GB2097098A - Making spring metal piston rings - Google Patents

Abstract

A method of manufacture of a piston ring 10, includes making a ring blank having an outer diameter which is greater, and an inner diameter which is less than that of the finished ring. The holes are drilled in the blank, and two substantially parallel pins are extended from retracted positions (in which they are aligned with the axes of the holes but out of engagement with the ring blank) to extended positions in which they engage in the holes without stressing the blank. A gap is cut in the blank, either before or after the drilling step, to afford two free ends, such that each hole is adjacent an end. The spacing of the pins is reduced to hold the ring blank in an at least partly closed position. One or more of said blanks are clamped in this position and their inner and outer peripheries are machined to be circular. <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 andlor 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 within its elastic limit to have "permanent set". The extent of this is that on assembly of the piston ring in 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-circular form 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 roller 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 tge 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 buy 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, drilling two holes in the ring blank, extending two substantially parallel pins set at a predetermined spacing from retracted positions, in which the pins are aligned with the axes of respective drilled holes and in which the pins are out of engagement with the ring blank, to extended positions in which the pins engage in the drilled holes to locate the ring blank in a condition in which the ring is unstressed by external forces, cutting a gap in the ring blank either before or after the drilling step to afford two free ends, the positions of the holes being such that each hole is adjacent an associated free end, reducing the spacing of the pins to hold the ring blank in a closed position, clamping one or more of said ring blanks in a closed position and then machining the inner and outer peripheries ofthe or each ring blank to be circular.

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

According to a third aspect of the invention there is provided, apparatusforthe manufacture of a piston ring from a ring blank having a diameteroverthe outer periphery which is greater than, and a diameter overthe inner periphery which is less than that required in the finished ring, the apparatus comprising drilling means movable towards and away from the ring blank for drilling two holes in the ring blank, two pins movable between a retracted position in which the pins are aligned with the axes of respective drilled holes and in which the pins are out of engagement with the ring blank, to extended positions for engaging in respective drilled holes to locate the ring blank in a position in which the ring blank is unstressed by external forces, cutting means for cutting a gap in the ring blank between the two holes either before or after the holes are drilled, the two holes being adjacent respective free ends of the ring blank formed by said cut, means for moving said pins to reduce their spacing so that the two free ends of the ring blank are in a closed position, clamp ing means for clamping a ring blank or blanks in a closed position and machining means for machining the inner and outer peripheries ofthe clamped ring blank or blanks circular.

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 and extending parallel to the axis ofthe 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 similarview of Fig. 3 but showing the piston ring blank with a gap cuttherein; 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.

Referring first to Fig. 1, a typical piston ring 10 has, when finished machined, an outside diameter of 60 mm, a thickness of 2.5 mm and a width of 2 mm. 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 crosssections. The piston ring may be a 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, for the purpose of correctly orientating the ring blank for machining. The side faces 11 arethen ground parallel to one 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 itis 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 offurther levers 57 and pins 58 to the position in which the pins 51 are extended. Extension ofthe 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 smallerthickness than the required finished dimensions ofthe ring, but has a cut-out portion 61 at 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 ofthe 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 operated, 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 loins 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. 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 bytwo 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 of the projecting pins 51, the ring blank is brough 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 ofthe 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 1 20" 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 boring 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 ccncentric 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 journailed 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 and the face 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 unit78 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 biank.

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 30, 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.

Instead ofthe 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 ofthe 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.

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

Claims (8)

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, drilling two holes in the ring blank, extending two substantially parallel pins set at a predetermined spacing from retracted positions, in which the pins are aligned with the axes of respective drilled holes and in which the pins are out of engagement with the ring blank, to extended positions in which the pins engage in the drilled holes to locate the ring blank in a condition in which the ring is unstressed by external forces, cutting a gap in the ring blank either before or after the drilling step to afford two free ends, the positions of the holes being such that each hole is adjacent an associated free end, reducing the spacing of the pins to hold the ring blank in a closed position, clamping one or more of said ring blanks in a closed position and then machining the inner and outer peripheries of the or each ring blankto be circular.

2. A method as claimed in claim 1 and comprising drilling the two holes simultaneously and then extending the two pins simultaneously.

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

4. . A piston ring when produced by the method of any one of claims 1 to 3.

5. Apparatusforthe manufacture of a piston ring from a ring blank having a diameter over the outer periphery which is greaterthan, and a diameter over the inner periphery which is less than that required in the finished ring, the apparatus comprising drilling means movable towards and away from the ring blank for drilling two holes in the ring blank, two pins movable between a retracted position in which the pins are aligned with the axes of respective drilled holes and in which the pins are out of engagement with the ring blank, to extended positions for engaging in respective drilled holes to locate the ring blank in a position in which the ring blank is unstressed by external forces, cutting means for cutting a gap in the ring blank between the two holes either before or after the holes are drilled, the two holes being adjacent respective free ends of the ring blank formed by said cut, means for moving said pins to reduce their spacing so that the two free ends of the ring blank are in a closed position, clamping means for clamping a ring blank or blanks in a closed position and machining means for machining the inner and outer peripheries of the clamped ring blank or blanks circular.

6. Apparatus according to claim 5 wherein the drilling means comprises apparatus for drilling two holes in the ring blank simultaneously and wherein means are provided for moving the pins simultaneously from the retracted positions to the extended positions.

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

8. 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 and extending parallel to the axis ofthe piston ring.