Power transmission belt tensile cord

Abstract: A tensile cord manufacture wherein polyester multifilament cords are heat treated and drawn to provide tensile cords (10) for use in power transmission belts (11). Prese­ lected characteristics of the original cord, parameters of heat treating and drawing, and characteristics of the formed tensile cords required to provide the improved features of the invention are set forth.

Description: Technical Field This invention relates to power transmission belts and in particular to the manufacture of tensile cords for use in such power transmission belts. Background Art In the conventional power transmission belt, such as an automotive fan belt, the looped belt is provided with longitudinally extending tensile cords. Such belts are conventionally formed of resilient material, such as rubber, and the tensile cords are formed of relatively low stretch, high strength materials. One known material for use in such tensile cords is polyester synthetic resin. An excellent example of such a synthetic resin advantageously adapted for such use is the polyester filament yarn sold by Celanese Corporation under the trademark Fortrel®. One form of such Fortrel® polyester resin manufactured and sold by Celanese Corporation is identified as the Type 111 polyester and has been utilized as tensile cords in such power transmission belts, as discussed above. The use of polyester tensile cords in power transmission belts is disclosed in U.S. Patent 4,522,614 of Hiroshi Matsuoka et al. As disclosed in that patent, an automatic tension-maintaining power transmission belt is formed by embedding polyester fiber cord having high relative viscosity and subjected to hot stretching prior to the embedment thereof in the rubber of the belt. Disclosure of Invention The present invention provides a method of forming a tensile cord for use in a power transmission belt, comprising the steps of: providing a cord of twisted polyester multi-filaments having an ultimate viscosity of less than 0.90; applying adhesive to the cord wherein the application of adhesive to the cord comprises the steps of applying isocyanate or epoxy to the cord, drying the cord by heating the cord to a temperature in the range of approximately 60°C to 100°C, and impregnating the isocyanate or epoxy coated and dried cord with resorcinol-formalin-latex solution; and drawing and setting the adhesive-coated cord in two steps at a temperature in the range of approximately 220°C to 240°C with a first draw ratio in the range of 1% to 1.7% and a second draw ratio in the range of 1% to 1.7%. Preferably, the method includes a final step of passing the adhesive-coated, drawn and set cord through a bath of rubber cement and heating the rubber-coated cord at a temperature in the range of approximately 100°C to 200°C. The initial twisted multi-filament polyester cord preferably has an original ultimate viscosity in the range of approximately 0.75 to 0.90, an original double refraction ratio in the range of approximately 0.16 to 0.195, an original Young's modulus in the range of approximately 50 grams/denier to 106 grams/denier, an original thermal stress in the range of approximately 0.10 to 0.60 grams/denier when heated to a temperature of 150°C for 8 minutes, an original stress ratio of approximately 0.85 to 0.91 with a load of 3 grams/denier applied for 2 minutes, and an original shrinkage under dry heat of approximately 4.0% to 9.0% when heated to a temperature of 150°C for 30 minutes. In a preferred embodiment, the adhesive-coated, drawn and set cord has a Young's modulus of at least 110 grams/denier, a thermal stress of more than 0.48 grams/denier when heated to 150°C for 8 minutes, a stress ratio of more than 0.925 with a load of 3 grams/denier, and a shrinkage under dry heat of less than 3.5% when heated to 150°C for 30 minutes. In another preferred embodiment, the adhesive-coated, drawn and set cord has a thermal stress of more than 0.52 grams/denier when heated to 150°C for 8 minutes, a Young's modulus of more than 120 grams/denier, a shrinkage under dry heat of less than 3.3% when heated to 150°C for 30 minutes, and a stress ratio of more than 0.93 with a load of 3 grams/denier. The invention also provides a power transmission belt tensile cord formed by a method according to the invention, after it has been incorporated into a rubber power transmission belt by vulcanization of the belt material, the cord, when removed from the vulcanized belt, having a Young's modulus of more than 120 grams/denier, a thermal stress greater than 0.49 grams/denier when heated to 150°C for 8 minutes, a stress ratio of greater than 0.925, and less than 3.5% shrinkage under dry heat when heated to 150°C. The tensile cord manufacture of the present invention is extremely simple and economical, while yet providing a highly improved tensile cord for use in power transmission belts and the like. Brief Description of the Drawings Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawing wherein: FIGURE 1 is a transverse section of a power transmission belt having tensile cords embodying the invention: FIGURE 2 is a transverse section of a multiple rib V-belt having tensile cords embodying the invention; FIGURE 3 is a schematic diagram illustrating the stops of manufacture of the tensile cord embodying the invention; FIGURE 4 is a graph illustrating the change in the stress ratio resulting from sequential loading of the cord with a load of 3 gram/denier; FIGURE 5 is a graph showing the relationship between the strength and strain of a cord before and after treatment; FIGURE 6 is a graph illustrating the relationship