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).
Article from 'The Autocar' magazine discussing the design, manufacture, and efficiency of different types of piston rings.
| Identifier | ExFiles\Box 16\1\ Scan122 | |
| Date | 7th September 1912 | |
| R.R. 235a THE AUTOCAR. September 7th, 1912. 437 The Design of Piston Rings. OCH/H The Best Form of Construction for the Highest Engine Efficiency. AFTER asking the owner of a car how his new purchase is behaving, we often hear the answer, “Oh! splendidly, better every day.” But what a disappointment that same man may experience when, say, two years later, he has the car overhauled, and finds that, instead of an improvement in its running, quite the reverse is apparent, and it is a matter of weeks before the car regains its lost efficiency. Fig. 1.—Showing a concentric ring split and closed. The causes of a car behaving in the manner described are numerous, but I only propose to deal with one of them, one which I consider of great importance, viz., piston rings. In the early days of motoring, very little importance was attached to piston rings, and it was considered sufficient by some makers of engines to turn piston rings in the lathe, cut fairly large gaps in them, and leave the fitting for the engine to do in the course of running. The result was that not only did the engine badly overheat, but the cylinders were oval in shape, Fig. 2.—Showing an eccentric ring split and closed. owing to the uneven and excessive pressures that its walls were subjected to by the irregularity of the ring. Fortunately, manufacturers nowadays pay considerably more attention to the design of piston rings. There are, however, many repairers who do not yet appreciate the need of accuracy in this direction, and for the benefit of those who contemplate having new rings fitted to their engines whilst the latter are being overhauled, I will point out the advantages and disadvantages of certain methods of manufacture. Fig. 3.—An eccentric ring finished to true circles inside and out after the ends have been closed. It is a generally accepted idea that if a ring be turned concentric inside and out and then split, the outward pressure on the cylinder walls, when the ring is closed and in its place, will not radiate from the centre. Now let us see exactly what this means. Looking at fig. 1, we shall see that if the ring be closed and placed in the cylinder, it will be bearing on the walls on three points only—A, B, and C. A piston ring in such a condition is, of course, impracticable for use in an engine, so the ring is made thicker in the first instance and then turned a true circle after being closed at the gap, thus giving a full bearing surface all round its circumference. Before doing this, however, it is the practice of many manufacturers to turn out the inner periphery of the ring eccentric to the outer, in order to equalise the pressures at all points of the ring, and although this does to some extent have the desired result, yet it is a far from perfect means to an end, and an inspection of fig. 2 will show that, although the pressures are far more even than with a concentric ring, yet imperfect pressure must result to some extent owing to the distorted shape of the inner periphery when the ring is closed. I do not suggest for a moment that rings of this description will not allow satisfactory running, yet with very little more expense a ring more nearly perfect can be manufactured, and has now been adopted by certain firms. That is, a ring turned to a perfectly true circle inside and out whilst the gap is closed, the outer circle being eccentric to the inner (fig. 3). It will be seen that in this case not only are the pressures more even on all parts of the ring, but less pressure is required to make the rings gas-tight, consequently there is less wear on the cylinder walls, more even wear and less heat is developed, resulting in higher engine efficiency. I would here point out that, although a ring machined with a lathe tool can be very perfect if care be used in the turning, yet it is impossible to prevent particles of metal from breaking away from its edges; and as its efficiency depends to a large extent upon the absolute truth of its upper edge, it is advisable to grind up the ring with a fine grit carborundum wheel. An equally important point is the accuracy of the fit of the ring in the groove, and the practice of fitting rings into grooves that are too large for them and that allow of the slightest vertical slackness is strongly disparaged. Turning now to the question of joining the ring, there are various methods, some of which are shown in fig. 4. Firstly, that of a diagonal cut across the | ||