GB2268268A - Determining the shape of a generally planar article - Google Patents

Abstract

The shape of a generally planar article, e.g. a piston ring (10), is determined by a method comprising positioning the article on a generally planar support surface (21) of a support (20), the article is positioned so that the outer peripheral edge (14) of the article overhangs substantially the entire outer peripheral edge (38) of the support surface (21). The method also comprises rotating the support (20) about an axis (24), and measuring the distances (42) between successive points on the edge (14) of the article and a fixed datum point (36) as said points come opposite the datum point (36) during the rotation. From the distances (42) and their relative orientations about the axis (24), the shape of the article is determined. <IMAGE>

Description

DETERMINING THE SHAPE OF A GENERALLY PLANAR ARTICLE This invention is concerned with a method of determining the shape of a generally planar article, e.g.

a disc or a ring such as a piston ring for an internal combustion engine or a compressor.

It is frequently desired to determine the shape of generally planar articles to ascertain whether their outer peripheral shapes have been correctly shaped. For example, piston rings for internal combustion engines are commonly formed of metal to a desired complex shape which is generally oval. The ring has two free ends which are closely adjacent to one another but do not touch so that a gap is formed in the ring. Each piston ring is intended to be received in a groove provided in the piston of an internal combustion engine to provide a seal between the piston and the cylinder in which the piston reciprocates.

The shape to which the piston ring is manufactured has to be precisely controlled so that the ring will provide an effective seal.

It is desirable to be able to determine a shape to which a piston ring has been manufactured, for example so that machines for making such piston rings can be adjusted.

However, present methods of determining such shapes are inaccurate and laborious. For example, such a piston ring can be inserted into a bore, and a light shone along the bore so that the gaps between the bore and the ring can be seen. This is essentially a manual operation which is able to detect grossly out-of-shape rings but is not generally accurate enough to satisfy normal accuracy requirements.

Another possibility is to encircle the ring with a flexible steel tape which is drawn tight around the ring to close the gap therein and measure diameters of the ring. This only gives limited information and is also laborious.

Another possibility is to rotate the ring with three spindles running on the outside of the ring. The central spindle is resiliently urged towards the ring and the movements of this spindle in moving around the ring can be detected and used to interpret the shape of the ring. This method gives information only about the mean radius of curvature on an arc defined by the other two spindles.

It is an object of the present invention to provide a method for determining the shape of a generally planar article, e.g. a piston ring, which is less laborious and more accurate than previous methods.

The invention provides a method of determining the shape of a generally planar article, the method comprising selecting a support which has a generally planar support surface which will fit completely within the area of said article, positioning the article on the support surface so that the outer peripheral edge of the article overhangs the support surface around substantially the entire outer peripheral edge of the support surface, rotating the support through at least one revolution about an axis thereof extending normally to the plane of the support surface, measuring the distances between successive points on the outer peripheral edge of the article and a fixed datum point as said points come opposite said datum point during rotation of the support surface, recording said distances and their relative orientations about said axis, and determining the shape of the article from said distances.

The method in accordance with the invention is simple to operate since the support can be selected in accordance with the particular type or size of article under consideration and once selected can be utilised for a large number of similar articles. Furthermore, positioning the article on the support is easy since it does not have to be precisely positioned. An operator can easily verify that the required overhang has occurred. Furthermore the method is found to give accurate results provided that the measuring step is carried out with sufficient accuracy.

This can readily be arranged since all the measurements take place at one location where highly accurate measuring apparatus can be located.

The article may be generally annular, in which case the support surface may have a circular outer edge which is larger in diameter than the hole through the article. If the article is a piston ring, it overhangs the support surface except where the gap occurs. The support surface may be divided into discrete portions, e.g. three portions forming a tripod, provided that all said portions are overhung by the article.

Preferably, the distances are measured at constant angular intervals about said axis. For example, a measurement can be taken after every 20 of revolution of the support. An encoder can be provided on the axis of the support to measure these angles.

Preferably, in determining the shape of a piston ring, the point of greatest symmetry on the shape detected is found and positions of other edge points of the ring in relation to this point are calculated from said distances.

Since the point of greatest symmetry should divide the ring into two identical halves, location of the point of greatest symmetry enables the overall symmetry of the ring to be assessed, e.g. by superimposing the two halves upon one another. Since the point of greatest symmetry should occur opposite the gap, an arc of the piston ring opposite to the gap can be searched for the point of greatest symmetry.

