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August, 1914 bearings and teeth. It will be readily seen that if the axis of. the pinion naturally tends to incline itself to the axis of parallelism with the axis of the driven gear, any arrangement which prevents, or tends to prevent this natural adjustment, must neces- sarily not only subject the teeth of the gear to unnatural and unnecessary stresses of considerable magnitude, but THE MARINE REVIEW pinion axis to assume its own position without restraint, will become more evident. If we were to assume the pinion made of rubber, it will be readily seen that even with a com- paratively small twisting force, the twist in-the pinion would become so great that the teeth at the end farthest from the point of application of the © criving force would fail to mesh with LG Must likewise subject the bearings to Similar stresses. It may be difficult without previous consideration to appreciate the effect of the torsional deflection, as this is Comparatively slight, and consequent- ly the natural tendency is to conclude that it is insignificant. However, if the pinion were assumed to be made of rubber or some equally elastic ma- terial, the necessity for permitting the the teeth of the driven gear, unless the axis of the pinion were free to tip sufficiently to permit the teeth at the end farthest from the point of appli- cation of the driving force to lag behind sufficiently to come into mesh. From the above it will be readily seen that unless the pinion is per- mitted to assume its Own aXiIS, the pressure on the teeth will be greater at one end of the pinion than at the 315 other, and consequently, though the pressure per unit length of pinion face may be sufficiently low if uniformly distributed, it will be sufficiently high locally to disturb the oil film between the teeth and cause undue friction and rapid wear. It will naturally occur to the reader that if the diameter of the pinion were made. sufficiently 'large in propor- tion to the power transmitted, the torsional deflection and consequent error between the natural axis of the pinion and parallelism may be in- significant, and this is- true within certain limits. Increasing the diameter of the pin- ion has serious drawbacks in marine work, since an increase in the diam- eter of the pinion means a larger in- crease in the diameter of the driven wheel, the increase in the latter be- ing greater than that of the pinion by the number of reductions of speed. In order to obtain the full benefit | of reduction gearing: in marine in- stallations, it is necessary to select the highest number of revolutions 'per minute permissible for the turbines, and as the propeller revolutions for maximum efficiency must be kept com- paratively low, the reduction ratio de- sirable is generally very large; and consequently any increase in the size of the pinion in order to reduce the torsional deflection, is accompanied by a considerable increase in the diam- eter of the large gears. Roughly, the weight of a reduction gear is pro- portional to the square of the diam- eter of the large gear plus the square of the diameter of the pinion, so it is evident that for a given ratio of re- duction the weight of the reduction gears increases approximately as the square of the diameter of the pinion. Reducing Revolutions of Turbine If it is attempted to avoid the abnormal increase in weight of the gear due to an increase in the diam- eter of the pinion for a given power transmitted by reducing the revolu- tions of the turbine, the result is that a considerable portion of the saving in weight in the gear will be counter- balanced by an increase in weight of the turbine, the weight of the tur- bines varying approximately inversely as the square of the revolutions per minute. Further, reducing the revolu- tions of the turbines below the maxi- mum permissible for any given power, results in a decrease in economy, since a turbine reaches its maximum econ- omy when the revolutions for the power developed are the maximum permissible. Likewise, the space occu- pied and the weight of the combined installation 'is considerably increased as the revolutions of the turbine are