GB2344628A

GB2344628A - Load indicator for a threaded fastener

Info

Publication number: GB2344628A
Application number: GB9924827A
Authority: GB
Inventors: William Eccles
Original assignee: BOLT SCIENCE Ltd
Current assignee: BOLT SCIENCE Ltd
Priority date: 1998-12-09
Filing date: 1999-10-21
Publication date: 2000-06-14
Also published as: GB9924827D0; GB9826947D0

Abstract

A load indicator comprises a ring gauge (1) that is free to spin in an unloaded condition. The ring gauge (1) is attached to a lower washer element (2) and an upper washer element (3). A disc spring (4) is inserted into the ring gauge (1) between the lower and upper washer elements (2,3). The components of the device are retained by the ring gauge (1) resulting in a unitary assembly. When a load on the load indicator is less than a predetermined amount, the ring gauge (1) can spin. When the load exceeds a predetermined amount, the disc spring (4) is compressed and the ring gauge (1) is held between the upper and lower washer elements (2,3) and cannot be rotated.

Description

LOAD INDICATOR FOR A THREADED FASTENER This invention relates to a load indicator for a threaded fastener.

Joints secured by threaded fasteners generally depend upon a preload provided by the fasteners to resist any forces applied to the joint. In many applications ensuring that the fasteners provide an adequate preload is vital in ensuring joint integrity and a trouble free service life. The preload being usually the crucial factor in ensuring that the joint will not fail when external forces are applied. The most common way of controlling a threaded fastener's preload is by controlling the tightening torque that is applied when the fastener is tightened. With this method the majority of the tightening torque is used to overcome friction in the threads and between the nut face, or bolt head, and the joint surface. Typically, 90% of the torque is used to overcome friction with only 10% being used to elongate (by a tension on the fastener exerted by the nut due to its tightening on the thread) the fastener to provide preload.

Because of this and variations in the coefficient of friction and thread quality between fasteners, significant scatter can occur in the preload achieved for a given tightening torque. Secondly, because of effects such as vibrational loosening, joint embedding and gasket creep, a fastener's preload can be reduced to below the required level even after it has been fully tightened so that the correct preload was initially achieved. There is normally no easy way to check whether or not there is the correct preload in the fastener either at the time the fastener is tightened or when the joint is in service.

According to one aspect of the present invention, a load indicator for a threaded fastener comprises first and second pressure elements which are urged apart by a resilient member, which pressure elements define an opening for receiving a threaded fastener, the load indicator also comprising an indicator portion which is operable to be held relative to the pressure elements when a load applied to the pressure elements is sufficient to deform the resilient member by a predetermined amount, the load indicator being otherwise movable relative to the pressure elements, wherein the load indicator comprises a unitary assembly.

The unitary assembly may remain intact before and during use and may remain intact after use.

The indicator portion may be a ring, which may be coaxial with the washer elements and the resilient member. The indicator portion may have a greater diameter than the washer elements and resilient member.

The indicator portion may retain the remaining parts of the load indicator in a unitary assembly. Projections of the indictor portion may engage projections of the washer elements, to thereby maintain the unitary assembly. Projections of the washer elements may be received in corresponding openings, which may be grooves, of the indicator portion. The openings may have a width to allow for axial movement of the projections therein. The projections of the washer elements may be arranged to bear against the edges of the openings when a load on the load indicator exceeds a predetermined amount. The openings may define a central projection, which may be arranged to be trapped between the washer elements when the load indicator is subjected to a predetermined level of load.

The resilient member may be a disc spring. The resilient member may have thickness less than the thickness of the central projection of the indicator portion.

One of the washer elements may be a bolt head. One of the washer elements may be a nut. An assembly comprises a load indicator as described in relation to the previous aspect, a nut and a corresponding bolt.

This invention relates to a preload indicating device that will provide an accurate indication of the preload in a fastener overcoming the preload scatter problems associated with torque tightening and many of the other methods that are used to tighten fasteners. This invention will also provide a simple means to check that the fastener's preload is being maintained after it has been installed and throughout the joint's service life. A secondary benefit of this invention is that it improves the stiffness characteristics of the joint so that the fastener is less prone to vibrational loosening and fatigue failure.

This invention relates to a device that is capable of providing an indication of preload in a fastened joint. The preload indicating device is for use with fasteners, which may be of the bolt, screw or stud types. In this specification the words"bolt", "screw"and"stud"are used interchangeably since such fasteners are designed to be elongated by tensile forces and the described invention can be applied to these type of fasteners. The preload indicating device incorporates upper and lower washer elements, a disc spring and a ring interposed between the two rigid washer elements.

