CA1043547A - Method of preloading and crimping fasteners - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method of assembling and preloading a joint assembly wherein a pin having a shank and a head at one end is disposed with the shank passing through aligned apertures of a plurality of workpieces to be joined. The shank of the pin extends beyond the surface of the workpiece and is provided with an irregular surface configuration. A deformable collar having an initially smooth surfaced internal bore is disposed about the extending shank and is snugged against the adjacent workpiece by a removably positioned means placed in abutting relationship against the free surface of the collar to lightly clamp the joint assembly together to remove free play in the joint. The collar is radially inwardly deformed by a crimping tool so that the initially smooth surfaced internal bore of the collar is deformed to assume a configuration complementary to the irregular surface configuration of the pin shank and upon continual application of the crimping force an axial elongation of the collar and the shank of the pin is effected to provide a predetermined tension or preload on the joint assembly. After the collar has been deformed, the means to snug the joint together initially is removed.

Description

~ ~ ~~i ~0~;~547 The present invention relates to a method of fastener assembly for joints and more parti.cularly to a fastener assembly method for joints adapted to place the joint in a stressed or preloaded condition. This application is a divisional of our Canadian Patent Application No. 195,863, filed March 25, 1974.
In certain applications it is desirable to preload a joint with a high residual tension or clamping force. When such a residual tension, also referred to hereinafter as preload or prestress is applied to a joint, the result is a joint which exhibits a high fatigue life and, also a tight rigid joint. In addition, it has been found that prestressed joints are able to withstand higher cyclical tensile loads before failing than a similar joint which has not been prestressed.
One of the known methods of preloading a joint assembly is by applying a torque of a predetermined magnitude to a nut and bolt assembly. The degree of prestressing achieved by this method is related to the magnitude of the torque applied. However, the actual degree of prestressing cannot be predetermined accurate-ly because the relationship between torque and preload is not always linear or predictable. This is due in part to the frictional effects between the bolt and nut, and also between the nut and/or bolt head and the surface of the workpiece. In ; addition, inordinately high torque values are required to achieve a high degree of preload.

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10~ 47 ~ Other kno~n methods to prestress a joint include use of a fastener such as a lock-bolt of the type disclosed in United States Patent No. 2,531,048 and high shear rivets of the type disclosed in United States Patent No. 2,355,580.
While use of the lock-bolt system in many instances permits the development of consistent, predeterminable high preloads, there is the disadvantage that the pintail is broken off and discarded as scrap. Where large diameter bolts or costly alloys are used, a lock-bolt system of fastening may prove to be needlessly expensive. The high shear rivet type of fastener requires the use of a hammer drive set and further, fasteners of this type can develop only relatively low pre-load forces in a joint.
Another known method for achieving controlled and uniform prestressing in a joint assembly is shown in Vnited States Patent No. 3,421,562. This patent shows the use of a bolt and a counter-bored nut wherein the bolt is provided with per~pheral grooves adjacent its free end. Some of these grooves form a screw thread and the remaining grooves pro~
vide an anti-rotation locking feature for the device. The nut is formed with an internal bore which is internally threaded over only a portion of its length and is adapted to threadably engage the bolt with the unthreaded counter-bored portion adjacent the workpiece. After the nut is torqued snug on the bolt, with a relatively low torque, the portion having the unthreaded counter-bore is crimped against the locking grooves. The crimping effects an elongation of this portion of the nut and, since the nut is fixed relative to the bolt by the engagement of the threaded portion of the nut bore and threaded bolt, the elongation of the nut causes 104;~5~'7 the bolt to elongate as well. This elongation in the bolt places it under tension which, together with the tension provided by the initial torquing, resu}ts in a joint assembly having a preload.
While this type of assembly can achieve high pre-stressed joint assemblies this method is not entirely satis-factory because of certain inherent disadvantages. One dis-advantage is the cost of manufacture in that a special nut must be prepared provided with a portion having a standard internal thread configuration and a counter-bored portion having a smooth internal bore thus necessitating additional manufacturing operations. In addition, the deviation from a desired predetermined preload when utilizing the fastener assembly shown in the aforesaid United States Patent No.
3,421,562 can be significantly greater than the deviation from a desired predetermined preload attainable with a fasten-er assembly according to the present invention. Thus,where a high degree of accuracy in achieving a predetermined pre-load is desired, use of a fastener assembly as shown in the aforesaid patent is not advantageous.
It is therefore an object of the present invention to provide an improved fastener assembly method for preloading joints which obviates the inherent disadvantages of prior art devices.
It is a further object of the present invention to provide a fastener assembly method which may be used to very accurately preload an assembled joint consistently to a desired predetermined preload.
It is another object of the present invention to provide a relatively simple and inexpensive method to pre-104~547 load a joint using means which may be inexpensively manufactured and readily assembled.
The method of the invention provides a joint assembly comprising a plurality of workpieces to be joined having substant-ially aligned openings, and a fastener member having a head and a shank portion. The method includes the steps of placing the shank through the openings so that said head engages the outer surface of one of the members and so that the shank projects beyond the free surface of a second workpiece, the projecting portiGn of the shank having a segment provided with an irregular surface configuration to provide a locking surface. A collar member having an initially smooth surfaced internal bore is placed about the projecting portion of the shank so that one end of the collar member is adjacent the second workpiece. The joint assembly is snugged together to eliminate free-play in the joint by bearing a removably positioned backup means against the other end of the collar member. Subsequently, the collar member is deformed radially inwardly toward the shank thereby to cause the collar member to flow inwardly and pack in about said irregular surface configuration to form a complementary configuration in the collar to lock the collar to the shank, and a radially inward deforming force is maintained on the collar to elongate this in an axial direction to react against said free surface of the second workpiece : and the removably positioned backup means to elongate the fastener shank to induce a preload on the joint assembly. Finally, the backup means is removed from the free end of the shank portion, so that the interlocking engagement between the collar member and the irregular surface configuration of the shank maintains the preload in the joint assembly.
The irregular surface configuration may be helical threads, annular locking grooves, a knurled surface, or a combination of these, and in one preferred embodiment is a standard thread ` 10435~7 configuration immediately adjacent the end o~ theshank.
The back-up means may be an internally threaded mandrel threadably engaged on the threaded portion of the shank to snug the joint together and remove any free play in the joint.

