Patent Publication Number: US-2022213922-A1

Title: Tie bolt and process for manufacturing a tie bolt

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority from Italian patent application no. 102019000006813 filed on 14 May 2019, the entire disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a tie bolt and to a process for manufacturing such tie bolt. 
     In particular, the present invention relates to a tie bolt for automotive applications and a process for manufacturing a tie bolt of such type. 
     BACKGROUND ART 
     A tie bolt of the known type is a single body having a shank and a head, which protrudes at one end of said shank. The shank is substantially a cylindrical body having a longitudinal axis of rotation, while the tie bolt head can have different shapes according to the specific use. Generally, the head protrudes both axially and radially outside the shank. In particular, a tie bolt has a length, i.e. an extension along the rotation axis, significantly greater than its diameter. As is known, a tie bolt can have a length greater than 150 mm. For example, in the case of automotive applications, a tie bolt can have a length greater than 200 mm, preferably about 300 mm. 
     In more detail, the present invention relates to a special tie bolt, i.e. a tie bolt made of a material lighter than steel, highly performing for automotive applications. In particular, the present invention relates to a tie bolt made of a material with high mechanical strength, for example made of titanium alloy. 
     In the common practice, different problems are encountered for manufacturing tie bolts. 
     In fact, generally the head of a tie bolt is made by forging, using automatic molding machines. However, the automatic molding machines generally in use are unable to make tie bolts having a shank of sufficient length for manufacturing a tie bolt. In this case, therefore, a molding machine other than the standard design should be provided, in order to be able to make the tie bolts. This can be conceived mainly for tie bolts made of steel or other materials of common application, but it is not convenient for the production of special tie bolts, i.e. made of material with high mechanical resistance (for example made of titanium alloy) given the difficulties of processing due to the type of material and the low quantities of tie bolts which are generally commissioned. 
     Alternatively, a tie bolt can be made by using a semi-automatic press, however also in this case the process is particularly expensive and slow. However also in this case there is the drawback of having to use equipment designed ad hoc different from the standard. 
     The problems indicated above increase when the tie bolt must be made of a material lighter than steel with high mechanical resistance, for example titanium and alloys thereof, due to the process specifications that these materials require. 
     DISCLOSURE OF INVENTION 
     The object of the present invention is to provide a process for manufacturing a tie bolt, in particular made of highly performing material, which allows the drawbacks mentioned above to be overcome. 
     The object of the present invention is to provide an improved tie bolt which allows the drawbacks described above to be overcome. 
     According to the present invention, a process is provided for manufacturing tie bolts as mentioned in the appended claims. 
     According to the present invention, a tie bolt of the improved type is provided as mentioned in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described with reference to the accompanying drawings, which illustrate some non-limiting embodiment examples, wherein: 
         FIG. 1  schematically shows a tie bolt according to the present invention; 
         FIG. 2  is an exploded view of  FIG. 1 ; 
         FIG. 3  shows, in section and on an enlarged scale, the detail III of  FIG. 1 ; 
         FIGS. 4 and 5  are schematic views and show respective different embodiments of a threaded tightening between a nut and a shank coupled to one another for forming a tie bolt according to the present invention; and, 
         FIG. 6  is a block diagram for illustrating schematically some steps of a process according to the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In  FIG. 1 , reference numeral  1  indicates as a whole a tie bolt  1  according to the present invention. 
     Advantageously, the tie bolt  1  is particularly suitable for tightening bodies subjected to high dynamic stresses, i.e. vibrations. For example, the tie bolt  1  is configured for tightening components of a vehicle engine or in a turbine engine. In other words, the tie bolt  1  according to the present invention can find application for example in the automotive and aerospace sector. The tie bolt  1  is configured for tightening components in extreme temperature and vibrational conditions. 
     Advantageously, the tie bolt  1  is configured for withstanding high temperatures, in particular temperatures higher than the melting point of steel, such as for example temperatures higher than 1300° C. 
     According to what illustrated in  FIG. 2 , the tie bolt  1  comprises a shank  2  and a nut  3 , i.e. two distinct bodies, which are tightened and locked together, as better illustrated below, so as to form a single body. In other words, the tie bolt  1  according to the present invention is not made in a single monolithic piece. 