between the belt running time and the axial load index; and FIGURE 7 is a graph illustrating the Young's modulus test. Best Mode for Carrying Out the Invention In the illustrative embodiment of the invention as disclosed in the drawing, a tensile cord generally designated 10 is shown for use in a power transmission V-belt generally designated 11 and in a multirib power transmission V-belt generally designated 12. In addition, such tensile cords may be utilized on other forms of power transmission belts, such as flat belts, etc. In each of these different belts, the tensile cord provides a longitudinal strength in supporting the resilient body material of the belts, which conventionally comprises rubber. It is desirable that the tensile cords maintain their configuration notwithstanding the tendency for elongation thereof resulting from the power transmission operation. As such belts transfer power between the belt surfaces and the drive system pulleys by frictional engagement therebetween, stretching of the belts causes a lowering of the frictional forces, with undesirable slipping and loss of efficiency in the power transmission. Such slippage further causes heating of the belt, resulting in early failure thereof. Thus, it is desirable to maintain the belt against undue stretching. The present invention comprehends an improved manufacture of the tensile cord notwithstanding high loads applied thereto and the heating thereof in the power transmission operation of the belt. The present invention contemplates the forming of a tensile cord by providing a cord of twisted polyester filaments having an ultimate viscosity in the range of approximately 0.75 to 0.90. The tensile cord is treated in such a way as to cause the stress relaxation of the cord to be low and the degree of thermal stress to be relatively high. The cord of the present invention further has a high Young's modulus and a small shrinkage under dry heat. The tensile cords 10 of the present invention have a low stretch, high strength, and small dimensional change with time. More specifically, the tensile cords of the present invention are formed of a polyester synthetic resin. Adhesive is applied to the tensile cords in two steps. In a first or pre-treatment step, fibers of the polyester resin are twisted to form a cord 13 which is passed through a bath 14 of adhesive, namely isocyanate or epoxy. The adhesive-coated cord is then passed through a pair of ovens 15 and 16 wherein the adhesive is dried. In the second step of applying adhesive, the adhesive-coated cord is passed through a bath of resorcinol-formalin-latex solution 17. The RFL-coated cord is then passed through a pair of ovens 18 and 19 wherein the cord is heated and concurrently stretched by suitably varying the rate of rotation of the input roller 20 and output roller 21 in oven 18, and input roller 22 and output roller 23 in oven 19. Oven 15 is preferably at a low temperature of approximately 65°C. and the cord is maintained within oven 15 for approximately 20 to 360 seconds and, more preferably, in the range of approximately 60 to 180 seconds. The cord is then passed to the second oven 16, which is maintained at a temperature in the range of approximately 60° to 100°C. for a period of time in the range of approximately 20 to 360 seconds, and preferably in the range of 60 to 180 seconds to effectively dry the coated cord. In the preferred embodiment, the isocyanate or epoxy is mixed with a solvent, such as toluene, which is effectively removed in the two-stage drying operation. The RFL-coated cord is maintained in the oven 18 for a period of time in the range of 20 seconds to 360 seconds, and preferably, in the range of 60 seconds to 180 seconds. Oven 18 is maintained at a temperature in the range of approximately 220° to 240°C., and the draw ratio effected in oven 18 is in the range of 1.0% to 1.7%. The partially drawn cord is then delivered to oven 19 and maintained in that oven for approximately 20 seconds to 360 seconds, and preferably for a period of time in the range of approximately 60 seconds to 180 seconds. The draw ratio in oven 19 is similar to that in oven 18, namely, from 1.0% to 1.7%. The drawn cord, with the RFL solution completely dried thereon, is then passed to a bath of rubber cement 24, and then passed through a final oven 25 wherein it is retained for approximately 20 seconds to 360 seconds, and preferably in the range of approximately 60 seconds to 180 seconds, at a temperature in the range of approximately 100° to 200°C., and preferably in the range of approximately 150° to 180°C. The completed tensile cord 10 is then wound on a suitable reel 26 for subsequent use in power transmission belts, such as belts 11 and 12 illustrated in Figures 1 and 2. The initial twisted multifilament polyester cord 13 preferably has the following characteristics: (a) an ultimate viscosity (eta) in the range of approximately 0.75 to 0.90; (b) a double refraction ratio (delta n) in the range of approximately 0.16 to 0.195; (c) a Young's modulus in the range of approximately 50 to 106 grams per denier; (d) a thermal stress (150°C., 8 min.) in the range 0.10 to 0.60 grams per denier; (e) a stress ratio (initial load 3 grams/denier for 2 min.) in the range of 0.85 to 0.91; and (f) a shrinkage under dry heat of approximately 150°C. for 30 minutes in the range of approximately 4.0% to 9.0%. The coated and drawn cord 10 delivered from oven 25 preferably has a Young's modulus of greater than 110 grams per Denier, a thermal stress (150°C. for 8 min.) of greater than 0.48 grams per denier, a stress ratio (initial load of 3 grams per denier) of