JP2014224554A - Bolt for evaluation of fastening characteristic, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide means capable of easily and inexpensively evaluating a fastening characteristic with high accuracy under a condition close to an actual use condition.SOLUTION: An evaluation bolt 7 is used to evaluate a fastening characteristic of a bolt for fastening fastened members 15, 17 by plastic region fastening, by using a strain gauge 1. The evaluation bolt 7 is manufactured by using a bolt according to specifications same as those of a mass-produced bolt actually used to fasten the fastened members 15, 17, as material. A strength of a hardened portion 23 including a strain gauge mounting area, of the evaluation bolt 7 is made higher than a strength of an area excluding the hardened portion 23, by hardening, and tempering thereafter. The thermally-refined portion 23 is formed on a columnar portion 13 between a seat surface of a head portion 11 and a screw portion 12 of the evaluation bolt 7. The strength of the hardened portion 23 is preferably 1.1 times or more a strength of the other region.

Description

  The present invention relates to a tightening characteristic evaluation bolt for evaluating a tightening characteristic of a metal bolt for fastening a member to be fastened by plastic region tightening using a strain gauge, and a manufacturing method of the tightening characteristic evaluation bolt It is about.

  Generally, when manufacturing or assembling an automobile or industrial equipment, various members to be fastened are fastened with bolts. And when it is required to fasten the fastened member with a bolt with high accuracy, that is, when it is required that the variation in the tightening axial force of the bolt is small, usually the plastic region tightening until the bolt is plastically deformed, For example, plastic region tightening by the rotation angle method is used (see, for example, Patent Documents 1 and 2). However, when such plastic region tightening is performed, plastic elongation, which is a permanent deformation, occurs in the bolt.

  On the other hand, in automobiles, industrial equipment, and the like, usually, for repair and inspection, disassembly and reassembly of a plurality of fastened members fastened with bolts are repeated. In this case, the plastic elongation of the bolt is accumulated every time the member to be fastened is fastened by plastic region fastening. When the plastic elongation of the bolt is accumulated in this way, the bolt tightening axial force for the same rotation angle gradually increases due to work hardening of the bolt material. And when the bolt tightening axial force reaches the ultimate tightening axial force, the bolt can no longer fully perform its function and must be replaced with a new bolt. For this reason, when using plastic region tightening for manufacturing or assembling automobiles or industrial equipment, it is necessary to accurately evaluate in advance how much the bolt can be reused.

Japanese Patent Laid-Open No. 10-299740 JP 2012-77797 A

  When the bolt is repeatedly tightened in the plastic region, it is necessary to stabilize the amount of plastic elongation generated in the bolt in each tightening of the plastic region, that is, to reliably generate a preset amount of plastic elongation. In order to stabilize the amount, it is necessary to set an appropriate tightening specification. In order to set an appropriate tightening specification, it is necessary to evaluate the tightening characteristics with high accuracy. To perform a highly accurate evaluation, the tightening characteristics under conditions close to actual use conditions are measured or evaluated. It is necessary to.

  However, at present, no means has been found that makes it possible to easily and inexpensively evaluate the fastening characteristics with high accuracy under conditions close to actual use conditions. The present invention has been made in view of such circumstances, and provides a means that makes it possible to easily and cost-effectively evaluate tightening characteristics under conditions close to actual use conditions. This is a problem to be solved.

  The fastening characteristic evaluation bolt (hereinafter referred to as “evaluation bolt”) according to the present invention, which has been made to solve the above-mentioned problems, has a fastening characteristic of a metal bolt for fastening a member to be fastened by plastic region fastening. Used to evaluate using strain gauges. This evaluation bolt has the same outer shape as a bolt (mass production bolt) that actually fastens the member to be fastened, and is formed of the same material as the bolt. In this evaluation bolt, the strength of the partial region (hereinafter referred to as “strengthening region”) of the evaluation bolt including the region where the strain gauge is mounted (hereinafter referred to as “strain gauge mounting region”) is high-frequency baked. By inserting, the strength of the region other than the reinforced region of the evaluation bolt (hereinafter referred to as “non-reinforced region”) is increased.

  In the evaluation bolt according to the present invention, it is preferable that the reinforced region including the strain gauge mounting portion is formed in a cylindrical portion (shaft portion) between the seat surface of the evaluation bolt and the screw portion in the bolt axial direction. In the evaluation bolt according to the present invention, the strength of the reinforced region is preferably 1.1 times or more than the strength of the non-reinforced region. Moreover, it is preferable that the length of the reinforced region in the bolt axis direction is 0.5 times or more the nominal diameter of the thread portion of the evaluation bolt (the outer diameter of the male screw).

An evaluation bolt manufacturing method for evaluating the tightening characteristics of a metal bolt for fastening a member to be fastened by plastic region tightening according to the present invention using a strain gauge includes the following steps. .
(1) A step of preparing, as an evaluation bolt material, a bolt having the same specification as that of a bolt used for fastening a member to be fastened (that is, a mass production bolt).
(2) A step of determining a strain gauge mounting portion of the bolt material for evaluation.
(3) A step of subjecting the reinforced region of the evaluation bolt material including the strain gauge mounting portion to induction hardening to increase the strength of the reinforced region over the strength of the non-reinforced region.