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~i 1(~ '7 The collar may be ~adiall~ inwa~dl~ defor~ed at a plur-ality of circumferentiall~ spaced locations by a crimping tool so that the initially smooth surfaced internal bore is deformed to conform to the irregular surface configuration on the shank of the pin.
A preferred method in accordance with the invention will now be described by way of example with reference to the drawings.
In the Drawings:
' Figure l is an elevational view partly in section showing 'the fastener assembly and joint during an initial stage of assembly according to the method of this invention;
Figure 2 is a view similar to Figure 1 showing the fastener assembly and joint at an intermediate stage of assembly;
Figure 3 is a view similar to Figures 1 and 2 showing the fastener assembly and joint after'final assembly; and - Figure 4 is a pérspectlve view showing the end of the fastener assembly after assembly.'' With reference-to the drawings and particularly Figure 1 -'a ~oint assembly lO is shown prior to the application~of assembling forces in accordance with the present invention. Joint 10 com- ' prises workpieces to be ~oined such as plate members 12 and 14 in face-to-face abutting relationship. Workpieces 12 and 14 include substantially aligned ùnthreaded thro'ugh bores or apertures 15 and i6 respectively therein to accommodate the shank 18 of a fastener member or pin 20. Fastener pin 20 includes a head member 22 at one end having an effective diameter larger than the diameter of the shank 18 and of through bore 15 to form a bearing clamping surface 24 to bear against the exterior surfacc of workpicce 12.
The axial length of the shank 18 of pin 20 is greater than the thickness of the workpieces 12 and 14 resulting in the end portion 26 of shank 18 extending beyond the free surface 28 of workpiece 14. Extending end portion 10~3547 26 of the pin shank 18 includes an intermediate segment 30 provided with an irregular exterior configuration 32 to provide a locking surface. Locking surface 32 may have any desired configuration such as standard helical ~crew threads, annular locking grooves, a knurled surface or any other irregular surface configuration which would form a loc~ing surface when a collar member is radially inwardly deformed so that material of the collar flows inwa~dly in locking contact with the irregular locking surface 32.
An outer end segment 34 is also provided on pin shank 18 and may be provided with a thread configuration 36 to mttingly engage with an internally threaded mandrel member 38 utilized to snug the joint assembly 10 together and to impart a slight preload on the joint assembly sufficient to remove any free play in the joint. The thread configuration 36 which extends b~yond the free surface of collar 40 after the joint assembly is snugged together need only be sufficient to provide a gripping contact with mandrel 38. Hence, only a maximum of three full thread forms need be provided.
The method of the present invention utilizes the technique of crimping a locking collar 40 as the means to impart a predetermined preload to the joint. The locking collar 40 is disposed adjacent the free surface 28 of workpiece 14 about intermediate segment 30 of the extending portion 26 of pin 18. Preferably, the locking collar 40 comprises a aollar body 42 having an initially smooth surfaced internal bore 44 through a substantial portion ~f the axial length of the collar body and may be provided with a counter-bored segment 46 adjacent the end 48 of the collar body to be placed in abutting relationship against the free surface 28 of the ~04;~S~7 workpiece. The exterior of the collar 40 includes a flanged segment 50 of maximum diameter and substantially coextensive with the counter-bored portion 46. Flanged segment 50 extends into an intermediate step-down portion 52 having a diameter slightly less than the diameter across the flange 50 and the collar body 42 terminates in a fu~ther step-down portion 54 having a diameter slightly less than the intermediate portion 52.
In assembling the joint 10 the pin 20 is inserted through the aligned bores 15 and 16 of workpieces 12 and 14, the collar 40 is disposed about the extending portion 26 of shank 18 and the joint is snugged together to remove free play and impart a ~light preload. In the preferred embodiment of the invention described herein the means to snug the joint together is an internally threaded mandrel 38 which is part of the crimping tool 56 and is threadably engaged on the threaded portion 36 of shank 18 just enough to induce a slight preload in the joint and eliminate free play. How~ver, it is to be expressly understood th2t any other removable means, for example a spring loaded collet or other means to engage the end of the collar body may be employed to induce the slight preload and snug the joint together. At this junctu0e in the assembly the crimping tool 56 is actuated to crimp or swage collar 40 radially inwardly to induce the predetermined preload on the joint.
Tool 56 includes an annular member on barrel 58 having a tapered circular mouth 60 to engage a crimping collet 62 positioned within barrel 58. The crimping collet 62 includes a plurality of resilient arcuate jaws 64 having tapered outer surfaces 65 which are complimentary to the ~043547 tapered mouth 60 or barrel 58. Each of the crimping jaws 64 include projections 66 and 68, respbctively, protruding radially inwardly and adapted to engage the outer surface of the collar 40.
The removable means to induce a slight preload in the joint and eliminate any free play, for example mandrel 38, is preferably an integral part of the crimping tool 56 so that the initial stage of assembly would be to engage the mandrel 38 on the threaded end 36 of the pin shank 18. The crimping tool is then actuated by moving barrel 58 toward the workpiece, as shown in Figure 2, so that the coaction between the tapered surfaces 60 and 65 result in radially inward movement of the resilient jaws 64 toward the collar 40.
During an initial stage of assembly, as shown in Figure 2, the radial inward movement of the jaws 64 of the crimping tool, as they contact the outer surface of the collar 40, impart a radially inward deforma~on of the material of the collar 40 as projections 66 and 68 progressively contact the outer surface of the collar. This initial deformt~ion of the collar is in a radial direction only and assures the collar material flowing radially inwardly and packing into the roots of the locking surface 32 in the intermediate segment 30 of the shank of pin 18. The deformation imparted to the collar by the projections 66 and 68 result in inward deformations or dimples 70 and 72, respectively, see Figure 4, on the outer surface of the collar.
Continued application of the radially inwardly directed deforming force by the crimping tool 56 after the ` - 1043547 c~llar material has packed into and around the locking surface of the pin shank, to insure a mating cooperating locking of the collar to the shank, results in a subsequent axial deform~tion of the collar which acts between the free surface 28 of the workpiece and the backpp màndrel 38 to effect an axial elongation of the pin, as shown in Figure 3.
Thus, as shown in Figures 2 and 3, the axial length of the pin in the joint assembly increases from a length Ll to a length L2 resulting in a net increase in the length of the pin L3.
It is this axial elongation of the pin caused by the crimping which ultimately determines the magnitude of the preload on the joint and it is the axial elongation of the collar 40 which provides the elongation in the shank of the pin and, therefore, determines the magnitude of the preload applied to the joint. After the desired preload condition has been reached, the crimping tool 56, including the backup mandrel 38, is removed resulting in a joint assembly having the locking collar, as ~hown in Figure 4, securely positioned and fixed on the extending portion of the pin shank by reason of the mating cooperation between the inwardly deformed surface of the collar and the locking surface provided on the pin shank. Inasmuch as the collar is fixed to the pin ahank by thi cooperating locking action, the preload induced in the joint is also fixed.
As shown in the preferred embodiment herein, collar 40 i5 provided with flange 50 adjacent the end of the collrr in bearing contact with the free surface 28 of the workpiece. Also a counterbore 46 may be provided at this end of the collar body to ac¢ommodate any unthreaded portion of the pin extending beyond surface 28 of workpiece 14. The 104;~S47 flange in collar 40 provides a beneficial result in that the flange prevents peel back of the collar from contact with the free surface 28 of the workpiece during the crimping operation.
By insuring the maintenance of maximum surface-to-surface contact b~tween the collar 40 and the free surface of the workpiece the loss of tensile strength of the joint is minimized.
As shown in the preferred embodiment, the shank 18 of pin 20 has an interference fit within the through bores 15 and 16 of the workpieces 12 and 14 with the interference fit providing an anti-rotation feature so that the shank of the pin is ~t rotated upon the installation and removal of the crimping tool and mandrel 38. However, it is to be understood that an interference fit ~wtween the collar shank and hhe through bores is not necessary and in some applic~tions would not be provided. In this event, an anti-rotation feature may be provided, such as a point drive or irregular surface configuration, to prevent rotation of the pin shank as the backup mandrel is installed or removed.
The fastener-and joint assembly produced by the present invention provides a highly effective method to control the preload induced in a joint with a high degree of accuracy.
This high degree of accuracy in attaining a predetermined preload is attributed to the control elongation of the collar 40 and the pin shank 18 attainable when a removable backup means such as mandrel 38 is used only to induce a slight initial preload to remove free play in the joint assembly. This assures that the force exerted during crimping is not wasted in drawing the workpieces together so that dis-tortion in the preload accuracy of the system is minimized 1(J4;~547 to a great extent. Also, because the locking collar and the backup nut are separate elements the crimping action allows for complete radially inward deformation of the locking collar before axial elongation begins. This action also increases the preload accuracy of the system and is not attainable through the fastening system wherein no removable back~p member is provided.
Thus, it is seen that the present invention provides a joint assembly which is of relatively simple construction, economical to manufacture and which provides an exceedingly accurate system ~or providing a predetermined preload on a joint assembly. It has also been found that the predetermined preload accuracy may be markedly improved by inducing more than a slight initial preload and in applications where the ultimate desired preload must be maintained to a high degree of accuracy such preload accuracy may be achieved by inducing a greater preload during the initial snugging step.
While the crimping tool i9 shown in the preferred embodiment as placing a pair of axially spaced deformations 70 and 72 on the collar body at a plurality of circumferentially spaced locations, it is expressly understood that a single row of circumferentially spaced deformations may also be employed as long as the deformation imparted to the collar is sufficient to lock the collar to the pin shank and effect an axial elongation of the shank. It is also to be understood that a simple crimping tool may be used and a removable means such as a nut may be employed to provide the initial preload and provide the reacting surface during crimping.

