GB2381056A

GB2381056A - Tension indicating fastener assembly

Info

Publication number: GB2381056A
Application number: GB0224101A
Authority: GB
Inventors: John Andrew Brian Hirst
Original assignee: JAMES WALKER GROUP Ltd; Rotabolt Ltd
Current assignee: JAMES WALKER GROUP Ltd; Rotabolt Ltd
Priority date: 2001-10-16
Filing date: 2002-10-16
Publication date: 2003-04-23
Anticipated expiration: 2022-10-16
Also published as: GB2381056B; GB0224101D0

Abstract

A screw threaded fastener assembly includes a screw threaded member (11) and an elongate member (12). The elongate member (12) has a first end inserted into a bore in the screw threaded member (11) and a second end spaced from the screw threaded member (11). There is an abutment member (13) between the second end of the elongate member (12) and the screw threaded member (11) which is relatively movable unless the screw threaded member (11) is tensioned beyond a given level when it is fixed in a sandwiched position between the second end of the elongate member (12) and the screw threaded member (11). The abutment member (13) is arranged to be urged away from its sandwiched position.

Description

Fastener Assembly This invention relates to a screw threaded fastener assembly and to a method of fastening that employs such an assembly. More particularly, but not exclusively, the invention relates to improvements in and modifications of the fastener assemblies of EP 0 049 537 A2, the description of which is incorporated herein by reference.

A known form of fastener assembly made in accordance with EP 0 049 537 A2 is shown in Figs la and lb. The assembly comprises an elongate member in the form of a pin 1 having a threaded end 2 which is inserted into an elongate cavity formed in a coaxial blind bore of a bolt 3.

The pin 1 has a head 4 at the opposite end to the threaded end 2 and an abutment member 5 is mounted between the head 4 of the pin and the head 6 of the bolt 3. The gap between the pin head 4 and bolt head 6 is sufficient to allow the abutment member 5 to rotate freely.

When the bolt 3 is tightened it is tensioned and its length therefore increases, thereby reducing the gap between the pin head 4 and the bolt head 6, until the abutment member 5 is gripped between the underneath of the pin head 4 and the top of the bolt head 6. When the abutment member is just gripped, that indicates that the tension in the bolt has stretched the bolt a given amount.

Thus by setting the gap between the bolt head 6 and the pin head 4 to a predetermined amount, the gripping of

the abutment member can be used to indicate a certain tension in the bolt 3. In use, a user tightens the bolt 3 while occasionally checking to see if the abutment member 5 is still free to turn and stops tightening when the abutment member is no longer free to turn. A further benefit of this design is that if for any reason the tension in the bolt 3 begins to fall for instance due to relaxation of a joint then this is indicated by the tension in the bolt falling and the abutment member 5 becoming free to rotate again. The bolt 3 can then be re-tightened before any damage occurs.

The working parts of the device are protected from corrosion and physical damage by a cap 7 fitted to the bolt head and covering the abutment member 5 and the pin head 4.

EP 0 049 537 A2 also describes an assembly where the bolt 3 is replaced by a stud with male screw threaded portions at both ends.

The fastener assembly just described is well proven and has been used in many different applications. However, it does have certain shortcomings, some of which are described below.

There is an element of uncertainty involved in deciding when the abutment member 5 is gripped. Different people have different strengths in the fingers and this leads to a variation in when the abutment member is judged to be gripped. This in turn leads to a variation in the resulting set tension in the bolt.

It is necessary to be able physically to reach the

bolt to check if the abutment member 5 is free to rotate.

Sometimes bolts are in such a position that it is difficult or impossible to reach the abutment member to try to turn it. Sometimes there may be so many bolts that testing them all may be a time consuming job.

The abutment member 5 locks at one preset load in the bolt and thus can serve as an indication of whether the tension in the bolt is greater or less than one reference value. This is sufficient for most applications, but sometimes it is a significant disadvantage that it is not possible to determine anything regarding the degree of over-tightening by testing the abutment member. In certain situations for example it is useful to provide a min. and max. tightening load point.

It is possible to remove the cover cap 7 and to tamper with the setting of the pin 1, hence changing the set tension of the bolt 3.

It is an object of the invention to provide a screw threaded fastener assembly that overcomes or mitigates one or more of the disadvantages referred to above.