  In the evaluation bolt manufacturing method according to the present invention, it is preferable to subject the strengthened region to a tempering process after the strengthened region is subjected to induction hardening. Here, the reinforced region including the strain gauge mounting portion is preferably formed in a cylindrical portion between the seat surface of the bolt material for evaluation and the screw portion in the bolt axial direction. In this evaluation bolt manufacturing method, it is preferable to perform induction hardening or tempering so that the strength of the reinforced region is 1.1 times or more the strength of the non-reinforced region. Moreover, it is preferable to set the length of the reinforced region in the bolt axis direction to 0.5 times or more the nominal diameter of the threaded portion of the bolt material for evaluation.

  The evaluation bolt according to the present invention has the same outer shape as a bolt (mass production bolt) actually used for fastening the member to be fastened, and is formed of the same material as the bolt. For example, the mass production bolt itself is used as a material for the evaluation bolt. For this reason, it is possible to evaluate the tightening characteristics under substantially the same tightening state as when actually tightening with bolts. Therefore, if the evaluation bolt according to the present invention is used, it is possible to easily and accurately evaluate the tightening characteristics of the bolt under conditions close to actual use conditions.

  In addition, the strength of the reinforced region including the strain gauge mounting portion is higher than the strength of the non-reinforced region such as the threaded portion by induction hardening. For this reason, in the tightening characteristic evaluation test of the evaluation bolt, when the member to be fastened is tightened with the evaluation bolt, the plastic deformation is generated in the portion around the strain gauge mounting portion even after the thread portion is plastically deformed. Absent. As a result, the axial force generated in the evaluation bolt can be accurately measured with the strain gauge, the tightening characteristics can be accurately evaluated, and an appropriate tightening specification can be set.

  According to the evaluation bolt manufacturing method of the present invention, a bolt having the same specification as that of a bolt actually used for fastening a member to be fastened (that is, a mass production bolt itself) is used as an evaluation bolt material. In other words, the bolt for evaluation manufactured by this manufacturing method is a partially modified version of the actually used bolt. Therefore, if the evaluation bolt manufactured by the manufacturing method according to the present invention is used, the tightening characteristics can be evaluated in substantially the same tightening state as when tightening with the bolt that is actually used. For this reason, it is possible to easily evaluate the tightening characteristics of the bolt with high accuracy under conditions very close to the actual use conditions at low cost.

  In addition, the strength of the reinforced region including the strain gauge mounting portion is increased from the strength of the non-reinforced region such as the threaded portion by induction hardening. For this reason, in the tightening characteristic evaluation test of the evaluation bolt, when the member to be fastened is tightened with the evaluation bolt, the plastic deformation is generated in the portion around the strain gauge mounting portion even after the thread portion is plastically deformed. Absent. As a result, the axial force generated in the evaluation bolt can be accurately measured with the strain gauge, the tightening characteristics can be accurately evaluated, and an appropriate tightening specification can be set.

It is a graph which shows the change characteristic of the fastening torque with respect to a fastening rotation angle, and a fastening axial force in the case of performing plastic area fastening by a rotation angle method. It is a graph which shows how the change characteristic of the fastening axial force changes with respect to the fastening rotation angle when the plastic region fastening and the release of fastening are repeated eight times by the rotational angle method. FIG. 5 is a graph showing the tightening axial force with respect to the tightening rotation angle and the change characteristic of the tightening axial force with respect to the tightening torque in association with each other when performing plastic region tightening by the rotation angle method. FIG. 4 is a schematic elevation view of a strain gauge attached to an evaluation bolt when performing plastic region tightening with an evaluation bolt. It is a perspective view of a nominal diameter bolt (bolt with the diameter of a cylindrical part and the nominal diameter of a thread part substantially equal). It is a perspective view of an effective diameter volt | bolt (bolt in which the diameter of a cylinder part and the effective diameter of a thread part are substantially equal). It is an elevational sectional view of an evaluation bolt equipped with a strain gauge in a state where two members to be fastened are fastened. It is a graph which shows the change characteristic with respect to the fastening rotation angle of the fastening axial force of the bolt for evaluation measured with a strain gauge. It is a perspective view of an evaluation bolt (effective diameter bolt) in which a thick shaft portion is provided in the vicinity of a strain gauge mounting portion of a cylindrical portion. FIG. 10 is an elevational cross-sectional view of the evaluation bolt shown in FIG. 9 in which a strain gauge is attached to the thick shaft portion in a state where two fastened members are fastened. It is an elevational sectional view of an evaluation bolt in which a strain gauge is attached to a tempered portion (strengthened region) of a cylindrical portion in a state where two fastened members are fastened. It is a graph which shows the Rockwell hardness (HRC) of four types of evaluation bolts in which the tempered part (strengthening area | region) was provided in the cylindrical part. It is a graph which shows the Vickers hardness (HV) of four types of bolts for evaluation in which the tempered part (strengthening area | region) was provided in the cylindrical part. (A) And (b) is a graph which shows the result of having performed the fastening characteristic evaluation test of the volt | bolt for evaluation in which the tempered part (strengthening area | region) was formed only by induction hardening. (A) And (b) is a graph which shows the result of having performed the fastening characteristic evaluation test of the bolt for evaluation in which the tempered part (strengthening area | region) was formed by induction hardening and subsequent tempering (215 degreeC). It is. (A) And (b) is a graph which shows the result of having performed the fastening characteristic evaluation test of the bolt for evaluation in which the tempered part (strengthening area | region) was formed by induction hardening and subsequent tempering (265 degreeC). It is. (A) And (b) is a graph which shows the result of having performed the fastening characteristic evaluation test of the bolt for evaluation in which the tempered part (strengthening area | region) was formed by induction hardening and subsequent tempering (300 degreeC). It is.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The evaluation bolt according to the present invention (tightening characteristic evaluation bolt) uses, for example, a strain gauge for tightening characteristics of a bolt for fastening a member to be fastened by plastic region fastening when manufacturing or assembling an automobile or industrial equipment. It is used for evaluation. First, the behavior of the bolt or the characteristics of the tightening characteristics when the plastic region is tightened by the rotation angle method will be described.

