Abstract:
A method of producing a hard wearing bearing surface on a component such as a cylinder or cylinder liner, by pressing an accurately screened hard powder in liquid medium into engagement with the surface and on a helical path, the pressure and powder size being such that oil retaining micro-grooves are formed and also that a great number of powder particles, which particles virtually constitute the bearing surface, are embedded.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to swept bearing surfaces, and is primarily but not exclusively concerned with components such as cylinders and cylinder liners for engines such as internal combustion engines for pumps and other machines. The invention can also be applied to parts with external bearing surfaces such as pistons, and to swept surfaces such as rotary piston engines with epitrochoid housings. An object of the invention is to produce bearing surfaces particularly but not exclusively of components of mild steel or cast or malleable iron, which are extremely hard wearing. 
     A further object of the invention is to produce bearing surfaces on components in which the material of the component itself virtually does not constitute the bearing surface. Thus materials which normally cannot be used e.g. for cylinder or cylinder liners such as malleable or soft iron, can be rendered with hard wearing surfaces. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a method of producing a cylindrical or other swept bearing surface on a component, which method consists in utilising a slurry or paste of abrasive or hard powder or grit screened accurately to maximum size, and in pressing the said slurry into engagement with said surface on a helical path from one end of said surface to the other repetitionally with a pressure such that grooves on criss-cross helical paths are cut in the surface and particles of the powder or grit embedded in the surface. 
     Thus in carrying out the process, a number of micro-grooves are cut into the surface on opposite hands by the particles and the particles will be embedded or welded in the surface. The grooves provide oil retention grooves and the particles a hard wearing bearing surface. It will be understood that the grooves are of very small dimensions and their spacing close, the depth and spacing depending on the particle size and speed of rotation and reciprocation of the tool relative to the workpiece. For example, the particle size and speed can be such that grooves in the region of 0.001 - 0.003 mm in depth width and spacing are formed. The grooves will not be in general, continuous but may extend only a short distance along the surface. It will be seen that a very large number of particles are embedded to form the bearing surface. It is possible, first to produce substantially continuous grooves with substantially no particle embedment and then follow up by using greater pressure, to cause particles to be embedded in the formed grooves. Pressure to cause embedment is in the region of 10 - 20 p.s.i. or more. 
     It has been found that in use, embedded particles remain in the surface and are not dislodged during operation of the engine or other machine. The grooves also serve for oil retention. 
     It has been found that by subjecting the surface to a second operation with a powder grit of smaller size, there is produced a low friction smooth finish and a hard wearing surface with a great number of particles embedded. This second run effectively smooths off the sharp peaks between the grooves without disturbing to any appreciable extent, the grooves cut by the first run, and it can result in the embedding of further particles in grooves on a helical path. It also will force in any particles further into the surface which project excessively and clean off sharp peaks on particles. Further, if any particles are loosely embedded they will either be forced in or removed. 
     The particles virtually form the bearing surface and provide a large proportion of such surface area e.g. 50%. 