According to the invention there is provided a screw threaded fastener assembly including: a screw threaded member including a screw threaded portion carried on a stem portion, an elongate member having first and second ends, the first end being inserted into an elongate cavity in the stem portion of the screw threaded member and connected to the screw threaded member, and the second end being spaced

from the screw threaded member, and an abutment member mounted between the second end of the elongate member and the screw threaded member, the abutment member being able to become fixed in a sandwiched position between the elongate member and the screw threaded member when the screw threaded member is in use and the stem portion is tensioned beyond a given level but also being able to be free to move when the level of the tension is reduced below the given level, wherein the abutment member is arranged to be urged away from its sandwiched position.

By arranging for the abutment member to be urged away from its sandwiched position, it becomes possible to control at a consistent level the moment at which the abutment member no longer moves from its sandwiched position. Thus the tension set in the screw threaded member can be more accurately set.

The abutment member is preferably rotatable about the longitudinal axis of the elongate member. It should be understood, however, that it is within the scope of the invention for the abutment member to be mounted in some other way, for example for linear movement.

A spring is preferably provided for urging the abutment member away from its sandwiched position.

Especially in the case where the abutment member is rotatable about the longitudinal axis of the elongate member, the spring is preferably a torsion spring. The torsion spring may be a metal wire spring or any other

flexible component that acts as a torsion spring. Instead of employing a spring, the abutment member may be arranged to be urged away from its sandwiched position by gravity.

For example, the stem of the screw threaded member may be disposed on a horizontal axis and the abutment member may be rotatably mounted about that axis and a weighted arm may be provided extending radially outwardly from the abutment member.

The assembly preferably further includes an indicating arrangement for indicating movement of the abutment member away from its sandwiched position. By providing an indicating arrangement, it becomes possible to establish simply by inspection whether or not the abutment member is still in its sandwiched position.

The indicating arrangement may take any of a wide variety of forms. It may for example merely be provided by a non-symmetrical shape of the abutment member, or it may include a pointer which is arranged to move when the abutment member moves away from its sandwiched position.

Usually a visual indicating arrangement is advantageous but it is also possible for the indicating arrangement to include an electrical device for indicating movement of the abutment member away from its sandwiched position and in that case the indication to an operator may or may not be a visual indication. The electrical device may be an electrical switch, for example a proximity switch.

In an especially preferred embodiment, the indicating arrangement is provided by portions of the abutment member

and of the screw threaded member which move from aligned positions to separated positions when the abutment member moves away from its sandwiched position. Preferably, at least one portion of the abutment member aligns with at least one portion of the screw threaded member in the sandwiched position of the abutment member. Preferably, said at least one portion of the screw threaded member is brightly coloured. In the especially preferred embodiment of the invention, the abutment member is rotatable about the longitudinal axis of the elongate member and one or a plurality of portions of the abutment may align with a corresponding one or plurality of portions of the screw threaded member in the sandwiched position of the abutment member, those portions being equiangularly spaced around the longitudinal axis of the elongate member.

The abutment member may be arranged to move to a limited extent when the level of the tension in the stem portion is reduced below the given level, and may be arranged to move to a greater extent when the level of the tension in the stem portion is further reduced by a given amount. In such an arrangement, the abutment member is able to indicate two tension levels which may correspond to minimum and maximum tensions for the stem portion of the screw threaded member.

Preferably, the abutment member includes a cover that covers over the second end of the elongate member. The cover might be an integral part of the abutment member or may simply be fixed thereto. Preferably the cover

completely covers over the second end of the elongate member.

By providing a cover that is part of the abutment member, it becomes possible to cover over the second end of the elongate member after installation of the elongate member in a factory and thereby to prevent subsequent tampering of the connection of the elongate member to the screw threaded member.

The cover preferably includes an opening through which the second end of the elongate member is accessible and a closure for closing over the opening. The opening on the cover can facilitate the initial installation of the elongate member in the screw threaded member while the installation of the closure, which may be bonded or riveted in place, may prevent any subsequent tampering with the adjustment. Thus, the closure may comprise a closure element permanently fixed in the opening in the cover.

Preferably, an 0 ring seal is provided between the abutment member and the screw threaded member.