  FIG. 1 shows a tightening torque T (hereinafter referred to as “torque T”) and a tightening axial force F (hereinafter referred to as “rotation angle θ”) with respect to a tightening rotation angle θ (hereinafter referred to as “rotation angle θ”) in the case of performing plastic region tightening by the rotation angle method. The change characteristic of “axial force F” is shown. In the example shown in FIG. 1, when the rotation angle θ becomes θy, the bolt reaches the yield point, and the yield tightening axial force Fy and the yield tightening torque Ty are generated. Therefore, the region where the rotation angle θ is equal to or less than θy is the elastic region tightening region, and the region exceeding θy is the plastic region tightening region. Θst represents the rotation angle θ corresponding to the snag torque point, and θu represents the rotation angle θ corresponding to the ultimate tightening axial force Fu or the ultimate tightening torque Tu. Note that the “snugging torque” is a linear relationship between the rotation angle θ and the axial force F by tightening with a relatively small torque before the tightening by the rotation angle method to eliminate each gap between the fastened members. This is a torque for causing a proportional relationship to occur.

  When fastening a member to be fastened, the bolt subject to the present invention is to be plastically deformed by tightening the member to be fastened until the rotation angle θ exceeds θy, for example, θc, that is, to perform plastic region fastening. Are planned bolts. In this case, it is possible to perform high-precision tightening with a small variation in the axial force F of the bolt, but the bolt undergoes plastic elongation, which is a permanent deformation. Therefore, when the plastic region tightening and the releasing of the tightening are repeated with the same bolt, the plastic elongation of the bolt is accumulated every time the plastic region tightening is performed.

  When plastic elongation is accumulated in this way, the axial force F with respect to the same rotation angle θ gradually increases due to work hardening of the bolt material. When the axial force F of the bolt reaches the ultimate tightening axial force Fu, the bolt can no longer fully perform its function and must be replaced with a new bolt. Therefore, when the plastic region tightening is repeated with the same bolt, the number of times that the tightening can be performed (the number of reuses) is one less than the number of times when the ultimate tightening axial force Fu is generated. When the bolt axial force F reaches the ultimate tightening axial force Fu, if the bolt is further tightened in the plastic region and the tightening is released, the axial force F gradually decreases and eventually the bolt breaks. Will do.

  FIG. 2 shows how the change characteristic of the axial force F with respect to the rotation angle θ changes when the plastic region tightening and the tightening release are repeated eight times using the same bolt. In FIG. 2, Δθ represents a rotation angle corresponding to the plastic elongation amount of the bolt when the first tightening is performed. In the example shown in FIG. 2, the axial force F reaches the ultimate tightening axial force Fu when the eighth tightening is performed. Therefore, with this bolt, the number of times that the plastic region can be tightened without any problem is seven times. Note that when the axial force F reaches or near the ultimate tightening axial force Fu, a local cross-sectional reduction occurs in the minimum cross-sectional portion of the bolt (in the case of an effective diameter bolt), and the actual stress at that portion (True stress) increases rapidly. The two-dot chain line in FIG. 2 indicates the actual stress that increases in this way. Bolts with increased actual stress cannot be used because they may cause fatigue failure or delayed failure.

  Thus, in plastic region tightening, it is necessary to stabilize the amount of plastic elongation generated in the bolt in each plastic region tightening, that is, to reliably generate the set amount of elongation. It is necessary to set a proper tightening specification. In order to set appropriate tightening specifications, it is necessary to evaluate the tightening characteristics with high accuracy. To perform high-precision evaluation, the tightening characteristics are measured or evaluated under conditions close to actual use conditions. It is necessary. In particular, the coefficient of friction between the bolt and the member to be fastened or the member to be screwed (for example, a nut), the yield tightening axial force, the ultimate tightening axial force, etc. are items that greatly affect the setting of the tightening specifications. It is necessary to measure or evaluate accurately.

