DESCRIPTION STEEL CORD CONVEYOR BELT FIELD OF THE INVENTION [0001] 5 The present invention is a technique relating to a conveyor belt with steel cords embedded in it as cores. BACKGROUND OF THE INVENTION [0002] Any discussion of the prior art throughout the specification should in no 10 way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. [0002A] Conventionally, in order to prevent global warming, various attempts have been made to reduce the amount of CO 2 emission. The attempts include the 15 technical development of fuel consumption improvement for the reduction of automotive exhaust gas. The attempts also include energy saving for buildings, factories, plants, and other facilities. As part of such grappling, an attempt has been made to make energy savings for belt conveyors, that is, to reduce the power consumption for the conveyors by lowering the running resistance to the 20 belts of the conveyors. Specifically, a conveyor belt has been put to practical use in which core canvas is embedded, and in which reinforcing canvas is embedded on one or each side of the core canvas at a widthwise central portion of the belt so as to increase the rigidity of this portion (refer to Patent Document 1, for example). When this belt passes over a three-point carrier roller, the 25 central portion of the belt is kept roughly flat without bending. This lowers the run-over resistance of the carrier roller, thereby lowering the running resistance. Patent Document 1 proposes the use of a rubber material having a low loss factor in viscoelastic characteristics and a low coefficient of friction in order to further lower the run-over resistance of the carrier roller. 30 Patent Document 1:JP 2004-18202 A (Page 3, FIGS. 1 and 2) 2 DISCLOSURE OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION [0003] Patent Document 1 discloses a technique for a conveyor belt with canvas embedded in it as a core, but there are demands for lowering the running resistance to conveyor belts with steel cords embedded in them as cores. [0004] The problem with the present invention is to provide a steel cord conveyor belt capable of saving energy by lowering the running resistance to the belt by lowering the run-over resistance of a carrier roller, which forms a large proportion of the running resistance. MEANS FOR SOLVING THE PROBLEM [0005] The steel cord conveyor belt described in claim 1 is characterized in: (1) that the strength Kn (N/mm) of the belt and the sectional area A (mm 2 ) found from the nominal diameter of the steel cords of the belt satisfy 90 Kn/A, 150; (2) that the under cover rubber of the belt is low loss factor rubber having a loss factor of 0.08 or less; and (3) that a reinforcing fabric is embedded in the under cover rubber. [0006] As stated above at (1), the strength Kn of the steel cord conveyor belt of claim 1 and the sectional area A satisfy 90LKn/A 150. This enables the relationship between the nominal cord diameter and the cord pitch of the belt to lower the run-over resistance of a carrier roller, and to be suitable for jointing. [0007] 3 Specifically, when the steel cord conveyor belt passes over the carrier roller, the under cover rubber deforms compressively. Because the under cover rubber is viscoelastic, the deformation causes an energy loss. The energy loss arouses the run-over resistance of the carrier roller to the belt. [0008] The satisfaction of 90 Kn/A reduces the nominal cord diameter and the cord pitch, thereby making it possible to disperse the stress concentrating between the steel cords and the carrier roller. The dispersed stress is relaxed. The stress relaxation lowers the contact pressure between the under cover rubber and the carrier roller, thereby restraining the deformation of the rubber. As a result, the run-over resistance of the carrier roller decreases. [0009] The satisfaction of Kn/A; 150 enables the nominal cord diameter and the cord pitch to avoid having a relation to each other that makes the jointing substantially impossible. Specifically, if Kn/A were greater than 150, the nominal cord diameter and the cord pitch would be too small, so that the distance between the steel cords could not be secured at the joint. As a result, the jointing would be impossible. Alternatively, the arrangement of the steel cords at the joint would be very complex for sufficient strength. This would lengthen the joint, so that the jointing would be substantially impossible. From the viewpoints of jointing operability and belt saving, it is preferable that the joint should have sufficient jointing strength and be short. From the viewpoint of jointing operability, it is preferable that the joint be simple in structure. [0010] The relationship between the nominal cord diameter and the cord pitch is defined by Kn/A (belt strength / sectional area) for the following reason. The belt strength Kn can be found from the tensile strength Fbs of each of the steel cords, the width b of the steel cord conveyor belt, the number n of steel cords embedded in the belt, and the cord pitch p by the following numerical expression. 