OX MACHINERY.

1?7

the face of the teeth, where they are in contact, is too much inclined to theradius, their mutual friction is not much affected, but a great pressure on theira xes is produced; and this occasions a strain on the machinery, as well as anincrease of the friction on the axes.

If it is desired to produce a great angular velocity with the smallest possibleQuantity of wheclwork, the diameter of each wheel must be between threea ud four times as great as that of the pinion on which it acts. Where thepinion impels the wheel, it is sometimes made with three or four teeth only;but it is much better in general to have at least six or eight; and consideringthe additional labour of increasing the number of wheels, it may be advisableto allot more teeth to each of them than the number resulting from the calcu-lation ; so that we may allow 30 or 40 teeth to a wheel acting on a pinion of 6°r 8. In works which do not require a great degree of strength, the wheelshave sometimes a much greater number of teeth than this; and on the other hand,an endless screw or a spiral acts as a pinion of one tooth, since it propels the wheelthrough the breadth of one tooth only in each revolution. For a pinion ofSl x teeth, it would be better to have a wheel of 35 or 37 than 3d; for eachtooth of the wheel would thus act in turn upon each tooth of the pinion, andthe work would be more equally worn than if the same teeth continued to^oet in each revolution. The teeth of the pinion shquld also be somewhatWronger than those of the wheel, in order to support the more frequent recur-le nce of friction. It has been proposed, for the coarser kinds of wheelwork,*'° divide the distance between the middle points of two adjoining teeth intoparts, and to allot 1(3 to the tooth of the pinion, and 13 to that of the' v heel, allowing 1 for freedom of motion.

r I'he wheel and pinion may either be situated in the same plane, both beingc °nunonly of the kind denominated spur wheels, or their planes may form an^Sle : in this case one of them may be a crown or contrate wheel, or both of^hem may be bevilled, the teeth being cut obliquely. According to the rela-Magnitude of the wheels, the angle of the bevil must be different, sothe velocities of the wheels may be in the same proportion at both endstheir oblique faces: for this purpose, the faces of all the teeth must be di-rec ted to the point where the axes would meet. (Plate XV. Fig. 193, 194 .)

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