GB2097097A - Making piston rings - Google Patents

Abstract

A method of manufacture of a piston ring, includes making a ring blank having an outer diameter which is greater, and an inner diameter which is less than that required in the finished ring, cutting a gap in the ring blank to afford two free ends and at least partly closing the gap by the application of force only to each of the free ends. One or more blanks 10 are clamped in this configuration between a stationary clamping member 60 and an annular clamping member 87 forming part of a machining head 78. The interior and exterior peripheries of the clamped blank or blanks are machined simultaneously to be circular, by boring and turning tools 91, 92 carried on inner and outer parts 79, 81 of the machining head. <IMAGE>

Description

SPECIFICATION Piston rings, and method and apparatus for their manufacture This invention relates to piston rings for reciprocating engines or compressors and to methods of their manufacture.

It will be appreciated that when a piston ring, particularly (but not exclusively) a compression ring having a single gap, of either a plain rectangular cross-section or any other known or convenient cross-section, is installed in an engine, the outer periphery of the ring should be circular and should have an outward resilient load so that it maintains sealing contact with the cylinder bore around the entire periphery. When the ring and bore are new the gap should be small or substantially closed. It is also desirable that as the cylinder bore and/or piston ring wear during operation, the resilient sealing contact between the ring and the bore should be maintained around the entire periphery (although it will be obvious that as the diameter of the piston ring increases by say 0.5% of the diameter, the gap will increase correspondingly).Moreover, to promote good sealing and minimise friction, it is desirable for the outward resilient load to be as uniform as possible around the periphery.

In the following specification and claims, the term 'diameter' is used in relation to the inner and outer peripheries of a ring blank even though these peripheries of the ring blank may not be precisely circular but may, prior to machining, be slightly oval or elliptical. Nevertheless these peripheries of the ring blank will be approximately circular and so the term 'diameter' will, for convenience, be used. Thus, references in the specification and claims to the 'diameter' of the ring blank are to be construed accordingly.

The term 'closed' as used herein in relation to the condition of a ring blank having a gap cut therein is defined as a condition of the ring blank in which the gap is reduced, as compared with a position of the ring blank when the ring blank is unstressed by external forces, or in which the free ends are lightly in abutment with one another. In most cases, however, there is still a small gap between the free ends in the 'closed' condition. The finished ring when contracted to fit into a cylinder bore may be in a 'closed' condition, as defined.

The term 'circular' as used herein in relation to the machining of a clamped ring blank or blanks is to be construed as meaning circular within the tolerances of which the means used for machining are capable.

Hitherto, in order to solve this problem, piston rings have been made by one of two methods, which may be outlined as follows: In the first method the piston ring is machined to have both its inner and outer diameters circular before cutting the gap; the gap is then cut and the piston ring is heat formed by placing it on a mandrel and subjecting it to a high temperature so that the ring is expanded with its elastic limit to have a "permanent set." The extent of this is that on assembly of the piston ring is an engine with the gap closed, the outer diameter is circular and the ring has an inherent resilient load which maintains it in contact with the cylinder bore.

The second known method is to turn the piston ring before cutting the gap so that its inner and outer diameters have a predetermined non-circular form, usually determined by the shape of a cam on a machine of which the cutting tool follows the movement of the cam follower. A predetermined gap is then cut, so arranged that on assembly, when the gap is closed, the external periphery of the ring is circular and has an outward resilient load.

The latter method has the disadvantage that the calculations of the non-circularform assume that the material is homogeneous, and in practice this is not always so.

It has been proposed in British Patent Specification 709,246 to provide a machine for forming a piston ring, comprising a device having a plurality of rollers adapted to be spaced round the outer periphery of the ring blank for constricting and holding the ring blank, and a grinding wheel for grinding the outer peripheral surface of the ring blank. This machine has many serious disadvantages. Firstly, the free ends are not maintained at the same predetermined distance apart throughout the machining step. Secondly, instead of the ring blank between its free ends being allowed to take its natural form, allowing for internal stresses and non-homogeneity of the material, it is constricted by a number of, e.g.

twelve, spaced rollers, so that the periphery will have a small but significant waviness, with a smaller radius at the point of contact of the rollers and a larger radius between the rollers. Thirdly, the ring blank is not clamped between clamping members against its parallel faces, to prevent it from moving while being machined. Fourthly, the ring blank in this prior art is ground to size progressively around the periphery; therefore the part of the ring which has been ground, and which therefore has a smaller radius than the part not yet ground, must move in an outward direction to maintain contact with the rollers.This implies that the centre of the ring blank moves progressively, as the ring blank is moved round to bring successive positions of its circumference into position to be ground; but unless the centre of the ring blank remains in the same portion, the grinding of the periphery will not result in a ring of which the outer surface is a true circle.

