Page:EB1922 - Volume 30.djvu/66

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ing rods, etc., with good results: Carbon 35%; Nickel 3-5%; Chromium o-6-l %.

Difficulties attended the manufacture of crank-shafts for 12-cyl- inder engines which, in order to reduce the overall length, employed roller main bearings. At first such crank-shafts were produced from billets twisted through 120 at the main journal, which provided only 3 in. length in which to effect the twist, necessitating so high a twisting temperature that no subsequent heat treatment could restore the structure to a uniform and satisfactory condition. The use of a billet of double width involving a twist of only 60 was then tried, with improved but not entirely satisfactory results. Finally such crank-:;hafts were produced from a billet first pressed or crinkled to a general crank-shaft form to provide a continuous grain flow throughout journals, webs and pins, and finished finally by drop stamping and twisting, where necessary, the main journal through 60 degrees.

The elimination of all sharp corners, as in keyways and the under- cutting of webs in grinding journals and pins, was found to be of the utmost importance to prevent fatigue failure.

Rough machining before heat treatment was also required, es- pecially in the rotary single-throw crank with large variations in mass of section, to secure uniformity of condition.

Cylinders. In 1914 air-cooled cylinders were of mild steel for rotary and cast iron for stationary engines. The steel cylinders were machined from the solid billet; by 1916 forged blanks were used.

By 1915-6 cast-iron cylinders were cast from metal patterns and machine-moulded, and a close limitation of chemical composition adopted to secure clean casting of the thin sections, and to overcome distortion and cracking in running. To eliminate casting stresses cylinders were normalized after casting, and set aside for some weeks to " age " before machining.

For water-cooled engines having separate cylinders cast iron (with a sheet-steel jacket pressed to shape, and welded on, or a copper jacket electrically deposited) was used. To allow the jackets to expand, crinkles, both circumferential and round the exhaust valve seatings, and sparking-plug bosses were introduced, as the local expansion of the jacket differs from that of the cylinder when running.

Later, mild-steel cylinders turned from forged blanks were used m lieu of cast iron. Valve pockets, sparking-plug bosses, and thin sheet jackets were then welded on as first tried by Vickers in 1909.

Aero-engine cylinders are also cast together in one block for the sake of the rigidity of the cylinders one to another. At first, following motor-car practice, cast iron was used for this. Towards the end of 1916, however, aluminium, with its low weight and high heat con-

FIG. 20.

ductivity, took its place. The first prominent " Mono block " (see fig. 20) comprised a mild-steel cylinder liner complete with head and valve seats, screwed into an aluminium block which took four cylinders, and constituted a complete enclosed water-jacket. The liners were not in contact with the cooling water, and with bigger cylinders overheating and loss of contact between the liner and the surrounding aluminium jacket occurred particularly in the flat head. A natural development, therefore, was to remove the top of the liner, leave it open, and let the aluminium itself form the combustion head of each cylinder. Two difficulties then had to be overcome : (i) The provision of a gas-tight joint between the top of the liner and the jacket and head; (2) the insertion of rings in the head to form valve seatings. The first was overcome by screwing the liner hard up against the shoulder in the head, and the second (which was achieved without distortion or burning of the seatings) by casting-in or ex- panding-in steel or hard bronze rings. To improve further the cool- ing of the cylinders, the lower portion of the aluminium jacket in contact with the liners was omitted, the liner being held only by a screw thread of some l-in. depth at the top and a rubber joint and ordinary lock nut ring at the bottom.

The form of aluminium cylinder head and jacket casting is complicated, and experiments, both as regards method of casting and choice of aluminium alloy, led to the selection of a mixture of

14-5% zinc, 2'5% to 3-0% copper, alloy with virgin i- The pouring temperature is 66oC. The percentage of

12-5%. to _ .

aluminium The pouring temperature is 66oC. The percentage of scrap is high, say, 10% to 15% in the simplest forms of block, and up to 30% or 40% for more complicated designs. To overcome the porosity of castings, stove enamelling of the interior of the blocks or the application of water-glass under pressure is used.

The Royal Aircraft Factory experiments in 1915 led the way in air- cooled stationary cylinder engines in the use of aluminium heads gilled for cooling, using a steel liner and inserted valve seatings. For rotary-engine cylinders in one instance a thin steel liner was shrunk into a finned aluminium shell which formed a jacket, the head of steel being secured to the liner with a plain metal-to-metal joint by bolts from the head to the crank-case, thus securing the cylinder as a whole.

Cylinders of all types before erection on engines are tested inter- nally to 450-500 Ib. hydraulic pressure, and for the jackets to 30-40 pounds.

Connecting Rods. Connecting rods, as regards material, followed crank-shaft practice in the standardization of plain nickel chrome steel, heat-treated to give 50-60 tons' tensile strength.

The 6-cylinder and early 8- and 12-cylinder types conformed to motor-car practice in the use of solid H " section shanks and white-metal big-ends, without a bronze bush, the cap being held usually by four bolts or studs. To reduce th'e crank-shaft length of certain " V " type engines the connecting-rods on one side of the engine were provided with lugs to carry a wrist-pin, this wrist-pin, on one side of, and parallel to, the big-end bearing, carrying the auxil- iary connecting-rod. Alternatively to the same end a pair of rods superposed. In one case, a hollow circular sectioned shank carried an integral big-end, white-metalled internally and externally, the second rod, being fork-ended, oscillating on the sleeve formed by the first rod. The comparatively thin and flexible section of the inner rod sleeve, however, enhanced the difficulty of white-metalling and led to cracking in running.

A further development therefore (of square hollow sectioned shank) provided a bronze shell rigidly gripped by the forked ends of the outer rod, while the inner rod oscillates on the middle portion of the shell, which is white-metalled internally to provide the main big- end bearing, as shown in fig. 21.

FIG. 21.

Connecting-rods of rotary and radial engines consist usually of one master rod, ball or rollcr-bearinged, with the big-end enlarged to form circular lugs to secure wrist-pins carrying the plain or auxiliary type of rod of the remaining cylinders. One exception provided a big- end consisting of a separate lead bronze shell (in two halves bolted together) mounted on ball bearings and provided on the inside with white-metalled concentric grooves in which oscillate the concen- trically formed heels of the connecting rods.

Initially, the ordinary small-end bronze bush system with gudgeon pins fixed in the piston was used. Later, variations with loose bushes and loose gudgeon pins were developed, the pins_in the latter being secured endwise in the piston by wire circlips let into grooves on the outside edges of the piston bosses.

Rough machining before heat treatment is necessary on the rotary type master-rod stamping which has a large big-end mass and a comparatively small stem section, to secure uniform structure and freedom from quenching cracks. The elimination of all sharp corners and abrupt changes of section is essential.

Main Bearings. Ball, roller and white-metal bearings are to be found in various types. The two former permit of high loading and reduce the length of the engine (bearing loads approximating to 100 % over normal practice being found to give a total life commensurate with the rest of the engine under service conditions). White-metal main bearings, usually bronze shelled, are secured either by separate loose caps bolted on or studded to the top half crank-case ; or, as in usual German practice, by the bottom half crank-case itself, which carries the lower halves of the whole of the crank-shaft bearings ; this adds to the rigidity and general strength of the engine, but increases the difficulty of production and fitting.

Valves. Valve breakage, originally a trouble, was almost eliminated by the standardization of valve steels and by stamping the valves