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Aviation History
1942
1942 - 0654.PDF
•282 FLIGHT MARCH 26XH, 1942 AMERICA'S ALLISON ENGINE forged alloy steel faced with Stellite, the valves themselves being of chrome nickel tungsten steel. The iniet valve seats are aluminium bronze alloy and the valves, which have hollow stems, are of low tungsten steel. A seven-bearing crankshaft of chrome nickel molybdenum steel with the throws counterbalanced is used, carried in plain hollow bearings except for one roller type for No. 7, as shown in the accompanying drawings. Removable light-alloy plugs pro vide an annular chamber as a passage for lubricant and permitting collection of foreign matter within each crankpin lug. The centre journal is 2i%in. long, the remainder iffin. by 3|in. diameter. At the front end the. airscrew reduction gear pinion is splined and the large roller race referred to is interposed between it and No. 6 crank throw. A vibration damper of the dynamic pendulum type is fitted at the rear end to minimise 2-node torsion vibration. At the rear end also is a splined gear wheel driving the spur gear oil pump, cooling pump, fuel pump, vacuum pump and generator. The engine is started through this gear. Connecting rods are ot I-section and forked. The bear ings consist of two steel shells with lead bronze lining for the crankpin journal. There is a lead bronze overlay on the central portion of the outside diameter. The small ends have bronze bushes. Pistons of forged Dural have flat heads with cross ribs inside to facilitate cooling. From the crown downwards there are two plain compression rings and two in one groove for oil control below the gudgeon pin. The pin is fully floating and retained by two circlips. Two stiffly constructed light-alloy castings with a hori zontal joint along the centre line of the shaft form the crankcase. The oil sump, also of light alloy, is bolted to the lower half of the crankcase, and oil is scavenged from both ends. Lubrication Oil is circulated by one pressure pump which, with the main scavenge pump, is arranged in unit at the right of the accessory housing. Oil is fed through an Auto-Kleau type strainer fitted with a safety by-pass valve opening at a pressure drop exceeding 100 lb.,/sq. in. From the The overhead valve gear of one cylinder. strainer outlet one large tube in the upper half of the crankcase connects to drilled passages conveying oil to each main bear ing- Journals and crankpins are hollow and interconnected to convey oil to the connect ing-rod bearings, from which it is thrown to lubricate the cylinder walls and gudgeon pins. - The valve gear is lubricated by oil pressure through the camshaft locating h^Mk ing, and flows through the hollow shaft^^p the eight plain bearings, and thereafter is drained to the crankcase. A second tube in the lower half of the crankcase conveys oil to the large-diameter airscrew shaft plain bearing and the propeller governor pad. The single-stage single-speed super charger is combined in the accessory hous ing. The impellor, of 9|in. diameter, is driven at 8.77 times crankshaft speed, and comprises two parts, one having 15 radial blades, the other 15 curved vanes. These two parts are maintained in their matched relationship by means of common splines on the impellor shafts. The fuel/air mixture flows from the impellor through a diffuser passage having six curved vanes, then through a volute casing or scroll leading to the single delivery duct located between the cylinder blocks. This is continued to the centre of the engine, where it is branched to connect to four manifolds each serving three cylinders. In the branch feed in each cylinder block a trap is pro vided to prevent flames from backfire reaching the super charger. Traps are composed of thin coil strips of alternate flat and corrugated bronze approximately iin. wide and mounted in a steel frame. Accessory Drive The main accessory drive shaft provides flexibility in its 4oin. length. It is located directly above the crankshaft in the upper crankcase and supported in four plain bear ings. This shaft is driven at twice engine speed from the airscrew shaft reduction gear, and from the opposite end are driven supercharger, camshafts, two tachometers, two distributors, two gun synchronising gears and the magneto. The airscrew governor and the secondary scavenging oil General arrangement of the long-nose V-type twelve-cylinder Allison engine of 1710 cubic inches capacity.
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