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B' G is the length of crank.
B' D is half radius of axle.
B' B equal half the throw of valve.
B' C equal half the throw of cut-off eccentric.
Draw the line, E F, perpendicular to the line of eccentric rod, and tangent to the lap and lead circle, and when it intersects the throw of valve at the points, F M and B M, is the centre of the eccentrics; the cut-off eccentric, c o, is set on the line of crank when the throw of eccentric intersects tha line.
Fig. 52 is the same construction as fig. 51, work
ing direct to the valve, without the intervention of rock arm, only the centres of eccentrics are on the left side instead of right, and the forward motion eccentric is below, and the backward motion ecceptric above.
E F line drawn at right angle with eccentric rod. A B' lap and lead.
B B'throw of valve.
B' C throw of cut-off eccentric. 124
B' D radius of axle.
For any other angle of cylinder or eccentric rod, the construction is precisely similar. The angle of connecting rod, being so trifling, is not taken into consideration in practice.
Tire-Bars, Lengths required to make Inside Diameter.
The alloy of 2 zinc and 1 copper may be crumbled in a mortar when cold. The ordinary range of good yellow brass, that files and turns well, is from about 41 to 9 oz. to the pound. Brazing solders-3 copper and 1 part zinc, (very hard;) 8 parts of brass and 1 zinc, (hard;) 6 parts brass and 1 tin, 1 zinc, (soft.) Solder for iron, copper, and brass, consists of nearly equal parts copper and zinc. Muntz's metal-40 parts zinc, 60 copper. Any proportions between the extremes of 50 zinc and 50 copper, and 37 zinc and 63 copper, will roll and break at the red-heat; but 40 zinc to 60 copper are the proportions preferred. Large bells-4 oz. to 5 oz. of tin to 1 lb. of copper. Tough brass for engine work—11⁄2 lb. tin, 11⁄2 zinc, and 10 lbs. copper. Brass for heavy bearings-21 oz. tin, 1 oz. zinc, and 1 lb. copper. Babbit's metal-1 lb. copper, 5 lbs. regulus of antimony, and 50 lbs. tin. Melt copper first, add the antimony with a small portion of the tin, charcoal being strewed over the metal in the crucible to prevent oxidation.
ON THE SAFETY VALVE AND LEVER.
THE apertures for safety valves require no nice calculations. It is only necessary to have the aper
ture sufficient to let the steam off from the boiler as fast as it is generated, when the engine is not at work.
The safety valve is loaded sometimes by putting a heavy weight upon it, and sometimes by means of a lever with a weight to move along to suit the required pressure.
When the whole weight is put on the valve, to find the pressure to each square inch:
Multiply the square of the diameter of the valve by 7854, and this product will give the area, or number of square inches in the valve.
And if the whole weight upon the valve, in pounds, be divided by the number of square inches in the valves, the quotient will give the number of pounds pressure to each square inch in the valve.
Ex.-If a weight of 40 lbs. be placed upon a valve, the diameter of which is 3 inches, what will be the pressure to each square inch?
32 x 78547 square inches; then, 40 ÷ 7 = 5 lbs. per square inch.
ON THE SAFETY VALVE LEVER.
This being a lever of the third order, it may be calculated as follows; and also the weight of the lever will be considered; for when the lever is large and the valve is small, the weight of the lever is such as to produce a very sensible pressure upon the valve to each square inch. But previous to the
calculation, it will be necessary to make the following remarks.
Since the fulcrum is at one end, and the power or the action of the steam between that end and the movable weight, (see fig. 2, where F is the fulcrum, A is the point where the steam acts, and W the movable weight,) some have taken A for the fulcrum, and thereby have committed very great errors; for, according to this rule, a weight put on at twice the distance from A that A is from F, would, if the weight of the lever were not considered, be twice its own weight upon the valve; whereas, if it had been reckoned from F, its real fulcrum, it would be three times its own weight upon the valve.
It has been shown, and indeed it is almost selfevident, that if we have two, three, or four times, &c. the leverage, we will have two, three, or four times &c. the effect produced respectively, the weight remaining the same. Therefore divide the length of the lever by the distance between the fulcrum and valve, and the quotient gives the leverage; and the leverage, multiplied by the weight, gives the whole weight upon the valve; and this product, divided by the number of square inches in the valve, gives the weight per square inch. Or, if the weight per inch be known, multiply the number of pounds per square inch by the number of square inches, and