It is an object of the invention to mitigate or overcome these disadvantages.

According to a first aspect of the invention there is provided, a method of manufacture of a piston ring, including the steps of making a ring blank of which the diameter over the outer periphery is greater, and the diameter within the inner periphery is less than that required in the finished ring, cutting a gap in the ring blank to afford two free ends, holding the ring blank in a closed position by the application of a force only to each of the free ends of the ring blank, clamping one or more of said ring blanks between a stationary clamping member and an annular clamping partforming part of a machining head, the machining head being movable towards the ring blank or blanks to bring the annular clamping part into engagement with the ora ring blankto clamp the ring blank or blanks, and then machining simultaneously the interior and exterior peripheries of the clamped ring blank or blanks to a circular, by respective boring and turning tools carried on respective inner and outer parts of the machining head, said inner and outer parts of the machining head being coaxial with, and arranged respectively within and outside the annular clamping part, being movable axially relative to said annular clamping part, and being rotatable relative to the annular clamping member to perform said machining step.

According to a second aspect of the invention there is provided, a piston ring when manufactured by the method of the first aspect of the invention.

According to a third aspect of the invention there is provided, apparatus for the manufacture of a piston ring from a ring blank having a diameter over the outer periphery which is greater than, and a diameter within the inner periphery which is less than that required in the finished ring, the ring blank having two free ends formed by the cutting of a gapin the ring blank, the apparatus comprising means for holding the two free ends ofthe ring blank in a closed position by the application of a force only to each of the free ends of the ring blank, a machining head having an annular clamping part and being mountedformovementtowardsthe ringblankto bring the annular clamping part into engagement with the ring blank to clamp the side faces of the ring blank between said annular member and a stationary clamping member, the machining head also including inner and outer parts carrying respective boring and turning tools, said inner and outer parts being coaxial with, and arranged respectively within and outside, the annular clamping part, being movable axially relatively to said annular clamping part, and being rotatable relativelytothe annular clamping part to perform said machining step.

According to a fourth aspect of the invention there is provided, a piston ring for an internal combustion engine, the piston ring having a gap cut therein to afford two free ends and there being a hole in each free end extending parallel to the axis of the piston ring.

An embodiment of the invention will now be specifically described, by way of example, with reference to the accompanying drawings, of which:- Fig. 1 is an elevation of a piston ring blank; Fig. 2 is an elevation of the piston ring blank of Fig.

1 showing holes drilled in the ring and a gap being cut in the piston ring blank; Fig. 3 is an elevation, partly cut away, of part of an apparatus for closing and holding piston ring blanks; Fig. 4 is a cross-sectional view of the apparatus of Fig. 3 showing a drilling unit of the apparatus; Fig. 5 is a similarviewto Fig. 3 but showing the piston ring blank with a gap cut therein; Fig. 6 is a cross-sectional view of the apparatus in the condition shown in Fig. 5 and showing a sawing unit; Fig. 7 is a similar view to Figs. 3 and 5 but showing the piston ring blank in a position to be clamped and machined and a pair of arms for centering the ring; Fig. 8 is a side elevation, partly in cross-section, of a machine for clamping, boring and turning a piston ring blank.

In piston ring practice it is conventional to refer to the radial dimension of the ring as the thickness and the axial dimension of the ring as the width.

The ratio diameter to thickness of a piston ring typically varies from 20:1 to 30:1 depending on the material.

Referringfirstto Fig. a typical piston ring 10 has, when finish machined, an outside diameter of 60mm, a thickness of 2.5mm and a width of 2mm. It is shown and described as a compression ring of plain rectangular cross-section, though it may be of any of a large number of known or convenient cross-sections. The piston ring may be compression or scraper ring, though it may also be, in suitable cases, an oil control ring, for example, a plain ring of a multi-piece oil control ring.

The ring blank may be cast, or produced by any known or convenient method and preferably has a small degree of ovality, though in certain cases-e.g.

where the required gap is small - may be truly circular; the ring blanks thus produced will have a machining tolerance, the reason for which will become apparent; thus the outer diameter will be larger and the inner diameter smaller than the finished dimensions, each by, say, 1 mm. The ring blank may be formed with a notch or projection 12 at a point on its circumference, forthe purpose of correctly orientating the ring blank for machining. The side faces 11 are then ground parallel to on another and to the required width.

