GB2169705A - Measuring separation between two bodies - Google Patents

Abstract

A method of obtaining such a measure, particularly for the purpose of making strain measurements on specimens to which the bodies are spot welded, involves inserting a blade (24) between the bodies (20) and effecting tangential contact therewith in two different angular orientations by rotating the blade (24) about its longitudinal axis (26). Measurement of the angle 2A between the two angular orientations provides a measure of the separation between the bodies and may be used to determine the strain induced in the specimen by an applied load, measurements of the angle 2A in this event being taken before and after the application of the load. <IMAGE>

Description

SPECIFICATION Measuring separation between two bodies this invention relates to a method of obtaining a measure of the separation between two spaced bodies and has particular application to the measurement of strain in circumstances where the strain are of larger magnitude and a high degree of spatial resolution is required.

According to one aspect of the present invention there is provided a method of obtaining a measure of the separation between two spaced bodies, said method comprising using said bodies to limit movement of an angularly movable measuring element from a predetermined angular orientation such that the degree of said angular movement is related to the separation between the bodies.

According to a second aspect of the present invention there is provided a method of obtaining a measure of the strain induced in a component by the application of loads thereto, said method comprising: (a) locating on a surface of said component two bodies in spaced relation to one another; (b) while the component is unstressed or subject to a relatively low load, using said bodies to limit movement of an angularly movable measuring element from a predetermined angular orientation such that the degree of angular movement is related to the separations between the bodies; (c) repeating step (b) while the component is subjected to a strain-inducing load; and (d) analysising the angular displacements of said measuring element to derive a measure of the strain induced by the load.

The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view illustrating a specimen whose strain field is to be investigated; Figure 2 is a schematic view illustrating an array of spherical bodies attached to the specimen in the vicinity of the neck portion of a pulled T-connector; Figure 3 is a schematic view illustrating the use of a contact element in obtaining a measure of the spacing between a pair of spherical bodies within the array; Figure 4 is a schematic view illustrating the effect of curvature on spacing between the spherical bodies; and Figure 5 is a schematic view illustrating use of a sensor for obtaining a measure of the local radius of curvature of the surface to which the spherical bodies are attached.

Figure 6 is a schematic longitudinal section of a separation-measuring device according to the invention; Figure 7 is a cross-sectional view illustrating the measuring element of the device; Figure 8 is an underside plan view.

Referring first to Fig. 1, there is shown a loaded specimen which is used for simulating the loads to which components of a decay-heat heat exchanger of a liquid metal cooled fast breeder reactor is subject in use. In such a heat exchanger, a number of pipes are connected to a header through the agency of welded connections with pulled T-joints formed in the header. In the specimen of Fig. 1, the components 10 and 12 respectively simulate the header and one of the pipes, the pipe 12 being butt-welded to a pulled T-portion 14 by the circumferential weld 16.

The neck of the pulled T-portion 14 is of arcuate section and is an area of particular interest in terms of investigating the strain field induced when the pipes attached thereto undergo lateral deflections, as illustrated by the arrow 18 in Fig. 1, with the header portion 10 immovably clamped at the positions indicated by reference numeral 19. In practice such deflections arise as a result of temperature differentials which develop in the heat exchanger but, in the specimen, loads of known magnitude are applied by suitable means, using the pipe 12 as a lever so as to produce known displacement, see angle W (Fig. 1).

It will be appreciated that such loads will tend to induce tensile and compressive strains in the neck region of the portions 14 and, in practice these strains are of large magnitude and this, in conjunction with the high temperature involved, renders conventional strain techniques, eg. strain gauges, unsuitable. Moreover, strain gauges do not afford a very satisfactory degree of spatial resolution within the strain field.