     Advantageously, the shank  2  is made of a material with high mechanical strength. Advantageously, the shank  2  is made of a material chosen within the following group of materials: titanium, titanium alloys; nickel-chrome alloys generally known as INCONEL®; X1CrNiMoAlTi12-11-2 stainless steel generally known as MLX17®; X1NiCrMoAlTi12-10-2 stainless steel generally known as MLX19®; and stainless steel with a PH (Precipitation Hardening) class ranging from 13 to 8 or from 15 to 5 or from 17 to 4; steel with composition Si, 19.00-21.00 Cr, 33.00-37.00 Ni, 9.00-11.00 Mo, 1.00 max. Ti, 0.01 B, 1.00 max Fe, Bal Co generally known as MP35N®; steel comprising nickel and cobalt generally known as MARAGING® and/or VASCOMAX®, for example MARAGING300 or AERMET®100; AISI4340 and AFNOR30NiCrMo16 steel, and stainless steel of the AISI300 series and of the AISI400 series. 
     Advantageously, the nut  3  is made of a material with high mechanical strength. Advantageously, the nut  3  is made of a material chosen within the following group of materials: titanium, titanium alloys; nickel-chrome alloys generally known as INCONEL®; X1CrNiMoAlTi12-11-2 stainless steel generally known as MLX17®; X1NiCrMoAlTi12-10-2 stainless steel generally known as MLX19®; and stainless steel with a PH (Precipitation Hardening) class ranging from 13 to 8 or from 15 to 5 or from 17 to 4; steel with composition Si, 19.00-21.00 Cr, 33.00-37.00 Ni, 9.00-11.00 Mo, 1.00 max. Ti, 0.01 B, 1.00 max Fe, Bal Co generally known as MP35N®; steel comprising nickel and cobalt generally known as MARAGING® and/or VASCOMAX®, for example MARAGING300 or AERMET®100; AISI4340 and AFNOR30NiCrMo16 steel and of the AISI300 series and of the AISI400 series. 
     According to an embodiment, the shank  2  and the nut  3  are made of the same material. 
     According to a further embodiment, the shank  2  and the nut  3  are made of different materials. In this case, advantageously, it is possible to choose the material of the shank  2  and of the nut  3  according to the specific application. For example, it is possible to choose the material of the shank  2  and of the nut  3  according to the coefficient of friction that is desired, in order to guarantee the desired fixing. Or, it is possible to select the material of the shank  2  and of the nut  3  so as to use a lighter material for a component less stressed during use, for example for the nut  3 , so as to obtain a tie bolt  1  which is lightened as much as possible and which allows at the same time the full tightening capacity. 
     For example the nut  3  is made of aluminum, or an alloy thereof, and the shank  2  is made of titanium, or an alloy thereof, in this case advantageously: the tie bolt  1  has a reduced weight (aluminum has a density lower than the density of titanium); the locking between shank  2  and nut  3  is safer since the coefficient of friction for aluminum on titanium (&gt;0.4) is generally higher than the coefficient of friction for titanium on titanium (&gt;0.3); the shank  2  made of titanium guarantees the mechanical resistance of the tie bolt  1  to vibrations and high temperatures. 
     According to what shown in  FIG. 4 , the shank  2  is an at least partially cylindrical body having a longitudinal axis of rotation X. The shank  2  has a head end  4 , which is configured for coupling with the nut  3 , and a foot end  5  opposite the head end  4 . The shank  2  has a length l 1 , i.e. an extension along the longitudinal axis X of at least 150 mm. 
     Preferably, for automotive applications, the shank  2  has a length l 1  greater than 200 mm, generally about 300 mm. According to the illustrated example, the shank  2  is a cylindrical body having a constant outer diameter dr along the entire length l 1  thereof. According to a variant not shown, the shank  2  can have some portions with a different diameter (greater or smaller) than the outer diameter dr at the head end  4  and/or the foot end  5 . 
     According to a variant not shown, the shank  2  can have a section with a different shape, for example with a polygonal profile. 
     Advantageously, the head end  4  has a threaded head portion  6 . 
     According to the illustrated example, the shank  2  has a further threaded portion  7 . In the illustrated example, the further threaded portion  7  is made at the foot end  5 , therefore hereinafter it is identified as foot portion  7 . According to a variant not shown, the further threaded portion  7  is made in a different position, for example in an intermediate position, of the shank  2 . 
     Advantageously, the nut  3  has a longitudinal axis of rotation X and a cavity  8  substantially coaxial with said longitudinal axis X ( FIGS. 2 and 3 ). The cavity  8  has an internal threaded portion  9  (shown in  FIG. 3 ). 
     Advantageously, the nut  3  and the shank  2  are configured so that the minimum length of the gripping thread is such that the yield strength Td of the nut  3  is substantially equal to the yield strength Tg of the shank  2 . 
     The yield strength Td of the nut  3  is given by the following relationship: 
     