4 [0011] Kn = Fbs X n/b ----- (1) The inverse b/n of n/b in the numerical expression (I) is equivalent to the cord pitch p although there is a slight error because the steel cord conveyor belt has edges. The belt strength Kn can also be found by the following numerical expression (11). [0012] Kn = Fbs/p ---- (II) If the tensile strength Fbs of each of the steel cords is constant, that is, if the cords are equal in diameter, the specified belt strength Kn can be secured by varying the cord pitch p. If the cord pitch p is constant, the tensile strength Fbs is proportional to the sectional area A of the steel cords. In this case, the specified belt strength Kn can be obtained by varying the sectional area A. If the steel cords differ in wire structure, they differ in real sectional area and tensile strength Fbs even for the equal nominal cord diameter. However, because the difference in wire structure is considered to exert little influence, the diameter of the steel cords is specified by nominal cord diameter. [0013] The relationship between Kn (belt strength) and A/p (sectional area / cord pitch) in JIS (Japanese Industrial Standards) and ISO Standards is shown in TABLES 1 (JIS) and 2 (ISO Standards). FIG. 1 is a scatter diagram of the relationship shown in TABLES 1 and 2. In FIG. 1, the axes of ordinate and abscissa represent A/p (sectional area / cord pitch) and Kn (belt strength) respectively. [0014] The scatter diagram of FIG. 1 shows that Kn (belt strength) and A/p (sectional area / cord pitch) have a nearly linear relationship with and are proportional to each other. Therefore, considering that the specifications for a belt are designed generally on the basis of belt strength, the relationship between the nominal cord diameter and the cord pitch p is defined by Kn/A (belt strength / 5 sectional area). The reason for the definition is that, if the relationship were defined by A/Kn (sectional area / belt strength), for example, this value would be a decimal, which is hard to handle. [0015] [TABLE 1] BELT STRENGTH CORD PITCH [mm] CORD DIAMETER [mm] CORD SECTIONAL AREA [N/mm] / CORD PITCH 500 10 2.8 0.62 630 10 3.0 0.71 800 10 3.5 0.96 1,000 12 4.0 1.05 1,250 12 4.5 1.32 1,600 12 5.0 1.64 2,000 12 6.0 2.36 2,250 12 6.3 2.60 2,500 15 7.2 2.71 2,800 15 7.6 3.02 3,150 15 8.1 3.43 3,500 15 8.6 3.87 4,000 15 9.2 4.43 4,500 16 10.1 5.00 5,000 16 10.6 5.51 5,400 17 11.5 6.11 6 [0016] [TABLE 2] BELT STRENGTH CORD PITCH [mm] CORD DIAMETER [mm] CORD SECTIONAL AREA [N/mm] / CORD PITCH 1,000 10 3.7 1.07 1,250 10 4.2 1.38 1,400 10 4.4 1.52 1,600 10 4.7 1.73 1,800 10 4.9 1.88 2,000 10 5.2 2.12 2,250 10 5.6 2.46 2.500 10 5.8 2.64 2,800 12 6.7 2.94 3,150 12 7.2 3.39 3,500 12 7.4 3.58 As stated above at (2), the under cover rubber is low loss factor rubber having a loss factor of 0.08 or less, in addition to the structure (1). This makes it possible to restrain the energy absorption into the under cover rubber, thereby making it possible to reduce the energy loss caused by the deformation of the rubber. As a result, the run-over resistance of the carrier roller can be lowered greatly. [0017] The loss factor (loss tangent), which is found in accordance with the general guidelines of JIS (Japanese Industrial Standards) K6394 on (how to find the dynamic properties of) vulcanized rubber and thermoplastic rubber, is 0.08 or less. The low loss factor rubber does not easily absorb energy. The loss factor (tano) can be found from storage elastic modulus E' and loss elastic modulus E" by tana = E"/E'. TABLE 3 shows the conditions for finding the loss factor. 