greater than approximately 0.925, and a shrinkage under dry heat (150°C. for 30 min.) of less than approximately 3.5%. The tensile cord further has, in accordance with the present invention, the following characteristics after it has been incorporated into a power transmission belt by vulcanization of the belt material. The cord 10, when removed from the vulcanized belt, preferably has a Young's modulus of greater than approximately 120 grams per denier, a thermal stress (150°C. for 8 min.) of greater than 0.490 grams per denier, and a stress ratio (initial toad of 3 grams per denier) of greater than approximately 0.920. The Young's modulus characteristics set forth above were determined in accordance with the Japanese testing procedure identified as JIS L-1017 (1983). This testing procedure is similar to the U.S. standard ASTM D 885 M (1986) procedure, except that the initial modulus of grams per denier is equal to where P is measured in grams per denier and is the load at the point of maximum tangency, as shown in Figure 7, D is the nominal linear density, L is the length of the specimen in millimeters, and L' is the length of TH. The thermal stress is determined by use of the Japanese standard test JIS L 1017 (1983), which is similar to the U.S. Standard ASTM D 885 M (1986), but wherein the shrinkage force (grams per denier) is equal to (F-F') + D , where F is the maximum force developed by the specimen in grams, F' is the pre-tension load in grams, and D is the nominal linear density. The shrinkage under dry heat is determined by means of the Japanese standard JIS L 1017 (1983), which is similar to the U.S. standard ASTM D 885 M (1986), but wherein the shrinkage in percent equals L-L' + L x 100 , where L is the initial length of the specimen and L' is the final length of the specimen. The stress ratio determination is illustrated in Figure 4. As shown therein, a load of 3 grams/denier is imposed on the cord for 2 minutes. The load is then removed for 1 minute. This sequence is repeated two additional times and the value of Ft at point 27 is determined 2 minutes after the weight is removed. The power transmission efficiency of a power transmission belt is related to the Young's modulus and stress ratio, and for optimum operating conditions, it is preferred that the Young's modulus and the stress ratio be high. In the present invention, the Young's modulus is preferably greater than 120 grams/denier, and the stress ratio is greater than approximately 0.920. Further more specifically, it is desirable that the stress ratio be greater than 0.93. A high thermal stress causes the belt to more tightly engage the pulley as a result of heat generated in the driving operation of the belt. Thus, as indicated above, the thermal stress, in accordance with the invention, of the tensile cords is preferably greater than 0.490 grams/denier at 150°C. and more specifically, is preferably greater than 0.52 grams/denier. It is further desirable that the shrinkage under dry heat of the tensile cord prior to its use in the belt be low and, in the illustrated embodiment, the shrinkage under dry heat is preferably less than 3.5% and more specifically, less than approximately 3.3%. The characteristics are further illustrated in graphic form in Figures 5 and 6. In Figure 5, the relationship between the strength and strain of the tensile cords is illustrated. This relationship is shown for the conventional cord and cord No. 3 of Table 2. Curve 31 illustrates the strength/strain characteristics of the conventional tensile cord, and curve 28 illustrates the characteristics thereof after being subjected to a heat treatment in accordance with the invention. Curve 30 illustrates the strength/strain characteristics of cord No. 3 before the heat treatment, and curve 29 illustrates the characteristics after subjection to the heat treatment of the present invention. Figure 6 illustrates the improved useful life of belt No. 3 as compared to that of the comparison belt. Thus, Figure 6 shows the relationship between the axial load index of these two belts and the running time in hours, with the axial load measured after the conventional belt was run for 1 hours shown as 100 on the index scale. In running this test, the V-belts were entrained between a drive pulley having a pitch diameter of 107 mm. and a driven pulley having a pitch diameter of 100 mm., with a tension pulley having a pitch diameter of 70 mm. being engaged with the belts to maintain the tension thereof. The belts had a pitch line length of 1010 mm. originally, and the load imposed on the driven pulley on 8 ps. The driver pulley was run at 3600 rpm, and the ambient temperature was 25°C. The axial load imposed on the tension pulley was measured at the indicated times. Tables 1 and 2 set forth below illustrate the characteristics of six different tensile cords made in accordance with the invention in comparison with a conventional tensile cord. Thus, the tensile cord manufacture of the present invention provides a highly improved tensile cord for use in power transmission belts having substantially improved operating characteristics and maintained dimensional stability so as to provide long troublefree life of the power transmission belt in which it is incorporated. The improved results are obtained by a novel heat setting/drawing of a polyester cord having preselected characteristics. The characteristics of the tensile cord material before treatment and the characteristics thereof after being incorporated in a power transmission belt are preferably caused to be within parameters discussed above so as to provide the improved manufacture.

