GB2124331A - Manufacturing piston rings with inserts - Google Patents

Abstract

A base piston ring material 11 and a wear-resistant material 12 are fed towards rotary disc electrodes 31 and 32 from different directions. Electric current is supplied to the electrodes 31, 32 so that the materials 11, 12 become welded together as they pass between the electrodes, thereby forming a welded assembly which is then curled by rollers 61 to 65 to the desired radius of curvature of the final piston ring. The curled assembly is cut to form a split piston ring, which is then subjected to heat treatment. In an alternative embodiment (Figure 7, not shown) the materials 11 is subjected to preliminary bending prior to reaching the electrodes 31, 32. <IMAGE>

Description

SPECIFICATION Process for manufacturing a piston ring This invention relates to a process for manufacturing a piston ring (e.g. for an internal combustion engine), and is particularly concerned with piston rings having a wear-resistant layer on their peripheral surface.

A piston ring is required to be wear resistant, particularly at its outer peripheral surface which defines a sliding surface for contacting a cylinder liner. It is, therefore, usual to provide a wearresistant layer 12 on a base piston ring material 11, as shown in cross-section in Figure 1 of the accompanying drawings. The layer 12 is usually formed by surface coating (realised by chromium plating or spray coating of molybdenum, a ferroalloy, ceramics or the like) or by hardening from heat treatment, such as soft or ionic nitriding, surface remelting or surface layer melting and alloying. When the layer is formed by surface coating, it is however not satisfactorily bonded to the base material and easily peels or breaks.

Hardening by heat treatment is low in productivity, either because a long time is required, or because some methods (such as surface melting) render the surface so rough as to require post-treatment even though the heat treatment itself may be carried out in a short time.

Some conventional piston rings have a different material embedded therein, for example a ferrox insert or a soft metal such as copper or lead. These piston rings are manufactured by filling a groove in a base piston ring material with powder of the said different material, or with a paste obtained by kneading the material with a binder, and baking the whole. Alternatively, a sheet or ribbon of wire rod material may be press-fitted or upset in the piston ring groove.

The material which can be embedded in these piston rings is, however, limited to a soft material which easily undergoes plastic deformation, or a material which can be baked at a low temperature.

These materials are primarily employed to improve the draping and lubricating properties of piston rings, and are not expected to be resistant to wear.

Thus according to conventional techniques it is not possible to embed a wire rod of hard material (such as martensitic high-chromium stainless steel) in a piston ring groove, because the material cannot easily undergo plastic deformation and therefore cannot be press-fitted or upset in the groove. Furthermore, no baking or other method is possible for fixing the hard material to the piston ring.

It is an object of the present invention to provide a process whereby a hard material may be effectively incorporated into a piston ring.

According to the invention, there is provided a process for manufacturing a piston ring which comprises the steps of: (a) feeding a base piston ring material and a wire rod separately from one another towards a joining station; (b) passing the base piston ring material and the wire rod between a pair of rotary disc electrodes at the joining station to bring the base piston ring material and the wire rod into intimate contact with each other between said electrodes; (c) supplying an electric current to the electrodes to weld the base piston ring material and the wire rod together and thereby form a welded assembly; (d) curling the welded assembly; (e) cutting the welded assembly to form a split piston ring; and (f) heat treating the split piston ring.

The invention will now be further described, by way of example only, with reference to the remaining figures of the accompanying drawings, in which: Figure 2 is a schematic front elevational view illustrating a first embodiment of a process according to the present invention; Figures 3 to 6 are cross-sectional views showing various modifications which can be made to the process of Figure 2; and Figure 7 is a similar view to Figure 2, but illustrating a second embodiment of a process according to the present invention.

Referring first to Figure 2, a piston ring is produced from a base piston ring material 11 in the form of a web-like wire rod, and a second web-like wire rod 1 2 made of wear-resistant material which is used to form the outer periphery of the piston ring. The base material 11 is fed by roller 2'1 from a stock (not shown) and is supplied to a rotary disc electrode 31 through a guide 22.

The second wire rod 12 is fed by guide rollers 41 and feed rollers 42 to a rotary disc electrode 32.

The speeds at which the base material 11 and the second wire rod 12 are fed depend upon the degree of their thermal deformation, but the second wire rod 1 2 is preferably fed faster than the base material 11 so that a piston ring of better performance may be obtained, as will be explained later.