     Various proposals have been made in the past for grooving bearing surfaces such as those of cylinders or liners to ensure their oil retaining properties. Such known proposals include etching, sand blasting, and also producing helical grooves utilising a slurry of a hard powder or grit. However in all previous proposals, the operation was such that oil retaining pockets or grooves were formed, and it was essential to ensure that any substance used for pocketing or grooving be removed from the component so that the component itself remained completely unadulterated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying diagrammatic drawings illustrate a cylinder liner produced according to the invention and a means for treating a piston surface. In the drawings: 
     FIG. 1 is a longitudinal section of part of the liner, 
     FIG. 2 a fragmentary section through the cylinder wall, after particle embedment, 
     FIG. 3 a view similar to FIG. 2 after the finishing operation, 
     FIG. 4 shows in side view a lapping tool for treating a piston and 
     FIG. 5 is a sectional plan of FIG. 4. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring firstly to FIGS. 1 - 3, it will in the first place, be understood that these views are purely diagrammatic, bearing in mind that in practice the grooves are close-spaced and of a depth and width of between 0.001 and 0.003 mm and the particles can constitute a large proportion of the final surface area. In FIG. 1, the cylinder 10 will be set up with a conventional lapping tool with spring-loaded lapping sticks and arranged so that the tool is rotated and reciprocated relative to the cylinder axis and within the cylinder, whilst a slurry of silicon carbide in liquid medium is poured in at the top of the cylinder and collected at the bottom and re-cycled. The grit is of a screen size of in the region of 220 and should be as free from sharp peaks as possible. The action results in the grit particles cutting grooves 11 on helical paths and on opposite hands to produce a diamond pattern. An initial run with a light pressure e.g. 10 p.s.i. or less may be done to start grooving without substantial particle embedment, and subsequently the pressure increased so that particles increase the groove depths and widths and finally become embedded or welded in as at 12. Generally a particle is embedded at the end of a noncontinuous groove and a following particle will then commence to cut a following groove. The initial pressure can be great enough to result in embedment without prior grooving, i.e. the one operation both grooves and embeds the particles, it being understood that the pressure used and length of time the process continues will be dependent mainly on the material of the cylinder. 
     The operation results in the production of a great number of helical grooves and the embedment of a great number of particles. The grooves may be continuous or some may extend only around a part of the surface, the length of time the operation is carried out and the pressure being the controlling factors. 
     The method according to the invention is not intended to size the cylinder bore or other component as there is no appreciable stock or surface removal, the powder or grit grooving the surface. Thus, the method can be applied to engine cylinder blocks, or wet and dry type cylinder liners of metal or cylinders of ceramic or like material. 
     A fragmentary section of the cylinder after the above operation is shown diagrammatically in FIG. 2. As will be seen, particles 12 are shown embedded in grooves 11, some of which particles may project slightly from the general surface. In many applications surfaces so treated are acceptable, but preferably the surface is subjected to a second similar operation but using a grit of smaller size (screen size 400-500). This second operation has the effect of smoothing off the peaks of the cylinder surface between grooves, removing sharp edges of particles and also tends to force particles if upstanding further into the surface. Moreover, it is found that should any particles be loosely embedded by the first operation, the second operation will either force them into the surface or remove them. FIG. 3 shows a finished surface part which is substantially flat, is grooved and has a multiplicity of close-spaced hard wearing particles therein. 
     As previously mentioned the invention can be applied to external bearing surfaces such as those of pistons. As shown in FIGS. 4 and 5, a piston P is mounted on a rotary member 13 by a locating plate 13a to rotate with the member 13. The piston is concentrically within a stationary holder 14, which carries a plurality of lapping sticks 14a spring loaded to the piston surface. The sticks also can move longitudinally axially with respect to the holder and are spring loaded downwardly by springs 15, so that they are reciprocated by a swash plate 16 and the springs. 
     The slurry is poured in between the holder and piston and is re-cycled and the rotary reciprocating motion produces the criss-cross grooving and particle embedment. 
     It will be understood that an arrangement of lapping sticks will be used to treat internal cylindrical surfaces as shown in FIG. 1. In this case, the blades extend radially outwardly from a holder. Preferably the cylinder is stationary and the holder both rotates and reciprocates. 
     It is also possible to press the particles into the surface using a spring loaded roller or rollers to force in the particles by a rotary action or rotary and reciprocating action relative to the surface under treatment. Further, a lapping tool with flexible or rigid lapping sticks with a negative or positive leading edge rake angle can be used. In the case of a roller or rollers, the leading edge or sides may be chamfered or radiussed to provide a gradual increasing force or load to the particles. 
     The grit or particles can be poured in a liquid medium or the surface can be pre-coated with particles applied in a suspension or as a paste. 
     An important feature of the invention is that components of mild steel or of malleable or soft iron of low quality can be treated so as to produce components with satisfactory hard wearing bearing surfaces as it is the embedded particles which constitute the bearing surface of a component. In the case of soft iron, particles may become embedded in soft pockets without prior grooving.