GB2074066A - Torque-limiting screw-thread fastener and method of manufacture - Google Patents

Abstract

A torque screw, whose head is twisted off when a given torque level is applied, has a primary driving head 14 in the form of an external twelve-point tool-engageable surface which is joined to a secondary head 16 by a torque control neck. The neck is roll-formed to a diameter smaller than the twelve point head and has a cross-sectional area smaller than that of the threaded shank (12). The screw is produced from a cold- headed cylindrical blank on which an external thread is formed. The thread may be rolled before, or simultaneously with, the neck rolling. <IMAGE>

Description

SPECIFICATION Torque limiting screw thread fastener and method of manufacture Field of the invention This invention relates to threaded fasteners and more particularly, it relates to torque limiting fasteners of the type in which the driving head is twisted off when a predetermined value of torque is applied.

Background of the invention In the use of threaded fasteners, such as bolts and machine screws, it is common practice to tighten the fasteners to a controlled value of torque in order to achieve the most effective use of the fastener. The useof a torque wrench which measures or limits the applied torque is commonplace. It is also known to design the fastener itself so that it is inherently torque limiting; in such a fastener, the driving head for tightening the fastener is adapted to break off at a predetermined value of torque when the fastener is driven into mating engagement with a coacting screw thread member.

With the torque limiting type of fastener, there has been a problem of manufacturing in a cost effective manner and achieving consistent accuracy in establishing the predetermined value of torque limit. The conventional torque limiting fastener has a primary driving head connected with the shank through a neck having a cross-section which is smaller than the head or the shank; a secondary driving head for removal of the fastener is usually provided between the primary head and the shank so that the neck is disposed between the two heads. In order to obtain a specified value of torque limit with good accuracy, the common practice is to form the neck by a machining operation to a diameter with close tolerances. The disadvantage of this conventional torque screw and method of manufacture is that the machining operation imposes high costs in the manufacturing.

Other forms of torque limiting fasteners have been proposed in an effect to achieve lower manufacturing costs while providing the desired accuracy in torque limiting. In one prior arttorque limiting fasteners, as disclosed in Reiland patent 3,812,757, the primary driving head and the neck which joins it to the secondary driving head, as well as the shank, are formed by cold heading of the metal. The torque limiting structure, however, is not the neck; instead, the primary driving head is formed as a socket and the wall of the socket is dimensioned so that it has a predetermined breaking strength. This type of fastener is disadvantageous because the torque at which breakage of the socket occurs may depend upon the fit of the torque applying tool.Further, in some applications, such as construction machinery, socket heads are undesirable because of the tenden cyofthesocketsto become fouled in a dirty environment. Another prior art torque limiting fastener is shown in the patent Schuster et al 3,498,174.

In this fastener, the head is a socket head and the primary head is hollow with either an internal or external tool engaging surface; the neck between the heads is the torque limiting structure.

A general object of this invention is to provide a torque limiting fastener which overcomes the disadvantages of the prior art.

Summary of the invention In accordance with this invention, a threaded fastener with an external tool engaging surface is provided having a torque limiting structure which exhibits a high degree of accuracy of torque control without the need for a machining operation in the manufacture. This is accomplished by providing a roll-formed neck between a primary driving head and the shank as the torque control structure. More particularly, the primary driving head is a twelvepoint head and a roll-formed neck of reduced cross-section is provided between the primary driving head and the shank. The entire fastener may be manufactured by cold-heading and roll-forming which enables the maintenance of close tolerances and a known condition of the metal after being worked.Preferably, the torque limiting fastener is of the type comprising a unitary body with a threaded shank and primary and secondary driving heads disposed axially of the shank. The primary head is disposed outwardly of the secondary head. The body is formed by cold-heading a cylindrical metal blank to provide primary head having an external twelve-point tool engaging surface which joined to the secondary head by a neck. The neck is rollformed to a diameter smaller than the twelve-point head and has a cross-sectional area smaller than that of the threaded shank so that the neck has a torsional breaking strength less than that of the shank.

A more complete understanding of this invention will be obtained from the detailed description that follows taken with the accompanying drawings.

Description of the drawings Figure 1 is an elevation view of the torque limiting fastener according to this invention, Figure 2 is an end view of the fastener, Figure 3 is an elevation view of a cylindrical blank from which the fastener is formed, Figures 4, 5and 6areviews in elevation of intermediate blanks which may represent stages in forming the fasteners from the cylindrical blank, Figure 7 shows a die and punch for forming the blank of Figure 3 into the intermediate blank of Figure 4, Figure 8 shows a die and punch for forming the intermediate blank of Figure 4 into the intermediate blank of Figure 5, Figure 9 shows a die and punch forforming the intermediate blank of Figure 5 into the intermediate blank of FigureS, and Figure 10 depicts a roll-forming tool for providing the torque control neck on the torque limiting fastener of this invention.

