Abstract:
Described herein is a nut assembly including a nut and a fastener insert wherein at least one of the components is formed from a nitrogen strengthened stainless steel which is resistant to galling. In addition to reduced galling, fastener inserts formed from the nitrogen strengthened stainless steel exhibits good corrosion resistance and a yield strength which is almost twice that of fastener inserts formed from 304 type stainless steel.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    This is a continuation-in-part of U.S. patent application Ser. No. 09/498,821, entitled “Anti-Galling Fastener Inserts”.  
     
    
     
         [0002]    The present invention relates to nut assemblies and, more particularly, to anti-galling nut assemblies including a nut and a fastener insert disposed within the nut for securing a threaded fastener.  
           [0003]    Nut assemblies come in a variety of shapes, sizes, designs and materials. Many nut assemblies include not only a fastener such as a bolt, pin or screw, but also will include a fastener insert to be positioned within the nut. One specific type of fastener insert useful in association with a threaded fastener is the helically coiled wire insert as described in U.S. Pat No. 2,672,070 entitled “Wire Coil Screw Thread Insert for Molded Material”, for example.  
           [0004]    Helically coiled fastener inserts generally do not exhibit staking, locking or swaging and does not require keying in place, which tends to greatly reduce stress which would otherwise be transferred to the nut. While such helically coiled wire inserts are generally useful as anchoring mechanisms for threaded fasteners in order to be used in high strength applications, such inserts must be formed from high strength materials. Heretofore, 302/304 stainless steels have been used to manufacture fastener inserts.  
           [0005]    Due to the intimate connection between the threads of the fastener and the insert, shearing forces are exerted on the fastener insert upon attachment of the fastener. As a result of the shearing forces, fastener inserts formed from 302/304 type stainless steels have exhibited a propensity to galling despite such stainless steels being considered among the hardest in the industry.  
           [0006]    Although galling and wear may occur under similar conditions, the types of deterioration involved are not similar. Galling occurs as a result of movements occurring along a metal-to-metal contact in a localized area which results in grooving and self-welding of the metals at the localized areas. This, in turn, leads to seizure of the metal parts.  
           [0007]    On the other hand, wear is synonymous with abrasion and can result from metal-to-metal contact or metal-to-nonmetal contact. Such wear is characterized by relatively uniform loss of metal from the surface, as contrasted to localized grooving with consequent metal build-up, as a result of rubbing a much harder metallic surface against a softer metallic surface.  
           [0008]    An article by Harry Tanczyn, entitled “Stainless Steel Galling Characteristics Checked” in STEEL, Apr. 20, 1954 points out that stainless steel sections at a relatively high hardness level or with a substantial difference in hardness, exhibit better resistance to galling than the combination of two soft members. This may be explained by the theory that the hardened sections deform elastically near the contact points under loading, while the softer pieces yield plastically for a significant distance beneath the contact points. During movement, the hardened surfaces apparently recover elastically with decrease in pressure, and this motion tends to sever any metallic welding.  
           [0009]    Among the numerous prior art steels currently available, the austenitic Type 304 is suited to a variety of uses involving welding and fabrication, but the galling and wear resistance of this steel are poor and the metal is likely to fail when subjected to such conditions.  
           [0010]    In view of the perceived problems with galling, the art has recently turned to cobalt bearing and high nickel alloys which are known to fight wear and galling.  
         SUMMARY OF THE INVENTION  
         [0011]    It is therefore a primary object of the present invention to provide nut assemblies which tend to limit, if not eliminate, galling.  
           [0012]    To accomplish this objective, among others, the present invention relates to a nut assembly comprising a nut including a threaded barrel for receiving a threaded fastener and a fastener insert which assists in securing the fastener within the nut. At a minimum, the fastener insert will be formed from a nitrogen strengthened stainless steel alloy and, optionally, both the insert and nut can be formed from the alloy. The stainless steel alloy preferably will include a positive amount up to a total of about 0.8% nitrogen. More particularly, the present invention relates to a nut assembly including a fastener insert or both the fastener insert and nut formed from a nitrogen strengthened stainless steel alloy comprising: a) from about 0.05 to 0.15% carbon; b) from about 5.0 to 12.0% manganese; c) from about 2.0 to 6.0% silicon; d) from about 12.0 to 20.0% chromium; e) from about 6.0 to 12.0% nickel; f) from about 0.02 to 0.8% nitrogen; with the remainder being iron.  