       
         
           
             
               T 
               ⁢ 
               d 
             
             = 
             
               
                 σ 
                 d 
               
               ⁢ 
               
                 π 
                 4 
               
               ⁢ 
               
                 
                   ( 
                   
                     0 
                     , 
                     9 
                     × 
                     
                       ∅ 
                       
                         n 
                         ⁢ 
                         d 
                       
                     
                   
                   ) 
                 
                 ^ 
               
               ⁢ 
               2 
             
           
         
       
     
     wherein: 
     σ d  is the yield stress of the material with which the nut  3  is made; 
     Ø nd  is the nominal diameter of the thread f 2  of the nut  3 . 
     The yield strength Tg of the shank  2  is given by the following relationship: 
     
       
         
           
             Tg 
             = 
             
               
                 σ 
                 g 
               
               ⁢ 
               
                 
                   sin 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   30 
                 
                 
                   0 
                   , 
                   75 
                   × 
                   π 
                   × 
                   
                     ∅ 
                     
                       n 
                       ⁢ 
                       g 
                     
                   
                   × 
                   
                     l 
                     min 
                   
                 
               
             
           
         
       
     
     wherein: 
     σ g  is the yield stress of the material with which the shank  2  is made; 
     Ø ng  is the nominal diameter of the thread f 1  of the shank  2 . 
     l min  is the minimum thread length f 1  of the shank  2  gripping the nut  3 . 
     It is known that the length of a thread is given by the following relationship: 
     
       
      
