A method of increasing strength of an internal thread in a threaded hole and a thread treatment tool

Abstract

Invention relates to a method of increasing strength of an internal thread (1) in a threaded hole of part subjected to cyclic load configured for use in a reciprocating piston engine, the method comprising determining an axial region (2) of the internal thread (1) which will house an external thread (6) of an attachment bolt (4) to form a bolt joint with the internal thread (1) when the bolt joint is used for securing function in internal thread, and applying axial force to a thread area (5) at an end in the axial region (2) of the internal thread (1), the axial force is applied in a same direction as applied when tightening the attachment bolt (4), when the internal thread (1) is used for securing function, and setting the applied axial force such that yield strength of the material of the force transmission rod is exceeded and a negative residual stress is created at the thread area (5). Invention relates also to a tread treatment tool (7).

Description

P38472PC00 / JRI 2024121912451A method of increasing strength of an internal thread in a threaded hole and a thread treatment toolTechnical field5

[0001] The present invention relates to a method of increasing strength of an internal thread in a threaded hole which is to be used under cyclic load in a reciprocating piston engine.

[0002] The present invention relates to a thread treatment tool for increasing strength of a thread in a threaded hole.Background art

[0003] In an internal combustion piston engine force transmission rods are subjected to high alternating tension-compression loads. A big end bearing of a force transmission rod is typically made of two-part structure where a rod cap, or a15 lower part of the force transmission rod is removably attached to the force transmission rod by threaded bolts and threaded holes in the big end bearing. In such a structure, in a treaded hole, the last threads are posed to the highest stresses.

[0004] Publication GB2535280A discloses a force transmission rod in which a big end bearing cover is screwed to a threaded bores in the big end of the force20 transmission rod. In order to minimize component stresses in the region of the last thread turn of the respective threaded bore the big end has, on its side which faces away from the force transmission rod bearing cover, a recess in the region of the respective threaded bore is arranged which recess is directed in the direction of the force transmission rod big end and is eccentric with respect to thread25 axis of the threaded bore.

[0005] An object of the invention is to provide method of increasing strength of an internal thread in a hole of a force transmission rod by means of which strength of the threaded holes in the big end of the connection rod is increased. It is also an object to provide a tool for practising the method according to the30 invention.P38472PC00 / JRI 2024121912452Disclosure of the Invention

[0006] Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention. Embodiments of the invention are associated with5 various advantages and / or technical effects.

[0007] According to an aspect of the invention a method of increasing strength of an internal thread in a threaded hole which is to be used under cyclic load in a reciprocating piston engine, the method comprising determining an axial region of the internal thread which will house an external thread of an attachment bolt to form a bolt joint with the internal thread when the bolt joint is used for securing function in the threaded hole, and applying axial force to a thread area at an end in the axial region of the internal thread, the axial force is applied in a same direction as applied when tightening the attachment bolt, when the internal thread is used for securing function in the15 threaded hole, and setting the applied axial force such that yield strength of the material of the threaded hole is exceeded and a negative residual stress is created at the thread area.

[0008] The application provides solutions for enhancing the yield strength of internal threads by applying controlled axial forces to specific thread areas, thereby20 creating negative residual stresses that improve the overall integrity of the thread. The method involves the use of a specialized thread treatment tool designed to engage with the internal thread while ensuring that the applied force exceeds the yield strength of the threaded hole material. This approach addresses the limitations of conventional methods that may not adequately reinforce the threads, leading to potential failures in securing attachment bolts during engine operation.

[0009] By selectively applying axial force to a specific thread area, the method ensures that the strength improvement is targeted where it is most needed, thus optimizing the treatment process without compromising the integrity of the entire thread.30

[0010] The method is advantageously applied to bolt joints in an internal combustion piston engine, which are subjected to cyclic load - both 2 and 4 stroke engines. It is particularly advantageous to use in threaded hole in piston rods which connects piston to power line of the engine.P38472PC00 / JRI 2024121912453

[0011] According to an aspect of the invention axial length of the thread area to which axial force is applied comprises two to four rounds of threads, at an end of the axial region of the internal thread.5

[0012] Limiting the axial length of the treated area concentrates the strengthening effect to the area which requires the treatment, ensuring a precise application of the strengthening process.

