Abstract:
A method of hardening metal parts, such as a rotor or drive link, and parts thus produced. Preferably the method includes imparting residual compressive stresses into the metal parts and uses various small ball type structures to create small compressions in the surfaces of the metal parts. The compressions apply residual stresses to the parts, which strengthen the metal. The substantial uniform ball type structures are pressed into the metal part to control the application of stress into the part and maintain substantial uniform properties within the metal part, which resists future stresses as the part is used in its desired machinery and/or processes.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application is a Non-Provisional Utility application which claims benefit of co-pending U.S. Provisional Patent Application Ser. No. 60/733,221 filed Nov. 3, 2005, entitled “METHOD FOR IMPARTING RESIDUAL COMPRESSIVE STRESS IN METAL PARTS” which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to increasing the wear characteristics of metal. More specifically, this invention relates to increasing the fatigue life, or hardening the metal, by imparting compressive stresses, such as residual compressive stresses, in the metal.  
         [0003]     It is known in the art to use small spherical items, such as shot or balls, to change various characteristics in the properties of a work piece. In most examples this shot is sent, or fired through the air, by a machine that imparts kinetic energy into the shot and directs the shot at the work piece. The shot then impacts against the work piece. This process is typically known as shot peening.  
         [0004]     Various attempts in the art have been made in order to try and improve this shot peening process. For example, U.S. Pat. No. 4,689,921 discloses improving a ceramic turbine rotor by rotating the rotor in abrasive grindstones to round the edges of the rotor.  
         [0005]     U.S. Pat. No. 5,125,191 discloses putting a work piece into a closed chamber and moving the work piece or chamber in an abrasive medium located in the chamber to adjust the characteristics of the work piece. This patent discusses by using the abrasive nature of the medium to generally smooth the work piece. This patent hones or abrades machine parts but does not impart any compressive stress on the work piece or affect the overall fatigue life of the work piece.  
         [0006]     U.S. Pat. No. 6,490,899 and U.S. Patent Application Publication 2003/0115922 generally disclose the use of ultrasonic energy to peen the blades of a rotor. These disclosures are generally focused on the external surfaces of those blades.  
         [0007]     U.S. Pat. No. 4,974,434 discloses a controlled shot peen method and a device for bending or leveling a work piece by shot peening. This patent fails to recognize the benefits of imparting compressive stresses in a work piece or the capability of increasing the fatigue life of internal components of a work piece.  
         [0008]     U.S. Pat. No. 6,009,980 discloses a ductal iron vehicle hub and a method for producing the same. This patent uses the conventional method of “shooting” the shot at the work piece and does not teach, and fails to realize the benefit of, adding compressive stresses to the internal components of a work piece.  
         [0009]     The advantage of having the capability of improving the fatigue life on the internal components of a metal part can be very beneficial in the application of rotors as used in engines or motors. The rotor typically has numerous teeth that project inwardly towards the axis. These teeth are used to transmit the torque and power from one portion of the motor to the next, typically from the shaft to the drive output. At any given moment only one internal tooth of the rotor could be engaged with the drive link. As such, a large amount of stress can be applied to any one individual tooth numerous times during the operation of the motor. It therefore becomes beneficial to increase the useful life, durability, and fatigue resistance of the individual internal teeth on a rotor.  
         [0010]     What is needed then is a rotor having internal components that have been processed to increase the wear characteristics thereof and a method of producing such a rotor. Preferably this rotor has been processed by the application of compressive stresses to the individual teeth of a rotor while the method is cost effective, consistent, and reliable. Preferably the method could produce a drive link that has improved wear characteristics such that the drive link and rotor, two parts of a motor that typically experience the greatest levels of stress, can be improved such that the overall useful life and potential efficiency of the motor is increased. This needed rotor, drive link, and method of producing the same is lacking in the art.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     Disclosed herein is a method of hardening metal parts, such as a rotor or drive link, and parts thus produced. Preferably the method includes imparting residual compressive stresses into the metal parts and uses various small ball type structures to create small compressions in the surfaces of the metal parts. The compressions apply residual stresses to the parts, which strengthen the metal. The substantial uniform ball type structures are pressed into the metal part to control the application of stress into the part and maintain substantial uniform properties within the metal part, which resists future stresses as the part is used in its desired machinery and/or processes.  