       l=p×n 
       f  
      
     
     wherein: 
     p is the pitch of the thread, which according to the known art is the longitudinal distance between two consecutive threads of the same helix; 
     n f  is the number of threads. 
     On the basis of the above formulas, once defined by the project, for example, the materials of the nut  3 , of the shank  2  and the value of the nominal diameter Ø ng  it is possible to determine the minimum length of the thread f 1  coupled with the thread f 2  of the nut  3 . 
     According to the illustrated example, the nut  3  has an operating portion  10 , which is configured for being engaged, in use, by a tool (not shown) to rotate the nut  3  around the longitudinal axis of rotation X. 
     According to the illustrated example, the nut  3  has a flange  11  which protrudes radially outside the operating portion  10 . The flange  11  is substantially a thin body and has a head surface  12 , which faces the operating portion  10 , and an opposed foot surface  13 . According to the illustrated example, the nut  3  has a collar  14  which protrudes axially from the foot surface  13  of the flange  11  and is coaxial with the longitudinal axis X′. According to the illustrated example, the collar  14  has a diameter greater than the diameter dr of the corresponding shank  2 . According to the illustrated example, the cavity  8  faces the outside of the nut  3  through an opening  15  made at the collar  14 . 
     Advantageously, the collar  14  allows centering and align the shank  2  with the nut  3 . According to the illustrated example, the collar  14  is threaded internally. According to a variant not shown, the collar  14  is not threaded. 
     According to a further variant, not shown, the collar  14  has a seat sized so as to obtain a coupling with the head portion  6  with a hole basis-shaft basis type of precision, so as to provide for the alignment of the shank  2  with respect to the nut  3 . 
     According to a variant not shown, the nut  3  can be devoid of the collar  14 . 
     In addition or as an alternative to the above, the alignment between shank  2  and nut  3  can be obtained by means of the flange  11 , which in this case is made in such a way to have a seat sized so as to obtain a coupling with the head portion  6  with a hole basis-shaft basis type of precision. 
     As shown in  FIGS. 2 and 3 , the nut  3  is dead. 
     In other words, the cavity  8  is not through, i.e. it faces the outside through a single opening  15 . 
     As shown in  FIG. 3 , the shank  2  is screwed into the cavity  8  so as to form the tie bolt  1 . 
     As illustrated in detail in Figures from  3  to  5 , the head portion  6  of the shank  2  has a thread f 1  and the internal threaded portion  9  of the cavity  8  of the nut  3  has a thread f 2 . 
     The threads f 1  and f 2  are configured for coupling and tightening to one another. Advantageously, the threads f 1  and f 2  are self-locking. In other words, the thread f 1  and the thread f 2  are configured for generating, during the screwing between shank  2  and nut  3 , a joint which opposes the unscrewing. 
     Advantageously, the cavity  8  has a housing  19 A which is substantially, as shown in the figure, a bottom portion of the nut  3 . Advantageously, the nut  3  is dead, i.e. it has a bottom wall  23  which is substantially transverse to the longitudinal axis X′ and closes the cavity  8  so that the nut  3  is substantially a cup-shaped body. Advantageously, the head portion  6  has an end portion  19 B which protrudes axially from the thread f 1 . The end portion  19 B is configured for fitting inside the housing  19 A. 
     Advantageously, the end portion  19 B is placed in contact, in use, against the bottom wall  23 . In this way, it is possible to obtain a pre-load force Fc, as better illustrated below, which will allow fitting the thread f 1  with the thread f 2 , guaranteeing the locking. 
     According to the illustrated example, the bottom wall  23  is delimited by a bottom surface  24  which faces the inside of the cavity  8 . The bottom surface  24  is substantially conical and coaxial with the longitudinal axis. 
     Advantageously, the inclination of the bottom surface  24  with respect to the longitudinal axis allows adjusting the pre-load force according to the degree of screwing of the shank  2  inside the nut  3 . 
     Advantageously, the inclination of the bottom surface  24  allows facilitating the alignment between the shank  2  and the nut  3 . 
     The thread f 1  and the thread f 2  are chosen within a group comprising a plurality of pairs of self-locking profiles.  FIGS. 4 and 5  show some examples of possible profiles of self-locking threads. It is noted that  FIGS. 4 and 5 , by way of example, show some known profiles of self-locking threads. According to some variants, not shown, the threads f 1  and f 2  can have different profiles. 