Referring next to Figs. 2 to 6, the ring blank 10 in full annular form is loaded into a unit where it is initially located by resting on a projecting rod 48 (which may if desired engage with a notch 12 in the ring blank) and centralised by means of two symmetrically-disposed ways or guides 49. The unit comprises a pair of slides 50 mounted to move in converging slideways 52. Two pins 51 are provided, one on each slide and the pins 51 can be retracted and extended through guide bore 53 in the slides 50.

For this purpose, the pins 51 are attached to pivoted end portions of pivoted levers 55. The latter are loaded by springs 56 to the position in which the pins 51 are retracted, and may be moved by means of further levers 57 and pins 58 to the position in which the pins 51 are extended. Extension of the pins 51 is effected by means of an actuator 59 acting on the levers 57.

The unit has a base 60 which is substantially annular, and has the same mean radius and slightly smal lerthickness than the required finished dimensions of the ring, but has a cut-out portion 61 the top (see Fig. 3). The surface of the base 60 is co-planar with the surface of the ends of the slides 50 in which the guide bores 53 are formed.

The axes of rotation of two drills 62 are aligned with the axes of the guide bores 53 (see Fig. 4). The drills 62 are driven in any convenient manner, and the drilling unit 63 incorporates a ring blank clamping plate 64 and an associated actuator 65, which clamps the ring blank 10 against the surface of the base 60 and of the ends of the slides 50, before drilling. The drilling unit 63 is then advanced and oper ated, to drill two holes 13 in the ring blank 10 at a predetermined spacing, such that the holes 13 will be close to the free ends of the ring after a gap is cut, and approximately midway between its inner and outer peripheries. As the drills 62 are retracted, the actuator 59 is operated to cause the two pins 51 to extend, engaging one in each hole 13.To facilitate this, the pins 51 may be of slightly smaller diameter than that ofthe drills 62. Thus the ring blank 10 is located in a position in which the ring blank 10 is unstressed by external forces.

The drilling unit 63 is then retracted, and a sawing unit 66 is next employed (see Figs. 5 and 6). A ring blank clamping plate 67 and actuator 68 clamp the ring blank 10 against the surface of the base 60 and of the ends of the slides 50, as described above with reference to Figs. 3 and 4. The orientation rod 48 is withdrawn by operation of actuator 48a (Fig. 4) and ways 49 are also withdrawn. The gap is then cut, for example by two circular saws 69 (see Figs. 2 and 6), positioned one on each side of the axis of symmetry, so that the portion of the blank 10 containing the notch or projection 12 is removed. The clamping plate 67 is then released.

The two slides 50 are then moved on their converging slideways 52, until their opposed faces 71, which serve as stops, come into contact with one another. Thus, by means ofthe projecting pins 51, the ring blank is brought to the closed position, in which the gap is reduced (or the free ends 15 are lightly in abutment with one another). Moreover, the ring blank 10 is, by this movement of the slides 50, brought to a position in which it is substantially in register with the base 60 (though the radial thickness of the base 60 will be less than that of the ring blank, to allow for machining). The ends of the slides 50 substantially fill the cut-out portion 61 (and the radial thickness of these ends will also be less than that of the ring blank).

With the slideways symmetrically disposed on each side of a vertical plane, the motion of the two slides 50 may be synchronised by means of a horizontal bar 45 engaging in a corresponding groove 44 in the slides 50, a vertical arm 46 attached to the horizontal bar being raised or lowered by means of a suitable pneumatic ram 47. The bar 45 is trapped between the rear face of the casing in which the slideways 52 are formed and a cover plate, the front of the slides 40 being flush with the front of the bar 45.

With the ring blank 10 in its closed position, supported against the base 60 by pins 51, it is then centralised by means of a centralising unit shown in Fig.

7 which consists of a pair of meshing gears 74 mounted to rotate in bearings (not shown) and carrying respective arms 73. An actuator 75 is connected to one of the gears 74 to cause the arms 73 to move symmetrically, by virtue of the meshing teeth of the gears 74, on either side of a plane in which lies the desired position of the centre of the ring blank 10, said plane being normal to the plane of base 60. The free ends 76 of the arms are arranged to contact the ring blank 10 at positions angularly spaced by approximately 120"from the free ends 15 supported on pins 51,so as to centralise the ring blank 10.

