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Power transmission shaftRelated Patent Categories: Metal Treatment, StockPower transmission shaft description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070251606, Power transmission shaft. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This is a Divisional Application which claims the benefit of Pending U.S. patent application Ser. No. 10/153,865, filed May 24, 2002 which also claims the benefit of priority from Japanese Patent Application Nos. 2001-158691 filed May 28, 2001; 2001-158766 filed May 28, 2001. The disclosures of the prior applications are hereby incorporated herein in their entirety by reference. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to a power transmission shaft to be used as, for example, a drive shaft or a propeller shaft, which constitutes a part of a power transmission system in an automobile. [0004] 2. Description of the Related Art [0005] In general, there are several kinds of transmission shafts that constitute a power transmission system of an automobile. The shafts include a drive shaft for connecting between an engine and a wheel-bearing device, a propeller shaft for transmitting power from a transmission to reduction gears, and so on. Each of these shafts has a coupling member such as a spline on the shaft-end. The power transmission shafts may be broadly classified in the group of solid shafts made of solid bars and the other group of hollow shafts made of steel pipes or the like, according to their basic structures. [0006] Conventionally, solid shafts have been used as power transmission shafts for automobiles. In recent years, for responding to the needs for higher function of automobiles, the sound insulating properties of a cabin to keep quiet, and the like, there are increasing demands of providing a power transmission shaft with various kinds of characteristic features, such as light weight, compactness, and comfortability against NVH (noise, vibration, and harshness), in addition to strength and durability. In addition, there is also required to improve the torsional rigidity of shafts for increasing the controllability and direct feeling of automobile at the time of start. In this case, for improving the torsional rigidity, there is an idea of increasing the diameter of the shaft. However, it will effect an increase in costs because of increasing the weight and the cutting amount of a coupling portion. In addition to the above demands, there is a need for adjusting the natural frequency of automobile for avoiding the noise produced by a resonance between vibrations of an engine and a shaft while the automobile runs. For adjusting the natural frequency, there is an idea of attaching a dumper or the like on the power transmission shaft. However, it will lead to an increase in costs because of increasing the number of structural components and the number of assembling steps in the manufacturing process. [0007] As a consequence of considering the above demands in terms of functions, there is an increasing tendency to make greater use of hollow shafts instead of the solid shafts. The hollow shafts can be broadly divided into integral-type and joined-type. The integral-type hollow shaft comprises a middle pipe part having the largest outer diameter and shaft parts integrally formed on the opposite ends of the pipe part. The shafts parts are made of the same material as that of the middle pipe part and a coupling portion such as a spline is formed on the outer periphery of each shaft-end. On the other hand, the joined-type hollow shaft comprises a pipe part and shaft parts. These parts are shaped separately and are then joined together using friction pressure welding, electric welding, or the like. [0008] Comparing with the solid shaft, the integral-type or joined-type hollow shaft has a reduced section modulus, while the maximum shear-stress thereof operative to the hollow shaft is large. Therefore, there is a possibility of a decrease in the shear strength of the hollow shaft. [0009] In some cases, an electro-resistance-welded tube having a wall thickness with an extremely high accuracy and an extremely stable strength is used as a power transmission hollow shaft. The welded tube is comprised of two or more pipe parts. The pipe parts are made of a steel material having a good dimensional accuracy and a good finishing accuracy and are butt-joined in a straight line using electric resistance welding. Therefore, the welded portion of an electro-unite part of the welded pipe, which extends in the axial direction, tends to be broken, leading to a decrease in the strength of the power transmission shaft. [0010] In addition, the integral-type hollow shaft for power transmission is typically formed by, for example, a swaging in which the diameter of an element tube is reduced by stamping in the radial direction thereof at high speed, while rotating the tube around the axis; or a press working in which the diameter of an element tube is reduced by inserting the element tube into a die. The hollow shaft formed by such a plastic working of the swaging or the like may have a plastic flow of the raw material into the inner radial at the time of reducing the