Claims: A method of forming a tensile cord (10) for use in a power transmission belt (11,12), comprising the steps of: providing a cord (13) of twisted polyester multi-filaments having an ultimate viscosity of less than 0.90; applying adhesive to the cord (13) wherein the application of adhesive to the cord (13) comprises the steps of applying isocyanate or epoxy (14) to the cord (13), drying the cord (13) by heating the cord (13) to a temperature in the range of approximately 60°C to 100°C, and impregnating the isocyanate or epoxy coated and dried cord (13) with resorcinol-formalin-latex solution (17); and drawing and setting the adhesive-coated cord (13) in two steps at a temperature in the range of approximately 220°C to 240°C with a first draw ratio in the range of 1% to 1.7% and a second draw ratio in the range of 1% to 1.7%. A method according to claim 1 including a final step of passing the adhesive-coated, drawn and set cord (13) through a bath of rubber cement (24) and heating the rubber-coated cord (13) at a temperature in the range of approximately 100°C to 200°C. A method according to claim 1 wherein the initial twisted multi-filament polyester cord (13) has an original ultimate viscosity in the range of approximately 0.75 to 0.90, an original double refraction ratio in the range of approximately 0.16 to 0.195, an original Young's modulus in the range of approximately 50 grams/denier to 106 grams/denier, an original thermal stress in the range of approximately 0.10 to 0.60 grams/denier when heated to a temperature of 150°C for 8 minutes, an original stress ratio of approximately 0.85 to 0.91 with a load of 3 grams/denier applied for 2 minutes, and an original shrinkage under dry heat of approximately 4.0% to 9.0% when heated to a temperature of 150°C for 30 minutes. A method according to claim 1 wherein the adhesive-coated, drawn and set cord (10) has a Young's modulus of at least 110 grams/denier, a thermal stress of more than 0.48 grams/denier when heated to 150°C for 8 minutes, a stress ratio of more than 0.925 with a load of 3 grams/denier, and a shrinkage under dry heat of less than 3.5% when heated to 150°C for 30 minutes. A method according to claim 1 wherein the adhesive-coated, drawn and set cord (10) has a thermal stress of more than 0.52 grams/denier when heated to 150°C for 8 minutes, a Young's modulus of more than 120 grams/denier, a shrinkage under dry heat of less than 3.3% when heated to 150°C for 30 minutes, and a stress ratio of more than 0.93 with a load of 3 grams/denier. A power transmission belt tensile cord (10) formed by a method according to any of claims 1 to 5, after it has been incorporated into a rubber power transmission belt (11, 12) by vulcanization of the belt material, the cord (10), when removed from the vulcanized belt (11, 12), having a Young's modulus of more than 120 grams/denier, a thermal stress greater than 0.49 grams/denier when heated to 150°C for 8 minutes, a stress ratio of greater than 0.925, and less than 3.5% shrinkage under dry heat when heated to 150°C.

Inventors: KITAHAMA KOJI, MASHIMO SATOSHI, TAKEDA KAZUHIRO, YAMAGUCHI TOSHIO, KITAHAMA, KOJI, MASHIMO, SATOSHI, TAKEDA, KAZUHIRO, YAMAGUCHI, TOSHIO
Applicants: MITSUBOSHI BELTING LTD, MITSUBOSHI BELTING LTD.
Application year: 1987
Main classification: F16G5
IPC codes: F16G1, F16G5