GB2107018A

GB2107018A - Bolt tension indicator

Info

Publication number: GB2107018A
Application number: GB08227148A
Authority: GB
Inventors: John Melvyn Ager
Original assignee: Glynwed Ltd
Current assignee: Glynwed Ltd
Priority date: 1981-09-29
Filing date: 1982-09-23
Publication date: 1983-04-20

Abstract

A bolt tension indicator in the form of a washer with a plurality of integral raised portions deformed out of the plain portion and being yieldable plastically to permit re- deformation back into substantially the plain portion. So that the indicator can be made relatively cheaply to be used merely as an indicator that some degree of bolt tightening has occurred, the integral raised portions (10) are of cylindrical shape and are backed by cylindrical recesses (14) of the same diameter as said raised portions, a load on the washer tending to re-deform the raised portions back into the plain portion (12) creating a shear force acting across a relatively narrow band (16) of metal behind each raised portion. <IMAGE>

Description

SPECIFICATION Bolt tension indicator The invention relates to bolt tension indicators.

Bolt tension indicators are used, for example, in the civil engineering industry to indicate visually that the required tensions in the fastenings of a structure have been achieved. Such indicators have been used for a number of years. By their use there is a definite visual indication of tension in the fastenings and this enables inspection to be undertaken at any convenient time after the fastenings have been tightened. Such indicators are usually preferred to the use of torque wrenches for the tightening of high strength bolts because torque wrenches take no account of the condition of the nut or bolt threads. They measure only the torque applied to the bolt or nut concerned and not the resultant tension in the bolt.

Various kinds of bolt tension indicators have previously been used. For example, there has been provided a pre-load indicating washer of hardened and tempered metal having a plain portion and a plurality of integral raised portions pressed or otherwise deformed out of the plain portion at circumferentially spaced intervals thereof and being yieldable plastically to permit redeformation thereof back into substantially the plain portion of the washer, the number of the raised portions and their size being such that the re-deformation of said portions back into substantially the plain portion of the washer under the clamping pressure of an associated bolt or other fastener will indicate a required tension in said fastener.

Such pre-load indicating washers have generally worked quite well but since they are made of hardened and tempered metal they are somewhat expensive. There are some applications where a more inexpensive item is required and which although not indicating the attainment of a particular tension in the fastening is indicative of some tightening having taken place.

According to the invention, there is provided a bolt tension indicator constituted by a metal washer having a plurality of integral raised portions pressed or otherwise deformed out of the plain portion at circumferentially spaced intervals thereof and in spaced relation to the inner and outer peripheries of the washer each, integral raised portion being of cylindrical shape and being backed by a cylindrical recess of the same diameter as the diameter of said integral raised portion, the depth of the recess and of the integral raised portion being somewhat less than the thickness of the plain portion of the washer whereby, when the washer is subsequently in use and subjected to a load tending to re-deform the integral raised portion back into the plain portion of the washer there is created a shear force acting across a relatively narrow band of metal extending from the bottom of the cylindrical recess to the junction of the cylindrical raised portion with the planar surface from which it has been deformed.

The integral raised portions will preferably be located nearer to the inner periphery than to the outer periphery of the washer. The washer may be of circular outline shape. Alternatively, it may be of square outline shape.

In order that the invention may be fully understood and readily carried into effect, the same will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig. 1 is a horizontal section through a bolt tension indicator embodying the invention, Fig. 2 is a plan view thereof, Fig. 3 is a scrap view drawn to a somewhat enlarged scale, of a part of the indicator which will presently be referred to, Fig. 4 is a graph which plots load against deformation of a bolt tension indicator embodying the invention and a pre-load indicating washer of a previously known form, and Fig. 5 is a partly cut away perspective view of a possible modification which will presently be described.

Referring now to Figs. 1 and 2 of the drawings, the bolt tension indicator there illustrated is constituted by a metal washer having a plurality of integral raised portions 10 pressed or otherwise deformed out of the plain portion 12 at circumferentially spaced intervals. The integral raised portions are also spaced from the inner and outer peripheries of the washer, although being much nearer to the inner periphery than to the outer periphery. Each integral raised portion 10 is of cylindrical shape and is backed by a cylindrical recess 14 of the same diameter as the diameter of said integral raised portion.

