GB2083233A

GB2083233A - Tensile test grips for packaged optical fibre

Info

Publication number: GB2083233A
Application number: GB8027269A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-08-21
Filing date: 1980-08-21
Publication date: 1982-03-17
Also published as: GB2083233B

Abstract

For performing tensile tests on packaged optical fibre, the fibre package is held between a pair of test grips each having a capstan 1 around which the fibre package is wrapped in a helical groove 4 before being wrapped around a circumferential shelf 5 against which it is clamped by a capping disc 8. <IMAGE>

Description

SPECIFICATION Tensile test grips for packaged optical fibre This invention relates to the design of test grips for performing tensile tests on lengths of packaged optical fibre.

For many applications of optical fibre an important parameter is the change of optical performance brought about by the imposition of tensile stress. In order to be able to make measurements of such changes it is necessary to grip the fibre in two spaced apart regions and apply tension over the intervening region. Clearly the gripping must be sufficiently tight to prevent slippage at the values of tension required to be in investigated. Normally this gripping will involve applying lateral compressive forces to the fibre. These forces are themselves liable to damage the optical performance of the fibre and hence it is necessary to design test grips which can support without slippage a relatively large tensile force with a relatively low compressive force.

Since the strength of a freshly drawn glass optical fibre deteriorates relatively rapidly if its surface is left exposed to atmospheric attack, it is general practice to arrange for the fibre to be provided with a protective coating on line with the drawing operation. Thus tensile tests are normally performed not on bare optical fibre, but upon packaged fibre consisting typically of the bare fibre encased in one or more plastics sheaths.

According to the present invention there is provided a pair of low-compression tensile test grips for gripping a packaged optical fibre in two spaced apart regions so as to be able to subject the intervening region to tensile stress, each of which test grips includes a fixed cylindrical capstan provided with a helical groove extending several turns around its curved surface and extending into a rebate that forms a circumferentail ledge at the junction of the curved surface of the capstan with one of its end faces, wherein the capstan is provided with a capping disc against said one of the end faces of the capstan to co-operate therewith such that the rebate defines a channel whose width is less than that of the groove.

Afeature of this design is that it is relatively compact and this is particularly useful when testing is required in special environments of controlled temperature and humidity provided within a confined space.

There follows a description of one of a pair of test grips embodying the invention in a preferred form.

The description refers to the accompanying drawings in which Figure 1 depicts an exploded perspective view of one test grip, Figure 2 depicts the test grip capstan in greater detail, and Figure 3a to 3g depict progressive cross-section details of the groove run-off of the capstan of Figure 2.

Referring to Figures 1 and 2, a substantially cylindrical metal capstan 1 is rigidly attached to a metal bracket member 2 by means of a pair of bolts 3. This particular capstan is designed for performing tests on optical fibre having a diameter in the range 100 to 400 microns encased in primary and secondary plastics coatings providing an overall package diameter of 1 mm, and the capstan has a diameter of about 6cm and a height of about 2cm. The packaged fibre can readily be bent to a significantly tighter radius but these dimensions provide a good compromise, allowing enough fibre to be held in contact with the capstan by somewhere in the region of five whole turns of fibre to be accomodated in a helical groove 4 formed in the curved surface.This groove leads directly into a circumferential ledge 5 provided by a rebate formed in the capstan at the junction of its curved surface and its outer end face 6.

Secured to this outer end face 6 by means of a bolt 7 is a metal capping discS having the same diameter as the main portion of the capstan 1. A pair of dowel pins 9 mounted in the end face 6 of the capstan engage in co-operating holes in the capping disc so as to prevent its rotation with respect to the capstan.

Just ahead of the point in the circumference where the groove 4 enters the ledge 5 the adjacent portion of the curved surface of the capping disc is provided with a groove 10 extending, at a somewhat steeper pitch than groove 4, from the inner end face to the outer. The final part of the assembly comprises a leaf-spring clip 11 secured buy a bolt 12 in position on the end face of the disc to retain an end of fibre in the groove 10.

The groove 4 is preferably Vee shaped in cross section and dimensioned so that the packaged optical fibre is pinched between the two walls of the Vee. Typically, for 1 mm diameter packaged fibre, at 30 angle is provided between the two walls of the Vee which are 1 .5mm apart at the surface.

The fibre is actually clamped by the action of capping disc pressing it against the ledge 5, and thus the extent of this ledge in the axial direction must be made somewhat less than the diameter of the packaged fibre.

The run-off of the groove 4 into the rebate forming the ledge 5 is arranged to provide a smooth transition and typically subtends about 30 of the circumference. Figures 3a to 3e illustrate details of typical cross-sections over the length of the run-off.

Figure 3a depicts a cross section just before the run-off, and shows a full height wall between the groove 4 and the ledge 5. Figures 3b, 3c and 3dshow how this wall height is progressively reduced. In the section of Figure 3e the wall is eliminated, while the sections of Figures 3f and 3g show the wall progressively reaching full height in respect of the next circuit of the groove.

To use the grip, the free end of a length of the fibre package to be tested is wound round the capstan 1, in its groove 4, and is then led round almost the full circumference of the shelf 5. The capping discS is then mounted on its dowels 9, and the bolt 7 is tightened to clamp the fibre package against the shelf 5. The protruding end of the fibre package is led through the groove 10, and is lightly held in position by means of the spring clip 11. A second grip is then applied to the fibre package in a similar way, and then the two grips are mounted such that the fibre package extends in a straight line from the groove in the capstan of one grip to the groove in the capstan of the other Alternatively, particularly with longer lengths of fibre package, it may be led over one or more pulley wheels or the like from the one test grip to the other.

Claims

1. A pair of low-compression tensile test grips for gripping a packaged optical fibre in two spaced apart regions so as to be able to subject the intervening region to tensile stress, each of which test grips includes a fixed cylindrical capstan provided with a helical groove extending several turns around its curved surface and extending into a rebate that forms a circumferential ledge at the junction of the curved surface of the capstan with one of its end faces, wherein the capstan is provided with a capping disc and means for removably securing the capping disc against said one of the end faces of the capstan to co-operate therewith such that the rebate defines a channel whose width is less than that of the groove.

2. A pair of test grips as claimed in claim 1 wherein each capping disc is mounted on its associated capstan by keying means that prevent its rotation with respect to that capstan.

3. A pair of test grips as claimed in claim 2 wherein said keying means is provided by dowels.

4. A pair of test grip as claimed in claim 1,2 or 3 wherein each capping disc is provided with a groove in its curved surface leading from one face of the disc to the other.

5. A pair of test grips as claimed in claim 1,2,3 or 4wherein the groove of each capstan is a Veegroove.

6. A pair of test g rips each substantially as hereinbefore described with reference to the accompanying drawings.

7. A method of subjecting a length of packaged optical fibre to tensile stress including the step of gripping the packaged optical fibre in two spaced apart regions with a pair of test grips as claimed in any preceding claim wherein the axial extent of the ledge on the capstan is less than the diameter of the fibre package.

8. A method as claimed in claim 7 wherein the test grips are mounted such that the fibre extends in a straight line from the groove in the capstan of one test grip to the groove in the capstan of the other.