  FIG. 3 shows the change characteristic of the axial force F with respect to the torque T and the change characteristic of the axial force F with respect to the rotation angle θ in association with the plastic region tightening by the rotation angle method. The graph on the right side of the vertical axis in FIG. 3 shows how the change characteristic of the axial force F with respect to the torque T changes depending on the friction coefficient μ between the bolt and the member to be fastened or the screwed body. . In FIG. 3, μmin represents the minimum value of the friction coefficient μ, and μmax represents the maximum value of the friction coefficient μ. As is apparent from FIG. 3, the axial force F with respect to the same torque T increases as the friction coefficient μ decreases, and decreases as the friction coefficient μ increases. For example, when the bolts are tightened with the same snag torque Tst, the generated axial force F is F1 if the friction coefficient μ is the minimum value μmin, and F2 (<F1) if the friction coefficient μ is the maximum value μmax.

  The graph on the left side of the vertical axis in FIG. 3 shows the change characteristics of the axial force F with respect to the rotation angle θ when the plastic region is tightened by the rotation angle method. In this plastic region tightening, first, tightening is performed with a snug torque, and then tightening is completed by performing plastic region tightening with control by a rotation angle method. Here, when the friction coefficient μ is small, for example, when it is the minimum value μmin, the axial force F generated when tightened with the snag torque Tst is F1. Thereafter, when the bolt is further tightened, the axial force F reaches the yield tightening axial force, but the rotation angle increment corresponding to this tightening is Δθ1. Further, when the bolt (or a member to be screwed) is further rotated and tightened, plastic elongation is generated accordingly. In this case, the amount of plastic elongation to be generated in the bolt by the first tightening is determined in advance, and the bolt (or the screwed body) is rotated and tightened so that the amount of plastic elongation is generated.

  On the other hand, when the friction coefficient μ is large, for example, when it is the maximum value μmax, the axial force F generated when tightening with the snag torque Tst is F2. Thereafter, when the bolt is further tightened, the axial force F reaches the yield tightening axial force, but the increment of the rotation angle corresponding to this tightening is Δθ2. As apparent from FIG. 3, the rotation angle increment θ2 when the friction coefficient μ is the maximum value μmax is larger than the rotation angle increment Δθ1 when the friction coefficient μ is the minimum value μmin by α.

  Here, for example, it is assumed that the tightening specification is set such that after tightening with the snag torque Tst, further tightening is performed until the rotation angle increment becomes Δθ1. When the tightening specifications are set in this way, if the friction coefficient μ is the minimum value μmin, the axial force F reaches the yield tightening axial force by tightening the rotation angle increment to Δθ1, so that the member to be fastened is appropriately Can be fastened. However, if the friction coefficient μ is the maximum value μmax, in order to reach the axial force F to the yield tightening axial force, it is necessary to perform tightening with an increment of the rotation angle of Δθ2, and thus the rotation according to the tightening specification. If tightening is performed with an angle increment of Δθ1, the axial force F does not reach the yield tightening axial force, and the fastened member cannot be fastened properly.

  Thus, since the relationship between the rotation angle θ and the yield tightening axial force depends on the friction coefficient μ between the bolt and the member to be fastened or the screwed body, the tightening specification depends on the actually used bolt and the target bolt. It is necessary to set according to the coefficient of friction μ between the fastening member or the member to be screwed. Therefore, in the present invention, the evaluation or experiment of the tightening characteristics for setting the tightening specifications of the bolt is performed by using the bolt (mass production bolt) by cold forging actually used and the member to be fastened actually used. I have to. That is, an evaluation or experiment is performed in accordance with the friction coefficient μ between the actually used bolt and the member to be fastened or screwed member.

  FIG. 4 shows, for example, the measurement of the bolt strain and the axial force F when evaluating or experimenting the tightening characteristics for setting the tightening specifications for the bolts having the tightening characteristics as shown in FIGS. The strain gauge used for is shown. The strain gauge 1 is connected to the grid 3 via a connection tab 4 and a carrier 2 (carrier) made of an insulating material, a grid 3 made of a linear or foil-like electric resistor mounted on the carrier 2, and a connection tab 4. Two lead wires 5 connected to each other are provided. As will be described later, the strain gauge 1 is attached to the cylindrical portion of the evaluation bolt, and is used to measure the strain generated in the evaluation bolt, and hence the axial force F.