           [0013]    Still more preferably, at least one of the nut or fastener inserts is formed from a nitrogen strengthened stainless steel alloy comprising a) from about 0.07 to 0.12% carbon; b) from about 7.0 to 10.0% manganese; c) from about 3.0 to 5.0% silicon; d) from about 14.0 to 18.0% chromium; e) from about 7.0 to 10.0% nickel; f) from about 0.06 to 0.3% nitrogen; with the remainder being iron.  
           [0014]    According to a highly preferred embodiment, the nut and/or fastener insert will be formed from a nitrogen strengthened austenitic stainless steel alloy comprising: a) from about 0.08 to 0.1% carbon; b) from about 7.0 to 9.0% manganese; c) from about 3.5 to 4.5% silicon; d) from about 16.0 to 18.0% chromium; e) from about 8.0 to 9.0% nickel; f) from about 0.08 to 0.18% nitrogen; with the remainder being iron.  
           [0015]    After forming the alloy into a nut using a conventional nut making machine, the nitrogen strengthened nuts are generally inventoried for later use. Similarly, the fastener inserts are formed from the alloy as a round wire shaped to a diamond cross section which is subsequently coiled to form the insert. The resulting nut assemblies should have excellent anti-galling characteristics at both ambient and elevated temperatures. Further, the nut assemblies should have good corrosion resistance and a room temperature yield strength. The nitrogen strengthened stainless steel nut assemblies of the present invention also provide excellent oxidation resistance and excellent impact strength, particularly at sub-zero temperatures.  
       
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a blown apart perspective view of an anti-galling nut assembly in accordance with the teachings of the present invention;  
         [0017]    [0017]FIG. 2 is an assembly view in cross-section of the nut assembly of FIG. 1;  
         [0018]    [0018]FIG. 3 is an assembly view in cross-section demonstrating the nut assembly and threaded fastener securing an object;  
         [0019]    [0019]FIG. 4 is a perspective view of a free running helically wound insert for use in a nut assembly;  
         [0020]    [0020]FIG. 5 is a perspective view of a self-locking helically wound insert for use in a nut assembly; and  
         [0021]    [0021]FIG. 6 is a cross-sectional view taken along line  6 - 6  of FIG. 5 demonstrating a locking convolution.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Referring to FIGS.  1 - 2 , there is shown an anti-galling nut assembly  10  including a nut  12  and a fastener insert  14 . According to the present invention, at least one of the components, i.e., the nut  12  or fastener insert  14  formed from a nitrogen strengthened stainless steel alloy as described below. The fastener insert  14 , which may be of a variety of shapes and sizes, is shown to be in the form of a helically coiled wire to be used in association with a threaded fastener  16 . As shown in FIG. 3, the insert is positioned within a threaded barrel  20  of the nut to secure an object  30  in combination with the fastener  16 .  
         [0023]    Preferably, the fastener inserts  14  are manufactured such that they are larger in diameter, before installation, than the threaded barrel  20  of the nut to ensure that they become firmly secured. For helically wound fastener inserts, it is preferable that the coils have about a 60° internal screw thread convolution which can accommodate virtually any standard threaded bolt or screw. Additionally, the fastener inserts may include removable or break away tangs  32 , as shown most clearly in FIGS. 4 and 5, including a detent  28  which is useful for facilitating removal of the tang after insertion.  
         [0024]    The alloys employed to form the anti-galling fastener inserts of the present invention have been analyzed using a “button and block” galling test to rank various stainless steel alloys for their galling tendencies. According to the procedure, a dead load weight was applied in a Brinell Hardness Tester on two flat, polished surfaces (10-20 micro-inches). Buttons having a 0.5 inch diameter were slowly rotated by hand 360° under the load and then examined for galling at a 7× magnification. If no galling was apparent, new specimens were tested at higher stresses until galling was observed. The “threshold” galling stress was selected as the stress midway between the highest non-galled stress and the stress where galling was first observed. As illustrated in Table 1 below, the alloys of the present invention had values of greater than 50, thereby significantly outperforming all other stainless steel alloys tested for galling characteristics.  
