Power transmission belt and its manufacturing process

Abstract

A power transmission belt (1, 101) comprising a body (2, 102) made of a first elastomeric material, a plurality of cords (3, 103) embedded in the belt body, a plurality of ribs (5) or teeth (106), and a back (8, 107), wherein said back has a plurality of geometric shapes (20) arranged in regular series, each geometric shape being defined by: a plurality of recesses (21) alternating with projections (22), each recess having a length of between 0.05 mm and 5 mm, a depth of between 0.05 mm and 2 mm and a width of between 0.05 mm and 0.5 mm; or a closed curved line (30) defining at least one recess (31) having an average depth between 0.01 mm and 2 mm.

Description

[0001] "POWER TRANSMISSION BELT AND ITS MANUFACTURING PROCESS"

[0002] Cross-Reference to Related Applications

[0003] This Patent Appl ication claims priority from Italian Patent Application No . 102024000025284 filed on November 11 , 2024 , the entire disclosure of which is incorporated herein by reference .

[0004] Technical Field

[0005] The present invention relates to a power transmission belt and, in particular, to a belt for use in power transmission systems for use in vehicles and to the relative transmission system .

[0006] State of the Prior Art

[0007] The present invention is preferably used for the transmission of motion in a transmission group of a motor vehicle .

[0008] The transmission of motion in motor vehicles preferably occurs by using synchronous , also called toothed, power transmission belts .

[0009] In motor vehicles , power transmi ssion belts are also used to drive accessory members of an engine , also known as multi-rib or poly-V belts , which are now used in some cases even in electric vehicles . An accessory transmission generally comprises a drive pulley connected to a crankshaft of an internal combustion engine o f the vehicle , at least a second and a third pulley connected for example to an alternator and to an accessory, for example a hydraulic pump, and a transmission belt for connecting the pulleys to one another .

[0010] Such belts generally comprise a body made of elastomeric material , preferably of EPDM for the poly-V belts and of HNBR for toothed belts , a plurality of resistant thread-like inserts embedded longitudinally in the body, and a coupling portion integrally connected to the body and comprising a plurality of teeth which are transverse to the sliding direction of the belt in use in the case of toothed belts or ribs along the sliding direction of the poly-V belt .

[0011] Today ' s engines require that the transmission members and in particular the transmission belts are capable of having an increasingly longer average li fe while having to operate in high temperature conditions and with much greater mechanical stresses .

[0012] It is known that in transmiss ions with flexible elements , a torque exchange occurs by means of friction forces present between the surfaces in contact with the belt and the pulley .

[0013] The friction between two bodies in contact is described by the following formula :

[0014] Fa= f * Fn with

[0015] Fa= Friction force , tangent to the contact surface

[0016] Fn= Normal force , orthogonal to the contact surface f = friction coef ficient ( CoF = Coef ficient of Friction)

[0017] The power transmission between two surfaces in contact is only possible i f there is adhesion between the surfaces such as to prevent slippage .

[0018] In particular, for the contact between belt and pul ley extending for an alpha winding arc , Euler ' s theory applies , which sets the limit relationship that there must be between the taut branch force ( T ) and the loose branch force ( t ) that arise on the two branches at the entrance and exit of a pulley so that the belt adheres to the pulley : T / t = eA(f*alpha) , resulting in:

[0019] T = taut branch belt force

[0020] T = loose branch belt force f = coefficient of friction between belt and pulley alpha = wound arc

[0021] The onset of such forces on the two branches (T>t) is due to the exchange of twisting torque calculable with the formula

[0022] Torque = (T-t ) *R, where R is the radius of the pulley in question.

[0023] The value of the coefficient of friction plays a fundamental role in the power exchange between a belt and a pulley. Slipping conditions between the surfaces, also deriving from overly low CoF values, as well as not fully satisfying the torque transmission function, also cause a deterioration, firstly, of the surface of the flexible element (for example, overheating and consequent ageing of the rubber, or abrasion of the same rubber surface in contact) and a reduction in the useful life of the belt.