  5 and 6 respectively show a nominal diameter bolt 6 and an effective diameter bolt 7 that are generally used for fastening a fastened member. The nominal diameter bolt 6 has a head portion 8, a screw portion 9 in which a male screw is formed, and a columnar portion 10 (shaft portion) in which a male screw between the head portion 8 and the screw portion 9 is not formed. . The nominal diameter bolt 6 is a bolt in which the nominal diameter (outer diameter) of the threaded portion 9 and the diameter of the cylindrical portion 10 are substantially equal, and the effective sectional area of the threaded portion 9 is considerably smaller than the sectional area of the cylindrical portion 10. . On the other hand, the effective diameter bolt 7 has a head portion 11, a screw portion 12 in which a male screw is formed, and a cylindrical portion 13 (shaft portion) in which a male screw between the head portion 11 and the screw portion 12 is not formed. ing. The effective diameter bolt 7 is a bolt in which the nominal diameter (outer diameter) of the screw portion 12 and the diameter of the cylindrical portion 13 are substantially equal, and the effective cross-sectional area of the screw portion 12 is slightly smaller than the cross-sectional area of the cylindrical portion 13. . The present invention is intended for the effective diameter bolt 7 as shown in FIG. 6 and not for the nominal diameter bolt 6 as shown in FIG.

  FIG. 7 shows an effective-diameter bolt 7 (hereinafter referred to as “evaluation bolt 7”) as an evaluation bolt that fastens two fastening members in cooperation with a nut. As shown in FIG. 7, the screw portion 12 and the cylindrical portion 13 of the evaluation bolt 7 extend through the hole portion 16 of the first fastened member 15 and the hole portion 18 of the second fastened member 17. A nut 19 is screwed onto the threaded portion 12 of the evaluation bolt 7. The 1st to-be-fastened member 15 and the 2nd to-be-fastened member 17 are pinched | interposed by the head 11 and the nut 17 of the evaluation volt | bolt 7, and the compression force (pressing force) of a bolt axial direction is applied. As a reaction, a tensile force (that is, axial force F) in the bolt axial direction is applied to the evaluation bolt 7. A portion indicated by 12 a in the screw portion 12 is a “free screw” that is not screwed into the nut 19.

  The evaluation bolt 7 is formed with a hole 20 having a small diameter (for example, a diameter of 1 to 2 mm) extending in the bolt axis direction from the head 11 toward the bolt tip, and an adhesive is used in the hole 20. A strain gauge 1 is embedded. The strain gauge 1 is fixed with an adhesive to a portion slightly closer to the head 11 than the center of the cylindrical portion 13 in the bolt axial direction. The two lead wires 5 of the strain gauge 1 protrude outside the evaluation bolt 7 in order to connect the strain gauge 1 to an external electric circuit (not shown) such as a Wheatstone bridge circuit. Note that the strain gauge 1 may be fixed to the outer peripheral surface of the cylindrical portion 13 with an adhesive without being embedded in the cylindrical portion 13. However, in this case, it is necessary to provide a hole for pulling out the lead wire 5 to the head 11 of the evaluation bolt 7.

  Thus, since the strain gauge 1 is embedded and fixed in the cylindrical portion 13 of the evaluation bolt 7, when strain in the bolt axis direction occurs in the evaluation bolt 7, the grid 3 of the strain gauge 1 A strain corresponding to the strain of the evaluation bolt 7 is generated, and the electrical resistance of the grid 3 changes corresponding to the strain of the grid 3. This change in electrical resistance can be detected as a voltage via a Wheatstone bridge circuit (not shown) or the like, and the distortion of the evaluation bolt 7 from time to time can be measured based on this voltage. The strain of the bolt 7 for evaluation is converted into the axial force F by a relational expression showing the relationship between the strain amount and the axial force F set in advance using a generally known calibration method.

  Hereinafter, in the state shown in FIG. 7, the evaluation bolt 7 is rotated until the evaluation bolt 7 breaks the first and second fastened members 15 and 17 with the evaluation bolt 7 and the nut 19. The behavior or operation of the evaluation bolt 7 and the strain gauge 1 when tightening, that is, when performing plastic region tightening will be described. When the evaluation bolt 7 is rotated after the first and second fastened members 15 and 17 are tightened with the snag torque Tst, the screw portion 12 and the column portion 13 first meet the spring constant of the evaluation bolt 7. Elastic deformation occurs.

  When the evaluation bolt 7 is further rotated to increase the axial force F, the evaluation bolt 7 exceeds the elastic limit. As a result, first, only the screw portion 12 begins to be plastically deformed, and accordingly, the strength or hardness of the screw portion 12 increases due to work hardening of the material of the screw portion 12. When the strength or hardness of the threaded portion 12 is increased, the cylindrical portion 13 (the diameter of which is the effective diameter of the threaded portion 12, that is, the diameter before rolling) starts plastic deformation. Thereafter, when the evaluation bolt 7 is further rotated and the first and second fastened members 15 and 17 are tightened, the screw portion 12 that is the weakest portion (that is, the minimum cross-sectional portion) is broken.

  The solid line in FIG. 8 shows the relationship between the rotation angle θ and the axial force F actually generated when such plastic region tightening is performed. A broken line indicates a relationship between the rotation angle θ and the tightening axial force calculated based on the output of the strain gauge 1 (hereinafter referred to as “apparent axial force Fi”). As shown in FIG. 8, the cylindrical portion 13 is not plastically deformed until the vicinity of the point P at which the screw portion 12 exceeds the elastic limit, so that the actually generated axial force F and the apparent axial force Fi substantially coincide.