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for assembling a joint assembly comprising a plurality of workpieces to be joined having substantially aligned openings therethrough and a fastener assembly including a fastener member having a head and a shank portion extending through said openings with the head adapted to engage the outer surface of one of said workpieces, and a free end of said shank portion projecting beyond the free surface of another one of said workpieces, the projecting part of said shank portion including an irregular surface configuration to provide a lock-ing surface and to cooperate with a selectively reusable removably positioned abutment means, comprising the steps of:
placing a deformable collar member having an initially smooth surfaced internal bore about said projecting end of said shank portion so that one end of said collar member is adjacent the free surface of said another workpiece and the other end of said collar member is formed to initially bear on said abutment means;
snugging the joint assembly together to eliminate free play by advancing said abutment means on said free end of said shank portion so that said collar member bears on the free surface of said another workpiece and said abutment means initially bears on said other end of said collar member while the fastener member head is held against the outer surface of said one of said workpieces and said workpieces are held together;
deforming said collar member radially inwardly toward said shank portion while said abutment means bears on said other end of said collar member, causing material of said other end of said collar member to extend radially inwardly into inter-locking engagement with said irregular surface configuration and to extend axially against both said free surface of said another workpiece and said abutment means, axially elongating said collar member and said shank portion and preloading said joint assembly;
and removing said abutment means from said free end of said shank portion, whereby the interlocking engagement between said collar member and said irregular surface configuration maintains preload in said joint assembly.