[0024] The coefficient of friction is determined by various parameters, among which the type of materials in contact, the surface finish of the surfaces, the temperature, the presence of contaminants at the interface between the surfaces, the working hours of the system.

[0025] It is therefore important to have a coefficient of friction between belt back and pulley surface that allows obtaining optimal adhesion conditions for the transmission of motion.

[0026] In order to have an optimal performance, numerous methods have been tried to optimize the coefficient of friction. For both the working surface and the back, it is known to apply chemical treatments or cover the surface with a fabric, for example a knitted fabric, or to use both methods .

[0027] However, such solutions are often expensive and can give rise to other undesired ef fects , such as particular noise arising from the rolling of the belt / pulley surfaces and / or speci fic noise from adhesion / detachment phenomena between the surfaces , known as stick-slip noise .

[0028] Therefore , there is still a search for a way to optimi ze the adhesion conditions of the back of a belt also by means of the coef ficient of friction that is at the same time cost- ef fective , easy to manufacture and that does not introduce noise problems and satis fies the primary function o f torque or power trans fer both when the interface between the surfaces is dry and when it is wet .

[0029] Summary of the Invention

[0030] The obj ect of the present invention is the manufacturing of a toothed or poly-V transmission belt which solves the aforementioned problems and which in particular allows having a higher coef ficient of friction in both dry and wet conditions .

[0031] The aforesaid obj ect is achieved by a belt according to claim 1 and by a process according to claim 14 .

[0032] Brief Description of the Figures

[0033] - Figure 1 is a schematic partial view of a poly-V belt ;

[0034] - Figure 2 is a schematic partial view of a toothed belt ;

[0035] - Figure 3 is a photo of a first embodiment of the back of a belt according to the invention;

[0036] - Figure 4 is a photo of a second embodiment of the back of a belt according to the invention; - Figure 5 is a photo of a third embodiment of the back of a belt according to the invention;

[0037] - Figure 6 is a photo of a fourth embodiment of the back of a belt according to the invention;

[0038] - Figure 7 is a schematic depiction of a geometric shape present on the surface of the back of a belt with the relative circumferences inscribed and circumscribed to a geometric shape and

[0039] - Figure 8 is a schematic depiction of a geometric shape present on the surface of the back .

[0040] Description of the Invention

[0041] "Main elastomer" means that it is present in the blend forming the body of the belt for more than 50 wt% calculated on the total weight of all the elastomers in the blend, therefore excluding all other non-elastomeric components of the belt .

[0042] " First elastomeric material essentially consists of" means that in addition to all the usual additives , it is possible to add small percentages of other polymers or copolymers to the blend without negatively af fecting the chemical compatibility between the body blend and the other elements forming the toothed belt and therefore without departing from the scope of the present invention .

[0043] "Treatment" means a composition of materials forming the treatment that comes into contact with a material to be treated to confer new properties to the material to be treated .

[0044] In the following, percentage is always understood as a percentage by weight unless otherwise speci fied .

[0045] With reference to Figure 1 , reference numeral 1 indicates a poly-V belt comprising a body 2 comprising a first elastomeric material , a plurality of resistant threadlike inserts 3 embedded longitudinally in the body, also called "cords" in the following, and a coupling portion 4 integrally connected to the body and comprising a plurality of ribs , also referred to in the following with the term V- shaped ribs 5 side by side and alternated with V-shaped grooves 6 that form the working surface 7 . The belt 1 further has a back 8 de fined by the surface opposite the surface with the V-shaped ribs or the toothing .

[0046] Preferably the body 2 is made of a blend comprising one or more elastomeric materials and numerous additives . The elastomeric material ( s ) are indicated overall for convenience in the following as " first elastomeric material" .