  However, when the screw portion 12 starts to be plastically deformed beyond the elastic limit, the screw portion 12 is work-hardened, and the main plastically deformed portion is transferred from the screw portion 12 to the cylindrical portion 13. For this reason, the apparent axial force Fi calculated based on the output of the strain gauge 1 rapidly increases. Therefore, under such circumstances, the axial force F in the plastic region tightening cannot be accurately measured. For this reason, it is not possible to evaluate the proper tightening characteristics of the evaluation bolt 7 and thus the mass production bolt actually used, and it is not possible to set an appropriate tightening specification.

  In view of such facts, the inventor of the present application has a strain gauge mounting region, that is, a partial region of the cylindrical portion 13 of the evaluation bolt 7 including a portion where the strain gauge 1 is mounted or disposed (hereinafter referred to as “strain gauge vicinity region”). If the strength or hardness is increased to prevent plastic deformation, the cylindrical portion 13 does not plastically deform in the vicinity of the strain gauge 1 when the plastic region is tightened, and the axial force F and the apparent axial force It was considered that Fi almost coincided.

  Thus, as shown in FIGS. 9 and 10, it is conceivable to increase the strength of the cylindrical portion 13 of the evaluation bolt 7a by increasing the diameter in the vicinity of the strain gauge. For example, it is conceivable that the diameter of the vicinity of the strain gauge of the cylindrical portion 13 having substantially the same diameter as the effective diameter of the screw portion 12 is increased to the nominal diameter. Here, the strain gauge 1 is located in a high-strength strain gauge vicinity region 22 (hereinafter referred to as a “thick shaft portion 22”) having an enlarged diameter. For this reason, even if the plastic region is tightened until the evaluation bolt 7 is broken, plastic deformation does not occur in the thick shaft portion 22. As a result, it is possible to prevent an increase in the apparent axial force Fi due to plastic deformation, and therefore it is possible to make the actually generated axial force F and the apparent axial force Fi substantially coincide with each other.

  Meanwhile, the contour shape of the cylindrical portion 13 (22) of the evaluation bolt 7 provided with such a thick shaft portion 22 is generally formed by cutting. In addition, the screw part 12 is shape | molded by rolling using a metal mold | die. In this case, since the seat surface of the evaluation bolt 7 is formed by cutting (turning), an annular groove is formed on the seat surface. For this reason, the friction coefficient μ is different from that of mass-produced bolts manufactured by cold forging and actually used. When the friction coefficient μ is different between the evaluation bolt and the mass production bolt in this way, as described above (see paragraph [0026]), according to the friction coefficient μ between the actually used bolt and the fastened members 15 and 17. It is not possible to conduct proper evaluation or experiment.

  Therefore, as shown in FIG. 11, in the present invention, the evaluation bolt 7 has the same outer shape or outline as the mass production bolt for actually fastening the fastened member, and is formed of the same material as the mass production bolt. Yes. That is, the mass production bolt itself is used as a material for the evaluation bolt 7. The strength or hardness of the strain gauge vicinity region 23 is higher than the strength of the region other than the strain gauge vicinity region by the induction hardening process and the tempering process (hereinafter, this region is referred to as the “tempered portion 23”). .)

The evaluation bolt 7 provided with such a tempered portion 23 (strengthened region) is manufactured through the following steps, for example.
(1) A bolt having the same specifications as the bolt used for fastening the first and second fastened members 15 and 17 (that is, a mass production bolt itself) is prepared as an evaluation bolt material.
(2) Determine the strain gauge attachment site of the bolt material for evaluation.
(3) The tempered portion 23 is formed by subjecting a predetermined partial region of the evaluation bolt material including the strain gauge mounting portion to induction hardening treatment.
(4) A tempering process is performed on the tempered region 23.

  Thus, in the evaluation bolt 7, the strength or hardness of the tempered portion 23 is higher than the strength of the region other than the tempered portion 23. Here, in the induction hardening treatment and the tempering treatment, the strength or hardness of the tempered portion 23 is, for example, 1.1 to 3.0 times the strength or hardness of a region other than the tempered portion (preferably 1. 1 to 2.0 times). The length of the tempered portion 23 in the bolt axis direction is, for example, 0.5 to 3.0 times (preferably 0.5 to 2.0 times) the nominal diameter of the threaded portion 12 of the evaluation bolt 7. Set to range.

  Hereinafter, with reference to FIGS. 12 to 17, an experimental bolt (cylinder head bolt) is manufactured using a mass production bolt (M10 bolt) as an evaluation bolt material, and the experimental results of measuring or evaluating the tightening characteristics are described. To do. In view of the fact that the prototype bolt that was prototyped was deformed not only in the threaded part but also in the cylindrical part (shaft part) during its tension, the tempered part including the strain gauge mounting part of the mass production bolt (strengthened region) ) Is subjected to induction hardening only, or induction hardening and tempering treatment, and the tempered part is strengthened or hardened to tighten the plastic zone. ) To prevent plastic deformation. In this experiment, four types of evaluation bolts with different tempering presence / absence or tempering temperatures were prototyped, the hardness of these evaluation bolts was measured, and a strain gauge was attached when the plastic zone was tightened. It was determined whether plastic deformation occurred in the region.