     According to the example shown in  FIG. 4 , the thread f 1  has two sides, hereinafter identified with side  16 I and  16 II connected to one another by a head fitting  16 III, which according to the illustrated example is flat. The sides  16 I and  16 II are substantially mirror-like to each other, i.e. they have an equal and opposite inclination with respect to a plane π (illustrated with a dashed line in  FIG. 4 ) perpendicular to the longitudinal axis X. 
     Furthermore, the longitudinal extension of the sides  16 I and  16 II is substantially the same. 
     Whereas, the thread f 2  is asymmetrical, i.e. the thread f 2  has two sides, hereinafter identified with  18 I and  18 II, connected to oner another by a head fitting  18 III, which according to the illustrated example is flat. The sides  18 I and  18 II have different inclinations with respect to a plane π 1  (illustrated with a dotted line in  FIG. 4 ) perpendicular to the longitudinal axis X′. Furthermore, the sides  18 I and  18 II have different longitudinal extensions. In more detail, the side  18 I forms an angle α with the plane π 1 , while the side  18 II forms an angle β with the plane π 1 . In particular, the angle α is smaller than the angle β. 
       FIG. 5  shows a variant of a combination of the threads f 1  and f 2 . According to the example illustrated in  FIG. 5 , the thread f 2  has a further fitting portion  18 IV, which connects the side  18 II with the adjacent side  18 I. The portion  18 IV is inclined with respect to the respective portion  18 I and is configured for coming into contact and creating a shoulder, during use, for the thread f 1 . 
     In the examples illustrated in  FIGS. 4 and 5 , the thread f 1  (male) is symmetrical and the thread f 2  (female) is asymmetrical. According to a variant not shown, the thread f 1  (male) is asymmetrical and the thread f 2  (female) is symmetrical. 
     Advantageously, by providing threads f 1  and f 2  with self-locking profiles, once tightened on the shank  2 , the nut  3  is indissolubly fixed to the shank  2  itself and forms a tie bolt  1 , which behaves like a tie bolt  1  of known type manufactured in one piece. 
     Advantageously, the fact that the nut  3  is dead, i.e. it has the bottom wall  23 , allows obtaining a pre-load force Fc between nut  3  and shank  2 , which in particular acts on the threads f 1  and f 2  locking them to one another. The pre-load force Fc is the force resistant to the thrust exerted by the end portion  19 B on the bottom wall  23  following the insertion and screwing of the head portion  6  inside the cavity  8 . 
     According to a variant not shown, the thread f 1  and the thread f 2  can have normal profiles, i.e not self-locking. In this case, the shank  2  and the nut  3  are fixed to one another by interference. In particular, exploiting a hot/cold connection, as better illustrated below. 
     Advantageously, the fact that the shank  2  and the nut  3  are both made of titanium alloy increases the locking effect between nut  3  and shank  2 . In fact, titanium has a static and dry coefficient of friction &gt;0.3 (for example the static and dry coefficient of friction for Titanium Alloy Ti-6Al-4V (Grade 5) on Titanium Alloy Ti-6Al-4V (Grade 5) is about 0.36) this allows obtaining a particularly firm and resistant locking between the nut  3  and the shank  2 . In other words, the fact that the nut  3  and the shank  2  are made of a titanium alloy increases the safety of the fixing of the components to one another. 
     Advantageously, the fact that the shank  2  and the nut  3  are made of different materials can allow increasing the locking effect between the two. For example, if the shank  2  is made of titanium and the nut  3  is made of aluminum, this allows obtaining a static and dry coefficient of friction &gt;0.4 (for example the static and dry coefficient of friction for Titanium Alloy Ti-6Al-4V (Grade 5) on Aluminum Alloy 6061-T6 is approximately 0.41). 
     A process for manufacturing a tie bolt  1  according to the present invention is described here below, referring also to the block diagram illustrated in  FIG. 6 . 
     The process involves a step of arranging a shank  2  (schematized with the block A) and a step of arranging a nut  3  (schematized with the block B). The process also involves a step of tightening, locking, the nut  3  to a threaded head portion  6  of the shank  2  (schematized with the block C). 
     The step of arranging a shank  2  and the step of arranging a nut  3  can be performed both simultaneously and in succession. 
     The step of arranging a shank  2  involves the sub-steps of:
         arranging a bar  20 A of metallic material, in particular a high resistance metal (sub-block IA), advantageously the bar  20 A is calibrated;   obtaining, in particular by cutting, from the bar  20 A a shank  21 A with a predetermined length l 1  (sub-block IIA), advantageously, the length l 1  substantially corresponding to the length of the final shank  2 ; and   making a thread f 1  along the head portion  6  for obtaining the shank  2  (sub-block IIIA).       