7 [0018] [TABLE 3] TEST CLASSIFICATION (NON-RESONANT) FORCED VIBRATION TECHNIQUE TEST APPARATUS SMALL TEST APPARATUS DEFORMATION METHOD STRETCHING SPECIMEN FORM STRIP MEAN STRAIN 5 % FREQUENCY 1 Hz TEST TEMPERATURE 60t In addition, as stated above at (3), the reinforcing fabric is embedded in the under cover rubber. This makes it possible to disperse the stress concentrating in the under cover rubber between the steel cords and the carrier roller. The dispersed stress is relaxed. The stress relaxation lowers the contact pressure between the under cover rubber and the carrier roller. The pressure lowering restrains the deformation of the under cover rubber, thereby reducing the energy loss. The loss reduction lowers the run-over resistance of the carrier roller. [0019] It is preferable that the reinforcing fabric should bend or curve easily around the trough angle of a trough type carrier roller when the steel cord conveyor belt passes over the roller. For example, it is preferable that the reinforcing fabric be made of flexible and highly stretchable nylon fiber. [0020] The steel cord conveyor belt described in claim 2 is characterized in that the reinforcing fabric is embedded over the whole width of the belt. [0021] The stress concentrating between the steel cords and the carrier roller of the steel cord conveyor belt of claim 2 can be dispersed over the whole width of the under cover rubber so as to be relaxed. The stress relaxation improves the effect of lowering the contact pressure between the under cover rubber and the carrier roller. 8 [00221 The steel cord conveyor belt described in claim 3 is characterized in that the strength Kn of the belt and the sectional area A satisfy 100 Kn/A 130. The relationship between the nominal cord diameter and cord pitch of the belt of 5 claim 2 enables jointing with a length equivalent to the conventional length, while the relationship lowers the run-over resistance of the carrier roller. EFFECTS OF THE INVENTION [0023] The steel cord conveyor belt of one embodiment of the invention can 10 greatly lower the running resistance to it in comparison with the conventional steel cord conveyor belts, without complicating the jointing operation. This makes it possible to drive the belt by low motive power, enabling operation at a great saving of energy. Because the running resistance is lowered, the safety factor of the belt can be increased even though the belt is conventional in 15 strength. Because the steel cords are small in diameter, the whole thickness of the belt can be reduced, so that the use of the rubber material can be saved. [0024] The steel cord conveyor belt of another embodiment of the invention is improved in its effect of lowering the contact pressure between the under cover 20 rubber and the carrier roller. Accordingly, the belt can run with low resistance, so that its energy saving effect can be improved. [0025] While the steel cord conveyor belt of another embodiment of the invention has jointing operability equivalent to that of the conventional steel cord conveyor 25 belts, it brings about an energy saving effect, a safety factor improvement and a rubber material saving effect that are similar to those brought about by the steel cord conveyor belt of the above described embodiments. 9 BEST MODE OF CARRYING OUT THE INVENTION [0026] Steel cord conveyor belts embodying the present invention will be described below with reference to FIGS. 1 - 8. [0027] As shown in FIG. 2(a), which is a transverse section, a steel cord conveyor belt 1 has steel cords 2 embedded in it along its whole length. The cords 2 are laid in the longitudinal directions X in which the belt 1 extends. The cords 2 are spaced at a constant pitch in the transverse directions Y in which the transverse section of the belt 1 extends. The cords 2 are sandwiched between upper cover rubber 1 a and under cover rubber 1 b, which is the low loss factor rubber compounded as shown in TABLE 4. [0028] [TABLE 4] COMMON RUBBER (WT %) LOW LOSS FACTOR RUBBER (WT %) NR 100 60 BR -- 40 FEF (N550) -- 20 HAF(N330) 50 25 OIL 10 5 STEARIC ACID 1 1 ZINC OXIDE 5 5 AGE RESISTOR 2 2 VULCANIZATION ACCELERATOR 1 1 SULFUR 1.5 2.5 The low loss factor rubber has a loss factor tana of 0.05, which is found in accordance with the general guidelines of JIS (Japanese Industrial Standard) K6394 on (how to find the dynamic properties of) vulcanized rubber and thermoplastic rubber. TABLE 5 shows the conditions for finding the loss factor tanm. 10 [0029] [TABLE 5] TEST CLASSIFICATION (NON-RESONANT) FORCED VIBRATION TECHNIQUE TEST APPARATUS SMALL TEST APPARATUS DEFORMING STRETCHING SPECIMEN SHAPE STRIP MEAN STRAIN 5% FREQUENCY 1 Hz TEST TEMPERATURE 60t The low loss factor rubber is a type of rubber that does not easily absorb