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792 a slow moving propeller. It was really the problem of the slow speed ship which brought about the development of marine tur- bine gearing, and now that the mechanical difficulties have been overcome, the direct coupled marine turbine is likely to be largely displaced by the geared turbine in all classes of vessels.

The first example of marine turbine reduction gearing appears to have been in 1897, in connexion with a twin screw launch, in which the Parsons Marine Steam Turbine Co. fitted a IO-H.P. tur- bine driving the two shafts by means of helical gearing having a speed ratio of 14:1. The result appears to have been entirely satis- factory. Other experiments followed, and in 1909, the " Vespa- sian," a cargo vessel of 4,350 tons displacement, was fitted 1 with geared turbines driving a single propeller. This vessel had pre- viously been equipped with triple expansion reciprocating engines of the usual type, and before these were removed they were put into perfect order, and very careful tests were made to determine the efficiency and performance of the vessel. The geared turbines drove the same shaft and propeller as the engines had done and were sup- plied with steam from the same boilers. The power developed was about 1,000 H.P. and the shaft ran at 70 revs, per minute, the gear reduction ratio being 19-9:1. The installation of the turbines resulted in an increase of about one knot in speed for the same coal consumption, and the results of the trials were highly satisfactory in every respect, and convincing as to the advantages of geared turbines over reciprocating engines. After the " Vespasian " had run 18,000 m. in regular service, the pinion was examined and found to be in perfect condition, the wear not exceeding 0-002 inches. (See Trans. I.N.A. 1910 and 1911.)

The success of the " Vespasian " led to rapid developments. In 1910 the British Admiralty adopted gearing, the torpedo boats " Badger " and " Beaver " being the first warships to be equipped with geared turbines. In these vessels each L.P. turbine drove its shaft directly, but the H.P. and cruising turbines were geared to a forward extension of the turbine spindles. At full load about 3,000 H.P. were transmitted through each set of gearing. Six years later complete gear drives had become the standard practice for British war vessels of all types and by 1920 some 652 gears, transmitting an aggregate of 7,280,000 shaft H.P., were fitted, or on order for the royal navy (Tostevin, Trans. I.N.A. 1920).

The appended particulars of H.M. battle cruiser " Hood," of 144,000 shaft H.P., which was completed in 1920, will indicate the development of gearing for turbines and will at the same time indicate the proportions which have been adopted.

Gearing, of H. M.S. "Hood." Horse-power of H.P. turbine Horse-power of L.P. turbine. Revs, per minute H.P. turbine Revs, per minute L.P. turbine Revs, per minute propellers. Diameter of pitch circle, in H.P. pinion Diameter of pitch circle, in L.P. pinion Diameter of pitch circle, in gear wheel Number of teeth H.P. pinion Number of teeth L.P. pinion. Number of teeth gear wheel ....

Circular pitch, in

Normal pitch, in

Helical angle of teeth

Effective width of pinion face, in. . Number of teeth engaging .... Total length of tooth contact, in H.P. pinion Total length of tooth contact, in L.P. pinion Load in Ib. per in. on total \ H.P. Width of tooth face ( = P) / L.P.. Value of K in formula P = K VP.D. \ H.P. Value of K in formula P = K ^p.D. / L.P. Velocity of pitch line ft. per second

TURBINES, STEAM

17-500 18,500

1,497 1,098

2IO

20-174 27-5I I43-787

55 75 392 I-I533 0-9985

29 '57'

73-25

36-6

128-8

132-9

965 1030

196 132

The earliest practice with regard to marine, gearing was to use a helical angle of 23 in conjunction with a normal pitch of 0-75 inches. Subsequently a helical angle of 45 which had been found successful in the De Laval gears was adopted with the idea of securing quieter running, but modern practice favours an angle of about 30, as teeth cut at this angle will run silently, while their less inclina- tion to the axis of the shaft results in increased efficiency and greater effective strength. The usual angle of obliquity is 145 ', and the normal pitch except for the very largest gears is nearly always 0-583 inches. The permissible pressure in Ib. per in. of axial length of the pinion is determined by the formula P = KVD in which D is the pitch diameter of the pinion in in. and K is a constant which has a value usually between the limits of 160 and 230. This formula represents the practice of the Parsons Co., who have a preponder- ating experience on these gears. There is reason for believing, how- ever, that the pressure might be made more directly 'proportional to the pitch diameter. A circumferential velocity of 150 ft. per second on the pitch line has been successfully employed, and it is possible that this might be exceeded with safety.