diameter of the element tube. Thus, there is a tendency in which the inner radial surface of the hollow shaft become wrinkled. Such a wrinkle may become the origin of breakage, causing a decrease in the strength of the power transmission shaft. SUMMARY OF THE INVENTION [0011] An object of the present invention is to provide a power transmission shaft allowing an improvement in the strength and allowing a stable torsion fatigue strength. [0012] As technical means for attaining the above object, a first aspect of the present invention is to provide a power transmission shaft comprising coupling members respectively provided on opposite ends of a pipe part made of a steel material, wherein the steel material includes 0.30-0.45% by weight of carbon (C), 0.05-0.35% by weight of silicon (Si), 1.0-2.0% by weight of manganese (Mn), 0.05% by weight or less of aluminum (Al), 0.01% by weight or less of sulfur (S), and the remainder, iron (Fe) and unavoidable impurities, and the pipe part has an electro-unite portion that extends in an axial direction, the electro-unite portion and neighborhood thereof being hardened by a hardening treatment so as to have a Rockwell hardness HRC of 45 or more. Here, the hardening treatment may be preferably a high-frequency induction hardening and tempering treatment. Here, the term "neighborhood of the electro-unite portion" means that a portion within 5 mm far from the middle to the opposite ends in the circumferential direction of the electro-unite portion. [0013] In this embodiment, a steel material in which the amount of each of the above components (C, Si, Mn, Al, and S) is defined in the above range is used and its electro-unite portion and neighborhood thereof are hardened so that the Rockwell hardness HRC thereof can be 45 or over. Therefore, the hardness of the pipe to be required as a power transmission shaft can be satisfied. Such a hardness makes sure of a stable torsion fatigue strength, providing a power transmission shaft of an elongated useful life and a high reliability. In addition, an electric-resistance welded tube is used as a steel pipe to be used as the subject pipe for stably ensuring the shaft strength. Thus, the power transmission shaft can be hardly broken at an electro-unite portion thereof, preventing a decrease in the strength of the pipe. [0014] A second aspect of the present invention is to provide a power transmission shaft with coupling members integrally formed on opposite ends thereof, which is formed from a steel element tube by a plastic working, comprising an inner diametrical surface which is subjected to a hardening treatment. Preferably, the hardening treatment may be a high-frequency induction hardening and tempering treatment. The hardening treatment on the inner diametrical surface can be performed by arranging a coil for high-frequency induction heating on the inner diametrical side of the power transmission shaft. Alternatively, the hardening treatment from the outer diametrical surface can be performed by arranging such a coil for high-frequency induction heating on the outer diametrical side of the power transmission shaft. In the hardening treatment with high-frequency induction hardening and tempering, the surface-portion hardness of the inner diametrical surface is a Rockwell hardness HRC of 35 or more. Here, the term "surface portion" means that, for example, about one fourth of the wall thickness of the power transmission shaft. [0015] According to the present invention, as described above, the inner diametrical surface is subjected to the hardening treatment, so that it becomes possible to ensure a hardness to be required for the power transmission shaft. In addition, such a resulting hardness allows to prevent the generation of wrinkle on the inner diametrical surface by a plastic working to be effected as an origin of breakage. As a result, the power transmission shaft that ensures a stable torsion fatigue strength and having a high reliability and a long useful life can be obtained. [0016] Furthermore, by applying a predetermined residual compression stress on the outer diametrical surface of the power transmission shaft, the residual compression stress increases. As a result, it becomes possible to further increase the torsion fatigue strength. Such a residual compression stress can be easily applied by a shot peening treatment. In addition, the residual compression stress may be preferably 750 MPa or more. BRIEF DESCRIPTION OF THE DRAWINGS [0017] In the accompanying drawings: [0018] FIG. 1 is a partially cross-sectional front view of a power transmission shaft as one of preferred embodiments of the present invention, which is provided as an integral-type hollow shaft; [0019] FIG. 2 is a partially cross-sectional front view of a power transmission shaft as another preferred embodiment of the present invention, which is provide as a joined-type hollow shaft; [0020] FIG. 3 is a radial cross-sectional view of an electro-unite portion of a pipe (i.e., an electric-resistance welded tube); [0021] FIG. 4 is a table of the results of an examination of torsion fatigue strength; Continue reading about Power transmission shaft... 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