The base material 11 and the second wire rod 1 2 are brought into intimate contact with each other by the disc electrodes 31 and 32. An electric current is supplied to the electrodes 31 and 32, either continuously or discontinuously, so that the resulting heat may melt those portions of the base material 11 and the second wire rod 12 at which they are to be joined together. The molten portions then become cooled by the heat capacity of the piston ring itself, thereby welding the base material 11 and the second wire rod 12 to each other.Such melting occurs in those portions of high electrical resistance at which the base material 11 and the second wire 12 rod are joined, and does not extend to the outer surfaces of the latter, although the base material and the second wire rod may be affected by the heat to some extent.

The welded assembly of the base material 11 and the second wire rod 12 is guided by guide rollers 5 and subsequently progressively bent or curled into a true circle by five rollers, i.e. three rollers 61, 62 and 63 defining a curling station, and two further rollers 64 and 65 provided for ensuring curling stability. The amount of this curling is equal to that which is usually done for forming a conventional steel piston ring. Although five rollers are illustrated in Figure 2, it is possible to perform the curling operation using only three.

If the position of the roller 63 is changed, it is possible to form the welded assembly in the shape of a cam, and if the rollers 61, 62 and 63 are displaced axially (i.e. back and forth in Figure 2), it is possible to form the welded assembly in the shape of a coil.

The welded assembly is then cut to yield a split ring. This ring is heat treated so that the welding stress may be removed, along with the influence of heat remaining in the base material 11 and the second wire rod 12, and the internal stress created by the curling operation.

It is advisable to preheat the base material 11 and the second wire rod 12 before welding by the rotary disc electrodes 31 and 32, to control the thickness of the welding nugget 1 9 (see Figure 3) formed during the welding operation.

The invention can also be carried out by, for example, winding the welded assembly on a mandrel assembly instead of using curling rollers.

The final heat treatment is performed to remove any welding or curling stress and any influence of heat produced for welding. However, if a hardening or heat treatment is employed for expansion or contraction, as is customary in the art of piston ring production, then such heat treatment intended solely for stress removal may be unnecessary, since it is achieved by such customary treatment.

Where the finished piston ring is to be used in an ordinary engine, the base material 11 is required to be of higher machinability and plastic workability than the second wire rod 12 although it may have a lower wear resistance than the latter. The use of a high carbon or low alioy high carbon steel is, therefore, preferred. Austenitic stainless steel or any other steel of high corrosion resistance may be used where the piston ring is intended for a diesel or EGR engine.

A material of high hardness is selected for the second wire rod 12, which forms the outer peripheral surface of the finished piston ring, in order to satisfy the desired requirements for wear resistance, particularly scuffing resistance.

Specific examples of the material include matensitic stainless steel, high carbon alloy steel and high speed steel.

A large amount of plastic deformation takes place for forming a straight wire rod into a ring having a diameter of 50 to 150 mm, and creates extremely high internal stresses in the welded joint and in the second wire rod 1 2. Because the second wire rod 1 2 is usually very hard and liable to embrittlement there is a possibility that cracks may develop in the rod 12 and also in the welded joint. It is, therefore, advisable to feed the rod 1 2 at a higher speed than is the case for the base material 11, and to rotate the electrode 32 at a higher peripheral velocity than the electrode 31 (e.g. by forming the electrode 32 with a larger diameter), so that compressive stress may be applied to the second wire rod 1 2 during welding.

The above-described process enables a strong welded joint to be obtained which is comparable to an ordinary seam welded plate, with a surface roughness which is not greater than that of the material used. The process can, however, be modified in the manner described below to obtain certain advantageous effects.

As illustrated in Figure 3, the base material 11 may be formed with a groove 1 3 into which the second wire rod 12 is welded. This will result in the upper and lower surfaces 14 and 1 5 of the piston ring material 11 being uniform to ensure high machinability and enable precise working.

The second wire rod 12 is welded only to the bottom 16 of the groove 13, so that a nugget 1 9 formed by the welding operation does not extend up the side wall of the groove 13. Therefore, the outer periphery of the piston ring does not have a rough surface, nor does it have an uneven surface, which would otherwise be created if the welding nugget 1 9 were exposed.

The base piston ring material 11, to which the second wire rod 1 2 has been welded in the groove 13, is drawn along its outer periphery when curled, as shown in Figure 4. The lateral edges of the second wire rod 1 2 are thus brought into intimate contact with the side walls of the groove 1 3. Therefore, if any clearance exists between the rod 12 and the side walls of the groove 13 during welding, this clearance will be removed when the welded assembly is formed into a ring, so that the rod 12 becomes tightly fitted in the groove 1 3.