energy. Accordingly, this rubber reduces the energy loss caused when the conveyor belt 1 runs over a carrier roller. This makes it possible to lower the run-over resistance. [0030] A reinforcing fabric 3 made of nylon fiber is embedded in the intermediate layer of the under cover rubber 1 b over the whole width of the rubber. Stress concentrates in the rubber 1 b between the steel cords 2 and the carrier roller. The fabric 3 disperses the concentrated stress over the whole width of the rubber 1 b, thereby relaxing the stress. The stress relaxation lowers the contact pressure between the rubber 1 b and the carrier roller. The pressure lowering restrains the deformation of the under cover rubber 1 b. The deformation restraint reduces the energy loss caused when the conveyor belt 1 runs over the carrier roller. The energy loss reduction lowers the run-over resistance. Because nylon fiber is flexible and highly stretchable, the belt 1 bends or curves easily around the trough angle of a trough type carrier roller when the belt runs over the roller. Because nylon fiber has high strength, the belt 1 is highly durable as well. [0031] The strength Kn of the conveyor belt 1 and the sectional area A of the steel cords 2, which is found from their nominal diameter, satisfy 905Kn/A;51 50. If 11 this condition is satisfied by Kn/A (belt strength / sectional area), it is possible to reduce the nominal cord diameter and the cord pitch within ranges which are suitable for the joint, and in which the run-over resistance of the carrier roller decreases. [0032] Specifically, when the conveyor belt 1 passes over the carrier roller, the under cover rubber 1 b deforms compressively. Because, generally, the under cover rubber 1 b is viscoelastic, the deformation causes an energy loss. The energy loss arouses run-over resistance when the belt 1 runs over the carrier roller. As a result, the energy loss constitutes a factor of the running resistance to the belt 1. Because the belt strength Kn and the sectional area A satisfy 90;Kn/A5 150, the stress concentrating between the carrier roller and the steel cords 2 can be dispersed to be relaxed. This lowers the contact pressure between the under cover rubber 1 b and the carrier roller, thereby restraining the deformation of the under cover rubber 1 b. As a result, the run-over resistance of the carrier roller decreases. [0033] If Kn/A were less than 90, the nominal cord diameter and the cord pitch could not be small for energy saving. If Kn/A were greater than 150, the nominal cord diameter and the cord pitch would be so small that the distance between the steel cords 2 could not be secured at the joint. This would make the jointing impossible. Alternatively, this would require a three or more step overlap joint for sufficient strength. As a result, the arrangement of the steel cords 2 would be very complex, and the joint would be long, so that the jointing would be substantially impossible. [0034] A description will be provided below of the results of running tests in which the resistance exerted to the conveyor belt 1 when the belt ran over the carrier roller was measured. 12 [0035] As shown in FIG. 2(b), the test apparatus 10 used for the running tests has a drive pulley 11 and an idler pulley 12, which have a diameter of 500 mm and are spaced by 3,000 mm. An endless conveyor belt 1 runs over the pulleys 11 and 1 2. A flat carrier roller 13 is positioned on the carrier side and fixed on a carriage 14. The carriage 14 can reciprocate along the conveyor belt 1, which runs over the pulleys. A load cell 15 is positioned on the side of the carriage 14 that is adjacent to the drive pulley 11. This makes it possible to measure the load applied to the carriage 14 when the carriage is pushed in the direction in which the belt 1 runs. A load device 16 for applying a vertical load on the belt 1 is positioned over the carrier roller 13. Rollers 17 are positioned on the side of the load device 16 that comes into contact with the belt 1. This makes it possible to apply the vertical load without affecting the running of the belt 1. [0036] The conveyor belt 1, which had a width of 500 mm, was fitted on the test apparatus 1. While the belt 1 was running at a belt speed of 170 m/min, a vertical load of 5 kN/m (widthwise per meter) was applied, and the load on the carriage 14 was measured to find the resistance to the belt 1 running over the carrier roller 13. The running tests were carried out on four conveyor