[0047] The belt body 2 advantageously comprises as main elastomer within the first elastomeric material an elastomer selected from the group consisting of natural rubber (NR) , polychloroprene ( CR) , acrylonitrile butadiene (NBR) and related hydrogenated elastomers known as hydrogenated acrylonitrile butadiene (HNBR) or zinc salts of hydrogenated acrylonitrile butadiene grafted with esters of unsaturated carboxylic acid, polyisoprene , styrene-butadiene rubbers , ethylene-alpha-olef in elastomers , EPDM, polyurethane , fluorine elastomers , ethylene-acrylic elastomers (AEM) , butyl bromine , polythene chlorosul fonate ( CSM) or alkylchlorosul fonate , chlorinated polythene , epoxidi zed natural rubber, SBR, carboxylated NBR, carboxylated HNBR, ACM and blends of these compounds .

[0048] The body 2 preferably comprises as a first or as a further elastomeric material at least one polyolefin copolymer or a rubber containing acrylonitrile units . More preferably the first elastomeric material comprises an elastomer selected from the group consisting of EPM (ethylene propylene monomer) , EPDM (ethylene propylene diene monomer) , NBR (acrylonitrile butadiene rubber) , HNBR (hydrogenated acrylonitrile butadiene rubber) , ZnHNBR (acrylonitrile butadiene rubber with zinc salts) , XHNBR (peroxide vulcanized hydrogenated acrylonitrile butadiene rubber) .

[0049] In particular, among the rubbers containing acrylonitrile units, those preferred are NBR, HNBR and modified HNBR, for example HNBR modified with zinc salt or ZnHBR or XHNBR. In particular, EPDM is preferred among the polyolefin copolymers.

[0050] Even more preferably the body 2 comprises EPDM.

[0051] Even more preferably it essentially consists of EPDM.

[0052] In particular, the embodiment is preferred wherein the body comprises more than 60 wt% with respect to the total weight of elastomer, more preferably more than 70% of EPDM.

[0053] Alternatively, the embodiment wherein the body essentially consists of EPDM is also preferred.

[0054] In addition to the elastomeric materials, the body bend can comprise conventional additives such as, for example, reinforcing agents, fillers, pigments, stearic acid, accelerators, vulcanizing agents, antioxidants, activators, initiators, plasticizers, waxes, pre-vulcanization inhibitors, antidegradants, process oils and the like, various kinds of reinforcing fibres such as cotton, aramid, nylon and of various sizes dispersed in the belt body.

[0055] A covering material, preferably a fabric 9, more preferably selected from the group consisting of woven fabric or non-woven fabric or knitted fabric, is generally arranged on the working surface corresponding to the ribs .

[0056] Preferably the fabric 9 is treated with an elastomeric material . Preferably the fabric 9 is a knitted fabric .

[0057] Knitted fabric has proven to be particularly preferred when it has a so-called "weft knitted" or "j ersey" construction .

[0058] The weight of the fabric 9 is preferably between 50 and 500 g / m2, more preferably between 150 and 300 g / m2, for example 250 g / m2.

[0059] In a di f ferent embodiment of the invention, a toothed belt is indicated, as a whole , by 101 in Figure 2 . The belt 101 comprises a body 102 . A plurality of resistant longitudinal thread-like inserts 103 , also alternatively called cords , are embedded in the body 102 .

[0060] The belt further comprises a toothing 104 composed of a plurality of teeth 106 that in use form the working surface 105 , namely the surface that meshes on a corresponding pulley of the transmission system .

[0061] The toothed belt further comprises a back 107 opposite the surface having the toothing 105 .

[0062] Preferably the body 102 comprises a first elastomeric blend comprising a first main elastomeric material .

[0063] Advantageously the belt pitch is between 3 mm and 20 mm, more preferably between 5 mm and 14 mm, for example 9 . 525 , 8 or 7 for vehicle applications .

[0064] The body 102 of the toothed belt advantageously comprises a first elastomeric material selected from the group consisting of natural rubber (NR) , polychloroprene ( CR) , acrylonitrile butadiene (NBR) and related hydrogenated elastomers known as hydrogenated acrylonitrile butadiene (HNBR) or zinc salts of hydrogenated acrylonitrile butadiene grafted with esters of unsaturated carboxylic acid, polyisoprene , styrene-butadiene rubbers , ethylene-alpha- olefin elastomers , EPDM, polyurethane , fluorine elastomers , ethylene-acrylic elastomers (AEM) , butyl bromine , polythene chlorosul fonate ( CSM) or alkyl-chlorosul fonate , chlorinated polythene , epoxidi zed natural rubber, SBR, carboxylated NBR, carboxylated HNBR, ACM and blends of these compounds .

