From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Cylinder bore wear, piston materials, and comparative data on piston ring designs.
| Identifier | ExFiles\Box 132\5\ scan0131 | |
| Date | 3rd February 1939 guessed | |
| 10 CYLINDER BORE WEAR inaccuracies, for instance, the oil used by bores maintained round to within 0·0005 inch and straight to 0·001 inch is just one-half that used by bores round to 0·0015 inch and straight to 0·002 inch. The taper of the piston skirt should be large at the bottom. Owing to tolerances, the skirt may be 0·0005 inch smaller at the bottom than near the rings, or it may be 0·001 inch larger at the bottom than near the rings. The oil used with a reverse taper is double that used with a proper taper. Assuming that the allowable tolerance for diametral tension of the oil ring is 9 to 13 lb., the difference is a 50 per cent increase in the oil used with the low-tension ring. Regarding the ovality of the piston, tolerance is usually allowed for the difference between the major and minor axis. The difference in oil used as between 0·004 inch ovality and 0·008 inch ovality is 40 per cent in favour of the small ovality. The result obtained when all the good combines in one engine and all the bad in another, accounts for the wide difference found in engines built to the same specifications, e.g. variations from 300 to 2,000 miles per quart of oil. An evaluation of the fundamentals such as is given above is an extensive and laborious undertaking, and requires complete control of all possible dimensions in many engines, so that each value can be rightly determined. With modern tooling, accuracy of the bores is a simple matter to attain, but this should be taken into consideration from the standpoint of hardness of the cylinder block iron. Materials for Cylinder Blocks and Pistons. The choice of cylinder block iron has not been reasonable. Because cylinder wear has been prevalent, a composition is used which, although its hardness adds nothing to bore life, is machined with difficulty. To offset wear by hardness alone requires the hardness of nitrided steels or their equivalent. Materials that resist wear do not lend themselves to modern cylinder block castings. The present material used in cylinder blocks not only shortens the life of tools, but distorts while being processed. Honing is difficult because this type of material is liable to be picked up in the hone and scratches the bores. The compatibility of piston ring iron with cylinder bore iron requires far more study in England. There is an undetermined difference between English and American piston ring iron. The English iron gives a higher nominal value of elasticity than the American iron, though the latter is tough and, apparently, is more compatible with the cylinder iron with which it is used than the former. This is shown by the sensitivity of English high-tension rings to cold scuffing. So far, this has been overcome by swilling the piston and rings in lard oil before assembly and by lowering the break-in loads. With rings proportioned to American standards, a high nominal modulus of elasticity means nothing. Toughness is to be preferred. Incipient damage done by cold scuffing may result in serious hot scuffing. Hot or cold scuffing may be started by aluminium oxide crystals which, as pointed out by Professor Finch, are as hard as the sapphire. In fact, such crystals may be in the main responsible for a type of bore wear associated with aluminium pistons, though C. G.{Mr Griffiths - Chief Accountant / Mr Gnapp} Williams has not been able to detect any difference in the bore wear between iron and aluminium pistons. The fact that manufacturers are anodizing or tinplating 90 per cent of the production of aluminium pistons, or are preparing to treat them in this way, must, however, throw some doubt on such findings. Nevertheless, too much piston ring hot scuff has occurred with coated pistons for the piston material solely to be blamed. Such scuffing follows blow-by.