2. The method as defined in claim 1 in which said shank is provided with external screw threads at its free end and said removably positioned abutment means comprises a member thread-ably engaged on said shank to snug said collar member and joint assembly together.

3. The method as defined in claim 1 in which said irregular surface configuration on said shank comprises a screw thread configuration and said collar is deformed radially inwardly by crimping said collar at a plurality of circumfer-entially spaced locations to cause said collar material to flow inwardly and pack about said screw thread configuration.

4. The method as defined in claim 1 wherein said step of deforming said collar radially inwardly comprises crimping said collar at a plurality of circumferentially spaced locations in at least two axially spaced positions along said collar.

5. The method as defined in claim 1 wherein said step of deforming said collar radially inwardly comprises crimping said collar at an intermediate portion thereof and at an end portion having a diameter less than said intermediate portion, said crimping being performed at a plurality of circumferent-ially spaced locations about said portions, and wherein said crimping is carried out without any crimping of a flanged segment lying adjacent said another workpiece, said flanged segment being of diameter greater than that of the intermediate portion so as to preclude peel back of said collar body from said another workpiece during crimping.

6. The method according to claim 5, wherein said shank is placed through said openings so that the portion thereof projecting through said openings includes an uninterrupted part adjacent said another workpiece and includes said segment with irregular surface configuration, and wherein said collar member is placed on said projecting portion with a counter-bored seg-ment thereof, substantially co-extensive with said flanged segment, surrounding said uninterrupted part.