GB2052079A

GB2052079A - Improvements Relating to Tension Indicating Bolts

Info

Publication number: GB2052079A
Application number: GB8020646A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-06-25
Filing date: 1980-06-24
Publication date: 1981-01-21

Abstract

A tension-indicating bolt 10 has an axial bore 12 in which a plug 13 is fixed. A pin 17 is received in the bore 12 and passes through a bore in the plug 13. The plug is stuck in place with its inner end abutting a stop 33 and holding the end of the pin bottomed in the bore when the bolt is un-tensioned. The tension in the tightened bolt 10 is indicated by the relative positions of the outer end surface 22 of the pin and an adjacent surface 21 of the plug. The relative positions of the two surfaces 21 and 22 can be indicated with any suitable gauge. To facilitate the application of a gauge, the plug 13 has an externally screw threaded head 14. A method of fitting the plug 13 using adhesive to avoid exerting pressure on the pin 17 is described. In a modified embodiment the pin is held bottomed by a compression spring acting between the plug and a stop on the pin; Fig. 3 (not shown). <IMAGE>

Description

SPECIFICATION Improvements Relating to Tension Indicating Bolts The present invention relates to tension indicating bolts, the term bolt being used for brevity to encompass all screw threaded fasteners which act in tension, whether headed or unheaded.

The importance of tensioning bolts sufficiently to prevent metal fatigue and eventual bolt failure in vibrational situations has long been known.

See, for example, "Pre-tensioning for Preventing Fatigue Failure in Bolts", W. A. P. Fisher et al, AircraftEngineering June 1952 page 160. This article warns of the need to ensure that the tension is maintained in service.

Very many proposals have been made for establishing and checking bolt tensions. The commonest remains use of a torque wrench but it is frequently inaccurate and cumbersome to use as a checking tool. Later assembled parts have to be removed to get enough clear access to the bolts to use the wrench. A number of alternative proposals have been based upon the use of a pin in a bore in the bolt, which pin is fixed relative to the bolt at their end but does not partake of the extension of the bolt under tension. The inner end of the pin has been fixed in the bottom of the bore, e.g. by adhesive or by screwing in. The relationship between the outer end of the pin and a reference surface on the bolt provides a means of checking bolt tension. Methods involving the use of gauge blocks and air gauges are proposed in U.S. Specifications 3,248,923 and 3,561,260 respectively.The use of a dial indicator gauge is described in "Bolt Preload-How can you be sure it's right?" by A. S Comford, Machine Design 6th March 1975, page 78.

On the other hand there are various proposals to make use of an annular disk spring, often called a Belleville washer, which can be caused to click through its unstable flat configuration when the correct tension is reached. In particular my UK Patent Specification 1,213,734 provides the disk spring between the bolt and a head on the aforementioned pin. Although it has been demonstrated that such a disk spring can give a very sensitive indication, various problems exist.

One is the expense of building the device into every bolt. Another is maintaining correct tolerances in mass production such that the disk spring clicks at the correct tension. Another is that the disk spring is subjected to severe stress reversals and elastic drift occurs, whereby the indicated tension drifts.

The object of the present invention is to provide a much simplified construction of the bolt, which is thus cheaper and easier to manufacture.

According to the present invention, there is provided a tension indicating bolt comprising a plug fixed in a bore in the bolt, the inner end of the plug being part way along the bore, leaving a reference length of the bore beneath the plug, a pin in the bore in the bolt and in a bore through the plug, and a stop on the part of the pin in the reference length of the bore, serving to retain the pin in the bolt, the arrangement being such that, when the pin is bottomed in the bore in the bolt, the outer end of the pin and an adjacent surface of the plug indicate the tension in the bolt by their relative positions. The plug preferably has an externally threaded head for screwing a gauge tool thereon. The pin should have substantially the same coefficient of expansion as the bolt.

In this invention the plug retains the pin; no fixing at the inner end of the pin is necessary. The gauge tool itself can ensure that the pin is bottomed when a measurement is made, although the pin can be bottomed by a compression spring acting in the bore between the plug and the stop on the pin.

In order to achieve the requisite accuracy without any complex manufacturing techniques, it is preferred not to use a spring at all but to have the inner end of the plug abut the stop when the bolt is under no tension. If the plug and pin are both ground to correct lengths and the stop is correctly positioned along the pin, all critical dimensions are in the pin and plug and the boring of the bolt is not a high precision step. A technique is described below whereby the plug can be fixed with exactly the correct amount of insertion to abut the pin stop without putting a compressive load on the pin.

The gauge length of the pin (from inner end to stop) is preferably a standardised dimension.

Different bolt sizes can be catered for by a range of different plug lengths and corresponding overall pin lengths. In practice a range of three lengths will suffice for practically all requirements.

The pin can protrude from the plug in the untensioned bolt by such an amount that it becomes flush with the plug when the bolt is correctly tensioned.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-section of a bolt embodying the invention, Figure 2 is a cross-sectional view of a tool used to fit the plug in the bolt, and Figure 3 is a cross-section of a second bolt embodying the invention.

Referring to Figure 1, a bolt 10 with a head 11 has a blind axial bore 12 whose inner end 30 forms a stop for a steel pin 1 7, e.g. of the same steel as the bolt. The bore 12 is counterbored to form a radial relief bore 31 and a recess 32 for the head 14 of a steel plug 13. the plug head is externally right-hand threaded.