The second wire rod 12 and/or the groove 13 in the base material 11 is preferably formed with chamfered corners 7, as shown in Figure 5. The chamfered corners 7 prevent the aforementioned welding nugget from reaching the outer peripheral surface of the piston ring, and remove from the outer periphery of the second wire rod 12 any acute edge which could do damage to a cylinder liner. The chamfered corners 7 also define grooves in which a lubricant may be held. It is preferable that the chamfered corners 7 are formed on the base material and the second wire rod as a preliminary measure to the welding and curling operations, although they could alternatively be formed after the piston ring has been produced.

The base material 11 may be formed with one or more projections 71, as shown in Figure 6, to ensure improved welding of the second wire rod 1 2 thereto. These projections are formed, e.g. by rolling, prior to the welding operation.

Figure 7 shows another embodiment of this invention, wherein the base material 11 is curved with a prescribed radius of curvature before it is supplied to the rotary disc electrodes 31 and 32 and the welded assembly is curled by the curling roller 61, 62 and 63. This prevents internal stresses from arising in the welded joint and in the second wire rod 1 2 when the welded assembly is curled by the rollers 61, 62 and 63. In this connection, it is possible to curve the base material 11 with a radius of curvature which is substantially equal to that with which the welded assembly is curled into a piston ring. This makes it possible to accomplish curling and welding substantially simultaneously.If any axial coiling speed exists in the direction of travel of the base material 11 through the electrodes 31 and 32, however, a twist stress is created in the welded joint. It is, therefore, advisable to perform the welding operation in a plane in which the piston ring is bent at least during the initial stage of curling.

The above-described processes enable high productivity by continuous production to be achieved (as is the case with ordinary steel piston rings), and wear resistance is obtainable by the provision on the outer peripheral surface of the piston ring of a material which is very hard, and substantially incapable of plastic deformation.

Moreover, the wear-resistant layer is joined to the outer peripheral surface of the piston ring with a high degree of strength.

Furthermore, a bimetal effect is produced which no ordinary surface coating can achieve. An ordinary surface coating having a thickness of, say, 0.1 to 0.5 mm does not exert any appreciable tension, surface pressure or other influence on the piston ring. In a piston ring having a welded construction as described above, however, the second wire rod 12 has a relatively large thickness which is usually at least 0.5 mm, so that the welding nugget 1 9 does not become exposed on the outer peripheral surface of the piston ring, and the second wire rod 12 is firmly welded in position. Therefore, the mechanical properties of the second wire rod 12 on the outer peripheral surface have a great influence on the piston ring.

In this connection, the present invention enables a difference between in thermal expansibility of the second wire rod 12 and the base material 11 to be utilised. More particularly, if the finished split ring is designed with a relatively large radius of curvature (i.e. a low-point split) and the second wire rod 12 has a higher co-efficient of thermal expansion than the base material 11, then the split ring will reduce its radius of curvature (i.e. a highpoint split) with an increase in temperature due to a rise in the speed of engine rotation, so that no fluttering will occur during such high-speed rotation.

In contrast to the above, the base material 11 may have a higher co-efficient of thermal expansion than the second wire rod 12 where the piston ring is intended for use in a two-cycle or diesel engine, in which some problems are anticipated with intake and exhaust ports or the split of the piston ring during high-speed operation of the engine. With an increase in the speed of engine rotation, the piston ring will increase its radius of curvature (i.e. it will have a low-point split) so that a reduction in wear and blowby may be achieved.

Claims (5)

1. A process for manufacturing a piston ring, comprising the steps of: (a) feeding a base piston ring material and a wire rod separately from one another towards a joining station; (b) passing the base piston ring material and the wire rod between a pair of rotary disc electrodes at the joining station to bring the base piston ring material and the wire rod into intimate contact with each other between said electrodes; (c) supplying an electric current to the electrodes to weld the base piston ring material and the wire rod together and thereby form a welded assembly; (d) curling the welded assembly; (e) cutting the welded assembly to form a split piston ring; and (f) heat treating the split piston ring.

2. A process as claimed in Claim 1, wherein in step (a) the wire rod is fed at a faster speed than the base piston ring material.

3. A process as claimed in Claim 1 or 2, wherein a surface of the base piston ring material which defines an outer peripheral surface of the piston ring has a groove in which the wire rod is received.

4. A process as claimed in Claim 3, wherein at least one projection if formed in said groove, to which the wire rod becomes keyed during step (c).

5. A process for manufacturing a piston ring, substantially as hereinbefore described with reference to Figure 2, or Figure 2 as modified by any one of Figures 3 to 6, or Figure 7 of the accompanying drawings.