[0013] According to an aspect of the invention axial length of the thread area to which axial force is applied is three rounds of threads.

[0014] By further reducing the axial length of the treated area the method allows for a more localized application of force, which can lead to a more pronounced increase in strength at the critical thread engagement zone.

[0015] In a development, the method further comprises guiding a thread treatment tool,15 the tool comprising a straight body elongated in an axial direction, wherein a first end of the body is provided with an external thread and a second end of the body is provided with a head for coupling the treatment tool with a force device, wherein the thread has a predetermined major diameter and axial length of which is shorter than the major diameter of the20 thread, and wherein the body comprises a unthreaded shank having a diameter, which shank diameter is smaller than the major diameter of the thread, into the internal thread by rotating the tool about its longitudinal axis in the internal thread, placing the external thread of the thread treatment tool axially to the thread area of the internal thread, which is at an end of an external thread section of an attachment bolt when the internal thread is used for securing function in the threaded hole, and applying axial force to internal thread via the external thread of the thread treat¬30 ment tool in a same direction as when tightening the attachment bolt to the internal thread, when the internal thread is used for securing function in the threaded hole, andP38472PC00 / JRI 2024121912454 controlling the force applied by the external thread of the treatment tool such that yield strength of the material of the threaded hole is exceeded and a negative residual stress is created at thread area of the internal thread.

[0016] The use of a thread treatment tool with a specific major diameter and axial5 length ensures that the force is applied accurately to the designated thread area, resulting in a consistent and controlled induction of negative residual stress.

[0017] The design of the treatment tool, including the unthreaded shank with a smaller diameter than the major diameter of the thread, facilitates easy insertion and removal from the internal thread, enhancing the efficiency of the treatment process.

[0018] According to an aspect of the invention when applying force by the external thread of the treatment tool to the threaded hole, yield strength of the material of the threaded hole is exceeded by 1 to 10%, preferably 2-8%.

[0019] By exceeding the yield strength of the material by a controlled amount of15 1 to 10%, the method fine-tunes the application of axial force to create an optimal level of negative residual stress without causing excessive deformation or damage to the thread. By exceeding the yield strength of the material by a controlled amount of 2 to 8%, the method fine-tunes the application of axial force even more precisely.20

[0020] The specified range for exceeding yield strength allows for a controlled plastic deformation that is sufficient to enhance the strength of the internal thread while maintaining the material's structural integrity and performance characteristics.

[0021] According to an aspect of the invention the external thread in the first end of the body of the tool has less than four times the pitch of the thread beginning from the end of the body of the thread treatment tool, and axial length of only less than four times the pitch of the internal thread is treated.

[0022] The method ensures precise treatment of the internal thread by limiting the axial length of the treated area, which may result in a more controlled and30 consistent thread strengthening process.

[0023] In an embodiment of the invention, the method of increasing strength of an internal thread of a threaded hole comprises determining an axial region ofP38472PC00 / JRI 2024121912455 the internal thread, applying axial force to a thread area, and exceeding yield strength of the material of the threaded hole by 1 to 10%, wherein the axial length of the thread area to which axial force is applied is three rounds of the thread.

[0024] The advantageous effects of this combination include the creation of a5 negative residual stress in the thread area, which enhances the fatigue resistance of the bolt joint in the threaded hole.

[0025] The method enhances the fatigue resistance of the internal thread by inducing negative residual stress, which counteracts the tensile stresses experienced during operation, thereby extending the service life of the threaded hole.

[0026] The method offers the advantage of synergistic interaction between the internal thread and the attachment bolt, resulting in improved joint integrity.

[0027] The method ensures the advantage of hardening the thread area, increasing its resistance to wear and fatigue over time.

[0028] According to the invention a thread treatment tool for increasing strength15 of a thread in a hole in a threaded hole which is to be used under cyclic load in a reciprocating piston engine, comprises a straight body elongated in axial direction, and a first end of the body is provided with an external thread and a second end of the body is provided with a head for coupling the treatment tool with a force device, and the external thread has a predetermined major diameter and20 axial length of which is shorter than the major diameter of the thread, and the body comprises a unthreaded shank having a diameter, which shank diameter is smaller than the major diameter of the thread.

