The adjustable friction-gearing (fig-. 45) of J. W. Howlett illustrates one of the many uses to which rubber has been effectively applied. The upper wheel A is composed of a V-edged rubber disc clamped between two metal lic plates. By screwing up the nut.B, which holds the parts together, the rubber disc is made to expand radially and to press more tightly in the groove of the larger wheel, thereby obtaining greater traction.
Figure 46, called an " ellipsograph," is more of an instrument than a mechanical movement. The traverse-bar carries two studs which slide in the grooves of the cross-shaped piece. By turtling the traverse-bar an attached pencil is made to describe an ellipse by the combined rectilineal motions of the studs in the grooves.
Figure 47 exhibits the so-called "flexible angle-coupling." The ends of a spiral spring are fastened to two shafts which may lie at any angle with each other; the flexibility of the spiral maintains the connection between the two shafts and transmits the rotations of one to the other. A wire rope connected in a similar manner accomplishes the same purpose.
To limit the number of revolutions of a wheel, as in winding a watch whose mainspring would be endangered by overwinding, is the purpose of the " Geneva stop " shown in Figure 48 (p. izo). It consists of a wheel (A) with a single slightly-projecting tooth, and of a star-wheel (B) with four concave teeth and one convex tooth (a, b) called the " stop." The operation of the movement is as follows: At each revolution of the wheel A its tooth engages one of the concave teeth of the wheel B, enters the tooth-space, and moves the wheel B to the extent of one tooth. When the convex tooth or stop a, b comes in contact with the tooth of wheel A, the latter is prevented from turning any farther by reason of the convex ity of the stop, which cannot pass the projecting tooth. The uncoilirig of the mainspring reverses the movement of the wheels until the tooth b comes into position on the opposite side, when, by rewinding the spring, the above-described operation is repeated.
To the mechanical movement shown in Figure 49 has been given the name " lazy tongs," which is a connected system of cross-links jointed at their crossings and angles. When the links are alike, the four-sided spaces are all uniformly enlarged or diminished by the rectilinear motion of the rod. Whatever motion is given to the link to the right (which is fixed at its junction with the next link) by the rod at its salient centre, will give a similar but much greater motion to the link and rod at the left.
A simple contrivance known as the "governor," by which advantage is taken of the ever-present gravitation and centrifugal force acting through a pair of weights and jointed levers, is shown in Figure 5o. The adapta tion to the steam-engine of this expedient, which was long in use for reg ulating the speed of millwork and other machinery, is due to the ingenuity of Watt. It consists of two heavy balls (B, B) attached to the extremities of the rods F, F which are jointed at E, and pass through a mortise in the vertical stem D, D'. The upper ends of the rods F, F are connected by links to a sliding collar (D) in which fits the horizontal lever I, G, K. This lever is jointed to a fulcrum-pillar at G, and to a link (A', I) which operates a valve (V) in the steam-pipe that supplies the engine. The motions can be easily traced through the various parts—increase of velo city throws the balls farther from the centre of motion, draws down the collar, raises the lever at I, and partly closes the valve. A reduction of velocity reverses these motions and produces an opposite effect. The property which renders this instrument so well adapted to its purpose is, that there is but one velocity at which the balls can remain in equilibrium. It governs the speed of the engine because the slightest change in thq velocity causes a considerable movement of the steam-admission valve.
(J. H. C.)