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Aviation History
1931
1931 - 1227.PDF
FLIGHT, NOVEMBER 20, 1931 The piston ring is small but vital, and it is only by the finest workmanship on the finest material that satisfactory rings are produced to stand up to work in our aero engines Piston Rings in the Making S [ir^HE majority of people, if they were asked bowCo III piston rings are made, would say that they III are just rings with slots in them, and areprobably cut off a machined-up cylinder by the hundred! Nothing is further from the fact. For themaking of an efficient piston ring is a highly-skilled and intricate business which necessitates many different opera-tions and, what is most important, a most elaborate in- spection system which will ensure that only the perfectrings are put upon the market. The main function of a piston ring is the maintenanceof a seal against the gas pressure in the cylinder. That is, it must not allow the compressed gas to percolate downthe cylinder walls past the piston. To fulfil such a func- tion it must retain sufficient elasticity to exert a uniformpressure at all points of its circumference on the cylinderwall for the whole of its life, and, moreover, while doing so.it must withstand the abrasive effects which such a pressureinduces. This naturally calls for avery special material which provides elasticity, strengthand resistance to abrasion without diminution over longperiods. The common or garden cast iron is not elastic,and may take quite an appre- ciable set under load, butcast iron of a very fine grain, and hence with the graphiteflakes in a very finely-divided condition, does come near thisdesirable elasticity. This fineness of grain canbest be obtained by the cen- trifugal casting process, and itis with this process that we are particularly concerned.The British Piston Ring Co.. whose trade name Brico isknown the world over, have PLUNGER FOR FORCING OUT ' CASTING MOULD \ CASTING t.v. Light Test for concentricity of piston rings. If the ring is not true then the light shows between the ring gauge (A) and the ring when this is rotated on the rollers (B). 3NT PLATE REGULATES INSICE OlA OF CASTING BORE OF MOULD REGULATES OUTSIDE DiA OF CASTING Diagram showing how the thickness of centrifugal castings- can be varied, i made a deep study of the pro-duction of high-grade centri- fugally-cast iron for manypurposes, and those who wish to know something aboutpiston rings cannot do better than visit their factory atHoi brook Lane, Coventry. Fur the production ofpiston rings and cylinder liners (these latter being aproduct of theirs which is coming into general use nowthat heavy motor transport has assumed such proportions)hollow tubes or pots are cast in a metal mould while this,latter is being rotated at high speed. Due to the centrifugalforce, the molten metal is thrown to the outer surface ofthe mould, where it solidifies, and may almost immediatelybe withdrawn as a cylinder, the thickness of which is con-trolled by the amount of metal poured in and by the size ofthe hole in the front plate which closes the mould.These pots are placed on suit- able lathes and turned down, both internally and ex-ternally, to the right size for the particular rings which are being made at the time. The pressure which a ring exerts on the face of the-piston wall is of great importance to its correct working. Too low a pressure means that the ring will take a longtime to bed in and, moreover, may allow an undue amount of oil to pass to the combustion chamber, with a conse-quent loss of efficiency in the engine. Too tight a ring will raise the total friction considerably, and create aneven greater loss of efficiency. At the Brico Works a test instrument is used which applies a load to the ring at theopposite ends of a diameter at right angles to the diameter through the joint ; this load may be converted intolb./per sq. in. by the following formula: — p = llit~' where/) — pressure in lb./sq. in. on the cylinder wall,Q = diametral load in lb. d = bore of cylinder in inches.h = width of ring in inches. It is essential that the ring, when in the cylinder, shouldfit as a perfect circle, and thus be able to exert a uniform pressure around the whole of its circumference. A little thought will show that if the ring was made by takinga circular concentric ring blank of larger diameter than the cylinder bore, and cutting out of it the requisite gap,then the result, when closed in .the cylinder, would be that the back of the ring only, for about 100 deg., would make Piston ring pressure gauge, a closing load being applied to an aero engine piston ring. 1157
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