Nut threaded fastener for applying a predetermined preload
Cross-Reference to Related Application 
     This application is a continuation of U.S. patent application Ser. No. 07/546,533, filed Jun. 29, 1990, now abandoned, which is a continuation of U.S. patent application Ser. No. 07/225,649, filed Jul. 25, 1988, now abandoned, which is a continuation of U.S. patent application Ser. No. 06/846,302, filed Mar. 31, 1986, now abandoned, which is continuation-in-part of U.S. patent application Ser. No. 06/702,811, filed Feb. 15, 1985, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to fasteners in general, and, more in particular, to female fasteners or nuts of the torque-limiting type. 
     In a standard threaded fastener system of a male threaded fastener and a female threaded fastener, the female fastener has internal threads that thread onto external threads of the male fastener. Wrenching surfaces of both fasteners accept tools that tighten them and clamp one or more workpieces together between them oftentimes with washers interposed in between. The combination of the fasteners and the workpieces are known as a &#34;joint.&#34; Male threaded fasteners are variously known as &#34;studs,&#34; &#34;screws,&#34; &#34;bolts,&#34; or &#34;pins;&#34; female threaded fasteners are variously known as &#34;nuts&#34; or &#34;collars;&#34; workpieces are sometimes called &#34;sheets&#34; or &#34;structural elements.&#34; 
     Fasteners bear loads along their axes, tensile loads, and radially of the axes, shear loads. Tensile loading always exists because of the clamping force applied by the pin and the collar to the sheets; this load is known as &#34;clamp-up&#34; or &#34;preload.&#34; When fasteners join two or more sheets and the sheets are loaded in their planes, one sheet may tend to slide over the other; when this loading of the sheets occurs, it is resisted by the fasteners, and the sheets load the fasteners in shear. Shear loads are transverse to the axes of the fasteners and transverse to the tension load. Cyclic loading of a fastener can produce fatigue failure. In aerospace applications fatigue failure is usually most critical in shear. 
     Adequate clamp-up or preload is absolutely necessary for a satisfactory joint A fastener adequately loaded by the reaction to the clamp-up load resists fatigue failure. Preload also helps the structural elements to resist fatigue failure. Accordingly, it is desirable to know the clamp-up load the fastener applies to a structure to be sure that a joint has adequate fatigue strength. Adequate clamp-up also avoids sheet slippage and fretting and insures against load shifting and joint failure. 
     Clamp-up load correlates to the resistance of a collar to further threading onto a pin and against a workpiece by the application of torque to the collar. As clamp-up force increases, the resistance to further threading increases, and the torque required to turn the collar increases. 
     This fact has been used in fasteners to develop a predetermined clamp-up load by termination of tightening through failure of a wrenching section on the collar. U.S. Pat. No. 2,940,495 to G. S. Wing and U.S. Pat. No. 4,260,005 to Edgar Stencel describe two types of such fasteners. 
     The Wing patent describes a collar extensively used in the aerospace industry. It has a wrenching section connected to an internally threaded section by a frangible breakneck collar. The collar breaks upon the application of a predetermined torque that corresponds to a desired clamp-up load. An acircular portion of the threaded section provides a thread lock by pressing tightly against the threads of the cooperating pin. A problem with this type of fastener is that it generates a waste piece: the wrenching section. The waste piece must be removed from the environment where the fastener is set. This type of fastener is also comparatively expensive because it requires a considerable amount of machining to make it, and the frangible breakneck must be held to very close tolerances to provide close tolerances in breakoff torques. 
     The Stencel patent describes a collar that has a plurality of circumferentially spaced lobes on its axial outside that serve as wrenching surfaces and in torque limitation. A wrenching tool, say a triangular shaped socket, has flats that engage flanks of the lobes and turn the collar with respect to the pin. Upon reaching a predetermined clamp-up load, the lobes fail in radial compression and merge into the body of the collar, and wrenching and tightening stops because the lobes no longer provide purchase for the setting tool. The Stencel collar produces a thread lock by a deformation of collar material radially inward of the lobes against the threads of a cooperating pin when the lobes fail. 
     Impact wrenches used in setting fasteners do so rapidly. The failures of the breakneck of the Wing fastener and of the lobes of the Stencel fastener occur over very few degrees of rotation, and, when an impact wrench is used, occur very rapidly. The rapid application of setting torques to a collar can result in loss of some desired preload through relaxation of the sheets; relaxation results from the continued deformation of the sheets after the initial loading. Sheet relaxation usually happens as a result of more than one fastener being necessary to pull all the parts together. Such deformation reduces the load per unit area and absolute loading because material moves away from the clamped zone. When the breakneck or the lobes fail, they fail at a torque corresponding to a desired preload. But the loaded sheets can relax and some of the preload lost. This relaxation is a time-dependent phenomenon, and with slower development of preload, relaxation and loss of preload will be less. 
     It may also be desirable to be able to change the preload even with the same collar. For example, when the sheets are not as strong in compression as some other sheets, it may be necessary to lower the compressive load on them. 
     In some applications secondary wrenching is desired in order to increase preload above design preload or to compensate for relaxation. Secondary wrenching is impossible in the standard configurations of the Wing and Stencel collars. These collars are also difficult to remove after they have been set because of the absence of wrenching sections. 
     In some applications because of space limitations straight-on wrenching is impossible and wrenching must be done from the side of the collar, say, with an open end wrench. 