For turbine gearing the British Admiralty specify that the pin- ion shall be made of oil-hardened nickel steel, containing not less than 3-5% of nickel and from 0-30 to 0-35% of carbon, with an ultimate tensile strength of 40 to 45 tons. The gear wheels are to be of steel of 31 to 35 tons ultimate tensile strength with 26% elongation in two inches.

It is essential that the teeth of turbine gearing shall be very effectively lubricated, and to insure this, oil under a pressure of from 5 to 10 Ib. per sq. in. issues in jets which flood the teeth immediately before they come into engagement. A further point of pri- mary importance is that the fitting and alignment of the gears must be as perfect as possible and great care must be taken to main- tain and insure these conditions. In America the practice has been adopted of carrying the pinion on a floating frame with the object of permitting a certain amount of self-alignment, but the required correction is of such a very small order of magnitude that the ad- vantages of the system are doubted by many engineers.

Gearing of British naval turbines is exclusively of the single reduction type, but double reduction gearing has been largely intro- 1 duced into cargo vessels during recent years, with the object of efficiently using turbine machinery for ships of comparatively low! speed without involving too large a reduction ratio for a single pair of gears. The general design follows mutatis mutandis that of single reduction gear.

Numerous tests have been carried out to determine the mechani- cal efficiency of gears of the kind described. The mechanical effi- ciency of a single reduction gear at full load should be over 98 %, and 98-5% has been recorded. With double reduction gear the efficiency is about 97-0%. These figures include bearing friction. No method of obtaining speed reduction by hydraulic or electrical methods has yet been devised which will approach the efficiency obtainable with mechanical gearing.

Fig. 12 gives a good idea of the shafts of the Cunard liner " Transylvania." built by Scotts Shipbuilding & Engineering Co. Ltd. An exactly similar set of machinery was fitted to drive the other shaft. The " Transylvania " was the first Atlantic liner to be fitted with geared turbines. The vessel had a length of 548 ft. and a gross! tonnage of 14,500. Each set of turbines and gearing was designed to develop and transmit 5,500 shaft H.P. and they drove the vessel at 16-75 knots. The turbines ran at 1,500 revs, per minute and 1 drove the propellers at 120 revs, per minute, the ratio of the gear- 1 ing being therefore 12-5:1. In the illustration the pinion in the foreground is driven by the high-pressure turbine, the steam from which operates the low-pressure turbine on the other side of the gear wheel. The astern turbine, consisting of an impulse wheel followed by a comparatively few rows of reaction blading, is seen i on the forward end of the low-pressure turbine. The size of the machinery is indicated by the fact that the gear wheel is 10 ft. < in diameter and 5 ft. wide.

THEORY OF THE STEAM TURBINE

Throughout the ensuing section, heat is expressed in foot pound centigrade units, and the symbols employed have the following meanings:

H =Total heat in one Ib. of steam.

H w = Total heat in one Ib. of steam at the supersaturation limit or Wilson line.

= Total heat in one Ib. of steam at the saturation line.

= Volume of one Ib. of steam in cub. ft.

= Volume of one Ib. of steam in cub. ft. at the Wilson line.

= Volume of one Ib. of steam in cub. ft. at the saturation line. V =Volume of one Ib. of steam in cub. ft. after an isentropic

expansion.

p = Absolute pressure in Ib. per sq. in. t, = Saturation temperature (centigrade). t = Efficiency ratio. r\ = Hydraulic efficiency.

=Thermodynamic head expended in isentropic expansion. U =Thermodynamic head expended in a practicable expansion, y = Index for adiabatic expansion. X = Index for an expansion at constant efficiency.

H. V V. V,