[0029] The provision of a head for coupling with a force device, such as pulling device allows for efficient power transfer and control during the thread treatment process, potentially increasing the tool's effectiveness.

[0030] The versatility of the thread treatment tool is enhanced by its compatibility with various embodiments, allowing for a wide range of applications and adaptability to different thread specifications.

[0031] The adaptability to tool design may facilitate the optimization of thread30 treatment for specific materials or thread geometries, leading to improved joint performance.P38472PC00 / JRI 2024121912456

[0032] According to an aspect of the invention the thread in the first end of the body has two to four rounds of threads at the end of the body.

[0033] The limited thread length on the first end of the body ensures that the tool engages with the internal thread in a controlled manner and precise axial loca¬5 tion.

[0034] According to an aspect of the invention the thread in the first end of the body has three rounds of threads.

[0035] The restriction of the thread length to three rounds of threads focuses the treatment on the most critical portion of the thread, which may lead to a more targeted strengthening effect and improved fatigue resistance of the treated thread.

[0036] According to an aspect of the invention that axial length of the shank is more than two times of the major diameter of the thread.

[0037] The design accommodates deeper threading applications, enabling the15 user to work with a wider range of workpiece thicknesses without the need for specialized equipment.

[0038] The proportionally longer shank provides greater reach for the tool, allowing access to threads in more confined or hard-to-reach spaces without compromising the structural integrity.20

[0039] The shank's length does not limit its compatibility, thus offering greater versatility in its application.

[0040] According to an aspect of the invention the tool is made of material obtained from the following list: hardened high tensile strength material, such as steel.

[0041] The choice of high-performance materials allows the tool to maintain its dimensional accuracy over time, ensuring consistent thread quality.

[0042] In an embodiment, a thread treatment tool for increasing strength of a thread in a hole in a threaded hole comprises a straight body with an external30 thread at one end and a head, suitable for use a hydraulic jack at the other end.P38472PC00 / JRI 2024121912457

[0043] The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features.5 The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.Brief Description of Drawings

[0044] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in whichFigure 1 illustrates a cross section of an attachment part of a force transmission rod for an internal combustion engine,Figure 2 illustrates a thread treatment tool according to an embodiment of the15 invention, andFigure 3 illustrates a stress-strain diagram illustrating the method according to the invention.Detailed Description of Drawings20

[0045] Figure 1 depicts schematically an example of a part subjected to cyclic load in a reciprocating piston engine, a cross section of an attachment part of a force transmission rod 20, which is configured to experience cyclic load when in use, particularly a big end housing of a connecting rod in a four-stroke engine or a piston rod attachment to a crosshead in a two stroke engine. The force transmission rod in the figure 1 is a connecting rod, of which a head of the connecting rod is shown here. The head of a connecting rod is divided into halves comprising a big end 21 of the connecting rod and a cap 22 of the connecting rod 20. The big end and the cap together attach the piston rod to a crank shaft 18. The cap22 is attached to the bid end by bots 108 which can be fitted to coaxial bolt holes30 23 in the cap 22 and in the big end. In the right-side hole 23 in the figure there is a bolt 4 arranged at its fastening position. The bolt 4 has a particular axial region where it has external thread 6 and likewise the hole 23 has an axial region whereP38472PC00 / JRI 2024121912458 it has an internal thread 1. When the attachment bold 4 is at its axial position, where the cap 22 is secured or fastened as intended during normal use to the big end 21 of the connecting rod, there is a certain axial region 2 of the internal thread 1 which houses an external thread 6 of the attachment bolt 4. For the5 present invention the end of the axial region 2 which houses the external thread 6 of the bolt 4, where an end of the bolt 4 is located is of particular interest.