         [0012]     Also disclosed is a method for producing a rotor. The method includes providing substantially spherical indenting elements and a metal rotor having an inner surface. The inner surface of the rotor includes a plurality of internal projections. The method includes forceably pressing the indenting elements against the inner surface of the rotor to induce compressive stresses in the inner surface to improve the fatigue life of the rotor.  
         [0013]     Preferably the method further includes forceably compressing the entire width of the inner surface of the rotor to induce the compressive stresses therein. Each internal projection can include a first and a second side wherein the indenting elements are forceably pressed against both sides of each internal projection to induce the compressive stresses. The inner surface of the rotor can further include a circumferential base section extending between each internal projection wherein the indenting elements are pressed against the base section. The indenting elements are preferably substantially spherical and can have a diameter in the range of approximately 0.5 millimeters to approximately 0.4 millimeters.  
         [0014]     In an alternate embodiment the method further includes using a drive link having an outer surface substantially corresponding with the inner surface of the rotor to press the indenting elements against the inner surface of the rotor to induce the compressive stresses in the rotor. The use of a drive link can induce compressive stresses in the outer surface of the drive link thereby improving the fatigue life of the drive link through the forceably pressing the indenting elements between the outer surface of the drive link and the inner surface of the rotor. The drive link can include an exterior circumference substantially mirroring the base sections of the rotor such that the indenting elements can be forceably pressed against the exterior circumference of the drive link.  
         [0015]     Also disclosed is an improved metal rotor having an inner surface with a plurality of internal projections wherein each internal projection has an initial fatigue life. The fatigue life of the internal projections of the inner surface of the rotor have been increased by inducing compressive stresses in the internal projections to improve the overall fatigue life of the internal projections and the overall fatigue life of the rotor. The internal projections of the inner surface of the rotor have been preferably compressed under a static load by spherical indenting elements.  
         [0016]     Also enclosed is an improved rotor and drive link combination wherein the rotor and drive link have been formed by subjecting the inner surface of the rotor and the outer surface of the drive link to compressive stresses to increase the fatigue life of the rotor and the drive link.  
         [0017]     It is therefore a general object of the present invention to provide an improved rotor.  
         [0018]     Another object of the present invention is to provide an improved drive link.  
         [0019]     Another object of the present invention is to provide an improved rotor and drive link combination.  
         [0020]     Still another object of the present invention is to provide an improved method for producing an improved rotor and/or drive link.  
         [0021]     Still another object of the present invention is to provide a method for subjecting internal surfaces of a rotor to compressive stresses to increase the fatigue life thereof.  
         [0022]     Another object of the present invention is to provide a method that concurrently subjects the internal surface of a rotor and the external surface of a corresponding drive link to compressive stresses to improve fatigue life of both parts.  
         [0023]     Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon reading of the following disclosure when taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0024]      FIG. 1A  is an exploded assembly view of a motor including an example of a drive link and rotor assembly having a rotor.  
         [0025]      FIG. 1B  is an end view of the engaging portion of a drive link with a rotor.  
         [0026]      FIG. 2  shows a magnified view of a rotor made in accordance with the current disclosure.  
         [0027]      FIG. 3  shows a magnified view of a drive link made in accordance with the current disclosure.  
         [0028]      FIGS. 4A-4F  show pictorial representations of a method of manufacturing a rotor and drive link in accordance with the current disclosure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     Looking generally at  FIG. 1A  a motor is shown and generally designated by the numeral  10 . The motor includes among other features a rotor assembly  12  having a rotor  14  and a drive link  16  having an engagement end  18 . The drive link  16  is normally attached to a shaft  20  which provides power to the drive link  16 . This general configuration of a motor is known in the art.  