Conveniently, the distances are measured optically so that contact with the article is not required. One possibility is to measure the distances by means of a laser beam which sweeps between said datum point and a point within the area of the support surface. The beam can sweep at a constant rate and the time during which light from the laser reaches a receiver during a sweep can be used to measure said distances, i.e. the distance between the datum point and the outer peripheral edge of the article at the point to be measured is proportional to the proportion of the laser light which reaches the receiver. This gives a particularly accurate measurement of the distance.

There now follows a detailed description, to be read with reference to the accompanying drawings of a method of determining the shape of a piston ring which is illustrative of the invention.

In the drawings: Figure 1 is a plan view of a piston ring; Figure 2 is a diagrammatic view of apparatus used in the illustrative method; Figure 3 is a diagrammatic view showing the principle of the operation of the apparatus of Figure 2; and Figure 4 is a graphical representation of the results obtained by the apparatus of Figure 2.

The illustrative method is for determining the shape of a piston ring 10 shown in Figure 1. The ring 10 is made by a complex bending operation from a strip of metal but could be made by machining or casting and has a gap 12 therein between 2 adjacent ends of the strip. The ring 10 has an outer peripheral edge 14 and an inner peripheral edge 16.

Figure 2 illustrates the apparatus used in carrying out the illustrative method. The apparatus comprises a support 20, a spindle 22 on which the support 20 can be mounted for rotation about a vertical axis 24 of the spindle 22, a motor 26 operable to rotate the spindle 22 about the axis 24, and an encoder 28 operable to detect the amount of the rotation of the spindle 22 and, therefore, of the support 20. The apparatus also comprises a scanning laser 30, a receiver 34 arranged to receive light from the laser 30 and a datum blade 36 mounted on a fixed support 37. The blade 36 is positioned to be co-planar with a ring 10 positioned on the support 20. There is, thus, a gap 42 between the blade 36 and the outer edge 14 of the ring 10.

The support 20 is a flat disc which has a circular outer peripheral edge 38 whose diameter is less than the minimum transverse extent of the piston ring 10. The diameter of the support 20 is, however, larger than the hole through the ring 10 so that it is great enough to enable the support 20 to support the ring 10, i.e. the support 20 is large enough that it will not fit through the space bounded by the inner peripheral edge 16 of the ring 10. The support 20 is one of a number of alternative discs which can be mounted on the spindle 22, each disc having a diameter appropriate to a particular size of piston ring.

The first stage of the illustrative method is to select the appropriate support 20 for the ring 10 in question and to mount it on the spindle 22.

Next, in the illustrative method, the ring 10 is positioned on an upper surface 21 of the support 20 which provides a generally planar support surface which will fit completely within the area of the riny 10. The ring 10 is positioned so that the outer peripheral edge 14 of the piston ring 10 overhangs the support 20 around the entire outer peripheral edge 38 of the support 20 except where the gap 12 in the piston ring 10 occurs. The piston ring 10 is shown positioned in this manner on the support 20 in Figure 2. It should be noted that it is not necessary to position the ring 10 centrally on the support 20 so long as overhang occurs all round the edge 14.

Next, in the illustrative method, the motor 26 is operated to rotate the support 20 through at least one revolution about the axis 24 which is a central axis of the support 20 extending normally to the plane of the ring 10 positioned on the support 20. The rotation has to be slow enough to avoid displacement of the ring 10 relative to the support 20. During, this rotation the gap 42 between the blade 36 and the outer peripheral edge 14 of the ring 10 varies in width.

The illustrative method also comprises measuring the distances between successive points on the outer peripheral edge 14 of the piston ring 10 and the fixed datum point provided by the blade 36 as said points come opposite said datum point during rotation of the support 20, i.e.

successive measurements of the gap 42 are made. These measurements are carried out using the laser 30 and the receiver 34. The laser 30 has an associated optical system arranged to cause the laser's beam 44 to sweep from side to side across a parallel-sidedXpath 46, the sweep moving from edge to edge of the path 46 at a constant rate. One edge of the path 46 impinges on the blade 36 and the other edge thereof impinges on the support 20. However, during part of each sweep the light from the laser 30 passes through the gap 42 and is received by the receiver 34 which is positioned on the other side of the gap 42. The amount of light which passes through the gap 42 depends on the width thereof and, hence, varies as the gap 42 varies during rotation of the support 20 around the axis 24.The receiver 34 is arranged to detect for how long in each sweep of the beam 44, light is received. This time is related to the width of the gap 42 and, hence, gives a measure of the distance between the blade 36 and the point on the edge 14. of the-ring 10 which is closest to the blade 36. In a modification of the illustrative method, the blade 36 could be dispensed with, and an edge of the path 46 used the datum point.

