Self-locking preload controlling nut

Abstract: The self-locking internally threaded nut has a deltoid shaped locking section (24) having circumferentially spaced protrusions (40). The protrusions (40) form a surface adapted to be engaged by a deltoid driver which tightens the nut onto a bolt to a predetermined axial load. At the predetermined load the protrusions (40) are plastically deformed inwardly on the nut so that the driver rotates freely on the nut. When the protrusions (40) are inwardly deformed, the deformed material impinges on the bolt, thereby rotationally locking the nut on the bolt.

Description: Background of the Invention This invention relates to self-locking fasteners providing predetermined axial loads. Threaded fasteners consist of an internally threaded member or nut and an externally threaded member or bolt. The nut has an internal thread that threads onto an external thread on the bolt. Surfaces of the nut and bolt are formed to receive wrenches or other tightening mechanisms. Turning the wrenches transmits torque to the fasteners to form a joint where the fasteners hold one or more workpieces tightly together. Another name for a bolt is a threaded pin, and a nut is sometimes referred to as a collar. Many environments in which fasteners are used require that the fasteners have extremely high integrity and strength. Use in vehicles and bridges are examples of such environments. Fasteners must often bear loads not only along their longitudinal axis, but radially of the axis. More particularly, when fasteners join together two or more workpieces and the workpieces are loaded in their planes with different loads, one workpiece tends to slide over the other. Fasteners passing through both workpieces become loaded in shear curing their resistance to this type of loading. Fasteners quite often must respond well in environments where they are cyclically stressed under conditions that could give rise to fatigue failure. A fastener with adequate axial load on it tends to resist fatigue failure and failure in shear. Axial loads arise by the clamping of workpieces between a head of the bolt on one side of the workpieces and the nut on the other side of the workpieces. Because adequate axial loads help fasteners resist failure, it is desirable to know the clamping or axial load the fastener applies to a structure. Axial load correlates to the resistance of a nut to further tightening onto a bolt and against the workpieces. As the axial force increases, the resistance to further tightening increases, and the torque required to turn the nut increases. This fact has been used in fasteners to develop a predetermined axial load. A desirable feature of a fastener is that it does not come apart in service. Various locking devices exist that keep nuts and bolts together. One deforms the thread of the nut so that it bears in radial compression against the thread of the bolt. The resistance to unthreading in this lock is purely frictional. The thread is commonly deformed at the factory in preference to the field, but field deformation has also been practiced. This type of thread lock is known as a prevailing torque thread lock. US-A-4,260,005 to Edgar Stencel discloses a self-locking nut that uses external lobes to accept a deltoid driver having a generally triangular or deltoid shaped socket to tighten the nut on a cooperating bolt or pin. The nut is preferably used with a bolt having axially extending flutes in the thread. Once a predetermined axial load exists in the joint being made, the lobes plastically deform and the driver can no longer turn the nut. Deformation of the lobes displaces material positioned radially inward from them into the flutes of the cooperating bolt to produce a thread lock. The thread lock results from material deforming into and across the bolt flutes. After lobe deformation material of the nut is in the flutes of the bolt, a mechanical or interference, thread lock exists. Lobe deformation is a function of setting torque applied through the driver. An improvement of the self-locking fastener of US-A-4,260,005 to Edgar Stencel is disclosed in US-A-5,092,726 to Roland A. Wheeler et al., on which the two-part form of independent claim 1 is based. The Wheeler et al. fastener nut has a deltoid-shaped surface at the free end of the locking section of the nut. Between the lobes of the nut the walls have axially tapered portions increasing in circumferential width toward the free end of the nut. Adjacent the lobes the walls of the nut are of constant thickness and there are sharp transitions between the lobes and the adjacent wall portions. The axially tapered wall portions of decreasing thickness eliminate wall interference with the driver and sticking of the nut to the driver. It is desirable to obtain higher axial loads with larger fasteners for applications in trucks, buses, railroad cars, sky scrapers, bridges and other industrial applications. It is also desirable to provide higher torsional resistance in the lobes prior to failure, to produce higher axial loads needed for larger industrial fasteners. Further, it is desirable to modify the shape and maintain ease of manufacture of the nuts while maintaining tolerances especially on a small scale basis. Further, it is desirable that, as the deltoid driver deforms the lobes, the lobes do not fold over thereby preventing cracking around the lobe. Thus, increasing the axial load that can be applied by the fasteners, eliminating cracking in the nut, simplifying manufacture, and increasing the flexibility of the shape and size of the nut are desirable to broaden the applications in which the fasteners can be used and to improve the function of the fasteners. Brief Summary of the Invention There is, therefore, provided in accordance with the invention a self-locking nut comprising a body having a threaded inner surface defining a bore therethrough; the body having a barrel portion with a locking section surrounding at least part of the threaded inner surface; the locking section having smaller wall thicknesses, adjacent larger wall thicknesses, and changes of thickness therebetween; and the larger wall thicknesses forming a tightening surface adapted to engage