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should be great enough at the point of intersection to resist shearing, but considering the shear at the intersecting point, the head wall will stand thinning down as the centre is approached, according to the following formula:

$$T_1 = .005 D \sqrt P$$,

in which T1 is the thickness in inches of the wall at the intersecting point; D the bore of cylinder in inches; P the maximum pressure in pounds per square inch. This is a contraction, or Thurston’s formula. The thickness of the head at the middle might be

$$T_2 = {P D \over {7200 \times .75}}$$,

in which P is the maximum pressure in pounds per square inch, and D the bore of the cylinder in inches. The change from the thick to the thin portion is to take place gradually. (See Fig. 2.)

Cast iron seems to be the best metal for cylinder construction, although steel castings as well as drawn steel tubes have been used. Cast iron for this purpose should be of an extra good quality, sometimes called “gun iron.” The analysis is about as follows:

Silicon. . . . . . . . . . . . . . . . . . . . . . . . . . 1.125 Phosphorous. . . . . . . . . . . . . . . . . . . . . 0.175 Sulphur. . . . . . . . . . . . . . . . . . . . . . . . . 0.120 Manganese. . . . . . . . . . . . . . . . . . . . . . . --- Carbon ﬁxed. . . . . . . . . . . . . . . . . . . . . 0.67 Carbon graphitical. . . . . . . . . . . . . . . .  2.90

Everything depends, however, upon the grade of "pig," the quality and quantity of “scrap,” the mold and the molder, the heat, the coke, time and location in the cupola. It is better by far to pay a responsible foundry man for good castings than to tell him how to “mix” for them and demand castings of the best quality at a price that will-barely suffice for "window sash weights.” There are various receipts for gun iron mixtures, each of which may be good, in view of the respective base metals.



The ﬁnished product in any event should test about as follows: Pounds per Square Inch. Tensile strength. . . . . . . . . . . . . . . . . . 27,500 Elastic limit. . . . . . . . . . . . . . . . . . . . . 10,850

The grain should be close, with no "chill," while cutting should indicate softness. It would not be advisable to ﬁgure upon the above strength in designing, but the foundry ought to be able to approximate the value given. Finished cylinders should be subjected to a hydrostatic test of 500 pounds per square inch. Do not use air pressure for this test—it is dangerous. All that is required is a small pump with a long lever, such as boiler inspectors employ in their work; a tested steam or water gauge, and some ammonia, pipe and fittings. These ﬁttings enable one to make tight joints quickly. In subjecting the finished cylinder to the test, the valves should be in place, else the high pressure will bear against the ﬂat, thin walls in the exhaust passageway outside the normal pressure zone—a risk that serves no good purpose.

Cylinders that will not stand the 500



pound hydrostatic test are not suitable for the purpose, but cylinders that will stand over 50 per cent. more than 500 pounds per square inch are too heavy for the purpose and should be made lighter; for, in all truth, there are times when a pound off the weight on the tires may be as good as $100 in the bank. There is just this difference between motor car and general machine practice: that in general machine practice, if a part is overstrong, it is pronounced “good”; but in motor car practice, if a part is overstrong, it is really "bad." What is wanted in cylinder construction is a deﬁnite but moderate factor of safety, and it is possible to realize just such a factor of safety.

In motor cars there are many opportunities to reduce weight, and by exercising skill and judgment it is oftentimes possible to effect large inroads. Among other things the "levers," bell cranks, etc., are generally too strong, excepting in some cases in which they are not harmonious in design, hence too strong in some parts and below the needed strength in others. There are three classes of levers known to mechanics, each of which has its special use, all of which are employed in motor cars in divers ways. Fig. 3 illustrates the principle of levers of the ﬁrst class.

In this lever the fulcrum lies between the point of applied force and the point of resistance or reaction. Hence:

$$F = {W \times l \over L} =$$ applied force in pounds.

$$W = {F \times L \over l} =$$ resultant reaction in pounds.

$$l = {F \times a \over {W + F}} =$$ length in inches from fulcrum to resistance.

$$L = {W \times a \over {W + F}} =$$ distance in inches from fulcrum to pull.

Fig. 4 illustrates the use of levers of the third class, in which the point of applied force is between the fulcrum and the point of resistance or reaction. Hence:

$$F = {W \times l \over L} =$$ applied force in pounds.

$$W = {F \times L \over l} =$$ resultant reaction in pounds.

$$l = {{F \times a} \over {W-F}} =$$ length in inches from fulcrum to resistance.

$$L = {W \times a \over W-F} =$$ length in inches from fulcrum to pull.

Fig. 5 illustrates the use of levers of the second class, in which the resisting point is between the fulcrum and the point of applied pull. Hence:

$$F = {W \times l \over L} =$$ applied force in pounds.

$$W = {F \times L \over l} =$$ resultant reaction in pounds.

$$l = {F \times a \over W-F} =$$ length in inches from fulcrum to resistance.

$$L = {{W \times a} \over {W-F}} =$$ length in inches from fulcrum to pull.

In the application of these formulae it is not necessary to conﬁne oneself to inch-pounds units. Any other system of units