but with a varying velocity ratio, for the pin F continually changes its distance B F from the axis

BC.

When the distance between the parallel axes is small, and the axis A D revolves uniformly, the angular velocity of the axis B C varies, very nearly, inversely as the distance, B F, of the pin from this axis.

The Eccentric Wheel.

81. This mechanism is usually employed to give motion to the slide-valve of the steam engine.

wheel so that the one may revolve freely within the other; EFD a frame connecting this loop with the extremity D of the bent lever D LG, turning on the fixed centre L. Now when the eccentric wheel revolves in the direction of the arrow, shown in the figure, the frame with the pin D is pushed to the right, and when the lob side of the eccentric has passed the line of centres, B and D, the frame with the pin D is drawn to the left, and so on. Thus the continuous rotation of the axis B

produces a reciprocating circular motion in the pin D. The stroke of the pin D will be equal to twice C B, or double the eccentricity of the wheel.

Cambs, Wipers, and Tappets.

82. Cambs are those irregular pieces of mechanism to which a rotatory motion is given for the purpose of producing, by sliding contact, reciprocating motions in rods and levers.

Fig. 58.

F

E

83. In fig. 58, в CD represents the camb, turning on its axis A, and giving a reciprocating rectilinear motion to the heavy rod E F, which is restrained to move in its rectilinear path by the guide rollers. The rotation of the axis A being in the direction of the arrow, the rod E F has an upward motion until the extreme point B of the camb comes in a line with the rod, then the portion BG of the camb allows the rod to fall, by its own weight, or by the action of a spring, until the point & comes in a line with the rod, and so on; thus one revolution of the camb, here presented, will cause the rod to make three upward and three downward strokes. By varying the curve of the camb, any law of motion may be given to the rod.

84. In fig. 59, the pin E of the rod is made to traverse a groove E G D, cut in the camb plate, so

that the pressure of the camb upon the pin pro

duces the downward stroke of

the rod as well as its upward

Fig. 59.

stroke. In this case the rod will only make one upward and one downward stroke in every revolution of the camb plate. The length of the stroke of the rod will be equal to the difference between AD and AG, where D is the point in the groove. furthest from the centre A, and G is the point nearest to it.

A

E

85. To find the curve forming the groove of a camb,

so that the velocity

ratio of the rod and

the axis of the camb may be constant.

Let A be the centre of the camb, and CABQ the direction

of the rod. From A as a centre, with any convenient distance A C, describe the circle CE DB N. On BA take B a equal to the length of the stroke of the rod; divide it into any

convenient number of equal parts, say five, in the points, b, c, d, e, and divide the semicircle B D E F G into the same number of equal parts by the radial lines, a D, A E, A F. From A as a centre, with a b, A c, A d, A e, as radii, describe circles cutting A D, A E, &c., respectively in the points g, k, l, m: then through these points draw the curve a g k l m c; and similarly in the semicircle BN C draw the

other curve a np c.

All lines drawn through the centre A of this curve are equal; thus a c I n = g p &c. Hence if the rod had two pins placed at a and c, the camb would revolve between them, and would cause the rod to make a downward as well as an upward stroke. This curve is the spiral of Archimedes.

By dividing the line B a into parts having a

varying ratio to one another, any proposed law of velocity may be given to the rod.

86. In fig. 61, the continuous rotation of the camb A E C, revolving on the axis A, gives an oscillating motion to the rod or lever F a, turning on the centre F. In one revolution of the camb the rod makes a double oscillation in the arc ɑ ɑ1. 87. Wipers. When the rod is to receive a series of lifts with intervals of rest, the camb is made into the form of projecting teeth, which are commonly called Wipers or Tappets.

88. In fig. 62, the revolving cylinder c has five

H

Fig. 62.

A

wipers upon its circumference, which give five downward strokes to the hammer, H, placed at the

extremity of the lever A H, in each revolution of the cylinder.