[0065] Advantageously, the first elastomeric material is present in the body blend as "main elastomer" , namely is present in the body blend for more than 50 wt% calculated on the total weight of all the elastomers in the blend, thus excluding all the other non-elastomeric components of the belt .

[0066] The body 102 preferably comprises as a first or as a further elastomeric material at least one polyolefin copolymer or a rubber containing acrylonitrile units .

[0067] More advantageously the copolymer ( s ) used as body blends are nitrile rubbers , advantageously they are acrylonitrile butadiene rubbers , known by the acronym NBR . Even more advantageously they are hydrogenated acrylonitrile butadiene or HNBR or even XHNBR, namely hydrogenated acrylonitrile butadiene and carboxylate .

[0068] For example , what are known as fully saturated HNBRs , and thus having a residual double bond percentage of more than 0 . 9% , can be used, but alternatively lower degree of saturation HNBRs such as , for example , what are known as partially saturated HNBRs having a degree of saturation of 4 % or 5 . 5% , can also be used .

[0069] Even more advantageously, in combination with a first elastomeric material also an elastomeric material is used formed by a blend of one or more copolymers , obtained starting from a diene monomer and a monomer containing nitrile groups where one or more of such copolymers are added with an acid or a salt of an unsaturated carboxylic acid . More advantageously the unsaturated carboxylic acid is methacrylic or acrylic acid and said salt is a zinc salt of methacrylic or acrylic acid . Even more advantageously a zinc salt of methacrylic acid is used . Even more advantageously the zinc salt of methacrylic acid is added in an amount between 10 and 60 phr .

[0070] For example , the elastomers sold by Zeon are advantageously used, with the names : ZSC 1295 , ZSC 2095 , ZSC 2195 , ZSC 2295 , ZSC 2295L, ZSC 2295R and ZSC 2395 .

[0071] In particular, it is possible to partially or entirely replace the aforementioned HNBRs , namely ZETPOL and / or THERBAN with a ZSC comprising an unsaturated carboxylic acid and zinc oxide and / or with THERBAN ART comprising a salt of the unsaturated carboxylic acid .

[0072] Mixed blends of polyolefins and rubbers containing acrylonitrile units are also preferred, more preferably blends containing a copolymer of ethylene with NBR or HNBR or the aforementioned modi fied HNBRs . For example , rubbers containing EPDM ( ethylene propylene diene monomer ) or EPM ( ethylene / ethylene monomer ) can be added to polymers containing acrylonitrile units in an amount preferably between 1 and 30% .

[0073] The cords 103 are formed by a plurality of threads or strands or yarns and each thread is formed by a plurality of filaments . Preferably the cords 103 are made of at least one material selected from the group consisting of glass fibres , aramid fibres , carbon fibres , PBO fibres .

[0074] Alternatively, the cords can be made of two di f ferent materials . Even more preferably at least the outer surface of the cords is made of glass fibres .

[0075] Advantageously, the toothed surface of the transmission belt 101 is covered with a covering 108 . The covering 108 is more advantageously selected in the group consisting of woven fabric, knitted fabric or non-woven fabric .

[0076] The covering fabric 108 of the toothing 105 or the optional covering fabric of the back 110 can consist of one or more layers and can be obtained by means of di f ferent weaving techniques , for example , by means of the weaving technique known as 2x2 twill .

[0077] According to an embodiment of the present invention the belt back 8 , 107 has a plurality o f geometric shapes 20 , 30 arranged in series , preferably regular .

[0078] The plurality of geometric shapes 20 , 30 is drawn or impressed on the surface forming deeper or elevated or raised areas with respect to the surface of a flat and smooth back and therefore defined in the following as the ideal back surface .