[0046] Figure 2 discloses a side view of a tread treatment tool 7 according to an embodiment the invention. The tool 7 be used for new connection rod as well as used one. The tool comprises a straight body 8 which is elongated in axial direction 9. The tool 7 has at a first end 10 of the body 8 a portion which is provided with an external thread 3. At a second end 11 of the body 8 the body is provided with a head 12 for coupling the treatment tool 7 with a pulling device that uses hydraulic pressure to lift or apply axial pulling force to the tool. Such a device can be called as a hydraulic jack. The external thread 3 has a predetermined major15 diameter 14 and axial length 13 which is shorter than the major diameter 14 of the thread, implying its purpose for engaging with the internal thread 1 of the bolt hole 23 (Figure 1). Between the head 12 and the external thread 3 the body is provided with a shank 16, which is a straight, unthreaded section of the tool between the head 12 and the external thread 3. The shank 16 has a diameter 17.20 Should the shank have cross section of other form than circular, the shank diameter 17 refers to smallest circle inside which the shank’s cross section fits. Thus, the shank is not necessarily of circular cross section. The shank diameter 17 is smaller than the major diameter 14 of the thread 3.

[0047] The external thread 3 in the first end 10 of the body 8 has preferably a length of less than five times the pitch of the thread beginning from the end of the body, opposite to the head 12 of tool 7. Depending on the practical application the thread may have a length of less than four times the pitch of the thread. Preferably the external thread comprises 2 - 4, preferably three rounds of threads i.e. its length is at least three times the pitch of the thread.30

[0048] Distance between the head 12 and the external thready 3 i.e. axial length of the shank is preferably more than five, more preferably ten times of the major diameter 14 of the thread. Length of the shank is selected to suit the purpose of the actual practical application.P38472PC00 / JRI 2024121912459

[0049] Preferably the tool is made of hardened high tensile strength material. Hardness of the material of the tool is greater than hardness of the treated material, for example more than 10 % of hardness of the treated material.

[0050] In the figure 1 on the left-side bolt hole 23 there is the thread treatment5 tool 7 according to an embodiment of the invention arranged at its position for use. The thread treatment tool is 7 positioned such that the external tread 3 is at position of the end of the axial region 2 which will house the external thread 6 of the bolt 4. The end of the axial region 2 is referred to as a tread area 5 in the figure 1. Axial location of the tread area 5 is determined by location of an end of threaded portion of attachment bolt 4 in the hole 23. This way, the axial region 2 of the internal thread 1 which will house an external thread 6 of an attachment bolt 4, an appropriate treating area of the internal thread 1 is determined.

[0051] Next, axial force, i.e. pulling force in the set up shown in the figure 2 is applied to a thread area 5 at an end in the axial region 2 of the internal thread 1.15 The axial force is applied in a same direction as applied when tightening the attachment bolt 4, when the internal thread 1 is used for securing function in the force transmission rod. The applied axial force is predetermined and set such that yield strength of the material of the force transmission rod in the internal thread 1 is exceeded and a negative residual stress is created at the thread area20 5.

[0052] After creating high yielding and high negative residual stress at thread area, the tool 7 is replaced by normal tightening bolt 4.

[0053] Negative residual stress refers to residual stress that is compressive in nature and remains within a material after it has been subjected to loading during25 the method. It is beneficial in enhancing fatigue resistance, delaying crack initiation, and improving wear resistance.

[0054] Figure 3 discloses a stress-strain diagram for an exemplary material illustrating the method according to the invention. The diagram shows elastic range by numeral 30 and plastic range by numeral 32. Yield strength of the material is30 indicated by the numeral 34. When the method is applied the yield strength 34 of the material of the threaded hole is exceeded by 1 to 10 %, as indicated by reference number 36. It has been found out that this provides excellent results particularly when applied to internal thread of a connection rod of an internalP38472PC00 / JRI 20241219124510 combustion piston engine. More precise control of negative residual stress is obtained by exceeding the yield strength by 2 to 8 %.

[0055] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred em¬5 bodiments, it is obvious to the skilled person that, along with the technical progress, the basic idea of the invention can be implemented in many ways. The invention and its embodiments are thus not limited to the examples and samples described above but they may vary within the contents of patent claims and their legal equivalents. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.REFERENCE NUMERAL LIST1 internal thread15 2 axial region3 external thread4 attachment bolt5 thread area6 external thread7 thread treatment tool8 body9 longitudinal axis10 first end11 second end25 12 head13 axial length14 major diameter15 shank diameter16 shankP38472PC00 / JRI 2024121912451118 crank shaft20 force transmission rod21 big end22 gap5 23 bolt hole

Claims

P38472PC00 / JRI 20241219124512Claims1. Method of increasing strength of an internal thread (1) in a threaded5 hole which is to be used under cyclic load in a reciprocating piston engine, the method comprising determining an axial region (2) of the internal thread (1) which will house an external thread (6) of an attachment bolt (4) to form a bolt joint with the internal thread (1) when the bolt joint is used for securing function in the threaded hole, and applying axial force to a thread area (5) at an end in the axial region (2) of the internal thread (1), the axial force is applied in a same direction as applied when tightening the attachment bolt (4), when the internal thread (1) is used for securing function in the threaded hole, and setting the applied axial force such15 that yield strength of the material of the threaded hole is exceeded and a negative residual stress is created at the thread area (5).

