Size of Bar Links.-Let D be the diameter of valve spindle, found as before.

Fig. 17, distance between centres of eccentric pins, 3 to 4 times throw of eccentrics.

Fig. 17, distance between eccentric rod pins

22 to 234 times throw of eccentrics.

Diameter of eccentric bolts (top end), at bottom of thread = 0.42 X D when of iron; and 0.38 X D when of steel.

=

These bars should be of a very good description of iron and case-hardened, or of steel; the latter is of course free from seaminess and stronger. The eccentric-rod pins of this kind of link (Fig. 17) are usually forged solid with the bars; but there is no absolute need of this, and it adds very much to the cost, both of manufacture and renewal when worn. Since the wear on these pins is limited to a very small portion of their circumference, it is not unusual to file away the parts which are not subject to wear, so as to admit of the brasses being closed when

worn. When loose pins are fitted, they should be steel, and hardened, so that all wear may come on the brasses which are capable of adjust

ment.

In another arrangement of bar link motion, the sliding block is divided, and on the outside, while the eccentric-rod ends are between the bars. This, while having some slight advantages, is on the whole very clumsy, and the block-pins wear badly; besides which, the link can be suspended only from the extreme end.

CHAPTER XII.

VALVE MOTION DIAGRAM.

THE practical man has no time for formulas when he can travel (to him) a much straighter road to what he wants. The plan is simple, and any one can understand why he does this or that;

he knows he is right, and he has no conclusions to jump at.

The cutside circle (Fig. 18) is the crank-pin

circle, the smaller one the eccentric's centre circle. The crank-pin circle is divided off into the inches of travel of piston, by a tram or pair of compasses set to represent the rod's length. S S, in Fig. 18, are the steam ports, E the exhaust port, and X X the bridges. Fig. 19, is a sectional elevation of the valve divided by the line a d. The valve drawing is cut out at the line fe, and placed on the line representing the valve-seat.

The laps and lead are laid off (from the pin C, the valve rod running directly from eccentric to

valve) a b from centre of circles, a. The line b c is erected perpendicular to centre line FC, and through where it cuts eccentric circle at ca line is drawn, as a B. This line is the centre line of eccentric, and the angle Ca B—the angle formed by eccentric, and crank c. This angle is constant in whatever position the crank-pin may be.

Place the valve, Fig. 19, on the seat so that line da coincides with line c b, and you have position of valve when crank is on centre. As the crank moves, in direction of arrow, from C, the valve moves from right to left, and back again, till right hand edge of valve comes to

edge of port, 2. The steam is now cut off, and, to find position of crank-pin, notice that the line a d on valve cuts the centre line, FC, at b, which extended down to small circle, cuts it at e. The line a e D is now evidently the centre line of eccentric. Taking the distance, B C, in a pair of compasses, and measuring back from D, we find the point A, which is the position of the crank-pin when steam is cut off. The figures on the large circle give the number of inches traveled when this occurs. The valve now continues to move from left to right till edge of exhaust cavity g, comes to edge of port h, when the exhaust commences. The centre line on valve da now cuts line FC at a; this, extended down, cuts eccentric circle at m. A line a m j G, now represents the centre line of eccentric; laying off the distance, B C, from G, we find the point H of crank-pin, when exhaust commences. The crank now travels on the lower half of its circle. The valve continues to travel from left to right, opening into the exhaust until the edge of exhaust cavity again comes to h (edge of steam port, the valve now traveling from right to left), the exhaust is closed and compression commences. This occurs, as before stated, when g, on valve, comes to h, on ports; the line da, on valve, then cuts the line F C at a, and the line a i K represents centre line of eccentric. Lay off from K the distance B C;