In accordance with a preferred feature of the invention, investigation of the strain field at high spatial resolution is achieved by attaching, by spot welding, an array of high precision steel balls 20 to the specimen in the area of interest, ie. the exterior neck surface of the portion 14. The balls 20 are all substantially identical size (typically 0.8 mm diameter) and they are spaced apart, preferably uniformly (typically a centre-to-centre spacing of 1.5 mm) in two mutually orthogonal directions, ie. circumferentially of the portion 14 and generally radially along the neck surface as it curves upwardly. In the specimen illustrated, two such arrays may be employed at diametrically opposite positions of the portion 14. An advantage of spot welding the steel balls to the specimen is that the integrity of the specimen is substantially unaffected.Fig. 2 illustrates a typical array of steel balls 20 within the strain field, the three rows being spaced apart circumferentially, ie. so that each horizontal row (as seen in Fig. 2) extends substantially radially of the portion 14 as it curves upwardly.

The strain measurements are, in effect, made by measuring the spacing between adjacent pairs of balls 20 prior to and during the application of a predetermined load 18. The method can be an indirect one as illustrated by Fig. 3 to which reference is now made. Fig. 3 shows a pair of balls viewed along a line perpendicular to the surface to which they are attached. The balls are each of radius r and the perpendicular spacing therebetween is d.

An accurately machined measuring blade 24, which is seen in cross section in Fig. 3, of thickness t is inserted into the space between the balls 20 and is turned about its longitudinal axis 26 until it makes tangential contact with both balls, as shown solid outline in Fig. 3. If the blade 24 is turned in the opposite direction about the axis 26, a second position of tangential contact with both balls is obtained, as shown in phantom outline in Fig. 3. if the angle of rotation between the two contact positions of the blade is 2A then it can be shown that: A=arc cos [(2r+t)/(2r+d)j By measuring the value of A before and after straining the specimen, the strain E can be derived from the formula:: IE=1-cos Al/cos A2 It will be noted that the above formula for the strain E is independent of the spacing d and requires measurement only of the angles although, if desired, these measurements could be used to derive the spacing between a pair of bodies so that such spacing can be monitored.

Reverting to Fig. 1, the blade 24 is connected to a high precision potentiometer 30 (or other encoding device) through a flexible coupling (not shown) which has torsional stiffness to avoid any angular resilience. Thus, the angular rotation of the blade 24 in moving between the position shown in Fig. 3 can be converted into an electrical parameter, such as voltage, and with suitable calibration, the voltage change measured, for example by a multimeter, may be used in computing the strains E according to the formula above.

The formula given above for strain E is valid only if the surface to which the balls are attached is substantially plane or if the ball radii are very much less than their centre to centre spacing. If however the surface is curved to a significant extent, the formula must be modified to take account of the influence of curvature on ball spacing.If, with reference to Fig. 4, the angle 2B reperesents the angle defined by the normals through neighbouring balls, it can be shown that the strain is given by: E - 1 -

1/(cos A2) + K sin B21 1/(cos A1) + K sin B1 where B, and B2 are the angles before straining and K is a constant for a given ball size and blade thickness and is given by the formula: K=2r/(2r+t) In practice, it can be assumed that B2 equals B, in order to simplify the procedure without too much loss of accuracy. In this event, it is only necessary for the angle B to be measured on one occasion, eg. before applying the strain inducing load.

Fig. 5 illustrates one means of measuring the angle 2B (and hence, in effect, the radius of curvature). The contact sensor 32 terminates in a flat tip 34 which is sufficiently wide to make tangential contact with two balls 20 simultaneously. The body of the sensor is of electrically conductive material and, in the vicinity of the tip, a contact 36 is provided which is also electrically conductive but electrically insulated from the main body. It will be seen that electrical continuity is obtained between the body of the sensor and the contact 36 when the tip makes proper contact with both balls, the conductive path being completed through the balls and the specimen to which they are spot welded. Thus, by incorporating the contact 36 and the sensor body in an electrical circuit, the making of tangential contact with the two balls may be indicated by a suitable signalling device. At this time, the inclination of the sensor 32 may be recorded.