a deltoid driver and deform radially inward when the nut is tightened on a thread to a predetermined axial load, so that upon such deformation the larger wall thicknesses substantially integrate into the body of the nut eliminating the tightening surface and terminating tightening of the nut, characterized in that the locking section is deltoid in cross-section and said changes of thickness between said smaller wall thicknesses and said larger wall thicknesses are gradual throughout the length of the locking section along the longitudinal axis of the nut. The gradual or smooth transitions between the wall portions of smaller and larger thicknesses prevent folding over and cracking of the nut during tightening, due to a sudden reduction in wall thickness on either side of the lobe. Avoiding folding over and cracking allows application of a higher axial load. In a preferred embodiment of the invention a relief groove is interposed between the locking section and the remainder of the barrel to prevent the larger wall thicknesses from deforming non-uniformly along their lengths. In this embodiment the locking section is located at one end of the nut and a second tightening surface adapted to engage a conventional wrench is located at the other end. These and other features and advantages of the present invention will appear from the following Detailed Description and the accompanying drawings in which similar reference characters denote similar elements throughout the several views. Brief Description of the Drawings FIG. 1 is an enlarged perspective view of a nut according to the present invention; FIG. 2 is an enlarged top view of the nut of FIG. 1; FIG. 3 is an enlarged cross-sectional view of a locking section of the nut of FIG. 1 taken along line 3-3; FIG. 4 is an enlarged top partial view of the nut of FIG. 1 with a driver placed over the nut illustrating the relief gap; FIG. 5 is a top view of the nut of FIG. 1 after the nut has been tightened onto a bolt by the driver; and FIG. 6 is a side view of the nut and bolt of FIG. 4 tightened onto a workpiece. Detailed Description Figures 1 and 2 show a self-locking nut. The body of the nut comprises a barrel 22 having a locking section 24. The locking section is preferably positioned at an end of the nut remote from the workpiece to be engaged by the nut. A conventional hexagonal wrenching section 26 is provided adjacent to the barrel at the other end of the nut. A cylindrical bore 28 passes centrally through the nut, and the inner surface 30 of the nut defining the bore is threaded. The internal thread is adapted to receive a threaded bolt 32 having a head 34 and a threaded portion 36 which has flutes 38 expending across the threads in the axial direction (FIG. 6). The head provides means to receive a wrench, or the fastener may have a hex socket for an Alien wrench or may have a Phillips screwdriver socket. Also, the socket may be in the same end as the thread. As the nut is tightened onto the bolt against a workpiece such as a pair of sheets (FIG. 6), the axial load applied between the bolt head and the nut increases up to a predetermined load. When the predetermined load is reached, the locking section deforms into the flutes, rotationally locking the nut onto the bolt. Collapse of the locking section prevents the nut from being further tightened onto the bolt. Referring additionally to FIG. 3, the preferred locking section 24 has a plurality of circumferentially spaced protrusions 40 which are connected by a pluralitv of walls 42. In the preferred embodiment shown, there are three equally spaced protrusions. The protrusions have a larger wall thickness which is greater than the smaller thickness of the walls which are adjacent thereto. Though the protrusions and the walls have different thicknesses, the thin walls gradually increase in thickness toward the protrusions, and the thick protrusions gradually decrease in thickness toward the walls. Therefore, the intersections, generally designated 43, between the walls and the protrusions are tangential, and thus, the intersections are smooth, without corners, notches, or other concave regions and without drastic changes in thickness. Further, the thicknesses of the walls do not change along the longitudinal axis 60 of the nut. In the preferred embodiment shown, there are three protrusions and three walls which form the deltoid shaped locking section. Thus, the transverse cross section (FIG. 3) of the locking section is deltoid in shape. The deltoid shape is generally the shape of a triangle with arcuate sides. The deltoid shape forms a tightening or wrenching surface which is adapted to be engaged by an approximately complimentary driver 44. To aid engagement of the nut by the driver, each protrusion has a chamfer 46. The chamfer provides an angled surface for the driver to slide over. Further, each of the walls has a radius of curvature illustrated by line 45, and each protrusion also has a radius of curvature illustrated by line 47. The radii of curvature of the walls and the radii of curvature of the protrusions intersect each other tangentially, so that as stated, the intersections 43 between the protrusions and the walls are smooth and without corners or concave regions. By selecting the size of the wall radii, the desired minimum wall thickness to promote locking while preventing the nut from sticking in the driver is obtained. The driver 44 shown in FIG. 4 is deltoid shaped and is thus approximately complimentary in shape to the locking section of the nut. Further, the driver is sized and the wall radii chosen so that there is a small relief gap 48 between the inner wall of the driver and the outer wall of the locking section. The relief gap is best seen when the midpoints of the driver walls are aligned with the thin wall portions of the locking section. During deformation of the locking section, the thin wall portions of the locking section tend to deform outwardly. The relief gap provides a space into which the thin walled portions of the nut can deform outwardly without contacting the driver. Thus, the deformed thin walled portions do not interfere with removal of the driver from the nut. To increase the space between