[0079] Therefore , geometric shape 20 , 30 means a part of the back surface 8 , 107 including a portion which is higher or lower than the rest of the back surface . Higher and lower refer to the ideal back surface that is in contact with relative pulleys or tensioners during the operation of the belt and therefore in the transmission system .

[0080] In a first embodiment illustrated in Figures 3 to 5 each geometric shape 20 includes a plurality of recesses 21 alternating with proj ections 22 .

[0081] Preferably the recesses 21 and the proj ections 22 are straight .

[0082] Preferably, the recesses 21 and the proj ections 22 are I-shaped .

[0083] Preferably the recesses 21 and the proj ections 22 defining a geometric shape are parallel with respect to one another within a geometric shape 20 .

[0084] Preferably the recesses 21 and the proj ections 22 are parallel and straight .

[0085] Preferably the recesses 21 and the proj ections 22 have substantially the same width along their entire length, although a smaller width is possible in the end portions as depicted in Figure 4 .

[0086] Preferably the recesses 21 and the proj ections 22 have substantially the same depth or elevation with respect to the ideal back surface, although a smaller depth or elevation is possible in the end portions .

[0087] Preferably, the recesses 21 and the proj ections 22 have substantially the same length .

[0088] Alternatively, the recesses 21 and the proj ections 22 are arranged in parallel successions with a length of each recess 21 or proj ection 22 decreas ing as depicted in Figure 5 .

[0089] Preferably the proj ections 22 have dimensions similar to the recesses 21 contiguous thereto .

[0090] A plurality of recesses 21 and proj ections 22 can define a geometric shape 20 as depicted in Figures 3 to 5 , but can also be arranged within a geometric shape the contour of which is delimited by a closed l ine defining a surface area having a di f ferent elevation or depth with respect to the ideal plane of the back surface .

[0091] Each recess 21 preferably has a length of between 0 . 05 mm and 5 mm, more preferably between 0 . 05 and 2 mm, even more preferably between 0 . 08 and 1 . 2 mm, a depth or elevation of between 0.05 mm and 2 mm, more preferably between 0.05 and 1.5 mm and a width of between 0.01 mm and 0.05 mm, more preferably between 0.02 and 0.04.

[0092] Preferably the projections 22 have a length of between 0.05 mm and 5 mm, more preferably between 0.05 and 2 mm, even more preferably between 0.08 and 1.2 mm, a depth of between 0.05 mm and 2 mm, more preferably between 0.05 and 1.5 mm and a width of between 0.01 mm and 0.05 mm, more preferably between 0.02 and 0.04.

[0093] Preferably the ratio between the widths of the projections 21 and those of the recesses 22 is between 0.1 and 10 mm, even more preferably between 0.2 and 5 mm.

[0094] The geometric shape can also be defined by a closed curved line defining a recess having an average depth between 0.05 and 5 mm, more preferably between 0.05 and 2 mm.

[0095] In other words, the depth of the closed lines corresponds to the distance between a plane tangent to the caps of the geometric shapes delimited by the closed curved lines and a plane passing through their bases.

[0096] The geometric shapes are preferably not arranged axially symmetrically with respect to the longitudinal axis of the belt.

[0097] The geometric shapes are preferably not even arranged symmetrically or specularly with respect to the latitudinal axis of the belt.

[0098] The geometric shapes are more preferably in the shape of a polygon.

[0099] The geometric shapes can advantageously be defined by a number of triangles equal to the sides of the polygon. If the polygons are regular, then all the triangles will be identical and isosceles. Preferably, the geometric shapes can be defined by a plurality of triangles , each with its alternation of recesses and proj ections to form a design or pattern called a field .

[0100] More preferably the geometric shapes can be defined by a number of triangles between 1 and 8 .

[0101] More preferably the polygon is defined by a triangle , a square , a rhombus or a rectangle .