The pin is formed with a stop 33 by half shearing the pin part-way along its length. When the bolt is not under tension, the inner end of the plug 13 abuts the stop 33 and the pin 17 is bottomed in the bore 12. The length of the pin between the bottom 30 of the bore and the inner end of the plug is precisely established gauge length, e.g. 25 mm. When the bolt is tensioned and the pin is bottomed on the bore, e.g. by a spring-biased plunger of a gauge tool screwed on the head 14, the pin end 22 will have moved inwardly relative to the face 21 of the plug. For preference, the movement is from a protruding relationship to aflush relationship.

In order to have the inner end of the plug just abut the pin stop 33, the plug must be inserted until the pin lightly bottoms and then stay fixed in position. This could be achieved by screwing the plug into the bore but this would require the bore and plug to be threaded which would increase manufacturing costs appreciably. It is therefore preferred to use an adhesive to stick the inner end of the plug in the bore, the plug being held against the stop on the pin while the adhesive sets.

One way to do this is illustrated in Figure 2. A flanged, internally threaded, split bush 35 is fitted into the head recess 32. A set screw 36 in the flange bears against one side of the split 39 in the bush and, when tightened, expands the bush which is thereby held firmly in the recess. The plug 13, with the pin 17 in it and with the adhesive on its inner end is then inserted. The head 14 of the plug screws into the bush 35, for which purpose the head can have a screwdriver slot, and is screwed in until the pin is felt to bottom. The bolt is left until the adhesive has set.

Then the set screw is backed off and the bush 35 is screwed off the plug head for re-use with another bolt. Bolts can be fitted with the pin and plug in this way on-site whereas known devices require factory fitting.

A clearance 37 under the plug head ensures that it is the stop 33 on the pin which determines the amount to which the plug is inserted. The clearance provided by the relief bore 31 ensures that the plug is only fixed at its inner end and its length is not altered by extension of the bolt. The inner end of the plug preferably locates substantially at the underside of the bolt head.

However, plug lengths to say 16 mm, 38 mm and 50 mm will cater respectively for bolts 9 mm-25 mm diameter, for bolts 25 mm-50 mm diameter and for bolts 50 mm diameter and above and for studs.

A protective cap is preferably screwed on to the plug head 14 and removed only when setting or checking the bolt tension.

Figure 3 shows a bolt 10 with a head 11 through which a bore 12 extends into the bolt. A steel plug 13 has an externally, right-hand threaded head 14 which may be about 8 mm in diameter. A standard diameter and thread are preferably specified. The plug has a plug end 1 5 of 4 mm diameter spaced from the head by a radially relieved portion 1 6. If the invention is to be used with studs, the portion 1 6 can be made longer to get the plug end 1 5 down into the tensioned part of the stud. The plug end 1 5 is lightly knurled and is a force fit in the bore 12.

A pin 17 of the same steel as the bolt is received in the bore 1 2 and passes through a bore in the plug 13. A small compression spring 18 acting between the plug and ears 19 stamped on the pin 1 7 holds the pin bottomed in the bore. The outer end 22 of the pin protrudes into a recess 20 in the end face 21 of the plug head 14. The recess is preferably a groove across the plug head for ease of machining.

After the plug has been forced into the bore and seated home, the end face 21 and pin head 22 are ground in a double operation either flush or with a recessing of the pin end 22 relative to the face 21.

Claims

1. A tension indicating bolt comprising a plug fixed in a bore in the bolt, the inner end of the plug being part way along the bore, leaving a reference length of the bore beneath the plug, a pin in the bore in the bolt and in a bore through the plug, and a stop on the part of the pin in the reference length of the bore, serving to retain the pin in the bolt, the arrangement being such that, when the pin is bottomed in the bore in the bolt, the outer end of the pin and an adjacent surface of the plug indicate the tension in the bolt by their relative positions.

2. A tension indicating bolt according to claim 1, wherein the plug has an externally threaded head for screwing a gauge tool thereon.

3. A tension indicating bolt according to claim 2, wherein there is a clearance between the underside of the head and the bolt.

4. A tension indicating bolt according to claim 1, 2 or 3, wherein the inner end of the plug abuts the stop when the bolt is under no tension.

5. A tension indicating bolt according to any of claims 1 to 4, wherein the inner end of the plug is fixed in the bolt bore by adhesive.

6. A tension indicating bolt according to claim 1,2 or 3, wherein the pin is bottomed in the bore in the bolt by a compression spring acting in the bore between the plug and the stop on the pin.

7. A tension indicating bolt according to claim 6, wherein the plug is a force fit in the bore of the bolt.

8. A tension indicating bolt according to any of claims 1 to 7, wherein there is radial clearance between the plug and the bolt bore except at the inner end of the plug.

9. A tension indicating bolt according to any of claims 1 to 8, wherein the coefficiency of expansion of the pin is substantially the same as the coefficient of expansion of the bolt.

10. A tension indicating bolt according to any of claims 1 to 9, comprising a removable protective cap on the plug to protect the outer end of the pin and the said adjacent surface when it is not required to measure the tension in the bolt.