Best mode for carrying out the invention Referring now to the drawings, there is shown an illustrative embodiment of the invention in a particu lartorque limiting fastener of the type commonly called a torque screw. The torque screw has a primary driving head joined to the shank through a secondary driving head with a roll-formed neck between the driving heads. The primary driving head has a particular external tool engaging surface, namely a twelve-point head, which facilitates the roll-forming of the neck. It will be appreciated as the description proceeds that the invention may be used for torque limiting fasteners of other configurations for various applications.

The torque limiting fastener of this invention is shown as a finished product in Figures 1 and 2. It comprises, in general, a unitary metal body having threaded shank 12 and a primary driving head 14for tightening the screw, i.e. for advancing the fastener into a coacting threaded part (not shown). It also comprises a secondary driving head 16 for loosening the fastener, i.e. retracting it from the coating threaded part. The primary head 14 and secondary head 16 are joined by a torque control neck 18 which transmits torque from the primary head to the shank.

The primary head 14 is a standard twelve-point head such as that defined in Fastener Standards, Fifth Edition, Industrial Fasteners Institute, Cleveland, Ohio, 1970. The head 14 is adapted to receive a conventional socket wrench as a torque applying tool. The external surface of the head 14 is provided with twenty-four external flats or facets 22 which are of equal size and are uniformly spaced around the circumference of the head. The facets intersect at twelve equally spaced points around the circumference and form equal internal angles of sixty degrees.

The standard twelve-point head has the characteristic that, for all nominal fastener sizes, the ratio of the width across the flats to the width across the corners (taking the minimum allowable dimensions) is substantially a constant value of 0.877. Accordingly, the various standard sizes of this head have the same degree of deviations from roundness. The twelvepoint head is found to be sufficiently close to being round that it can be held in form-rolling dies.

The secondary head 16 is a square head with external flats or facets 24 adapted to receive a torque applying tool for removing the fasteners. It will be understood that the secondary head 16 may comprise other head configurations, as desired.

The neck 18 is of circular cross-section and has a cross-sectional area which is less than the crosssectional area of the shank 12 and less than the cross-sectional area of the primary driving head 14.

It is noted that the neck 18 is of nonuniform diameter with the minimum diameter being approximately midway between the adjacent edges of the secondary head 16 and the primary head 14. The neck 18 defines an annular torque control groove 146 which, in cross-section is substantially circular but it could be of other shapes. The neck 18 is constructed of a roll-formed portion of the unitary body and it has a torsional breaking strength less than that of the shank and less than that of the primary driving head 14.

The torque limiting fastener has a predetermined torque limiting value equal to the torsional breaking strength of the neck 18 which is a function of the minimum diameter of the neck. When the fastener is put into use, it is tightened to the predetermined value of torque by applying sufficient driving torque to the driving head 14to cause the neck 18 to fail in shear, i.e. until the driving head is twisted off. If it is desired to loosen or remove the fastener, the secondary head 16 is engaged by a suitable torque applying tool.

The torque limiting fastener described above, is made in accordace with the following method which will be described with reference to Figures 3 through 10. The fastener is formed from a cylindrical metal blank 30, as shown in Figure 3. The blank 30 is upset or cold-headed in a suitable number of operations to produce an intermediate blank 60, as shown in Figure 6. These preliminary forming steps are suitable as follows. The blank 30 is placed in a die 70, shown in Figure 7, which comprises a platen 72 which contains inserts 74,76 and 78. A knock-out pin 80 is disposed in the bores of the inserts 76 and 78. A puch 84 comprises a platen 86 which contains an insert 88 having a knockout pin 90. An intermediate blank 40, as shown in Figure 4, is cold-formed in the punch 84 and die 70.It is noted that the intermediate blank 40 has a smooth shank portion 12a, a primary head portion 14a and a secondary head portion 16a.

The intermediate blank 40 is formed by a die 90 and a punch 92, shown in Figure 8, to produce the intermediate blank 50 of Figure 5. The die 90 comprises inserts 94,9698 and a knockout pin 102.

The punch 92 comprises inserts 104 and 106 and a knockout pin 108. The intermediate blank 50, as cold-formed in the die 90 and punch 92, comprises a smooth shank 12b, a primary head 14b having multiple facets 22b formed thereon. It also comprises a secondary head portion head 16bwhich is joined with the primary head by a neck portion 18b.