belts (practical examples 1 - 4) embodying the present invention, another conveyor belt (comparative example 2) according to the standard specifications, and five other conveyor belts (comparative examples 1 and 3 - 6) according to specifications some of which are modifications to the standard specifications. The practical examples 1 - 4 differed in cord pitch and nominal cord diameter. The thickness of the under cover rubber of each of the practical and comparative examples was set according to the diameter of the standard steel cord. In expectation of lowered running resistance, the strength of the conveyor belts of the comparative examples 4 - 6 and the practical examples was 2,250 N/mm, which is lower by one rank than the standard 13 specifications. TABLE 6 shows the specifications of the practical and comparative conveyor belts. [0037] [TABLE 6] COVER TYPE OF BELT CORD CORD BELT CANVAS JOINT RUNNING RUBBER UNDER STRENGTH DIAMETER PITCH STRENGTH REIN- LENGTH RESISTANCE THICKNESS COVER [N/mm] [mm] [mm] / CORD FORCE- Em] INDEX [mm] RUBBER SECTIONAL MENT UPPER X AREA UNDER COMPARATIVE 7X6 COMMON 2,500 6.7 17 70.9 NO 1.1 103 EXAMPLE 1 COMPARATIVE 7X6 COMMON 2,500 6.3 15 80.2 NO 1.5 100 EXAMPLE 2 COMPARATIVE 7X6 LOW-LOSS 2,500 6.3 15 80.2 NO 1.5 50 EXAMPLE 3 COMPARATIVE 7 X 5 LOW-LOSS 2,250 5.3 12 102.0 NO 1.5 47 EXAMPLE 4 COMPARATIVE 7X5 LOW-LOSS 2,250 4.7 10 129.7 NO 1.5 43 EXAMPLE 5 COMPARATIVE 7 X 5 LOW-LOSS 2,250 4.3 8 154.9 YES IM- EXAMPLE 6 POSSIBLE PRACTICAL 7 X 5 LOW-LOSS 2,250 5.3 12 102.0 YES 1.5 39 EXAMPLE 1 PRACTICAL 7X5 LOW-LOSS 2,250 5.0 10 114.6 YES 1.5 38 EXAMPLE 2 PRACTICAL 7X5 LOW-LOSS 2,250 4.7 10 129.7 YES 1.5 36 EXAMPLE 3 PRACTICAL 7 X 5 LOW-LOSS 2,250 4.5 9 141.5 YES 2.0 35 EXAMPLE 4 14 In TABLE 6, the common type of under cover rubber represents the type of rubber the main component of which is the natural rubber used for general-purpose common conveyor belts, and which has a loss factor tan o of 0.15. The compounding of this rubber is shown in TABLE 1. The measurement results of the running tests are described as running resistance indexes in TABLE 6. The running resistance indexes are shown on the basis of the load measured in the running test for the conveyor belt of the comparative example 2, with this load assumed as 100. [0038] As compared with the comparative example 2, the comparative example 1 is large in nominal cord diameter and cord pitch and has a high running resistance index. Therefore, it is inferred that increases in the nominal cord diameters and the cord pitches are factors for an increase in the run-over resistance of the carrier roller. [0039] The comparative examples 2 and 3 are equal in nominal cord diameter and cord pitch. However, as compared with the comparative example 2, the comparative example 3 has a half running resistance index. Therefore, it is inferred that the use of low loss factor rubber for the under cover rubber greatly lowers the run-over resistance of the carrier roller. [0040] As compared with the comparative example 4, the comparative example 5 has a low running resistance index. Therefore, it is inferred that the run-over resistance of the carrier roller decreases with the nominal cord diameters and the cord pitches. [0041] The comparative example 6 is too small in nominal cord diameter and cord pitch and requires three or more step overlap jointing. Accordingly, the structure 15 and jointing of this example are complex and not practicable. Therefore, no running test was carried out for this example. [0042] As compared with the comparative example 4, the practical example 1 has a low running resistance index. Therefore, it is inferred that the embedding of a reinforcing fabric in the under cover rubber is effective in lowering the run-over resistance of the carrier roller. [0043] The comparison of the practical examples 1 - 4 makes it inferred that a lower running resistance is exerted to a conveyor belt smaller in nominal cord diameter and cord pitch. As compared with the comparative example 2, the running resistance indexes of the conveyor belts of all the practical examples are 60 or fewer %. Therefore, it is inferred that the conveyor belts of the practical examples 1 - 4 greatly lower the running resistance to them. [0044] The study of these comparisons and the relationships between Kn/A (belt strength / sectional area) and the running resistance indexes makes it inferred that, if Kn/A ranges between 90 and 150, the nominal cord diameters and the cord pitches are sizes that greatly lower the run-over resistance of the carrier roller, that is, enable the conveyor belts to run with very low resistance. If the joint length of the comparative example 2 is assumed as a standard, these sizes practically enable jointing. In particular, if Kn/A ranges between 100 and 130, jointing is possible with a joint length equivalent to that of the comparative example 2. This is suitable because this makes it possible to keep the joint structure from being complex and the operability from lowering. [0045] The results of an FEM (finite element method) analysis will be described below. 