     The step of arranging a nut  3  comprises the sub-steps of:
         arranging a bar  20 B or wire (variant not shown) of metallic material, in particular a metal with high resistance (sub-block IB);   obtaining, in particular by cutting, a shank  21 B with a predetermined length (sub-block IIB); and   forging the shank  21 B, in particular by means of an automatic molding machine, so as to obtain a semi-finished product  22  substantially with the external shape of the nut  3  and having a cavity  8  (sub-block IIIB), advantageously the nut  3  is dead and has a bottom wall  23 ;   making a thread f 2  inside the cavity  8  for completing the construction of the nut  3  (sub-block IVB).       

     As previously mentioned, the manufacturing process also involves the tightening of the nut  3  on the shank  2 . The tightening step can simply comprise the sub-step of screwing the nut  3  onto the head portion  6 . In this case, the locking of the nut  3  on the portion  6  of the shank  2  is given by the interaction of the self-locking threads f 1  and f 2 . 
     Advantageously, during the tightening step, the threads f 1  and f 2  are screwed so as to bring the bottom portion  19 B into contact with the bottom surface  24  of the nut  3 . Advantageously, during the tightening step, the threads f 1  and f 2  are screwed to one another so as to push the bottom portion  19 B against the bottom surface  24 , so as to generate a pre-load force Fc which acts on the shank  2  and ensures the tightening between the shank  2  and the nut  3 . The pre-load force Fc is basically a function of the screwing level between the thread f 1  and the thread f 2 . 
     Advantageously, the pre-load force Fc generated on the shank  2  is given by the inclination of the bottom surface  24  with respect to the longitudinal axis X′ of the nut  3  (and consequently of the shank  2 ). 
     Optionally, the tightening step can involve a heat treatment step of the nut  3  and/or the shank  2 . For example, the tightening step can involve heating the nut  3 , so as to perform a coupling by hot/cold interference. 
     Optionally, according to the example shown in  FIG. 6 , a further threaded portion  7  is realized on the shank  2  having a thread f 3  (dashed IVA sub-block). According to the illustrated example, the further thread f 3  is made along a foot portion  7 . 
     According to a variant not shown, the thread f 3  can be made after the step of tightening the nut  3  on the shank  2 . 
     Advantageously, both the thread f 1  and the thread f 3  are made by rolling. Advantageously the nut  3  and the shank  2  are made of the same material and this for tribological attributes facilitates the locking by seizing, i.e. welding, of the thread f 1  on the thread f 2  by chemical affinities, especially with the titanium-based alloys. 
     In the illustrated examples, the thread f 1  and the thread f 2  are at a beginning. 
     The process of the type described above is simple and quick to implement and allows the use of machinery generally already in use by manufacturers of tightening components, such as screw manufacturers. 
     Advantageously, a tie bolt  1  of the type described above is made with significantly reduced costs and production time, compared to the tie bolts in a single piece and of a known type. 
     Advantageously, the tie bolt  1  of the type described above has a high fatigue resistance, in particular in the junction area between the shank  2  and the nut  3 . Therefore, advantageously, a tie bolt  1  of the type described above can be used in environments highly stressed to vibrations, such as for example the engine base of a vehicle. 
     The tie bolt  1  of the type described above is resistant to high temperatures. 
     Advantageously, a tie bolt  1  of the type described above guarantees the same reliability and the same tightening properties of a monolithic tie bolt  1  of a known type. 
     Advantageously, the fact that the shank  2  and the nut  3  are made of different materials (for example shank  2  made of titanium alloy and nut  3  made of aluminum alloy) allows obtaining a tie bolt  1  which guarantees high performance and, at the same time, is lightened.