The depth of each recess 14 and of each integral raised portion 10 is somewhat less than the thickness of the plain portion of the washer, as best seen in Fig. 3, Consequently, when the washer is subsequently in use and subjected to a load tending to re-deform the integral raised portion back into the plain portion of the washer, there is created a shear force acting across a relatively narrow band 1 6 of metal extending from the bottom of the cylindrical recess to the junction of the associated cylindrical raised portion with the planar surface from which it has been deformed. This narrow band of metal is less than one third of the thickness of the plain portion of the washer and it will be understood that it will have a relatively low threshold resistance to deformation of the associated integral raised portion back into the plain portion of the washer.Such a relatively low threshold resistance is shown graphically in Fig. 4 in which the line BTI results from the plotting of load against deformation for a bolt tension indicator embodying the invention. The arrangement is such that when the threshold resistance to deformation has been exceeded the integral raised portions yield rapidly back into the plain portion of the washer. (It will be seen however, that the washer does not yield purely in shear and that throughout their movement back into a plain portion of the washer the integral raised portions continue to support substantially the same load at which they started to yield.) The line LIW by way of contrast plots load against deformation for a pre-load indicating washer of a previously known form of comparable size, that is to say a pre-load indicating washer of hardened and tempered metal having a plain portion and a plurality of integral raised portions pressed or otherwise deformed out of the plain portion at circumferentially spaced intervals thereof and being yieldable plastically to permit redeformation thereof back into substantially the plain portion of the washer.In this previously known form of washer, the number of the raised portions and their size was such that the redeformation of said portions back into substantially the plain portion of the washer under the clamping pressure of an associated bolt or other fastener was such as to indicate a required tension in the fastener. It will be seen that over a very wide range the deformation is roughly proportional to the applied load. A bolt tension indicator embodying the present invention is not intended to indicate a required tension in the fastener but merely to indicate by a quick visuai check that some tightening of the fastener has taken place.

Thus there is provided a bolt tension indicator which by virtue of the fact that it is not made from hardened and tempered metal, and by virtue of the fact that, in use, there is created a shear force across a relatively narrow band of metal, has a relatively low threshold resistance to deformation.

However, various modifications may be made. For example, in the example illustrated in Figs. 1 and 2 the washer is shown to be provided with six integral raised portions but it will be understood that a greater or lesser number could be provided.

Referring now to Fig. 5, the modified form of bolt tension indicator there illustrated is basically similar to that previously described except that it is of square outline shape with rounded corners. A further difference is that it has only four integral raised portions 10, these being located in the narrowest regions of the washer although still being located in spaced relation to the inner and outer peripheries of the washer.

In a further variation of this last described embodiment, the corners of the otherwise square outline shape of the washer could be simply chamfered off at 450 instead of being rounded.

The extent of the chamfering off could be any convenient amount, and indeed such an amount as to produce a regular octagonal outline shape.

Claims

1. A bolt tension indicator constituted by a metal washer having a plurality of integral raised portions pressed or otherwise deformed out of the plain portion at circumferentially spaced intervals thereof and in spaced relation to the inner and outer peripheries of the washer, each integral raised portion being of cylindrical shape and being backed by a cylindrical recess of the same diameter as the diameter of said integral raised portion, the depth of the recess and of the integral raised portion being somewhat less than the thickness of the plain portion of the washer whereby, when the washer is subsequently in use and subjected to a load tending to re-deform the integral raised portion back into the plain portion of the washer there is created a shear force acting across a relatively narrow band of metal extending from the bottom of the cylindrical recess to the junction of the cylindrical raised portion with the planar surface from which it has been deformed.

2. A bolt tension indicator according to claim 1, in which the integral raised portions are located nearer to the inner periphery than to the outer periphery of the washer.

3. A bolt tension indicator according to either one of the preceding claims, in which the washer is of square outline shape.

4. A bolt tension indicator constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.