         [0030]     However, one of the improvements of the current disclosure is the rotor  14  having an inner surface  22  with a plurality of internal projections  24 , which can also be described as teeth  24 . The rotor  14  has an initial fatigue life and more specifically the teeth  24  have an initial fatigue life wherein the initial fatigue life of the teeth  24  of the inner surface  22  of the rotor  14  have been increased by the induction of compressive stresses as indicated by depressions  26  in  FIG. 2 . The manifestation of the compressive stresses by the depressions  26  in the teeth  24  improves the fatigue life of the teeth  24  of the inner surface  22  of the rotor  14 .  
         [0031]     As best illustrated in  FIG. 2 , the depressions  26  are preferably positioned along the entire width of the inner surface  22  of the rotor  14 . Additionally, the circumferential base section  28  of the inner surface  22  can also contain the depressions  26  thereby increasing the overall fatigue life of the inner surface  22 . Preferably both sides  30  and  32  of each tooth  24  have been introduced to the compressive stresses and exhibit the depressions  26 .  
         [0032]     Also included is the drive link  16  having an outer surface  34  substantially corresponding with the inner surface  22  of the rotor  14 . The outer surface  34  of the drive link  16  also includes initial fatigue life which has been increased by the induction of compressive stresses in the outer surface  34  as manifested by depressions  36  as best seen in  FIG. 3 . The external projections  38 , which can also be described as teeth  38 , of the engagement end  18  of the drive link  16  have been preferably subjected to compressive stresses along the full width of the engagement end  18  and on both sides  40  and  42  of each tooth  38 . The drive link  16 , and more specifically the engagement end  18  of the drive link  16  includes an exterior circumference  44  that substantially corresponds with the base section  28  of the rotor  14 . This exterior circumference  44  is also preferably subjected to the compressive stresses to increase the fatigue life thereof.  
         [0033]     Now referring generally to  FIGS. 4A-4F , an exemplary method of producing an improved rotor and/or improved drive link is shown. The method includes providing a rotor  14  having an inner surface  22  with the teeth  24 . Spherical indenting elements  46  ( FIG. 4C ), which can also be described as shot  46 , are also provided. The method includes forceably pressing the shot  46  against the inner surface  22  of the rotor  14  to induce compressive stresses in the rotor  14  to improve the fatigue life of the rotor  14 . The inducement of these compressive stresses is best illustrated by the depressions  26  as previously discussed.  
         [0034]     The method preferably includes forceably pressing the entire width of the inner surface  22  of the rotor  14  with the shot  46  as well as both sides  30  and  32  of the each tooth  24  as well as the base section  28  between the individual teeth  24  with the shot  46 .  
         [0035]     In a preferred embodiment the drive link  16 , with its outer surface  34  substantially corresponding with the inner surface  22  of the rotor  14 , is used to forceably press the shot  46  into the inner surface  22  of the rotor  14 . Additionally, the shot  46  can be pressed between the outer surface  34  and inner surface  22  such that both the outer surface  34  and inner surface  22  are compressively stressed, or prestressed, to increase fatigue life, durability, and wear characteristics of both the rotor  14  and drive link  16 .  
         [0036]     Alternately described, the method can include using a static load, or fixed load, to compressively stress, at individual locations, the inner surface  22  of a rotor  14  and the outer surface  34  of a drive link  16  in order to increase the wear characteristics, for example the fatigue life, of the rotor  14  and drive link  16 . This static load can be applied through the shot  46  that can create the depressions  26  and  36  in the rotor  14  and the drive link  16 , respectively.  
         [0037]     Referring back to  FIGS. 4A-4F , an exemplary method of producing the rotor and/or drive link can be described as follows. An assembly unit  50  is assembled to control the movement of the rotor  14  and drive link  16 . More specifically the assembly unit  50  can restrict and contain the rotor  14 , drive link  16 , and the shot  46  before, during, and after the application of the force used to impart the compressive stress in the parts  14  and  16 . The assembly unit  50  includes a bottom base  51  which can include a counter bore  54  on the center to allow various size rotors and drive links to be processed. A plunger  56  is placed into the counter bore  54  to establish a centering location for the rotor. This plunger  56  can include a plate  55  and stem  57  as shown.  