The procedure is repeated with one of the balls previously contacted by the sensor and another neighbouring ball to derive another angle of inclination for the sensor. From such measurements, the angle 2B for each pairing of neighbouring balls can be determined (which, in effect, also constitutes a measure of the radius of curvature of the surface to which the balls are attached).

Although the invention is described above with reference to the measurement of strain, other applications involving measurement of the separation between two bodies are possible. Moreover, while spherically shaped contact bodies are preferred, other shapes are feasible, eg.

cubical, with appropriate modification of the formulae to take account of different geometrical shapes.

Referring now to Figs. 6-8, the measuring device shown is intended for measurement of the spacing between a pair of bodies. In this case, the angularly movable measuring element (corresponding to the blade 24 of Fig. 3) does not make direct contact with the separated bodies. Instead, the measuring element makes indirect contact with the bodies tbrough the intermediary of a pair of contact members. Thus, as shown, the device comprises a pair of contact members 40 formed as levers which terminate at their free ends in thin contact blades 42 which can be inserted into the gap 44 between the bodies 46. The levers 40 are mounted by flexural pivots 48 at their other ends and are spring-biassed away from one another by a compression spring 50.

The measuring element is constituted by a pair of laterally projecting arms 52 on a rotatable spindle 54 which is screwthreadedly engaged with a plate 56 and passes with clearance through a bush 58 secured to the plate 56. The arms 52 may be formed by a washer inserted into an axial slot in the free end of the spindle 54. The radial extremities of the arms 52 each bear against a respective one of the contact members 40. The spindle 54 is biassed by a torsional spring 60 (see Fig. 8) in a direction in which the arms 52 are caused to bear constantly against the contact members 40 and consequently the angular position of the spindle 54 automatically follow changes in the spacing between the contact members 40 and hence blades 42.

The plate 56 is provided with a graduated scale (see Fig. 8) over which a pointer 62 moves in response to rotation of the spindle 54. Although the graduated scale are shown spaced uniformly, in practice a non-linear scale may be necessary since rotation of the spindle 54 is not linearly related to the variation in separation between the contact members. The scale may be calibrated to indicate the spacing between the outside faces of the contact blades 42, ie those faces which are intended to contact the bodies 44.

In use, a measurement of the separation between the bodies 44 is obtained by manipulating the device so that the blades 42 lie within the gap and then allowing the spring 50 to press the blades into contact with the bodies with consequent turning of the spindle 54. The spacing can then be read off the scale according to the position of the pointer 62.

In a modification, only one of the contact members 40 need be movable. The other may be fixed relative to the plate 56 in which event, the measuring element need only contact the movable contact member since it can be located at a fixed spacing from the fixed contact member.

In a further or alternative modification, the position of the measuring element may be adjustable lengthwise of the gap between the contact members 40 and the latter may be mounted so that the maximum measurable gap that can be bridged by the blades 42 varies according to the lengthwise position of the measuring element. In this event, a number of measuring scales may be associated with the pointer, each scale corresponding to a different position of adjustment of the measuring element.

As shown in Figs. 6-8, the measuring element is movable angularly about an axis extending generally lengthwise of the contact member 40. In an alternative arrangement, the measuring element and spindle carrying the same may be rotatable about an axis which is transverse to the contact members, ie. an axis extending into the plane of the paper as seen in Fig. 6.

Claims (21)

1. A method of obtaining a measure of the separation between two spaced bodies, said method comprising using said bodies to limit movement of an angularly movable measuring element from a predetermined angular orientation such that the degree of said angular movement is related to the separation between the bodies.

2. A method as claimed in Claim 1 in which said measuring element is caused to contact directly with at least one of said bodies.

3. A method as claimed in Claim 1 in which said measuring element is caused to make direct contact with both of said bodies.

4. A method as claimed in Claim 1 in which said measuring element is caused to make contact indirectly with both of said bodies through the intermediary of at least one of a pair of contact members between which the measuring element is located.