It should be understood that the invention is applicable to various screw threaded fastener assemblies where in use a stem portion of a screw threaded member is tensioned. For example, the screw threaded member may be a stud or a bolt, for example, as described in EP 0 049 537 A2. In the case where the screw threaded member is a stud, the elongate member may pass all the way through the stud; in that case an abutment member may be provided at each end of the elongate member and the

abutment of each abutment member against a respective end of the stub may be the only connection between the stud and the elongate member. In the case where the screw threaded member is a bolt, it may be similar to a conventional bolt apart from the provision of the elongate cavity for receiving the elongate member.

The nature of the connection of the elongate member to the screw threaded member may be of various kinds but conventionally the first end of the elongate member is screw threaded and is connected to an internally screw threaded portion of the elongate cavity in the screw threaded member.

The second end of the elongate member may project from the screw threaded member and may be defined by an enlarged head portion of the elongate member. It should be understood, however, that other arrangements are also possible: for example the elongate member may be received internally within a recess formed in the end of the screw threaded member. The second end of the elongate member may also not be enlarged: for example, a circular groove may be provided around the second end of the elongate member and the abutment member may engage in that groove.

The use of a cover that is part of the abutment member can be of advantage even when the abutment member is arranged without any bias applied to it and even when an indicating arrangement is not provided. Thus according to a second aspect of the invention, there is provided a screw threaded fastener assembly including:

a screw threaded member including a screw threaded portion carried on a stem portion, an elongate member having first and second ends, the first end being inserted into an elongate cavity in the stem portion of the screw threaded member and connected to the screw threaded member, and the second end being spaced from the screw threaded member, and an abutment member mounted between the second end of the elongate member and the screw threaded member, the abutment member being able to become fixed in a sandwiched position between the elongate member and the screw threaded member when the screw threaded member is in use and the stem portion is tensioned beyond a given level but also being able to be free to move when the level of the tension is reduced below the given level, wherein the abutment member includes a cover that covers over the second end of the elongate member.

The screw threaded fastener assembly described immediately above may also incorporate any of the other features already referred to in connection with the first form of the screw threaded fastener assembly.

The present invention also provides a method of fastening employing a screw threaded fastener assembly as defined above, in which, as the screw threaded member is fastened and the abutment member begins to become sandwiched between the elongate member and the screw threaded member, the abutment member is moved against an opposing bias into its sandwiched position.

By way of example certain embodiments of the invention will now be described with reference to the accompanying drawings, of which: Fig. la is a sectional side view of a known fastener assembly, Fig. lb is a plan view of the fastener assembly of Fig. la, Fig. 2a is a sectional side view of a first form of fastener assembly embodying the invention, Fig. 2b is a plan view of the fastener assembly of Fig. 2a, Fig. 3 is a plan view of an indicating arrangement that may be provided on the fastener assembly of Figs. 2a and 2b, Fig. 4a is a plan view of another indicating arrangement that may be provided on the fastener assembly of Figs. 2a and 2b, Fig. 4b is a side view of the indicating arrangement of Fig. 4a, Fig. 5 is a plan view showing the indicating arrangement of Figs. 4a and 4b applied to two fastener assemblies, both correctly tightened, Fig. 6 is a plan view of the fastener assemblies of Fig. 5, with one of the fastener assemblies loose, Fig. 7a is a plan view of an electrical indicating arrangement that may be provided on the fastener assembly of Figs. 2a and 2b, the fastener assembly being correctly tightened,

Fig. 7b is a plan view of the electrical indicating arrangement of Fig. 7a, the fastener assembly being loose, Fig. 8a is a plan view of another indicating arrangement that may be provided with the fastener assembly being loose, Fig. 8b is a plan view of the indicating arrangement of Fig. 8a, with the fastener assembly tightened to a minimum desired tension, Fig. 8c is a plan view of the indicating arrangement of Fig. 8a with the fastener assembly tightened to a maximum desired tension, Fig. 9a is a plan view of another indicating arrangement that may be provided on the fastener assembly of Figs. 2a and 2b, with the fastener assembly correctly tightened, Fig. 9b is a plan view of the indicating arrangement of Fig. 9a, with the fastener assembly loose, Fig. 9c is a sectional side view of part of the fastener assembly including the indicating arrangement, and Fig. 10 is a sectional side view of part of a modified form of the fastener assembly shown in Figs. 2a and 2b.