This disc spring is located between the upper and lower washers elements so that it will deflect when compressed by the preload applied to the joint through the fastener.

The disc spring is concentric to, and located within the inner diameter of a ring gauge member. The ring gauge is free to spin in the unloaded condition but is attached to the lower and upper washes so that a single assembly is achieved. Upon a predetermined level of deflection occurring within the disc spring, the deflection being calibrated to a fastener preload level, the ring is clamped between the upper and lower washers preventing it from being able to be rotated by a hand applied force. Being able to manually rotate the ring indicates that the correct disc spring deflection has not been achieved and hence the correct joint preload level has not been achieved.

It is another objective of this invention to provide a preload indicating device that may be utilised with a variety of bolt head styles and/or nut body styles. Another objective is to provide a device that is simultaneously capable of improving a joint's stiffness characteristics whilst providing an indication of a bolt's preload. Another advantage of the present invention is the ability to arrange a plurality of devices that have been set to differing levels of reload thereby enabling the user to be informed of increasing preload levels in the joint. It is also an object of the present invention to provide an economical and accurate means to determine the preload imposed by fasteners so that they will not, when installed, be either under stressed or overloaded to fracture. Another objective is to allow an accurate preload to be achieved from a fastener without the need of the use of special instrumentation or special wrenches.

This being achieved with the advantage of permitting checking for preload accuracy both prior to use and during the service life of the joint.

Specific embodiments of the invention will now be described by way of example with reference to the accompany drawings in which: Figure 1 shows a section through the preload measuring device in its unloaded condition; Figure 2 shows a plan view looking down onto the device; Figure 3 shows a section through the invention with the device in a compressed condition; Figure 4 shows an outside view of the preload measuring device; Figure 5 shows a modified embodiment of the invention consisting of serially arranged units of the preload measuring devices; Figure 6 shows a modified embodiment of the invention consisting of two disc springs arranged in series so the defection of the springs is increased for a given preload; Figure 7 shows a modified embodiment of the invention consisting of two disc springs arranged in parallel so the deflection of the springs is increased for a given preload; Figure 8 shows a modified embodiment of the invention where the preload measuring device is incorporated into flanged bolt; and Figure 9 shows a modified embodiment of the invention where the preload measuring device is incorporated into a flanged nut.

Referring first to figures 1 and 2, a preferred embodiment of the invention will include a ring gauge 1 that is free to spin in the unloaded condition. The ring gauge is attached to a lower washer member 2 and upper washer member 3. The method shown for achieving this is for a lip 8 to be incorporated into the upper and lower parts of the ring gauge 1. A lip 9 present on the lower washer member 2 and upper washer member 3 has a diameter slightly larger than the lip 8 diameter on ring gauge 1. The ring gauge 1 is pressed onto the upper washer member 3 by forcing lip 8 over lip 9. By careful control of the dimensions of the lip diameter 8 relative to lip diameter 9 only a small amount of force is necessary to achieve an assembled unit.

The tolerancing of the two diameters being selected so that the stresses present in the ring gauge 1 and upper washer member 3 are within their elastic limits so that permanent deformation does not occur. A recess 10 present on the ring gauge 1 allows the upper washer member 3 to be able to freely rotate relative to the ring gauge 1. A disc spring 4 is inserted into the ring gauge 1 and the lower washer member 2 pressed onto the ring gauge 1 in an identical manner as for the upper washer member 3. The assembled unit is now complete; the upper washer member 3 and lower washer member 2 being retained together as a single unit due to the diameter of the lip 8 on the ring gauge 1 being greater than the diameter of the lip 9 on the washers. A bolt 5 can now be inserted through the preload measuring device so that to clamp the device and joint member 6 to joint member 7. Referring to Figure 2, the central axes of upper and lower washer units 2 and 3, the ring gauge 1 and the bolt 5 are concentric to each other. The ring gauge 1 is able to be rotated relative to the lower washer member 2, upper washer member 3 and bolt 5 because in the unloaded condition all the force applied by the bolt 5 is transmitted into the upper washer member 3 into the disc ring 4 and into the joint via the lower washer member 2.

Referring now to Figure 3. As the bolt is tightened the preload on the joint increases and the disc spring 4 is compressed. The ring gauge 1 has a protrusion on its inside surface. At a predetermined amount of compression of the disc spring 4, the upper washer member 3 touches point 11 on the protrusion on the ring gauge 1 and the lower washer member 2 touches point 12 on the protrusion on the ring gauge 1.