         [0038]     Next the rotor is inserted such that the stem  57  passes through the opening  13  of the rotor  14 . The shot  46 , which can also be described as balls  46 , is inserted into the opening  13  of the rotor  14 . Preferably this shot  46  has a diameter in the range of approximately 0.5 millimeters to 4.0 millimeters. Next the shot  46  can be mixed, stirred, or otherwise shifted or moved to facilitate that the inner surface, or at least the bottom portion of the inner surface, is substantially covered with the shot  46 .  
         [0039]     Next a plunger rod  58  is inserted onto the stem  57  of the plunger  56  and turned such that the splines  60  of the plunger rod  58  engage with the teeth  24 , which can also be described as rotor splines  24 , of the rotor  14 . In a preferred embodiment the plunger rod  58  is replaced with a drive link  16  having an engagement end  18  which corresponds to the rotor  14 . A retaining cap  62  is placed over the base  51 , which can also be described as housing  51 , and secured into place with fasteners, such as bolts. If needed a plunger alignment sleeve (not shown) can be positioned around a plunger rod to ensure proper force transfer.  
         [0040]     Next the assembly unit  50  is positioned in the bay of a press  64 . Retaining cap spacers  66  can be applied around a plunger rod  58  to hold the retaining cap in position and to keep the cap  62  from separating from the housing  51 . The press  64  is then brought down and brought into contact with first the retaining cap spacer  66  and then the plunger rod  58 . The hammer  68  of the press  64  engages the plunger rod  58  and continues pressing until a desired force is reached. Preferably this desired force is 25 tons, but can vary according to application and the type of metal of which the rotor  14  and/or drive link  16  are comprised. Preferably this force is held for a desired period of time and then released. This length of time is preferably approximately 5 seconds but can vary as desired. Preferably the force is then reapplied for another time period to ensure the application of proper compressive stresses into the parts.  
         [0041]     In one embodiment this application of force in the assembly unit imparts the preferred compressive stresses in the bottom section of the rotor. The assembly unit  50  can then be disassembled, the rotor flipped, and the process repeated until the other half of the rotor has been processed. In an alternate embodiment this force is applied throughout the full rotor in a single application.  
         [0042]     Various tests have been performed on rotors made in accordance with the current disclosure. These tests determined that after more than 806,000 motor revolutions there was an absence of noticeable and/or discernible chipping, nicks, pitting, or cracks in the teeth of the rotors. This is typically unprecedented in the metal parts industry and can be explained by the theory that the compressive stresses have created depressions and resist the effects of imposed bending stresses from external loads during the operation of the motors. As such these surfaces treated are much harder and fatigue resistance is greatly improved.  
         [0043]     Some metal items made in accordance with the current disclosure went through more than 1,000,000 test cycles before developing a failure point, or a chip on one of its teeth. The applicants are unaware of any prior art metal rotors that achieve these levels. The test data measuring the level of residual compressive stress in a metal part, or rotor, showed marked improvement in comparison to conventional metal rotors. For example, the level of residual compressive stress in a rotor made in accordance with the current disclosure measured 50,000 psi. The level of residual compressive stress in conventional rotors that had not been subject to the current disclosure measured 0 psi. In increased levels of compressive stress are believed to directly equate to longer life of the part. The test data indicates that rotors and drive links made in accordance with the current disclosure could increase this level to 150,000 psi. Additionally, parts made in accordance with the current disclosure possessed a recognizably more uniform residual compressive stress distribution than conventional parts, which can reduce the likely of weak spots in the rotor.  
         [0044]     Additional benefits received from the exposure of the parts to the compressive stresses and the subsequent depressions formed in the parts therein could also lead to improved rotor and/or drive link performance. For example, these depressions are believed to provide locations where islands of contained fluid, such as oil or lubricant, were stored during operation of the motors. This in essence provides a self-lubricating surface that can reduce friction in the operation of the rotors, drive links and motors. This can further facilitate a longer fatigue life and reduced wear on the parts.  
         [0045]     Thus, although there have been described particular embodiments of the present invention of a new and useful METHOD FOR IMPARTING RESIDUAL COMPRESSIVE STRESS IN METAL PARTS, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.