5. A method as claimed in Claim 4 in which said contact members are adjustable towards and away from one another and are inserted into the gap between said bodies and then adjusted until they each contact a respective one of said bodies.

6. A method of obtaining a measure of the separation between two spaced bodies, said method comprising angularly orientating a measuring element between the two bodies such that oppositely facing portions of the measuring element each contact a respective one of the bodies, and determining, with respect to a predetermined angular orientation, the degree of angular displacement necessary to achieve such contact.

7. A method of obtaining a measure of the strain induced in a component by the application of loads thereto, said method comprising: (a) locating on a surface of said component two bodies in spaced relation to one another; (b) while the component is unstressed or subject to a relatively low load, using said bodies to limit movement of an angularly movable measuring element from a predetermined angular orientation such that the degree of angular movement is related to the separation between the bodies; (c) repeating step (b) while the component is subjected to a strain-inducing load; and (d) analysing the angular displacements of said measuring element to derive a measure of the strain induced by the load.

8. A method as claimed in Claim 7 in which the measuring element is employed in the manner claimed to any one of Claims 2-4.

9. A method of obtaining a measure of the strain induced in a component by the application of loads thereto, said method comprising: (a) locating on a surface of said component two bodies in spaced relation to one another; (b) while the component is unstressed or subject to a relatively low load; (c) determining, with respect to a predetermined angular orientation, the degree of angular displacement necessary to achieve such contact; (d) repeating steps (b) and (e) while the component is subject to a strain-inducing load; and (e) analysing the angular displacement so determined to derive a measure of the strain induced by the load.

10. A method as claimed in Claim 7, 8 or 9 in which there is an array of said bodies attached to said surface and in which the method steps are performed for a plurality of pairs of said bodies within the array whereby the spatial distribution of the strain can be ascertained.

11. A method as claimed in any one of Claims 1 to 10 in which said measuring element is angularly movable about an axis and is located so that said axis extends substantially normal to the separation distance between said bodies.

12. A method as claimed in any one of Claims 7 to 9 and claims dependent thereon in which said surface is curvilinear and in which the radius of curvature thereof in the locality of the or each pair of bodies is allowed for in the derivation of the said measurement.

13. A method as claimed in any one of Claims 1 to 12 in which said predetermined angular orientation corresponds to the orientation of said measuring element when a first position of contact directly or indirectly with said bodies, the measurement being made by bringing the measuring element into a second position of contact directly or indirectly with said bodies.

14. A method as claimed in any one of Claims 1 to 13 in which said bodies are of substantially indentical shape and dimension.

15. A method as claimed in Claim 10 in which said bodies within said array are spaced apart in two mutually orthogonal directions.

16. A method as claimed in any one of Claims 1 to 15 in which said bodies are spherically shaped,

17. A method as claimed in any one of Claims 12 and claims appendant thereto in which a measure of the radius of curvature at a specific location is ascertained by contacting the uppermost surfaces of first and second bodies at the location with a member having a tip capable of bridging only two of said bodies at one time, recording the angle of inclination of said member and repeating the procedure for said first body and third body at that location.

18. A method of obtaining a measure of the separation between two spaced bodies, said method being substantially as hereinbefore described with reference to the accompanying drawings.

19. A method of obtaining a measure of the strain induced in a component by the application of loads thereto, said method being substantially as hereinbefore described with reference to the accompanying drawings.

20. A device for measuring the separation between a pair of bodies, said device comprising a pair of contact members for insertion between said bodies, said contact members being adjustable relative to one another to enable them to be moved until they simultaneously contact a respective one of the bodies, a measuring element located between said contact members, said measuring element being movable angularly into engagement with at least one of said contact members such that the degree of angular movement necessary to obtain such engagement provides a measure of the separation between said contact member and hence said bodies, and means associated with the measuring element for providing an indication of the separation distance between said contact members.

21. A measuring device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.