This clamps the parts together preventing any further rotation of the ring gauge 1 relative to the upper washer member 3 and lower washer member 2. The prevention of the free rotation of the ring gauge 1 indicates that the correct predetermined level of preload has been achieved in the joint.

Figure 4 shows a side elevation of the invention showing the ring gauge 1 and upper and lower washer units 2 and 3. A knurled pattern 13 is present on the outside surface of the ring gauge. The purpose of the knurling being to improve grip onto the ring to allow it to be more easily rotated either by hand or with a tool.

The preload indicating device shown in Figure 5 consists of an upper ring gauge 14 and lower ring gauge 15 incorporated into a single unit. The unit contains a lower washer member 2 and upper washer member 3. Two disc springs 16,17 are separated within the unit by a central washer member 18. The unit is assembled in a similar manner to that described for Figure 1. The height of the inner protrusion 19 is larger on ring gauge 14 than on ring gauge 15. At a predetermined amount of compression of the disc spring 16, the upper washer member 3 clamps the upper ring gauge 14 to the central washer member 18 preventing any further rotation of the upper ring gauge 14 relative to the upper washer member 3 and central washer member 18. The locking of the rotation of the upper ring gauge 14 indicates that the correct predetermined level of preload has been achieved in the joint. The bottom ring gauge 15 is still free to rotate indicating that the bolt preload is within (or between) two predetermined levels. If further preload is applied the bottom ring gauge 15 is clamped between the central washer member 18 and lower washer member 2 preventing any further rotation. This indicates that too high a preload has been applied.

The preload indicating device shown in Figure 6 consists of a ring gauge 20, a lower washer member 2, two disc springs 21,22 and upper washer member 3. Two disc springs 21,22 are arranged in the manner shown so that the preload required to compress the springs will be greater than that required for a single spring alone for the same total deflections.

The preload indicating device shown in Figure 7 consists of a ring gauge 23, a lower washer member 2, tow disc springs 21,22 and upper washer member 3. The two disc springs 21,22 are arranged in the manner shown so that the preload required to compress the springs will be same as for a single spring yet twice the deflection will be achieved.

Figure 8 shows the preload indicating device incorporated into a flange headed bolt 24. The bolt 24 has its flange machined 25 so that its profile is identical to the upper washer member 3 shown in Figure 1. The ring gauge 1 is pressed onto the flange with the lower washer member 2 being pressed onto the ring gauge so that the disc spring 4 is contained within. The unit then works in an identical manner to the unit described in Figures 1 to 4.

Figure 9 shows the preload indicating device incorporated into a flanged nut 26. The nut 26 has its flange machined 27 so that its profile is identical to the upper washer member 3 shown in Figure 1. The ring gauge 1 is pressed onto the flange with the lower washer member 2 being pressed onto the ring gauge 1 so that the disc spring 4 is contained within. The unit then works in an identical manner to the unit described in Figures 1 to 4 The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment (s). The invention extend to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

CLAIMS 1. According to one aspect of the present invention, a load indicator for a threaded fastener comprises first and second pressure elements which are urged apart by a resilient member, which pressure elements define an opening for receiving a threaded fastener, the load indicator also comprising an indicator portion which is operable to be held relative to the pressure elements when a load applied to the pressure elements is sufficient to deform the resilient member by a predetermined amount, the load indicator being otherwise movable relative to the pressure elements, wherein the load indicator comprises a unitary assembly.
2. A load indicator as claimed in claim 1, in which the unitary assembly remains intact before and during use.
3. A load indicator as claimed in claim 1 or claim 2, in which the indicator portion is a ring which is coaxial with the washer element and the resilient member.
4. A load indicator as claimed in any preceding claim, in which the indicator portion retains the remaining parts of the load indicator in a unitary assembly.
5. A load indicator as claimed in claim 4, in which projections of the indicator portion engage projections of the washer elements, to thereby maintain the unitary assembly.
6. A load indicator as claimed in claim 4 or claim 5, in which projections of the washer elements are received in corresponding openings of the indicator portion.
7. A load indicator as claimed in claim 6, in which the openings have a width to allow for axial movement of the projections of the indicator portion therein.
8. A load indicator as claimed in 7, in wich the projections of the pressure elements are arranged to bear against the edges of the openings in the indicator portion when a load on the load indicator exceeds a predetermined amount.
9. A load indicator as claimed in claim 8, in which the openings define a central projection, which is arranged to be trapped between the pressure elements when the load indicator is subject to a predetermined level of load.
10. A load indicator as claimed in any preceding claim, in which the resilient member is a disc spring.
11. A load indicator substantially as described herein with reference to the accompanying drawings.