Chromium
Page 63
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plated Oil-control Rings
THE long-wearing properties of chromium!. plated rings when fitted in the top piston grooves are such that the necessity of replacement is now often determined by the useful life of the scraper rings, which normally wear at a much higher rate than chromed top rings. To meet the demand for improved oil-control rings, Hepworth and Grandage, Ltd., St. John's Works, Bradford, have developed a Hepolite chromium-plated twin-stepped scraper ring for replacement purposes (KSS.40) and have produced a ring of the same type (KSS.80) for worn engines and power units operating on oils of very low viscosity.
Each ring comprises two stepped segments which fit into a groove of standard size. The corners of the contacting surfaces are chamfered to prevent " treeing " of the plating, but the sealing ability and oil-control properties have not been impaired. The two scraping lands function independently and are not lifted away from the cylinder wall by piston rock or hydraulic pressure. Moreover, the chromium surfaces reduce friction for a given radial pressure. The wall pressure of the two contacting surfaces of the replacement ring is the same as that of the slotted-and-grooved ring. Holes below the groove provide a passage for the oil scraped from the cylinder wall by the lower segment of the pair, and the smaller amount of oil removed by the top segment escapes between the segments to the vent passage behind the ring. The minimum intersegment clearance is 0.002 in.
For oil control in worn engines it is necessary to raise the wall pressure by increasing the radial thickness, the free gap or the modulus of the material. The KSS.80 oil-control ring has narrow lands down to a thickness of 0.020 in., or less, which give the high contact pressure required, but the ring has a long wearing life by virtue of the special chromium finish.
By reducing the land thickness while operating with the same radial force, tilt contact pressure is raised, but does not increase the friction losses. The greater scraping potential which this provides (without attendant disadvantages) is particularly beneficial in engines being run on low-viscosity oils.
After 34,000 miles, the gap of the scraper rings in an A.E.C. 9.6-litre engine fitted to a city bus increased by 0.007 in., and the rings were refitted for further use.