Table 1 shows the conditions of induction hardening and tempering of the evaluation bolts used for the measurement of Rockwell hardness (HRC) and Vickers hardness (HV) hardness. Table 2 shows the conditions of induction hardening and tempering of the bolts for evaluation used for measurement or evaluation of the fastening characteristics. In Table 1 or Table 2, the tempering process is not performed for the prototype whose column of the tempering temperature is “−”. The tempering was carried out by holding the evaluation bolt prototype at each tempering temperature for 1 hour and then air cooling.

  FIG. 12 shows the results of measuring the Rockwell hardness (HRC) of the tempered part and the threaded part not subjected to heat treatment a plurality of times for the prototypes 1 to 4 of the evaluation bolt, and averaging the measured values. . Moreover, the result of having measured the Vickers hardness (HV) of the tempering part in the several site | part of a bolt radial direction about the prototypes 1-4 of the bolt for evaluation in FIG. In FIG. 13, the horizontal axis (X axis) indicates the distance (depth) from the surface of the cylindrical portion of the prototype of the evaluation bolt toward the center.

  According to FIG. 12, the Rockwell hardness (HRC) and hence the strength of the tempered portion of each evaluation bolt prototype is generally increased by 50 to 70% as compared to the unthreaded screw portion. I understand. According to the knowledge of the present inventor, the Rockwell hardness of the tempered portion of the evaluation bolt is 1.1 to 3.0 times (preferably 1.1 to 2.0 times) the Rockwell hardness of the threaded portion. If it is about the extent, it is considered that plastic deformation does not occur in the tempered portion even if the plastic region is tightened with the bolt for evaluation. Further, according to FIG. 13, when a tempered part is formed by induction hardening and tempering using a mass production bolt of M10 as an evaluation bolt material, the Vickers hardness is relatively low on the surface of the cylindrical part. Although it is low, it can be seen that the Vickers hardness is relatively stably increased within the range of 1 to 3.5 mm from the surface toward the center. Therefore, it is preferable that the strain gauge is mounted not on the surface of the cylindrical portion but inside.

  14 (a), 14 (b) to 17 (a), 17 (b), a tensile test was performed on evaluation bolt prototypes 5-12 using a universal material testing machine and an XY recorder. Results are shown. Specifically, these drawings basically show the change characteristics of the axial force F (Y axis) calculated based on the output of the strain gauge with respect to the displacement amount (X axis) of each evaluation bolt. Yes. However, in these tensile tests, the evaluation bolt was extended (tightened) even after exceeding the ultimate tightening axial force, and the displacement meter was removed immediately before the evaluation bolt broke, and the measurement of the displacement amount was stopped. Thereafter, the evaluation bolt is broken, and the axial force F is recorded (monitored) until after the evaluation bolt is broken. 14A and 14B are graphs for prototypes 5 and 6, respectively, and FIGS. 15A and 15B are graphs for prototypes 7 and 8, respectively. ) And (b) are graphs for prototypes 9 and 10, respectively, and FIGS. 15A and 15B are graphs for prototypes 11 and 12, respectively.

  In each of the graphs shown in FIGS. 14 (a), 14 (b) to 17 (a), (b), for example, the apparent axial force Fi as shown by the broken line in FIG. Not. However, as shown in FIGS. 14A and 14B, in the prototypes 5 and 6 that have not been tempered, the axis calculated based on the output of the strain gauge after the evaluation bolt broke. The force F does not return to 0 kN, and a residual strain (residual axial force) of 9 kN to 10 kN is generated. This is because, in the prototypes 5 and 6 that have not been tempered, a slight plastic elongation occurs in the tempered portion of the tempered portion when it is pulled to its ultimate tensile strength (extreme tightening axial force). It shows that.

  Further, as shown in FIGS. 15 (a), 15 (b) to 17 (a), (b), in the prototypes 7 to 12 subjected to the tempering process, after the evaluation bolt is broken, the strain gauge The axial force F calculated on the basis of the output of is returned to almost 0 kN. This indicates that no plastic elongation occurred in the prototypes 7 to 12 that had been tempered. If an evaluation bolt equipped with a tempered part that has been induction hardened and tempered is used, even if the tensile bolt is pulled beyond the tensile strength in the tensile test, the gauge is not stretched. It is considered that the portion where the quality is applied and the gauge is attached does not undergo plastic deformation even if it is pulled until the threaded portion breaks, and therefore the actually generated axial force F can be measured without hindrance.

