A train of toothed wheels, for transmitting motions in machinery. There are two sorts of gearing in common; viz. spur gear and beveled gear. In the former, the teeth are arranged round either the concave or convex surface of a cylindrical wheel in the direction of radii from the centre of the wheel, and are of equal depth throughout. In beveled gear, the teeth are placed upon the exterior periphery of a conical wheel in a direction converging to the apex of the cone, and the depth of the tooth gradually diminishes from the base. For the rules for setting out these wheels, and for the best form of teeth, we refer to the article Mill Work, and shall, in this place, only notice a new system of gearing, invented simultaneously by Mr. Dyer, in America, and by Messrs. M'Dougall, of Ferry Bridge, in Yorkshire, and communicated by the former gentleman to Mr. H. Burnett, who took out a patent for the same. The object of this invention is to obtain a great difference in the relative velocities of the wheel and pinion, by a construction which in some respects resembles what is called a worm wheel, or a wheel driven by an endless screw, but is at the same time free from the objections attending the ordinary arrangement of the wheel and endless screw.
The common application of the wheel and endless screw is in the direction of a tangent line, (or very nearly so) to the wheel to which it is applied, and the force exerted upon it or by it, is constantly in a direction coincident with its longitudinal axis, and never in that of its radius, whilst a rubbing action takes place between the threads of the screw and the teeth of the wheel, thus producing excessive friction, not only at the point of contact, but also against the pivot or resisting point of the screw. In the patent gearing, the axis of the screw, or spiral pinion, is not a tangent to the wheel, but lies in the same plane as the axis of the wheel in ordinary gear, whether it be spur gear or beveled gear. The consequence of this arrangement is, that the power of the screw or spiral is exerted in the direction of its radius; consequently, it can be driven by the wheel with the same facility with which the wheel drives it (difference of leverage, according to their respective radii, excepted), which is not the case with the ordinary wheel and screw.
Another advantage is, that there being but one point of bearing in action at once, and that uniformly on the line of centres, and that action passing uniformly over equal spaces, in equal times, and in the same direction; the action which takes place between the wheel and the spiral pinion is not a sliding, but a perfect rolling action, whereby nearly the whole of the friction which would otherwise occur, is avoided. One valuable property of this invention is. that any required strength may be given to the arbor, without affecting its power or velocity; this will be rendered evident by an inspection of Fig. 3 on the following page, in which the external lines e e and f f represent the sections of two arbors of very different sizes, and consequently strengths, while their mechanical action is precisely similar, for it will be seen that, notwithstanding the difference of magnitude in the two sections, they have but one common pitch-line, marked i i in both figures, so that both can work in the same wheel, and their power and velocity must be equal. Fig. 4 represents the arbor or pinion engaged with the tooth of a wheel.
Another most important advantage belonging to this system of gearing, is the simplicity which may be thereby introduced into machinery, and the consequent diminution of friction; for since each tooth of the wheel will be equivalent to an entire revolution of the arbor, a great saving may be effected; thus, for instance, if a common wheel has 100 teeth, and works into a pinion of ten leaves, the same power or velocity may he obtained by the new mode of gearing, by using a wheel of only ten teeth, working into one spiral groove upon the arbor; as an example of which Fig. 1 is a side, and Fig. 2 a front elevation of a regulator clock, which is capable of showing hours, minutes, and seconds, and will go for a whole year with once winding up, by a weight of only a few pounds. The curve of the spiral groove on the arbor may be found by covering the face of the wheel with a strip of thin paper, and marking the bevel of one tooth upon it, when the extension of that tooth may be cut off and wrapped round the arbor, which will give the form of the spiral, which may be marked and cut; but this is only a proximate method, and as the action of the improved gearing, like that of all other gearings, depends mainly upon the perfect working of the teeth and grooves, it is better to cut the groove on the arbor, in a regular and proper machine for cutting spirals and screws.