                                               TABLE 1                                                       Alloy of       Conditions &amp; Nominal   Type   Type   Type   Type   Type   Type   Type   Present       Hardness (Brinell)   410   416   430   440C   303   304   316   Invention                   Type 410   3    4   3   3   4   2   2    50+       Type 416   4   13   3   21    9   24    42    50       Type 430   3    3   2   2   2   2   2   36       Type 440C   3   21   2   11    5   3   37     50+       Type 303   4    9   2   5   2   2   3    50+       Type 304   2   24   2   3   2   2   2    50+       Type 316   2   42   2   37    3   2   2   38       Alloy of Present   50+    50+   36    50+   50+   50+   38     50+       Invention                  
 
         [0025]    Fastener inserts manufactured using the alloys of the present invention as well as inserts formed from Type 304 stainless steels were installed into aluminum blocks provided with #10-32 size bores for analysis.  
         [0026]    To analyze for resistance to friction forces, a comparison was made between “standard tapped hole assemblies” and assemblies incorporating the fastener inserts of the present invention. The so-called standard tapped holes included preparing Type 316 stainless steel test blocks with #10-32 size threaded bores which were electropolished and introducing both plain and chemically polished Type 316 stainless steel screws which were cleaned to electronic industry standards. While Type 304 stainless steel inserts were originally used in the standard tapped hole assemblies, the torque required for rotation of the screws varied so drastically from one sample to the next that no quantitative data could be compiled. As such, the analysis on the standard tapped hole assemblies were carried out without fastener inserts.  
         [0027]    The fastener of the present invention as well as those formed from various other alloys were disposed in identical aluminum test blocks with #10-32 size threaded bores. Both plain and chemically polished Type 316 stainless steel screws were used to measure the friction forces. Torque tests were conducted with test blocks perfectly aligned as well as misaligned by 2 and 4 degrees from perpendicular. The torque was recorded for twenty screws under each assembly with ten insertions for each screw.  
         [0028]    As should be understood by those skilled in the art, friction between a screw and a tapped hole can diminish the pre-load on the screw by absorbing some of the rotational torque during installation. In extreme instances, the friction during rotation can lead to galling and self-welding, thus, resulting in seizure of the fastener.  
         [0029]    The torque required to rotate the stainless steel screws in the standard tapped holes increased from the first to the tenth insertion and increased dramatically with an increase in the angle of misalignment. The calculated installation torque increased by as much as 77%.  
         [0030]    The torque required for rotation of the same stainless steel screws within the fastener insert formed from the nitrogen strengthened stainless steel alloys of the present invention were between 17% to 44% lower than the comparable standard tapped holes. Additionally the friction forces generated with an increase in the misalignment angle were significantly lower than the standard tapped holes. Perhaps most importantly, the required torque values for the fastener assemblies employing the inserts of the present invention were substantially uniform as compared to the standard. As such, the holding power of the screws from one application to the next should be relatively consistent.  
         [0031]    In view of the foregoing analysis, nut assemblies employing fastener inserts formed by the alloys herein described should theoretically demonstrate similar results as compared to those formed from other known stainless steel alloys.  
         [0032]    Referring to FIGS.  3 - 5 , two fastener insert embodiments, namely free running inserts and locking inserts are shown. By free running, it is meant that the insert disposed within the nut barrel has a substantially symmetrical helical coil over its entire length, whereas a locking insert includes at least one unsymmetrical convolution  34 , usually including straight segments  36  as shown in FIG. 5.  
         [0033]    The nut  12  of nut assembly  10 , which may be made on a conventional nut making machine and generally includes a barrel  20  of generally constant pitch and diameter throughout. Optionally, upon installing the insert  14  within the nut barrel  20 , the internal threads of the nut may be deformed at each end  38  and  38 A, respectively. By deforming the threads, this would further assure that the insert would not become disengaged from the nut, particularly during extreme or high torque applications.  
         [0034]    As noted above, at least one of the nut assembly components, i.e., the fastener insert and/or the nut are formed from an alloy comprising a) from about 0.05 to 0.15% carbon; b) from about 5.0 to 12.0% manganese; c) from about 2.0 to 6.0% silicon; d) from about 12.0 to 20.0% chromium; e) from about 6.0 to 12.0% nickel; f) from about 0.02 to 0.8% nitrogen; with the remainder being iron.  
         [0035]    While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fulfill the objects stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the spirit thereof.