     SUMMARY OF THE INVENTION 
     The present invention provides a female fastener that can be installed or removed with standard wrenches, say a hexagonal socket, or with a ball drive socket. When driven with a ball drive socket, lobes of the collar fail upon reaching a predetermined preload by one or more balls of the driver plowing a furrow through them. A ball drive socket can also act against the lobes to tighten the collar to a predetermined preload and leave Brinnell marks as a signature that tightening the desired amount has occurred. 
     In one form, the present invention contemplates a female fastener or nut having internal threads and a plurality of axially extending lobes with arcuate external surfaces. These lobes provide wrenching purchase for either spherically curved wrenching elements or flat surfaced wrenching tools such as hexagonal sockets or end wrenches. Troughs between the lobes permit the spherically curved wrenching elements to bear on the flanks of the lobes. 
     The lobes lie within an imaginary regular geometric figure having straight sides, say a hexagon. Each side of the figure strikes a tangent to the surfaces of two successive lobes so that the lobes can accept a flat sided wrenching tool. 
     In a particular form, a right cylindrical surface concentric with the axis of the nut defines the surface of the troughs, the lobes extending radially outward from this surface at regular intervals. The lobes themselves have surfaces lying on tangent cylinders arrayed in a regular pattern with the axes of the cylinders lying on a second cylindrical surface concentric with the axis of the nut and having a radius smaller than the radius of the defining cylinder for the trough surfaces. Preferably, the nut has six lobes. It is also preferred that the nut have a base below the lobes with a counterbore for receiving imperfect thread convolutions of a male fastener, as is standard in many nuts or collars. 
     The lobed fastener of this invention accepts wrenching by either a flat sided wrench or a ball drive. The fastener can be re-wrenched and removed by wrenching. When driven with a ball drive, the fastener signs when it has been tightened the desired amount either by Brinnell marks or circumferential furrows. 
     These and other features, aspects, and advantages of the present invention will become more apparent from the following description, appended claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows the preferred form of the nut of this invention in elevation; 
     FIG. 2 shows the nut of FIG. 1 in top plan; 
     FIG. 3 shows the nut of FIG. 1 in top plan being wrenched by a flat side wrenching tool; 
     FIG. 4 shows the nut of FIG. 1 in elevation set in a joint with circumferential furrows in its lobes; and 
     FIG. 5 shows the nut of FIG. 1 in elevation set in a joint with Brinnell marks in its lobes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show the preferred form of a nut 10 made in accordance with the present invention. In general, the nut has a wrenching section 12, a base 14, and a transition section 16. An axial bore 18 through the nut has a threaded section 20 and a counterbore 21. The nut has an axis 23. 
     The wrenching section has a plurality of wrenching lobes 22 extending axially the length of the section. Each lobe has an arcuate, axially extending external surface. Troughs 26 extend axially the length of the wrenching section between adjacent lobes; the external surfaces or troughs 26 lie on a common right cylinder. 
     FIG. 2 shows best the geometry of the lobes and troughs. Lobes 22 have external cylindrical surfaces defined by the surfaces of tangent cylinders 28. These cylinders have parallel axes 30 that lie on a cylinder 32 that is concentric with axis 23 of the nut; axes 30 are also parallel to axis 23. An arrow 36 is the radius of cylinder 32. The tangency of cylinders 28 is shown at 38. All of the other lobes also have generating cylinders that have the same relationship as cylinders 28 to each other and to the nut. 
     Troughs 26 fall on the surface of a right cylinder having an axis concentric with axis 23 and having a radius 40. Radius 40 is larger than radius 36, and so troughs 26 meet cylinders 28 well out from cylinder axis 30 towards the outside of the nut. 
     A tangent line 42 between adjacent lobes is spaced radially from the surface of troughs 26. Line 42 represents the position of a wrenching surface against the lobes of a flat sided wrenching tool such as an end wrench. 
     The top of the lobe has a slight chamfer 44 to facilitate the insertion of a wrenching tool onto the nut. 
     FIG. 3 shows nut 10 accepting a wrenching socket 50. The flats on the socket bear on the lobes about at tangency points 52 during tightening and at tangency points 54 during loosening. Troughs 26 lie on a cylinder inside of the wrenching surfaces of the socket and spaced from such surfaces. 
     When driven with a ball socket, the balls track on troughs 26 and bear against lobes 22. A suitable driver is shown in the parent to this application and a description of that driver is incorporated into this specification by reference. FIG. 4 shows a joint produced with a nut of the present invention and a circular driver that has plowed furrows 60 through lobes 22. The joint includes the nut, a bolt 62, and workpieces 64 and 66. The joint has a predetermined preload on it effected by a clamp-up force of nut 10 and the head of bolt 62 on sheets 64 and 66, respectively. This preload correlates with the failure of lobe material in circumferential compression by balls of the driver. 
     FIG. 5 shows a similar joint effected by a ball driver. However, instead of plowing furrow 60, the driver Brinnelled the lobes at 68 at a predetermined load. The Brinnelling represents the beginning of failure of the lobe, which if carried to completion would develop furrows 60. But Brinnelling 68 can correspond to a predetermined load. Brinnelling instead of furrows can result at the same preload that produces the furrows if the race diameter of the ball of the driver is smaller. Obviously, the preload with signature can be changed with appropriate changes in the race diameter of the drivers. The nut can be re-torqued to the same value with drive balls on a race of smaller diameter, and the furrow will deepen; this can be done again and again. The number of re-torquings can be determined by the drive sockets. This nut can also be re-torqued as necessary and with the re-torque new Brinnell marks added. 
     Brinnelling or the furrows sign the nut so that an inspector knows that it has been tightened a predetermined amount. 
     The present invention has been described with reference to certain preferred embodiments. The spirit and scope of the appended claims should not, however, necessarily be limited to the foregoing description.