The intermediate blank 50 is cold-formed by a die 110 and punch 112, shown in Figure 9, to produce the intermediate blank 60 of Figure 6. The die 110 comprises inserts 114, 116 and 118 and a knockout pin 120. The punch 112 comprises inserts 122 and 124 and a knockout pin 126. The intermediate blank 60 comprises the smooth shank 12c, a primary head portion 14c with multiple facets 22c and a secondary head portion 16c. The primary head portion and the secondary head portion are joined by a neck poirtion 1 8c which has the configuration of a twelve-point head.The intermediate blank 60, as shown in Figure 6, has a primary head portion 14c which is in its final dimensions and is the same as a primary driving head 14 of the fastener shown in Figure 1.The secondary head portion 16c of the intermediate blank 60 is formed to its final dimensions and is the same as the secondary driving head 16 of the torque screw shown in Figure 1. However, it remains to finish the forming of the neck portion 18e to its final dimensions.

The torque control neck 18 of the fastener shown in Figure 1 is formed to its final dimensions by form-rolling the intermediate blank 60 of Figure 6.

The form-rolling is preferably performed in a conventional manner by flat dies in a cold-forming operation as depicted in Figure 10. It utilizes a pair of flat dies 130 and 132 with the intermediate blank 60 disposed therebetween. The stationary die 130 is provided with a ridge 134 and the reciprocating die 132 is provided with an oppositely disposed ridge 136. The ridges 134 and 136 extend parallel to each other and in a direction perpendicular to the axis of the intermediate blank 60. The ridge 134, as illustrated, has a triangular cross-section 138 of small height at one end (the near end) and it has an arcuate cross-section 140, approximately semi circular, at the other end (far end). The ridge 136 has a triangular cross-section 142 of small height at the far end and an arcuate cross-section 144 of larger height at the near end.The blank 60 is loaded into position between the dies with the ridges 134 and 136 engaging the neck portion 1 8c and the respective triangular cross-sections 138 and 142. The reciprocating die 132 is advanced relative to stationary die 130 (into the paper, as viewed in Figure 10) and the blank 60 is rolled therebetween to progressively form the torque control groove 146 (Figure 1) in the surface of the torque control neck 18. This rolling displaces the metal of the neck 18 in such a manner that the diameter of the neck is reduced and the axial length of the neck is increased. Further, the material ofthe neck 18 is work hardened by the rolling operation.The work hardened material has a different grain structure than would be produced if the final dimensions of the neck were provided by metal removal by machining or grinding operations. The form-rolling operation affords good dimensional control so that the torque control neck 18 is held to close tolerance.

The blank 60 of Figure 6 may be provided with threads on the shank 12c by a separate thread rolling operation. With suitable dies the form-rolling operation on the neck portion 18c and rolling of the threads could be done at the same time. Alternatively, the threads may be provided by any conventional thread forming operation.

Although the description of this invention has been given with reference to a particular embodiment, it is not to be construed in a limiting sense.

Many variations and modifications will now occur to those skilled in the art. For a definition of the invention, reference is made to the appended claims.

Claims (7)

1. A torque limiting fastener comprising a unitary body having threaded shank and first and second driving heads, the driving heads being disposed axially of the shank with the first head being disposed axially outwardly of the second head, the second head being joined with said shank and having plural external facets adapted to be engaged by a torque applying tool for loosening said fastener, the first head having plural external facets adapted to be engaged by a torque applying tool for tightening said fastener, the first head being joined with the second head by a torque control neck having a cross-sectional area smaller than the cross-sectional area of the shank, said neck being constructed of a roll-formed portion of said body and having a torsional breaking strength less than that of said shank whereby the neck will fail in shear when a predetermined value of torque is applied to the first head.

2. The invention as defined in Claim 1 wherein said first head is a twelve-point head.

3. The method of making a torque limiting fastener of the type comprising a unitary body having a shank and first and second driving heads disposed axially of the shank with the first head being disposed outwardly of the second head, said method comprising the steps of: cold heading a cylindrical metal blank to provide an intermediate blank with said first head, said second head and said shank, said first head being formed with an external twelve-point tool engaging surface and being joined with said second head by a neck portion, said second head having an external tool engaging surface which is joined to said shank, and reducing said neck portion of said intermediate blank to a diameter smaller than said twelvepoint head by form-rolling to provide a torque control neck having a cross-sectional area smaller than that of the shank and a torsional breaking strength less than that of the shank.

4. The invention as defined in Claim 3wherein the step of reducing said neck portion is performed by rolling said neck portion between a pair of flat dies.

5. The invention as defined in Claim 4 wherein said neck portion has the surface configuration of said twelve-point tool engaging surface and said intermediate blank is held by the engagement of the dies with said neck portion.

6. A torque limiting fastener substantially as herein described and shown in the accompanying drawings.

7. A method of making a torque limiting fastener substantially as herein described with reference to the accompanying drawings.