1) Conveyor Belt Specifications 16 The conveyor belts of the practical examples 1 - 4 and comparative examples 1 - 6, which were used in the foregoing running tests. 2) Working Conditions Conveying capacity: 1,200 t/h Belt speed: 170 m/min Specific gravity: 1.6 Tension: 112.5 kN (equivalent to 225 kN over the whole width on a model of a 1/2 belt width) S. F. = 10 (set from the belt strength of 2,500 N/mm) Carrier roller diameter: 114.3 mm Carrier roller pitch: 1.5 m 3) Analysis Model FIG. 3 shows an analysis model, which has a three-point carrier roller 101 and a conveyor belt 102 running over the roller. The centerline of the belt 102 is cut along the length of the belt. Steel cords 103 are shown in FIG. 3. 4) Output Item The total value of the strain energy of one layer of the rubber element from the centers of the steel cords to the surface of the under cover rubber was output. The total value multiplied by a loss factor is loss energy, which has been confirmed correlated with running resistance. The loss energy of the comparative example 2 is shown as an index of 100. 5) Analysis Results TABLE 7 shows the results of the FEM analysis. 17 [0046] [TABLE 7] COVER TYPE OF BELT CORD CORD BELT CANVAS LOSS RUBBER UNDER STRENGTH DIAMETER PITCH STRENGTH REIN- ENERGY THICKNESS COVER [N/mm] [mm] [mm] / CORD FORCE- INDEX (mm] RUBBER SECTIONAL MENT UPPER X AREA UNDER COMPARATIVE 7 X 6 COMMON 2,500 6.7 17 70.9 NO 106 EXAMPLE 1 COMPARATIVE 7 X 6 COMMON 2,SOO 6.3 is 80.2 NO 100 EXAMPLE 2 COMPARATIVE 7 X 6 LOW-LOSS 2,500 6.3 15 80.2 NO 32 EXAMPLE 3 COMPARATIVE 7 X 5 LOW-LOSS 2,250 5.3 12 102.0 NO 26 EXAMPLE 4 COMPARATIVE 7X5 LOW-LOSS 2,250 4.7 10 129.7 NO 24 EXAMPLE 5 COMPARATIVE 7 X 5 LOW-LOSS 2,250 4.3 8 154.9 YES 13 EXAMPLE 6 PRACTICAL 7X5 LOW-LOSS 2,250 5.3 12 102.0 YES 18 EXAMPLE 1 PRACTICAL 7 X 5 LOW-LOSS 2,250 5.0 10 114.6 YES 17 EXAMPLE 2 PRACTICAL 7X5 LOW-LOSS 2,250 4.7 10 129.7 YES 15 EXAMPLE 3 PRACTICAL 7 X 5 LOW-LOSS 2,250 4.5 9 141.5 YES 14 EXAMPLE 4 As compared with the comparative example 2, the comparative example 1 is large in nominal cord diameter and cord pitch and has a high loss energy factor. 18 Charts of the Mises stress distribution on cross sections of the conveyor belts show that the red and yellow areas and the areas between them were wider in the comparative example 1 (FIG. 4(a)) than in the comparative example 2 (FIG. 4(b)), and that higher stress concentration occurred in the comparative example 1 than in the comparative example 2. Higher stresses are indicated in red, yellow, green and blue in that order. Therefore, it is inferred that increases in the nominal cord diameters and the cord pitches are factors for an increase in the run-over resistance of the carrier roller. [0047] There is no difference between the Mises stress distribution charts for the comparative examples 2 and 3 because the stress-strain characteristics, which are the input conditions for the FEM analysis, of the comparative examples 2 (FIG. 4(b)) and 3 (FIG. 5(a)) are nearly identical. However, because of the difference in loss factor that is made by the difference in the under cover rubber, the comparative example 3 has a loss energy index that is about 1/3 as compared with the comparative example 2. Therefore, because the nominal cord diameters and cord pitches of these examples are equal, it is inferred that the use of low loss factor rubber for the under cover rubber greatly lowers the run-over resistance of the carrier roller. [0048] As compared with the comparative example 4, the comparative example 5 has a low loss energy index. The Mises stress distribution charts show that the red and yellow areas and the areas between them were narrower in the comparative example 5 (FIG. 6(a)) than in the comparative example 4 (FIG. 5(b)), and that the stress concentrating in the comparative example 5 is relaxed as compared