  The evaluation bolt 7 having the tempered portion 23 has the same outer shape as the mass production bolt actually used for fastening the first and second fastened members 15 and 17 and is formed of the same material as the mass production bolt. ing. That is, the mass production bolt itself is used as a material for the evaluation bolt 7. For this reason, it is possible to evaluate the tightening characteristics under substantially the same tightening state as when actually tightening with bolts. Therefore, if this evaluation bolt 7 is used, it is possible to easily and cost-effectively evaluate the tightening characteristics of the bolt under conditions close to actual use conditions.

  Further, the strength or hardness of the tempered portion 23 is made higher than the strength or hardness of the region other than the tempered portion 23 by the induction hardening process and the tempering process. For this reason, in the tightening characteristic evaluation test of the evaluation bolt 7, when the member to be fastened is tightened with the evaluation bolt 7, even after plastic deformation occurs in the threaded portion 12, the portion around the strain gauge mounting portion is plastic. No deformation occurs. As a result, the axial force generated in the evaluation bolt 7 by the strain gauge 1 can be accurately measured, and the tightening characteristics can be accurately evaluated.

  According to this evaluation bolt 7, since the friction coefficient μ of the seating surface of the evaluation bolt 7 is equal to the friction coefficient μ of the mass production bolt, the axial force F generated when tightening with the snag torque is also the mass production bolt. Is almost the same as For this reason, an appropriate tightening specification can be set. In addition, it is possible to measure the tightening axial force F using the mass-produced bolts to each stage of the elastic range, yield tightening axial force, ultimate tightening axial force, and fracture tightening axial force. The accuracy of evaluation is improved, and appropriate tightening specifications can be set. Further, since a mass production bolt is used as the material for the evaluation bolt 7, the spring constant of the evaluation bolt 7 is the same as that of the mass production bolt. For example, an evaluation having a thick shaft portion 22 as shown in FIGS. The problem that the spring constant becomes larger than that of the mass-produced bolt does not occur unlike the bolt 7a, and the adverse effect of affecting the tightening angle does not occur.

  1 strain gauge, 2 carrier, 3 grid, 4 connection tab, 5 lead wire, 6 nominal diameter bolt, 7 evaluation bolt (effective diameter bolt), 8 head, 9 threaded portion, 10 cylindrical portion, 11 head, 12 Screw part, 13 Cylinder part, 15 1st to-be-fastened member, 16 Hole part, 17 2nd to-be-fastened part, 18 Hole part, 19 Nut, 20 Hole part, 22 Thick shaft part, 23 Tempered part (strengthening area).

Claims (9)

A tightening characteristic evaluation bolt for evaluating a tightening characteristic of a metal bolt for fastening a member to be fastened by plastic region tightening using a strain gauge,
It has the same outer shape as the bolt and is formed of the same material as the bolt,
The strength of the partial region of the tightening characteristic evaluation bolt including the portion to which the strain gauge is attached is higher than the strength of the region other than the partial region of the tightening characteristic evaluation bolt by induction hardening. Bolt for evaluating tightening characteristics.   The bolt for tightening characteristic evaluation according to claim 1, wherein the partial region is formed in a cylindrical part between a seating surface of the bolt for tightening characteristic evaluation and a threaded part in the bolt axial direction.   The bolt for tightening characteristic evaluation according to claim 1 or 2, wherein the strength of the partial region is 1.1 times or more of the strength of the region other than the partial region of the bolt for evaluating tightening characteristic.   The length in the bolt axis direction of the partial region is 0.5 times or more of the nominal diameter of the threaded portion of the bolt for tightening characteristics evaluation, according to any one of claims 1 to 3. Bolt for evaluating tightening characteristics. A method for producing a bolt for tightening characteristics evaluation for evaluating a tightening characteristic of a metal bolt for fastening a member to be fastened by plastic region tightening using a strain gauge,
Prepare bolts of the same specifications as the bolts used to fasten the fastened member as bolt material for tightening characteristics evaluation,
Determine the strain gauge mounting site for mounting the strain gauge in the bolting property evaluation bolt material,
Inductive quenching treatment is performed on a partial region of the bolting material for tightening characteristic evaluation including the strain gauge mounting portion, and the strength of the partial region is determined from the strength of the region other than the partial region of the bolt material for tightening characteristic evaluation. A method for producing a bolt for evaluating tightening characteristics, characterized in that it is enhanced.   6. The method of manufacturing a bolt for evaluating tightening characteristics according to claim 5, wherein after the partial region is subjected to induction hardening, the partial region is tempered.   The bolt for tightening characteristic evaluation according to claim 5 or 6, wherein the partial region is formed in a cylindrical part between a seat surface of the bolt material for tightening characteristic evaluation and a screw part with respect to a bolt axial direction. Manufacturing method.   6. The induction hardening process is performed so that the strength of the partial region is 1.1 times or more the strength of the region other than the partial region of the bolting property evaluation bolt material. The manufacturing method of the bolt for a fastening characteristic evaluation as described in any one of -7.   The length of the partial region in the bolt axis direction is set to 0.5 times or more the nominal diameter of the threaded portion of the bolting material for tightening characteristics evaluation, according to any one of claims 5 to 8, The manufacturing method of the bolt for bolting characteristic evaluation described in 2.

Images