{R.W. Bailey - Chief Engineer} When blow-by is less than 1 cu. ft. per min., hot scuffing does not occur. When blow-by is about 6 cu. ft. per min. at 4,000 r.p.m., which is not unusual, burned and scuffed rings result. Tests indicate that, with 6 cu. ft. per min. of blow-by, two hours' running at 4,000 r.p.m. will burn or scuff the rings. There cannot be any doubt that blow-by is the most destructive element in engine operation. Unfortunately, a perfect seal is accomplished by the physical characteristics—controlled by the proportions—of the rings. Piston rings will leave the bores at certain speeds and the speed at which this may happen depends on the tension of the ring, the degree of damping by the radial pressure pattern, and the fit of the ring in the bore. Design of Piston Rings. Fig. 11 illustrates a radial pressure pattern for good and poor rings. The opinion of a ring maker who manufactures 35 million rings per year—an opinion which is supported by all American ring makers supplying equipment to original manufacturers—may be quoted* :— "For piston ring shape is most important in obtaining good performance and long life. The most important portion of the ring form is the 180 deg. arc opposite the back half; that portion having the joint. The ring must be shaped so as to exert maximum pressure on the cylinder wall at the points, and yet have such a shape on either side of the joint as to exert sufficient pressure immediately after installation to inhibit blow-by.{R.W. Bailey - Chief Engineer} To accomplish this result has required considerable study of the proportions of radial wall thickness and free-joint opening to diameter. When these proportions are correct, the individually cast, would have worn themselves by the scrubbing action alone during this mileage if tension was the factor that promoted wear. Blow-by is the enemy of rings, and if not controlled, will be taken as an excuse to introduce new metals. The extreme pressure in aviation engines may require better ring material than is now used in motor cars, but if blow-by control is really studied for this type of engine, the more expensive materials will be worth while. * Teetor, R.{Sir Henry Royce} R.{Sir Henry Royce}, and Bramberry, H.{Arthur M. Hanbury - Head Complaints}, pamphlet published by Perfect-Circle Company, Hagerstown, Indiana, U.S.A. TABLE 1. COMPARISON OF HIGH- AND LOW-PRESSURE COMPRESSION PISTON RINGS Bore, mm. | Width, inches | Type | Diametral load to close gap, lb. | Type of ring 57 | 3/32 | [Square icon] | 7-10 | High pressure | 1/8 | [Square icon] | 2 1/4-4 | B.S.I. standard 61 1/4 | 3/32 | [Square icon] | 7-10 | High pressure | 1/8 | [Square icon] | 2 1/4-4 | B.S.I. standard 63 1/2 | 3/32 | [Square icon] | 7 (minimum) | High pressure | 1/8 | [Square icon] | 2.8-4.25 | B.S.I. standard 82 | 1/8 | [Square icon] | 9 (minimum) | High pressure | 5/32 | [Square icon] | 4.9-6.95 | B.S.I. standard 3 1/2 inches | 1/8 | [Square icon] | 12 (minimum) | High pressure | 5/32 | [Square icon] | 5.9-8.5 | B.S.I. standard TABLE 2. COMPARISON OF HIGH- AND LOW-PRESSURE SCRAPER PISTON RINGS Bore, mm. | Width, inches | Type | Diametral load to close gap, lb. | Type of ring 57 | 3/32 | [Grooved icon] | 7 (minimum) | High pressure | 1/8 | [Grooved icon] | 3 1/4-5 1/4 | B.S.I. standard 61 1/4 | 3/32 | [Grooved icon] | 7 (minimum) | High pressure | 1/8 | [Grooved icon] | 3 1/4-5 1/4 | B.S.I. standard 63 1/2 | 3/32 | [Grooved icon] | 7 (minimum) | High pressure | 1/8 | [Grooved icon] | 3.8-5 1/4 | B.S.I. standard 82 | 1/8 | [Grooved icon] | 10 (minimum) | High pressure | 5/32 | [Grooved icon] | 5 1/4-7 1/4 | B.S.I. standard 3 1/2 inches | 1/8 | [Grooved icon] | 10 (minimum) | High pressure | 5/32 | [Grooved icon] | 5 1/4-7 1/4 | B.S.I. standard In conclusion, it may be pointed out that the practical illustrations which are given in this paper can be repeated in many thousands of trucks and cars on the highways of the world. It is the author's opinion that good practice leading to a better result or more favourable cost is worthy of consideration, regardless of the origin of the idea. Opposition should be based upon other grounds than scepticism or a desire to be different regardless of cost. 15 | ||