2. A method according to claim 1 , characterized in that method comprises increasing strength of a threaded hole for an attachment bolt in a connecting rod of an internal combustion piston engine.20 3. A method according to claim 1 or 2, characterized in that axial length (13) of the thread area (5) to which axial force is applied comprises two to four rounds of threads, at an end of the axial region (2) of the internal thread (1).

4. A method according to claim 3, characterized in that axial length (6) of the thread area (5) to which axial force is applied is three rounds of thread.

5. Method according to claim 1 or 2, characterized in that the method comprising guiding a thread treatment tool (7), the tool comprising a straight body (8) elongated in an axial direction (9), wherein a first end (10) of the body (8) is provided with an external thread (3) and a second end (11) of the body (8) is provided with a head (12) for coupling the30 treatment tool with a force device (13), and the thread has a predetermined major diameter (14) and axial length (13) of which is shorter than the major diameter (14) of the thread, and the body (8) comprises aP38472PC00 / JRI 20241219124513 unthreaded shank (16) having a diameter (15), which shank diameter (17) is smaller than the major diameter (14) of the thread, into the internal thread (1) by rotating the tool about its longitudinal axis (9) in the internal thread,5 placing the external thread (3) of the thread treatment tool (7) axially to the thread area (5) of the internal thread (1), which is at an end of an external thread section (6) of an attachment bolt (4) when the internal thread (1) is used for securing function in the threaded hole, and applying axial force to the internal thread (1) via the external thread (3) of the thread treatment tool (7) in a same direction as when tightening the attachment bolt (4) to the internal thread (1), when the internal thread (1) is used for securing function in the threaded hole, and controlling the force applied by the external thread (3) of the treatment tool such that yield strength of the material of the threaded hole is exceeded and a nega¬15 tive residual stress is created at the thread area (5) of internal thread (1).

6. A method according to claim 1 or 4, characterized in that yield strength of the material of the threaded hole is exceeded by 1 to 10%.

7. A method according to claim 5, characterized in that the thread treatment tool (7) used in the method is according to any one of the claims 8 to 12.20 8. A thread treatment tool (7) for increasing strength of a thread in a threaded hole which is to be used under cyclic load in a reciprocating piston engine, characterized in that the tool comprises a straight body (8) elongated in axial direction (9), and that a first end (10) of the body (8) is provided with an external thread (3) and a second end (11) of the body (8) is provided with a head (12) for coupling the treatment tool with a force device (13), and that the external thread (3) has a predetermined major diameter (14) and axial length (6) which is shorter than the major diameter (14) of the thread, and that the body (8) comprises a unthreaded shank (16) having a diameter (15), which shank diameter (17) is smaller than the major diameter (14) of the thread.30 9. A thread treatment tool (7) according to claim 9, characterized in that the thread in the first end (10) of the body (8) has length of less than five times the pitch of the thread beginning from the end of the body (8).P38472PC00 / JRI 2024121912451410. A thread treatment tool (7) according to claim 9, characterized in that the thread in the first end (10) of the body (8) has two to four rounds of threads.

11. A thread treatment tool (7) according to claim 11 , characterized in that the thread in the first end (10) of the body (8) has three rounds of threads.5 12. A thread treatment tool (7) according to claim 9, 10 or 11 , characterized in that axial length (6) of the shank is more than five times of the major diameter (14) of the thread.

13. A thread treatment tool (7) according to claim 12, characterized in that axial length of the shank (16) is more than ten times of the major diameter (14)10 of the thread.

14. A thread treatment tool (7) according to claim 8, characterized in that the tool is made of hardened high tensile strength material, such as steel.

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