as said toothing pitch (P). A process according to claim 1 or 2, characterized in that for the accomplishment of said cylindrical tubular sleeve the tubular formation of a coating fabric (4) on which the cords and the raw elastomeric material (2) are wrapped is also provided. A process according to claim 1 or 2, characterized in that during said pressing step, a covering fabric (15) previously interposed between said moulding matrix (11) and the annular element is fastened to the inner surface of the annular element (10). A process according to claim 1 or 2, characterized in that before the pressing step a presetting of the loops (10a) of the semifinished product in the form of a strip (7) is carried out through application of a covering fabric (15) to the inner surface (10a) of the annular element (10). A process according to claim 8, characterized in that said covering fabric (15) is applied during said wrapping step through interposition of the covering fabric between the wrapping rollers (8, 9) and the annular element being worked. A process according to claim 1 or 2, characterized in that before the pressing step a mutual presetting of the loops (10a) formed from said semifinished product in the form of a strip (7) is carried out through application of at least a coating sheet to the outer surface (10c) of the annular element (10). The process according to claim 10, characterized in that the outer coating sheet is applied, after said wrapping step, to the annular element (10) wrapped over said wrapping rollers (8, 9). A process according to claim 1 or 2, characterized in that said cross-linking step of the elastomeric material (2) is obtained simultaneously with the pressing step, through heating of the moulding matrix (11) and opposite mould (12). A process according to claim 1 or 2, characterized in that said pressing step is carried out by successive stages to each of which the pressing of a portion of predetermined length of said annular element (10) corresponds. A process according to claim 1 or 2, characterized in that after said cross-linking step, a step involving the trimming of the edges of the obtained toothed belt is provided. A process according to claim 1 or 2, characterized in that after said cross-linking step, a circumferential cutting step of the annular element (10) is executed, in order to obtain a plurality of toothed belts of predetermined width. A process according to claim 1 or 2, characterized in that, before said pressing step, the sleeve (10) obtained from the wrapping step is submitted to a final cutting step executed following circumferential lines spaced apart from one another a distance corresponding to the width of the belts to be obtained. A process according to claim 1 or 2, characterized in that at the end of the preforming step said hollows have a depth corresponding to at least 1/3 of the height detectable on the individual teeth (3) of the finished belt. A process according to claim 1 or 2, characterized in that at the end of the preforming step, said hollows (5) are alternated with projections (3a) the transverse outline of which has a surface extension equal to at least 1/2 of the surface extension detectable on the tooth profile of the finished belt. A process according to claim 2, characterized in that the application of centripetal pressure during the preforming step occurs concurrently with the administration of heat to the sleeve being worked. A process according to claim 2, characterized in that for the sleeve manufacture, the application of centripetal pressure during the preforming step is carried out in an autoclave concurrently with the administration of heat to the sleeve over a period of time included between 1/6 and 1/2 of the time necessary to cause the complete cross-linking of the elastomeric material at the temperature conditions according to which the administration of heat occurs. A process according to claim 1 or 2, characterized in that a centripetal pressure is applied to the sleeve during a preforming step carried out in an autoclave concurrently with the administration of heat to the sleeve executed by submitting the latter to temperatures of a value included between 1/2 and 4/5 of the value needed for obtaining the complete cross-linking of the elastomeric material over a period of time corresponding to the duration of the administration of heat. A process according to claim 2, characterized in that the application of centripetal pressure during a preforming step occurs at a value included between 10 bar and 22 bar, over a period of time included between 2 and 15 minutes, while administering heat to the sleeve at a temperature included between 50°C and 205°C at the outside thereof, and included between 100°C and 140°C at the inside thereof. A process according to claim 1 or 2, characterized in that the elastomeric material viscosity after preforming is measured in Mooney unit following ASTM 1646-80 standard, said viscosity undergoing an increase not higher than 10% relative to the raw material. A toothed belt of elastomeric material comprising: at least a reinforced cord layer (1) of annular configuration; at least a layer of elastomeric material (2) incorporating said reinforced cord layer (1) and exhibiting, on an inner surface thereof, a plurality of teeth (3) extending transversely to the circumferential extension of the belt and spaced apart from each other the same distance according to a predetermined toothing pitch (P); said reinforced cord layer (1), incorporated into said at least a layer of elastomeric material, structured in the form of a continuous strip helically wrapped in several loops disposed in side by side relation and mating at the respective opposite edges, characterized in that it further comprises at least a covering fabric (15) applied to the inner surface thereof provided with said teeth (3), said covering fabric (15) heing structured in the form of a continuous strip hang the same width as the belt. A belt comprising an annular element hang a plurality of teeth spaced apart by grooves and incorporating longitudinal extending reinforcing cords (1), said belt being manufactured in at least two steps, respectively a first step wherein a strip of noncross-linked elastomeric material incorporating longitudinally extending reinforced cords and having teeth partially shaped is wrapped up helically according to a plurality of loops disposed in mutual side by side relation to obtain said annular element and a further step wherein by means of a matrix and an opposing mold member moved toward each other the interposed annular element is molded and cured to had the final shape of the teeth, characterized by the fact of comprising a fabric (15) having the sane width of the said plurality of loops disposed in mutual side by side relation, said fabric being applied to the plurality of teeth and grooves of said plurality of loops by mechanical pressure, the final thickness of the fabric originating a thickness of the belt, measured from the plane of the longitudinal cords and the centers of the grooves, corresponding to a predetermined value for obtaining a correct meshing with a toothed pulley. A belt according to claim 24 or 25, characterized in that it further comprises at least a coating fabric (4) applied to the inner surface thereof provided with said teeth (3), said coating fabric (4) being structured in the form of a continuous strip wrapped in several side by side loops mating at the respective opposite edges.

Inventors: DI CESARE DANILO, DI GIACOMO TOMMASO, MACCHIARULO VINCENZO, PETACCIA MARINO, DI CESARE, DANILO, DI GIACOMO, TOMMASO, MACCHIARULO, VINCENZO, PETACCIA, MARINO
Applicants: DAYCO PTI SPA, DAYCO PTI S.P.A.
Application year: 1992
Main classification: B29D29
IPC codes: B29D29, F16G1, B29C69