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Bolt and manufacturing method of bolt

Bolt and manufacturing method of bolt description/claims

The present invention relates to a bolt that is effective to be used in a place where a male screw is subject to a repetitive stress, and relates to a manufacturing method of the bolt.

An example of a bolt used in a place where a male screw is subject to a repetitive stress includes, for example, a rotor spindle bolt of a gas turbine or a stud bolt of an engine crank case. Referring to FIGS. 10 and 1A, description will be given below for a rotor spindle bolt of a gas turbine.

In FIG. 10, a reference numeral “100” denotes a rotor of a compressor of a gas turbine. The rotor 100 is configured by: a plurality of discs 103 that are fastened by a rotor spindle bolt (a fastening bolt for the gas turbine), that is, a bolt 101, and a nut 102; and moving blades 104 that are respectively fixed to the plurality of discs 103. The plurality of discs 103 are fastened by the plurality of bolts 101 and the plurality of nuts 102 that are disposed at approximately equal intervals in the circumferential direction of the discs 103.

In the rotor 100, thermal extension difference is generated associated with starting up and stopping of the gas turbine, and an elongation and a shrinkage are caused by projections of fluctuating loads due to thrust, a centrifugal force, or a rotation of the rotor 100 with a deformation under the weight thereof. Therefore, the fastening force of the bolt 101 and the nut 102 is varied. Thus, as shown in FIGS. 1A and 1B, a fluctuating stress (a tensile stress and a bending stress) is applied to an engaging portion of a male screw of the bolt 101 and a female screw of the nut 102, that is, a screw engaging portion 105. When the fluctuating stress is applied to the screw engaging portion 105, the screw engaging portion 105 tends to suffer from wear and fatigable damage. Therefore, improvement is necessary for a fatigue resistance (especially, a fretting fatigue resistance) of the screw engaging portion 105, especially, the male screw of the bolt 101.

Techniques that improve the fatigue resistance of screw fastening members (for example, a bolt and a nut, and a stud bolt) are conventionally presented (for example, Patent Document 1 and Patent Document 2). Description will be given below for a bolt that is a conventional screw fastening member. The bolt that is a conventional screw fastening member is a bolt formed by shaping a male screw that fits with a female screw, into a tapered shape in a direction along which the bolt is pulled (Patent Document 1). Another bolt is a bolt formed by molding a bolt with a round portion beneath the bolt neck thereof that has the curvature radius within a range of 1.2 to 3.0 times as large as that of a round portion beneath the bolt neck of the finished product bolt and by shaping the round portion beneath the bolt neck of the bolt by applying cold working such that the portion has a predetermined curvature radius (Patent Document 2).

However, the former (Patent Document 1) is a technique that equalizes the tensile stress applied on the male screw by shaping the male screw into the tapered shape in a direction along which the bolt is pulled, and is not a technique that increases the surface hardness of the male screw and applies a compressive residual stress to the male screw. Thus, improvement by the former of the fatigue resistance of the male screw is limited. The latter (Patent Document 2) is a technique that increases the surface hardness of the round portion beneath the bolt neck by applying cold working to the round portion beneath the bolt neck and applies a compressive residual stress to the round portion beneath the bolt neck, however, is not a technique that increases the surface hardness of the male screw and applies a compressive residual stress to the male stress. Therefore, similarly to the former, improvement by the latter of the fatigue resistance of the male screw is also limited.

Patent Document 1: Japanese Patent Application Laid-open Publication No. 1981-53651

Patent Document 1: Japanese Patent Application Laid-open Publication No. 07-180714

The problem that the present invention is to solve lies in that improvement of the fatigue resistance of the male screw is limited for a bolt that is a conventional fastening member.

The present invention (the invention of claim 1) includes a surface hardening portion that is disposed at least on the surface of the male screw to increase a surface hardness and to provide a compressive residual stress.

In the present invention (the invention of claim 2), the surface hardening portion is formed by a process in which the particles having a equal or smaller particle size of being capable of passing through a mesh with a mesh size of #150 are projected to at least the surface of the male screw.

The present invention (the invention of claim 3) includes the steps of cutting to form the male screw, and processing to provide on at least the surface of the male screw a surface hardening portion that has increased hardness and that is applied with a compressive residual stress.

In the present invention (the invention of claim 4), the processing step includes a surface treatment by which the particles having a equal or smaller particle size of being capable of passing through a mesh with a mesh size of #150 are projected to at least the surface of the male screw.

The bolt of the present invention (the present invention of claim 1) has the increased surface hardness of the male screw thereof and has a compressive residual stress applied to the surface of the male screw thereof, due to the surface hardening portion provided at least on the surface of the male screw thereof. Therefore, the bolt of the present invention (the present invention of claim 1) enables improvement of the fatigue resistance of the male screw thereof and, therefore, the bolt is effective to be used in a portion where the male screw is subject to a repetitive stress.

The bolt of the present invention (the present invention of claim 2) has the surface hardening portion that is provided by processing of applying particles each having the particle diameter that can pass through the mesh having the mesh size of #150 or a smaller particle diameter, to at least the surface of the male screw. Therefore, the bolt of the present invention (the present invention of claim 2) has the increased surface hardness of the male screw thereof and has the compressive residual stress applied to the surface of the male screw thereof while no influence is given to the improvement of the fatigue resistance of the male screw due to the roughness of the surface of the male screw. As a result, the bolt of the present invention (the present invention of claim 2) securely enables the improvement of the fatigue resistance of the male screw.

The manufacturing method of the bolt of the present invention (the present invention of claim 3) provides, using the step of processing, the surface hardening portion on the surface of the male screw of the bolt formed by the step of cutting. Therefore, due to the surface hardening portion, the surface hardness of the male screw is increased and a compressive residual stress is applied to the surface of the male screw. Therefore, the manufacturing method of a bolt of the present invention (the present invention of claim 3) enables improvement of the fatigue resistance of the male screw and, therefore, the method enables the manufacture of a bolt that is effective to be used in a portion that generates a repetitive stress to the male screw thereof.

According to the manufacturing method of the bolt of the present invention (the present invention of claim 4), the surface hardening portion is formed on the surface of the male screw of the bolt by processing of applying the particles each having the particle diameter that can pass through the mesh having the mesh size of #150 or a smaller particle diameter, to at least the surface of the male screw. Therefore, the manufacturing method of a bolt of the present invention (the present invention of claim 4) enables increase of the surface hardness of the male screw and application of the compressive residual stress to the surface of the male screw while no influence is given to the improvement of the fatigue resistance of the male screw due to the roughness of the surface of the male screw. As a result, the manufacturing method of a bolt of the present invention (the present invention of claim 4) enables manufacture of a bolt of which the fatigue resistance of the male screw can securely be improved.

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