Manufacture of piston rings
This is a continuation of application Ser. No. 227,296, filed Jan. 22, 1981, now abandoned. 
    
    
     This invention relates to the manufacture of piston rings, especially piston rings for internal combustion engines. 
     A process is known for the manufacture of cylinder liners for internal combustion engines in which particles of a hard grit such as silicon carbide are deliberately embedded in the internal wall surface of the liner in order to render the surface extremely hard-wearing. However, there has been no process for treating piston rings in a similar way and it is an object of this invention to provide such a process. 
     According to the invention, there is provided a method of treating piston rings to render their surfaces extremely hard-wearing, comprising assembling and retaining the piston rings on a mandrel, introducing the mandrel with the rings thereon into a corresponding cylinder, and subjecting the mandrel and cylinder to relative cyclical motion while a slurry of hard particles, especially silicon carbide particles, is poured in. 
     The cyclical movement may comprise simultaneous relative rotation and reciprocation and the cylinder or the mandrel, which is preferably tubular, may have holes in its wall to allow the slurry to pass through and flow away. 
    
    
     Methods of carrying the invention into effect will now be described by way of example and with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic part-sectional elevation of a cyclically moving mandrel, for carrying piston rings to be treated, inserted in a cylinder, 
     FIG. 2 is a similar elevation of an assembly in which the cylinder moves cyclically, and 
     FIG. 3 is a part-sectional pictorial view of the assembly of FIG. 2. 
    
    
     Referring firstly to FIG. 1, the mandrel consists of a vertical hollow tube 11 carried at its upper end by a shaft 14 of smaller diameter. The piston rings 12 are slipped over the tube 11 in batches of 20-50 to 1000 rings depending on the size of the ring, a bottom ring being received in a circumferential groove 13 at the lower end of the mandrel to retain the rings to be treated. At its upper end the mandrel shaft 14 has a lateral drive bar 15 and below this is a compression spring 16 which urges a pressure plate 17 down on to the batch of rings. At intervals along the length of the mandrel tube 11 holes 18 are bored equidistantly around the tube circumference. 
     The mandrel with the piston rings on it is inserted into a matching cast iron cylinder 10 and is rotated at constant speed (170 r.p.m.) and simultaneously moved repeatedly up and down within the cylinder, at the rate of 5 cycles of reciprocation per minute. At the same time a slurry of 220 mesh silicon carbide powder, in a liquid (oil), is poured into the cylinder. The piston rings, being themselves springy bear outwardly on to the cylinder bore surface and as a consequence become permanently impregnated with carbide particles from the slurry. 
     During this treatment, the holes 18 in the mandrel tube allow the slurry to flow away. 
     After one minute of such treatment the piston rings are found to be adequately impregnated with silicon carbide particles. The particles are mostly 5-10 microns in size and are generally to be found at the working surface of the ring, thereby giving an extremely hard-wearing piston ring. Preferably, a second similar treatment is carried out for a further one minute but using 400 mesh silicon carbide. This serves to cut back the peaks of the particles embedded during the preceding step. 
     FIGS. 2 and 3 show an arrangement in which the mandrel 20 carrying the piston rings 21 is in the form of a pillar fixed on a base plate 22 that in turn is secured to the base 23 of the honing machine. The rings, of which there may be any number from 10 to 1000, are retained on the mandrel by a circlip or keeper 24. The cylinder 25 is placed over the mandrel and rings and has a transverse driving bar 26 so that it can be reciprocated and rotated over the mandrel for which purpose the cylinder is made considerably longer than the ring-bearing portion of the mandrel. 
     As before, the slurry of silicon carbide powder is introduced into the top of the cylinder and in this case, holes 27 are provided in the cylinder to allow the slurry to escape.