Referring next to Fig. 8, a turning and bornng unit 78 is next aligned with the central axis of the held ring blank 10. The unit 78 as a whole is movable towards and away from the ring blank 10, for example on slides. The machining head of this unit has three concentric parts. The inner part 79 and the outer part 81 are secured together, and are mounted in a sliding member 82 by means of bearings 83. The outer part 81 carries a sleeve 84 slidable within it against a coil spring 85.

An intermediate annular clamping part 80 is jour nalled within the outer part 81, is rotatable relative to the inner and outer parts, and has a thrust bearing 86 between itself and the sleeve 84. The intermediate part 80 has a first annular portion 80a including an annular clamping face 87 of substantially the same mean radius and thickness as the base 60. The portion 80a has a convex part-spherical face 88 engaging with a concave part-spherical face 89 on a second annular portion 90 of the intermediate part 80, the common centre of these faces lying in the plane ofthe clamping face 87.

The inner part 79 has a boring tool 91 and the outer part 81 has a turning tool 92 at their ends near the annular clamping face of the intermediate part 80.

In operation, the unit 78 is advance towards the ring blank 10 andtheface 87 engages the ring blank 10 to press and clamp the ring blank 10 against the base 60 and the surfaces of the lower ends of slides 50 on which the ring blank 10 is held. The arms 73 are withdrawn either before this engagement or as the engagement occurs. Continued movement of the unit 78 clamps the side faces 11 of the ring blank 10 between the clamping face 87 and the base 60, which forms a stationary clamping member. The clamping force is partly supplied by the spring 85.

The inner and outer parts 79,81, including sleeve 84 and coil spring 85, are rotated by any convenient form of drive, and, as the unit 78 is advanced further, the boring tool 91 machines the inner periphery and the turning tool 92 machines the outer periphery of the ring blank 10 traversing across the width of the ring blank. The advance of the unit 78 compresses the spring 85 within the sleeve 84, and thus causes an increased clamping force to be exerted through the thrust bearing 86 and intermediate part 80 on the ring blank 10.

The part-spherical mating faces 88, 89 allow the face 87 to bear evenly on the ring blank 10 around its whole circumference even if the axis of the face 60 is not exactly in alignment with the axis of the intermediate part 80. The boring tool 91 and the turning tool 92 cut at respective points on the surfaces of the ring blank, the points lying on a common radius of the ring blank as they rotate around the ring blank.

This ensures that the tools apply no nett radial forces to the ring blank.

If desired, there may be provided a rough turning and boring unit, followed by a fine turning and boring unit; as a result of the retraction of part 80, the ring blank 10 will be unclamped between the operation of the former and of the latter unit. Any "features" such as grooves, steps, bevels, etc., may also be machined by means of a suitable tool in a similar way.

Piston rings with a circularity of better than three microns have been produced.

It has been found that with rings manufactured as described above with reference to the drawings, the resilient action of the ring against the cylinder bore is maintained around the entire periphery for a longer time, in terms of the wear of the cylinder bore, than with a conventional ring. For example, with a 60mm diameter ring, as described, a conventional ring may maintain good sealing contact for an increase in cylinder bore size of 0.15mm on diameter. It has been found that rings made in accordance with the present invention retain good sealing characteristics until about double this amount of wear has taken place.

In addition, the fact that the rings may be machined by normal turning to circular form, rather than on a special machine designed to give a particular non-circularform, tends to reduce the cost of machining.

Instead of the holes being drilled wholly through the axial width of the ring, where the ring is of sufficient width, the holes may be drilled part way only through the width. The criterion is that the pins, whether mounted on a hand tool or on a face of a machine tool, when inserted in the holes, should have sufficient bearing area to enable the gap to be maintained at the predetermined "closed" dimension. Moreover where the holes are drilled through the entire width of the ring, pins may be inserted in the holes with an interference fit, the projecting portion of the pins enabling the gap to be closed by a suitable tool or slides, and the projecting portion then being ground off.

It will be appreciated that before the ring blank 10 is clamped, it is free from distortions caused by radially directed forces and it may be clamped in this state, as described above. Alternatively, the ring blank may be deliberately radially deformed by predetermined radially directed forces before being clamped; the radial deformations being held by the clamping. The radial deformations are chosen so that, after the inner and outer peripheries have been machined to be circular and the ring blank unclamped to release the radial deformations, the peripheries of the ring have a required shape for the insertion of the finished ring in a cylinder bore. For example, finished rings may be produced in this way in which the free ends of the ring are deflected either inwardly, e.g. diesel engines, or outwardly, e.g. petrol engines.