Referring first to Figs. 2a and 2b, there is shown a screw threaded fastener assembly generally comprising a screw threaded member 11, an elongate member or pin 12 and an abutment member 13 fixed thereto.

The screw threaded member 11 includes a head 15 and a stem portion 16, the distal end of which is formed with a screw threaded portion 17. In those respects the member 11

is like a conventional bolt. The member 11 also, however, has an elongate cavity extending coaxially into the bolt from the head 15, the innermost end of the cavity being screw threaded.

The pin 12 has at one end a screw threaded portion 18 engaged in the screw threaded end of the cavity in the member 11 and at the opposite end a head 19.

The abutment member 13 is rotatably mounted around the pin 12 and is trapped between the head 19 of the pin 12 and the head 15 of the member 11. An 0 ring 20 is provided around the periphery of the abutment member 13, between the abutment member and the head 15 of the member 11, to prevent ingress of dirt and water. In the assembled state shown in Figs. 2a and 2b, a disc 27 is fixed in a corresponding opening in the top of the abutment member, so that the abutment member entirely closes over the head of the pin 12 and prevents a user gaining access to it.

As can be seen in Fig. 2a, the top of the head 15 of the screw threaded member 11 is slightly recessed to accommodate the lowermost portion of the abutment member 13.

A torsion spring 21 is mounted between the head 15 of the member 11 and the abutment member 13. The torsion spring has an upwardly cranked first end that engages in a blind hole 22 in the bottom face of the abutment member 13 and a downwardly cranked second end that engages in a blind hole 23 in the top face of the head 15 of the screw threaded member 11. Between its ends, the torsion spring

extends around the pin 12; in the example shown the spring has 1 turns, but it will be understood that a greater or smaller number of turns may be employed. A stop arrangement (not shown) is provided for limiting the rotation of the abutment member 13 relative to the member 11. The rotation may for example be limited to 1200.

During manufacture, before the disc 27 is fixed in place, the threaded portion 18 of the pin 12 is passed through the abutment member and into the member 11. The pin is screwed into the member 11 to a controlled extent such that the abutment member is a loose fit between the pin head 19 and the head 15 with a selected amount of axial play. Because the abutment member is a loose fit it naturally adopts a rotational position in which the torsion spring 21 is unstressed. Once the desired axial play of the abutment member is achieved, the disc 27 is bonded in place, preventing any further adjustment of the pin 12 relative to the member 11. The bolt is then fully manufactured and ready for use.

When the assembly is used, the screw threaded portion 17 of the member 11 is screwed into a threaded base, for example of a nut, and the stem portion 16 thereby subjected to tensile longitudinal stress, which causes a corresponding longitudinal strain in the member 11. That strain serves to draw the head 19 of the pin 12 towards the head 15 of the member 11, reducing and finally eliminating the axial play in the mounting of the abutment member 13.

While the abutment member 13 remains loose, it will, if

rotated by a user against the bias of the torsion spring 21, return to its unstressed position. Once the play is eliminated, however, the abutment member becomes sandwiched between the head 19 and the head 15 so that, if rotated by a user against the bias of the torsion spring 21, it will remain so despite the bias force of the spring urging the abutment member back to its original position.

The spring loading of the abutment member 13 provides a uniform repeatable means of judging when the abutment member is gripped during tightening of the member 11. When tightening the member the user occasionally partially rotates the abutment member 13 and then allows it to spring back to its rest position. The user continues to tighten the bolt until the abutment member 13 does not spring back after it has been rotated. The spring 21 provides a common measure of the force gripping the abutment member, which removes variation caused by different users trying to judge the degree of grip without such assistance.

The rotation of the abutment member 13 under the spring action described above may be used to provide a visual signal.

Fig. 3 shows a pointer 30 marked on the top of the abutment member 13. Any rotation of the abutment member moves the pointer 30. Figs. 4a and 4b show a similar arrangement but employing a larger pointer 31 which is integral with a ring 32 tightly fixed over the abutment member 13.

Figs. 5 and 6 show the use of the pointer 31 on two

adjacent fastener assemblies. When the assemblies are first tightened and the abutment members 13 become sandwiched, the pointers 31 are tightly fixed over their respective abutment members 13 in positions chosen so that the pointers 31 point towards one another. If, however, one of the assemblies loses its tightness, the abutment member 13 of that assembly rotates, as illustrated for the right hand assembly shown in Fig. 6. Whilst two assemblies are shown in Figs. 5 and 6 it will be understood that there may be a row of a multiplicity of assemblies or some other array of a multiplicity of assemblies.

