182 ALQUIST GEARING FOR SHIP PROPULSION. 
product shows that this expectation has been amply justified. Some of the gears 
which have been used have been very imperfect, both in the matter of material and 
workmanship, and have been used under extremely trying conditions. That they 
have not failed has afforded the strongest evidence of the general reliability of the 
method. 
The character of construction used in this gearing is shown by one of the draw- 
ings (Fig. 1, Plate 100). The gear is built up of a number of plates machined to a 
form which gives them the desired degree of lateral flexibility. These plates are put 
together, engaging solidly at the hub and also engaging on a narrow edge at the pe- 
riphery. When so built together they form a solid cylinder which can be spirally 
cut in the ordinary manner. After cutting, the edge engagements are relieved with 
a small dividing tool so that each disc operates independently and is free to deflect 
laterally under the side pressure which results from its diagonal engagement with 
the pinion. The parts are so proportioned that this lateral deflection can at no time 
involve fiber strains which could possibly cause destructive fatigue. A very small 
amount of this lateral deflection is sufficient to afford the desired distribution of load, 
and this amount can easily be given without approaching dangerous periodic 
strains. : 
To appreciate the value of this flexibility and load distribution, various peculi- 
arities of solid and spiral gearing must be considered. In the first place, where gears 
are inflexible, there must always be a tendency to increase strain at the loaded end 
of the pinion through torsional deflection of the pinion. There is also a tendency to 
inequality of strain on different parts of the surface through the lateral deflection of 
the pinion under load. These inequalities can be partially compensated by elevating 
the bearings or evening the pressure on the bearings, but this compensation can only 
be partial because the correction applies only to the two ends and not to the middle. 
Furthermore, the momentary and periodic strains on different points of solid spiral 
gearing may be seriously affected by vibrations of supporting structures, irregu- 
larities of machine work or gear cutting, and other causes. If for any reason such 
conditions cause any tooth or part of a tooth to receive periodically excessive 
strains, fatigue may result and a broken tooth may destroy the whole gear. 
To obviate the possibility of breakage under such conditions, it is often desirable 
in solid gearing to use relatively large teeth in order that these possible irregular 
strains on teeth or parts of teeth will not involve danger of breakage. For other 
reasons, however, the use of large teeth is distinctly undesirable in- spiral gearing. 
Spiral gears tend to engage by point contacts at or near the pitch line, and the ability 
of these point contacts to bear pressure without fatigue of the surface metal is 
governed largely by the flatness of the surfaces engaging rather than by the size of 
the teeth carrying these surfaces. The flatness of the surface is a function of the 
pinion diameter and not of the pitch. If we double the number of teeth in a spiral 
gear we have twice the number of driving points in action, and the flatness of all of 
these points is the same in both cases if the pitch diameters are the same. These 
matters are illustrated in the sketches in Fig. 1, Plate too. 