[0102] More preferably and as depicted in Figure 7 , when the geometric shape is a polygon, the dimension can be established by de fining an inner circumference of the polygon of diameter I and an outer circumference of the polygon of diameter E . It is defined as average radius Rm= (E+ I ) / 2 , represented by the dashed intermediate circumference in Figure 8 . According to the present invention, Rm preferably varies from 2 to 10 mm .

[0103] In the case of regular polygons , or with equal sides , it will result that the inner circumference is tangent to the sides and the outer circumference passes through the vertices as depicted in Figure 7 for a hexagon .

[0104] For example , as depicted in Figure 8 , the geometric figure 20 can be formed by a plurality of triangles , in particular in the case of Figure 8 there are six triangles forming a hexagon . Each triangle is defined by a plurality of recesses 21 and proj ections 22 .

[0105] I f the polygon is not regular, it will result that the inner circumference must at least pass through the innermost point of the polygon, for example tangent to one side .

[0106] The outer circumference must at least pass through the outermost point of the polygon or the outermost vertex .

[0107] More preferably the polygon comprises or is defined by a number of reces ses between 2 and 25 , more preferably from 5 to 25 , even more preferably between 5 and 15 .

[0108] The polygon can comprise two or more series of recesses arranged at di f ferent angles to one another with respect to the longitudinal axis of the belt .

[0109] The geometric shapes 20 overall form a "carbon f ibre" ef fect , in other words the succession of polygons seem to shape or have the appearance of a surface made of carbon fibres , although not made of carbon fibre .

[0110] In an alternative embodiment the geometric shapes 30 are defined by a closed curved line 31 , defining at least one recess having an average depth between 0 . 01 and 2 mm, more preferably of circular or elliptical shape .

[0111] Preferably, the closed curved line encloses an area between 0 . 01 mm2and 5 mm2.

[0112] Preferably the belt is a toothed belt 101 .

[0113] Alternatively the belt is a poly-V belt 1 .

[0114] Preferably, the poly-V belt 1 has a width of between 10 and 60 mm .

[0115] It is also possible to manufacture a belt comprising a back fabric and a further layer of rubber or treatment , preferably mainly comprising an elastomeric material on the outer surface of which the geometric shapes are drawn or impressed .

[0116] The back fabric can be for example a woven fabric, a knitted fabric or what is known as a tire cord .

[0117] For such purpose , it is preferable that the rubber layer or treatment is suf ficiently thick to allow the geometric shapes to be drawn or imprinted without bringing the fabric to the surface .

[0118] To manufacture the belt of the present invention, a step is included in the belt printing process in which a material , preferably a sheet of paper, pre-impressed and capable of transmitting an impression capable of forming geometric shapes of any shape on the surface of the belt back is added during the belt printing step .

[0119] It has been noted that by making geometric figures 20 , 30 on the surface of the back, it is possible to increase or even double the coef ficient of friction on the belt back, which leads to improving the performance of the belt both in the absence and in the presence of water in the interface between belt and pulley and therefore ultimately to obtain an increase in belt performance .

[0120] Examining the characteristics of the belt manufactured according to the present invention, the advantages that it allows obtaining are apparent .

[0121] By using a transmission belt , both poly-V and toothed, according to the present invention, considerable improvements were obtained and, in particular, the above problems could be overcome .

[0122] The invention will now be described by way of examples without being limited by them .

[0123] COMPARATIVE EXAMPLE 1 and EXAMPLES 2 and 3

[0124] Three poly-V belts are printed with an EPDM body and 6 ribs covered with a treated fabric . The cord is made of polyester .

[0125] The belt has no fabric on the ribs .

[0126] The width of the belt is 21 . 36 mm .

[0127] The first belt has a smooth back .

[0128] The second belt has a back with a carbon ef fect in which there are rectangular geometric shapes defined by a series of parallel recesses as depicted in Figure 3 .

[0129] The third belt has a back with geometric shapes defined by a series of ellipses as depicted in Figure 6.