In a further embodiment shown in Figs. 7a and 7b the rotation of the abutment member 13 caused by the spring 21 when the tension in the fastener falls is used to trigger an electrical switch 33. In this case an actuator is fixed to the abutment member 13 and this operates the electrical switch 33. The switch may be of any type but is typically a proximity switch. Hence all the fastener assemblies on critical equipment may be monitored continuously and from a central control room if required. The connections between the fastener assemblies and the control room may be wired or wireless connections. In a further optional refinement the switch 33 may be incorporated into the abutment member 13 and become an integral part of it. Alternatively, an inertia switch may be fitted directly to the abutment member 13 to sense its movement.

Figs. 8a to 8c illustrate a further embodiment of the invention in which the spring action is used to provide

indications of both minimum and maximum tension setting.

In the example shown the tension spring is able to rotate a maximum of 120 degrees but requires considerably less torque to rotate the first 60 degrees than is required to rotate the next 60 degrees. A mark 36 is provided on the top of the head 15 of the member 11 and a pointer 35 is provided on the abutment member 13. When the abutment member 13 is loose it adopts the position shown in Fig. 8a.

As a user tightens the fastener assembly, he rotates the abutment member 13 periodically through 60 degrees.

Initially the abutment member springs back to the position shown in Fig. 8a, but once a minimum tension in the stem portion of the member 11 is reached, that no longer occurs.

The user can now check that a maximum tension has not been exceeded by turning the abutment member 13 to the position shown in Fig. 8c. If the abutment member moves back from that position, then the maximum tension has not been reached. If, however, the member stays in the position shown in Fig. 8c, that indicates that the tension is at or above the maximum desired and the bolt should be loosened slightly. Thus, by applying simple tests on the abutment member 13, it is possible to keep between maximum and minimum tension settings.

Figs. 9a to 9c illustrate an especially preferred indicating arrangement. In this embodiment, an arcuate portion 40 of the head 15 of the member 11 are brightly coloured. The portion is shown shaded in Fig. 9b and lie just outside the cylindrical exterior of the abutment

member 13. The abutment member 13 has a segment 41 marked or formed on its upper surface which may also be brightly coloured. The brightly coloured segment 41 and arcuate portion 40 typically include an angle of 60 and there is one position of the abutment member 13 (shown in Fig. 9a) where the segment 41 aligns with the arcuate portion 40.

The abutment member 13 is arranged so that, when it is free to rotate, it is spring biased to the position shown in Fig. 9b. When the fastener assembly is tightened, a user rotates the abutment member 13 to the position of Fig.

9a where it is gripped while the fastener assembly remains correctly tightened. In this position the segment 41 and arc 40 are aligned. If, however, the fastener assembly loses sufficient tightness, the abutment member 13 will rotate under the bias of the spring 21 and the segment 41 and arcuate portion 40 will lose alignment, as in Fig. 9b, providing a clear indication that the fastener assembly has become loose.

The indicator arrangement of Figs. 9a to 9c will allow the loosening of a fastener assembly to be detected particularly easily. Consequently the loosening of a stationary fastener assembly can be detected with the naked eye at a distance, and it is also possible to detect the loosening of a moving fastener assembly with the naked eye, unless the assembly is moving very quickly. In the case of a rapidly moving fastener assembly, it may still be possible to detect loosening of the assembly: for example a stroboscope or high speed photography may be used.

Fig. 10 shows a screw threaded fastener assembly similar to that shown in Figs. 2a and 2b, but omitting the torsion spring. In that case, the abutment member 13 must be turned manually to check the tightness of the fastener assembly. The assembly does, however, have other advantages of the embodiment of Figs. 2a and 2b, notably the provision of a cover that is fixed to the abutment member 13 and prevents access after manufacture to the head 19 of the pin 12.

In the illustrated embodiments described above the screw threaded member is a bolt. It will be understood, however, that a stud may be provided instead. In that case a nut screwed onto an externally threaded end portion of the stud takes the place of the head 15 of the bolt.

