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Aviation History
1919
1919 - 1087.PDF
AUGUST 14, 1919 of the turbo-compressor here means that the radiator must be put elsewhere than at the front, most probably in the wings. At the rear of the turbine housing is the automatic control mechanism, which serves to maintain the turbine speed at the right value to supply the correct supercharge under any atmospheric conditions up to the limit of about 20,000 ft., which is the maximum height for which the Sherbondy machine is intended to be effective. As already explained, the turbine rotor and the air impeller are direct-connected on the same shaft, a labyrinth arrange ment being constructed on the shaft between the two rotating members to prevent pressure leakage between the compressor chamber and the turbine. The rotating part of the device is shown in Fig. 3. Fig. 4 is a sectional drawing of Mr. Sherbondy's last super charger, in which a number of minor changes were made over the two previous designs in order principally to over come the troubles due to the warping of the thin-gauge parts of the two earlier constructions. The impeller, A, is enclosed in an aluminium housing, H, which, throughout the length of the impeller blades follows the same taper, and is carefully machined inside to maintain a constant clearance between blades and housing of only twenty-five thousandths of an inch. The housing has the same "taper as the blades, but to a diameter a little larger than that of the impeller, where its sides become parallel for about another inch. This space acts as a diffusion chamber, after which the housing opens into an outer chamber, C, of circular section. This outer chamber is of constantly increasing diameter, tapering from an area of almost nothing until it finally reaches the area of the induction pipe. This may, perhaps, be better understood by referring to the front ele vation drawing in Fig. 2, which shows the outer chamber. D, very clearly. The point of juncture with the induction pipe is shown at J in Fig. 4. The impeller has ten blades, and is 9 ins. in diameter. The blades are tapered from a point a little over half-way out on the intake side, and converge to a relatively narrow tip width, as seen in the drawing. The entering edge of the blades is rounded off to prevent air shocks. A most interesting feature is the turbine rotor, which in the latest Sherbondy design is fitted with 72 buckets cut by a special process to the greatest accuracy. As will be readily- appreciated, the buckets all had to be of exactly the same weight so as to prevent unbalance of the rotor—a condition which would be very serious atjthe high speeds which the device is designed to attain. These buckets are assembled into sockets accurately machined in the periphery of the rotor proper, and expansion under the heat of operat:on serves to hold them in place most securely. Much research and experimentation was carried out by Mr. Sherbondy before he was able to fix upon an alloy which would be sufficiently strong and of a high enough heat- resistant quality to withstand the very high temperatures and stresses encountered in operation of the device. The turbine rotor was designed for an angular velocity of 780 ft. per second, which means a rotative speed of 31,050 r.p.m. The turbine nozzle angle is 210 45', and that of the buckets, 350 15', with a gas velocity at entrance designed to be about 1,950 ft. per second, and that at exit from the buckets, 750 ft. per second. This is a ratio of a little over 2.5 to 1. In designing the air inlet, it was desired to prevent all air shock so far as possible, and, therefore, the conical air inlet, G, Figs. 2 and 4, was provided with radial guide vanes which impart a certain velocity to the air and give it direction also before it comes in contact with the rapidly-rotating impeller blades. The designing of the bearings was also a tough problem, for they must withstand a variety of severe conditions in service, especially when the device is operating at or near its maximum effective speed. Reference to L and Lj in Fig. 4 will give an idea of the details of their construction. The rear bearing, L,, is subjected to most serious conditions in this design, in that it is in the path of the exhaust gases, whereas the front bearing, L, is in the centre of the air inlet part, and is under no severe heat conditions. Both bearings are constructed with spherical seats, which allow them to compensate automatically for any misalignment of the assembly, a possibility should the castings warp to any extent under the high temperatures. They are in effect small self- aligning bearings. The bearing proper between shaft and housing is of plain babbitt. At the front there is, in addition to the bearing just mentioned, a marine type thrust bearing which holds the rotating member against end-thrust caused by the pressure of the gauges going through the nozzles against the buckets of the rotor. FLIGHT Pressure feed is used for efficient lubrication of both bear ings, which are provided with an excess over actual require ments for obvious reasons. This oil is intended to be forced to these bearings through connection into the regular pressure feed line of the Liberty engine. Thus the Liberty oil pump supphes them, avoiding an added complication of having a special oil pump in the turbo-compressor unit itself. An oil seal is made against oil leakage around the shaft by pro viding a washer that is a loose fit on the shaft, and which has a spherical shape on one side. This side fits against a seat in the bearing chamber, against which it is held by a spring. This construction is very effective in preventing oil escapage into the turbine casing or blower housing, as the ca-?e may be. In several of the earlier Sherbondy designs, trouble was experienced with the gas casing, which had a tendency to warp and thus throw the turbine nozzles out of place to such an extent that they touched the turbine wheel in several instances. Other troubles were encountered due to warping of the nozzle ring and the misshaping of the nozzles them selves. In the last design, as seen in Fig. 4, however, these difficulties were overcome in large measure by bolting the nozzle ring to the compressor casing and permitting the gas casing to deflect as much as it pleased with no ill effect upon the running 01 the device. It is just such problems as these that Sherbondy has had to overcome throughout, for the intense heats encountered, the restricted space available for the unit and the necessity for extreme lightness are three diametrically opposed factors which must be compromised into the best possible assembly. At F in Fig. 4 is seen the diaphragm, which is the con trolling unit for operation automatically of the by-pass valves, V, 'which allow escape of exhaust in proportion to the supercharger speed required for efficient working of the turbo-compressor at any given altitude. Obviously, at ground level, these by-passes are open, whereas, at the max mum height, they should be entirely closed so that all exhaust energy is given to the turbine to operate it at top speed. In effect, the automatic control works upon the principle of differential pressure on the two sides of the diaphragm, which opens and closes a piston valve in the casting. This, in turn, operates a system of oil ports through which oil flows under pressure to open or close ports that govern the action of the small pistons of the by-pass valves. The The supercharger developed by E. H. Sherbondy for the Aircraft Production Bureau
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