Alternatively, rings having such inwardly or outwardly deflected ends may be made by machining a ring blank without radial deformations to be circular but of greater or lesser diameter respectively than the diameter of the cylinder bore.

Other variations within the scope of the invention will be apparent to those skilled in the art.

Claims (11)

1. A method of manufacture of a piston ring, including the steps of making a ring blank of which the diameter over the outer periphery is greater, and the diameter within the inner periphery is less than that required in the finished ring, cutting a gap in the ring blank to afford two free ends, holding the ring blank in a closed position by the application of a force only to each of the free ends of the ring blank, clamping one or more of said ring blanks between a stationary clamping member and an annular clamp ing partforming part of a machining head, the machining head being movable towards the ring blank or blanks to bring the annular clamping part into engagement with the or a ring blank to clamp the ring blank or blanks, and then machining simultaneously the interior and exterior peripheries ofthe clamped ring blank or blanks to be circular, by respective boring and turning tools carried on respective inner and outer parts of the machining head, said inner and outer parts of the machining head being coaxial with, and arranged respectively within and outside the annular clamping part, being movable axially relative to said annular clamping part, and being rotatable relative to the annular clamping member to perform said machining step.

2. A method as claimed in claim 1 wherein said machining step comprises rotating the boring and turning tools in synchronism in contact with respective points on the respectively, said points lying on a common radius of the ring blank, while rotating around the ring blank.

3. A method as claimed in claim 1 or claim 2 and comprising, before clamping the ring blank or blanks, centering the ring blank or blanks by moving two arms from a retracted position in which the arms are equally spaced on either side of a plane in which lies the required position of the centre of the closed ring blank or blanks, when clamped, and which is normal to the plane or planes of the ring blank or blanks, by equal amounts to a position in which the arms engage the ring blank or blanks to centre the ring blank or blanks.

4. A method of manufacture of a piston ring as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

5. A piston ring when manufactured by the method of any one of claims 1 to 4.

6. Apparatusforthe manufacture of a piston ring from a ring blank having a diameter over the outer periphery which is greater than, and a diameter within the inner periphery which is less than that required in the finished ring, the ring blank having two free ends formed by the cutting of a gap in the ring blank, the apparatus comprising means for holding the two free ends of the ring blank in a closed position by the application of a force only to each of the free ends ofthe ring blank, a only to each of the free ends of the ring blank, a machining head having an annular clamping part and being mounted for movement towards the ring blank to bring the annular clamping part into engagement with the ring blank to clamp the side faces of the ring blank between said annular part and a stationary clamping member, the machining head also including inner and outer parts carrying respective boring and turning tools, said inner and outer parts being coaxial with, and arranged respectively within and outside, the annular clamping part, being movable axially relatively to said annular clamping part, and being rotatable relatively to the annular clamping part to perform said machining step.

7. Apparatus as claimed in claim 6 wherein the annular clamping part includes a first annular portion having, at one end, a face for engagement with the ring blank and having at the other end a part spherical convex surface whose centre lies in the plane of the face, and a second annular portion having at one end a concave part-spherical surface with the same or substantially the same centre and radius of curvature as said convex part-spherical surface and in engagement with said convex part-spherical surface to allow angular relative movement of said first and second portions about said centre.

8. Apparatus as claimed in claim 6 or claim 7 wherein said annular clamping part is urged by spring means to a position in which a face of the annular clamping part extends axially beyond the boring and turning tools so blank, the annular clamping part engages the ring blank and retracts axially against the force of the spring means to clamp the ring blank between said annular clamping part and said stationary clamping member, further movement of the said annular clamping part relative to said boring and turning tools allowing the boring and turning tools to traverse the inner and outer peripheral faces of the ring blank or blanks to machine them.

9. Apparatus as claimed in any one of claims 6 to 8 and further including centering means comprising two arms movable from a retracted position in which the arms are spaced on either side of the plane in which lies the required positions of the centre of the closed ring blank, when clamped, and which is normal to the plane or planes of the ring blank, by equal amounts to a position in which the arms engage the ring blank to the centre the ring blank or blanks.

10. Apparatus for the manufacture of a piston ring as claimed in claim 5 and substantially as hereinbefore described with reference to the accompanying drawings.

11. A piston ring for an internal combustion engine, the piston ring having a gap cut therein to afford two free ends and there being a hole in each free end extending parallel to the axis of the piston ring.