Claims

Claims: 1. A screw threaded fastener assembly including: a screw threaded member including a screw threaded portion carried on a stem portion, an elongate member having first and second ends, the first end being inserted into an elongate cavity in the stem portion of the screw threaded member and connected to the screw threaded member, and the second end being spaced from the screw threaded member, and an abutment member mounted between the second end of the elongate member and the screw threaded member, the abutment member being able to become fixed in a sandwiched position between the elongate member and the screw threaded member when the screw threaded member is in use and the stem portion is tensioned beyond a given level but also being able to be free to move when the level of the tension is reduced below the given level, wherein the abutment member is arranged to be urged away from its sandwiched position.
2. An assembly according to claim 1, in which the abutment member is rotatable about the longitudinal axis of the elongate member.
3. An assembly according to claim 1 or 2, in which a spring is provided for urging the abutment member away from its sandwiched position.
4. An assembly according to claim 3, in which the spring is a torsion spring.

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5. An assembly according to claim 1 or 2, in which the abutment member is arranged to be urged away from its sandwiched position by gravity.
6. An assembly according to any preceding claim, further including an indicating arrangement for indicating movement of the abutment member away from its sandwiched position.
7. An assembly according to claim 6, in which the indicating arrangement includes a pointer which is arranged to move when the abutment member moves away from its sandwiched position.
8. An assembly according to claim 6, in which the indicating arrangement includes an electrical device for indicating movement of the abutment member away from its sandwiched position.
9. An assembly according to claim 8, in which the electrical device includes an electrical switch.
10. An assembly according to claim 6, in which the indicating arrangement is provided by portions of the abutment member and of the screw threaded member which move from aligned positions to separated positions when the abutment member moves away from its sandwiched position.
11. An assembly according to claim 10, in which at least one segment of the abutment member aligns with at least one portion of the screw threaded member in the sandwiched position of the abutment member.
12. An assembly according to claim 11, in which said at least one portion of the screw threaded member is brightly coloured.

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13. An assembly according to any preceding claim, in which the abutment member is arranged to move to a limited extent when the level of the tension in the stem portion is reduced below the given level, and is arranged to move to a greater extent when the level of the tension in the stem portion is further reduced by a given amount.
14. An assembly according to any preceding claim, in which the abutment member includes a cover that covers over the second end of the elongate member.
15. An assembly according to claim 14, in which the cover includes an opening through which the second end of the elongate member is accessible and a closure for closing over the opening.
16. An assembly according to claim 15, in which the closure comprises a closure element permanently fixed in the opening in the cover.
17. An assembly according to any preceding claim, in which an 0 ring seal is provided between the abutment member and the screw threaded member.
18. An assembly according to any preceding claim, in which the screw threaded member is a stud.
19. An assembly according to any of claims 1 to 17, in which the screw threaded member is a bolt.
20. An assembly according to any preceding claim, in which the first end of the elongate member is screw threaded and is connected to an internally screw threaded portion of the elongate cavity in the screw threaded member.
21. An assembly according to any preceding claim, in which

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the second end of the elongate member projects from the screw threaded member.
22. An assembly according to any preceding claim, in which the second end of the elongate member is defined by an enlarged head portion of the elongate member.
23. A screw threaded fastener assembly including: a screw threaded member including a screw threaded portion carried on a stem portion, an elongate member having first and second ends, the first end being inserted into an elongate cavity in the stem portion of the screw threaded member and connected to the screw threaded member, and the second end being spaced from the screw threaded member, and an abutment member mounted between the second end of the elongate member and the screw threaded member, the abutment member being able to become fixed in a sandwiched position between the elongate member and the screw threaded member when the screw threaded member is in use and the stem portion is tensioned beyond a given level but also being able to be free to move when the level of the tension is reduced below the given level, wherein the abutment member includes a cover that covers over the second end of the elongate member.
24. An assembly according to claim 23, further including the features of any of claims 14 to 22.
25. A screw threaded fastener assembly substantially as herein described with reference to Figs 2 to 10 of the accompanying drawings.

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26. A method of fastening employing a screw threaded fastener assembly according to any preceding claims, in which, as the screw threaded member is fastened and the abutment member begins to become sandwiched between the elongate member and the screw threaded member, the abutment member is moved against an opposing bias into its sandwiched position.