the inner wall of the driver and the outer wall of the locking section, alternative shapes of drivers, such as a conventional hexagonal shaped driver, could be used. Referring to FIGS. 5 and 6, to complete a joint with the disclosed nut and bolt, the bolt is inserted through holes 50, 52 in a pair of workpieces 54, 56, and the nut is threaded onto the bolt with the conventional wrenching end toward the head of the bolt and the workpieces as shown in FIG. 6. The deltoid driver is placed over the nut and used to tighten the workpieces between the nut and the bolt head. When the predetermined axial load is reached, the protrusions fail. As they fail, the driver plastically deforms the protrusions, forcing the material making up the protrusions inwardly on the nut and integrating the protrusions into the locking section, so that the protrusions are almost indistinguishable from the remainder of the locking section. Thus, the driver deforms the outer circumference of the locking section 24 from the deltoid shape shown in FIG. 2 to the generally circular shape shown in FIG. 5. As stated, the bolt preferably has a plurality of flutes 38 extending axially across the threads of the bolt into which the material is deformed. The deformation of material into the flutes creates an interference lock. The barrel of the nut body also includes a circumferential relief groove 58. The relief groove comprises a circumferential groove having a sufficient depth to axially separate the locking section and hence the protrusions from the rest of the barrel. Thus, the protrusions are axially separated from nut material that would prevent the protrusions from failing uniformly along their length which extends parallel to a longitudinal axis 60 of the nut. The diameter of the relief groove is greater than the diameter of the thin wall sections. The deltoid driver now rotates freely on the nut, and therefore, the driver cannot further tighten the nut. The material forced inwardly impinges on the thread of the bolt and into the flutes locking the nut from rotation relative to the bolt. The thin wall portions expand outwardly into the relief gap without impinging on the driver, and thus, the driver is easily removed from the nut. The protrusions determine the axial load applied to a joint made with the nut because the protrusions fail at a predetermined load. The load can be determined by the resistance of the locking section to deformation. One convenient way to adjust the value of the load is to adjust the size of the protrusion and consequently the length of the wall between the protrusions. Further, the length of the protrusions along the longitudinal axis 60 of the nut can be increased, thereby increasing the axial load applied by the fastener. Because of the smooth transition between the protrusions and the walls, the protrusions deform inwardly into the nut with little or no folding. Thus, no cracking occurs during tightening. The extra material in the protrusions and walls requires a high torque to deform the protrusions. Thus, the axial load applied by the fastener is increased over the axial load obtained by the prior art. Further, the deltoid shape is simple. It has no corners or complex contours. Therefore, it is easily manufactured, and the design is easily modified. Thus, a self-locking nut is disclosed which utilizes smooth transitions between a plurality of protrusions and a plurality of walls to prevent cracking, apply high axial loads, and provide a simple nut design that is easily modified. While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. It is, therefore, to be understood that within the scope of the appended claims, this invention may be practiced otherwise than as specifically described.

Claims: A self-locking nut comprising: a body having a threaded inner surface (30) defining a bore (28) therethrough; the body having a barrel portion (22) with a locking section (24) surrounding at least part of the threaded inner surface (30) ; the locking section (24) having smaller wall thicknesses, adjacent larger wall thicknesses, and changes of thickness therebetween; and the larger wall thicknesses forming a tightening surface adapted to engage a deltoid driver (44) and deform radially inward when the nut is tightened on a thread to a predetermined axial load, so that upon such deformation the larger wall thicknesses substantially integrate into the body of the nut eliminating the tightening surface and terminating tightening of the nut, characterized in that the locking section (24) is deltoid in cross-section and said changes of thickness between said smaller wall thicknesses and said larger wall thicknesses are gradual throughout the length of the locking section (24) along the longitudinal axis (60) of the nut. The self-locking nut according to claim 1, characterized in that the smaller and larger wall thicknesses intersect tangentially. The self-locking nut of claim 1, characterized in that the deltoid shaped locking section (24) is substantially triangular with arcuate sides. The self-locking nut according to claim 1, characterized in that the wall thickness is constant along a longitudinal axis (60) of the locking section (24). The self-locking nut according to claim 1, characterized in that the larger wall thicknesses are adapted to plastically deform. The self-locking nut according to claim 1, characterized in that the body further comprises a second tightening surface (26) adapted to engage a conventional wrench. The self-locking nut according to claim 1, characterized in that the body further comprises a relief groove (58) axially separating the locking section (24) from the remainder of the barrel portion (22). The self-locking nut according to claim 1, characterized in that the body further comprises a relief groove (58) axially separating the larger wall thicknesses from the remainder of the barrel portion (22). The self-locking nut according to claim 1, characterized in that the body comprises an end and the locking section (24) is located at the end.

Inventors: LOUW JOHN A, LOUW, JOHN A.
Applicants: FAIRCHILD HOLDING CORP, FAIRCHILD HOLDING CORP.
Application year: 1997
Main classification: F16B39
IPC codes: F16B39, F16B31