The illustrative method also comprises recording the distances measured by the receiver 34 and their relative orientations about said axis 24 as measured by the encoder 28. The distances and orientations are recorded by a memory 48 which is connected to the receiver 34 and to the encoder 28.

The illustrative method also comprises determining the shape of the ring 10 from the distances recorded by the memory 48.

Figure 3 diagrammatically illustrates how, in the illustrative method, the shape of the ring 10 is determined. Figure 3 a plot with the position of the axis 24 as the origin. Because of the rotation of the support 20, the gap 42 (represented by radial lines) varies as different points on the edge 14 of the ring 10 pass the blade 36. The relative positions of the blade 36 relative to the axis 24 can be considered as a circle centred on the axis 24 and the lengths of the radial lines illustrate the different sizes of the gap 42. Points 50 on the edge 14 are plotted by measuring the different measurements of the gap 42 from points on the circle representing the blade 36 towards the axis 24. The points on the circle 36 from which the lines 42 commence are determined from the signals from the encoder 28.Figure 3 shows measurements of the gap 42 at 10 intervals although, in practice, smaller angles are used, e.g. 2O. At the gap 12 in the ring. 10, the distance from the blade 36 to the edge 38 of the support 20 is measured, which is a known distance, so that the gap 12 is detected. Once the points 50 have all been plotted a line 52 can be fitted to them representing the shape of the duter edge 14 of the ring 10.

Because the centre of the ring 54 does not normally coincide with the axis 24 and because the position of the edges of the gap 12 may not have been precisely detected, in analysing the shape of the piston ring 10, the point of greatest symmetry 58 of the shape 52 is calculated. In order to find the point of greatest symmetry, an arc 56 centred on a point directly opposite the location of the centre of the gap 12 as detected is investigated until the point at which the two halves of the shape 52 between the gap 12 and said point are found to have greater symmetry than any other point of said arc 56. The point of greatest symmetry can then be positioned on an axis of a new plot and the remainder of the shape 52 positioned in relation thereto so that the shape 52 can be compared to a standard shape. Also, if desired, as shown in Figure 4, the two halves of the plot of the shape 52 can be superimposed upon one another. One half is shown by a full line 60 in Figure 4 and the other half by a broken line 62. Various statistical analyses can be carried out, e.g. the standard deviation between the lines 60 and 62 can be calculated.

Alternatively, the line 60 may represent half the shape 52 determined by the illustrative method and the line 62 may represent an ideal form of this shape. Similar statistical analyses can also be carried out in this case.

Claims (12)

1 A method of determining the shape of a generally planar article, the method comprising selecting a support which has a generally planar support surface which will fit completely within the area of said article, positioning the article on the support surface so that the outer peripheral edge of the article overhangs the support surface around substantially the entire outer peripheral edge of the support surface, rotating the support through at least one revolution about an axis thereof extending normally to the plane of the support surface, measuring the distances between successive points on the outer peripheral edge of the article and a fixed datum point as said points come opposite said datum point during rotation of the support surface, recording said distances and their relative orientations about said axis, and- determining the shape of the artic-le from said distances.

2 A method according to Claim 1, wherein the article is generally annular, and the support surface has a circular outer edge which is larger in diameter than the hole through the article.

3 A method according to Claim 2, wherein the article is a piston ring and is positioned on the support surface so that it overhangs the support surface except where the gap occurs.

4 A method according to Claim 3, wherein in determining the shape of the piston ring, the point of greatest symmetry on the shape detected is found and the positions of other edge points of the ring in relation to said point are calculated from said distances.

5 A method according to Claim 4, wherein an arc of the piston ring located opposite the gap therein is searched for said point of greatest symmetry.

6 A method according to any one of Claims 3 to 5, also comprising comparing one half of the ring with the other half thereof in order to determine how symmetrical the ring is.

7 A method according to any one of Claims 3 to 6, also comprising comparing the shape determined with an intended shape for the ring.

8 A method according to any one of Claims 1 to 7, wherein the distances are measured at constant angular intervals about said axis.

9 A method according to any one of Claims I to 8, wherein said distances are measured optically.

10 A method according'to Claim 9, wherein said distance are measured by means of a laser beam which sweeps between said fixed datum point and a point within the area.of the support surface.

11 A method according to Claim 10, wherein the time during which light from the laser reaches a receiver during a sweep of the laser, is used to measure said distances.

12 A method of determining the shape of a piston ring substantially as hereinbefore described with reference to the accompanying drawings.