[0130] The coefficient of friction test was performed according to the standard SAE J2432.2021 with the following values: poly-V belt with test profile K length 1200 mm, 6 ribs, width 21.36. The pulley material is Stainless Steel 303, installation voltage N 186.6, Temp. Box 22.7 °C. The driving speed is 1900 rpm, the braking torque gradient is 0.5 Nm / s and the final driven speed 0 rpm, corresponding to a slip ration on the driven side equal to 100%.

[0131] The belt with a smooth back has a Cof (coefficient of friction) of 0.59 in dry conditions and 0.4 in wet conditions with a water flow of 0.3 1 / min.

[0132] The belt with a carbon-effect back as depicted in Figure 3 has a Cof (coefficient of friction) of 1.02 in dry conditions and 0.6 in wet conditions.

[0133] The belt with an ellipsoid back has a Cof (coefficient of friction) of 1.09 in dry conditions and 0.7 in wet conditions .

[0134] As can be seen from the test, the belts according to the invention have a Coefficient of friction that is about twice that of those with a smooth back.

Claims

C L A I M S1. A power transmission belt (1, 101) comprising a body (2, 102) made of a first elastomeric material, a plurality of cords (3, 103) embedded in the belt body, a plurality of ribs (5) or teeth (106) , and a back (8, 107) , characterised in that said back has a plurality of geometric shapes (20) arranged in regular series, each geometric shape being defined by:- a plurality of recesses (21) alternating with projections (22) , each recess having a length of between 0.05 mm and 5 mm, a depth of between 0.05 mm and 2 mm and a width of between 0.05 mm and 0.5 mm or- a closed curved line (30) defining at least one recess (31) having an average depth between 0.01 mm and 2 mm.

2. The transmission belt (1, 101) according to claim 1, characterised in that said geometrical shapes (20) are not arranged axially symmetrically with respect to the longitudinal axis of said belt.

3. The transmission belt (1, 101) according to claims 1 to 2, characterised in that said geometric shapes (20) are in the shape of a polygon (20) .

4. The transmission belt (1, 101) according to claims 1 to 3, characterised in that said polygon (20) is a triangle, a square, a rhombus or a rectangle or a parallelogram.

5. The transmission belt (1, 101) according to claim 3 or 4, characterised in that having defined an outer circumference of the polygon having diameter E and an inner circumference of the polygon having diameter I, the averageradius Rm of the polygon is 2 mm < (E+I) / 2 < 10 mm.

6. The transmission belt (1, 101) according to any one of the preceding claims, characterised in that said polygon comprises a number of recesses between 5 and 25.

7. The transmission belt (1, 101) according to any one of the preceding claims, characterised in that the ratio of the widths of the projections (22) to the widths of the recesses (21) is between 0.2 and 5 mm.

8. The transmission belt (1) according to any one of the preceding claims, characterised in that said recesses (21) have the same dimensions as said projections (22) contiguous thereto.

9. The transmission belt (1, 101) according to any one of the preceding claims, characterised in that said geometrical shapes form a carbon fibre effect.

10. The transmission belt (1, 101) according to claim 1 or 2, characterised in that said closed curved line defines an area of circular or elliptical shape.

11. The transmission belt (1, 101) according to claim 1, 2 or 10, characterised in that said closed curved line encloses an area between 0.005 mm2and 5 mm2.

12. The transmission belt (101) according to any one of the preceding claims, characterised in that said belt is a toothed belt (101) .

13. The transmission belt (1) according to any one of claims 1 to 11, characterised in that said belt is a poly-V belt (1) .

14. The transmission belt (1) according to claim 13, characterised in that said poly-V belt (1) has a width of between 10 and 60 mm.

15. A manufacturing process of a power transmissionbelt (1, 101) comprising a body (2, 102) made of a first elastomeric material, a plurality of cords (3, 103) embedded in the belt body, a back (8, 107) and a plurality of ribs (5) or teeth (106) , characterised in that it comprises the step of arranging a material capable of transmitting an impression capable of forming geometric shapes on the surface of said back of the belt during the belt printing steps.

16. The manufacturing process of a power transmission belt (1, 101) according to claim 15, characterised in that said material is paper.

Images