with the comparative example 4. Therefore, it is expected that the run-over resistance of the carrier roller decreases with the nominal cord diameters and the cord pitches. [0049] 19 The comparative example 6 has the lowest loss energy index. The Mises stress distribution chart (FIG. 6(b)) for the comparative example 6 does not show a red area, a yellow area, and areas between the read and yellow areas. This means that the stress concentrating in the comparative example 6 is greatly relaxed. However, because the comparative example 6 is too small in nominal cord diameter and cord pitch, this example requires three or more step overlap jointing. Therefore, the structure and jointing of the comparative example 6 are complex, so that this example is not practical. [0050] As compared with the comparative example 4, the practical example 1 has a low loss energy index. The Mises stress distribution chart for the practical example 1 (FIG. 7(a)) substantially does not show a red area, a yellow area, and areas between the read and yellow areas. The Mises stress distribution charts for the practical example 1 and comparative example 4 (FIG. 5(b)) show that the stress concentrating in this practical example is relaxed as compared with this comparative example. Therefore, it is inferred that the embedding of a reinforcing fabric in the under cover rubber lowers the run-over resistance of the carrier roller. [0051] The comparison of the practical examples 1 - 4 makes it apparent that the loss energy indexes decrease with the nominal cord diameters and the cord pitches. The Mises stress distribution charts (FIGS. 7(a), 7(b), 8(a) and 8(b)) for the practical examples show that, for the smaller nominal cord diameters and cord pitches, the green and blue areas and the areas between them are wider, although the differences are small, and the stress concentration is more relaxed. In other words, it is inferable that the run-over resistance of the carrier roller decreases with the nominal cord diameters and the cord pitches. The loss energy indexes of the conveyor belts of all the practical examples were 60 or fewer % as compared with the comparative example 2. Therefore, it is inferred that the conveyor belts of the practical examples 1 - 4 greatly reduce the loss energy indexes. 20 [0052] The study of these comparisons, the relationships between Kn/A (belt strength / sectional area) and the loss energy indexes, and the Mises stress distribution charts makes it inferred that, if Kn/A ranges between 90 and 150, the nominal cord diameters and the cord pitches are sizes that greatly reduce the run-over resistance of the carrier roller, that is, make the running resistance very low. If the joint length of the comparative example 2 is assumed as a standard, these sizes practically enable jointing. In particular, if Kn/A ranges between 100 and 130, jointing is possible with a joint length equivalent to that of the comparative example 2. This is suitable because this makes it possible to keep the joint structure from being complex and the operability from lowering. BRIEF DESCRIPTION OF THE DRAWINGS [0053] [FIG. 1] A scatter diagram showing the relationships between nominal cord sectional area and cord pitch and between nominal cord sectional area and belt strength on the basis of the values specified by JIS and ISO Standards. [FIG. 2] (a) A transverse section of a steel cord conveyor belt embodying the present invention. (b) A conceptual diagram of the running test machine referred to at BEST MODE OF CARRYING OUT THE INVENTION. [FIG. 3] An analytic model for stress distribution analysis. [FIG. 4] (a) A Mises stress distribution chart for a comparative example 1. (b) A Mises stress distribution chart for a comparative example 2. [FIG. 5] (a) A Mises stress distribution chart for a comparative example 3. (b) A Mises stress distribution chart for a comparative example 4. [FIG. 6] (a) A Mises stress distribution chart for a comparative example 5. (b) A Mises stress distribution chart for a comparative example 6. [FIG. 7] (a) A Mises stress distribution chart for a practical example 1. (b) A Mises stress distribution chart for a practical example 2. 21 [FIG. 8] (a) A Mises stress distribution chart for a practical example 3. (b) A Mises stress distribution chart for a practical example 4. LEGENDS OF REFERENCE NUMERALS [0054] 1 steel cord conveyor belt (conveyor belt) 1 a upper cover rubber 1 b under cover rubber 2 steel cords 3 reinforcing fabric 22