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(146) Drawings of Valves.-In drawing valves of the indiarubber disc type (Figs. 169 and 170) it is sufficient to show two views, a sectional elevation and a plan, one-half of the plan to show the grating with the indiarubber removed, and the other half the guard. With stop and sluice valves (Figs. 172, 175, and 176) it is usual to draw two elevations and a plan, the front elevation and sometimes half the plan being in section. The order of drawing a valve of the type of Fig. 172 should be as follows, the proportions being first decided:

1. Draw centre lines of all views.

2. Draw seating in front elevation, to be bisected both by the centre line of the valve, spindle, and the centre line of the inlet and outlet branches.

3. Draw valve and spindle in position when closed.

4. Draw in inside and outside of valve casing on both sides of the centre line (this is to ensure a symmetrical section).

5. Draw in cover and stuffing box, allowing for clearance between cover and valve, and for a clearance above the screwed part of the spindle equal in length to the valve lift.

6. Draw division plates, flanges, and complete details.

EXAMPLES.

Make working dimensioned drawings of the following valves:

EX. 3.-Indiarubber disc valve with dished guard (Fig. 169). Diameter of rubber, 83"; thickness, "; depth of grating, 13"; diameter inside, 8"; outside, 91"; division bars, " thick at top; guard, 61" diameter outside, 11" extreme height; thickness, 3 to 5" ; screw for guard, 1" diameter. Calculate effective area through grating. Full size.

16

18

(Arrange grating with eight division bars in inner circle, and twelve in outer circle. Boss of grating and guard to suit screw, and to give overlap for rubber on boss of grating. About "to" clearance between top of rubber and guard. Ten or twelve holes in guard.)

8

EX. 4.-Gunmetal steam stop valve for pipes of 2′′ diameter to pattern of Fig. 172, screwed cover; valve, 3" thick; spindle, " diameter, valve and spindle in one; general thickness of metal of casing, "; distance outside flanges, 61". Full size.

5

(Spindle enlarged in diameter at screwed part; length of screw in cover forming nut for spindle not less than " diameter; nuts on cover and gland to be some standard size; proportions of stuffing box given in Ex. 3, Section xxiv.)

EX. 5.-Cast-iron steam stop valve for pipes of 4" diameter with outside screws and flanges at right angles (Fig. 175).

Gunmetal valve, seating, and gland, wrought-iron spindle, 1′′ diameter. Scale 9" = 1.

(Seating as in Fig. 1716, valve as Fig. 173, proportions of arrangements for outside screw in Fig. 174, top of valve seating on level with horizontal outlet, flanges as close in as convenient for nuts and proportioned same as for cast-iron pipes (see §§ 44-46); casing thickened around seating-see Problem lv. for method of obtaining points in the intersection of the horizontal branch with the main casing.)

STEAM ENGINE DESIGN.

SECTION XXVI.

INTRODUCTION.

THE object of the following sections is to give an elementary knowledge of the principles and practice of steam engine design as far as the construction of simple single-cylinder engines of ordinary type and of small power. It will be assumed that something is known of the uses of the different parts of such engines, and how the work of an engine is measured. The limits of the book will not permit of anything like a full consideration of the conditions which decide the proportions of many of the parts, such as connecting-rods and cranks, owing to their requiring a somewhat advanced treatment; but as far as possible the reasons for the sizes and arrangement adopted will be made clear, both from the standpoint of the stresses the parts have to stand and the practical requirements of construction. No consideration will be given to the design of such simple parts as have been dealt with in the preceding sections, it being presumed that such sections have been worked through, or will be referred to.

The majority of the illustrations are those of the parts of two separate engines, each constructed by eminent engineering firms, and each typical of their class. But in some cases additional designs are illustrated taken from engines constructed by other makers, so that a comparison can be made of the relative advantages and disadvantages of different arrangements of the same parts.

The questions appended to each section are so arranged that when all the "Examples A," or all the "Examples B," have been worked, the student will possess drawings of the leading parts of the following engines :

(147) Examples A.-A high-speed vertical engine, having the cylinder inverted above the crank, and suitable for the direct driving of pumps or dynamos. The cylinder is supported by two turned steel columns in front, and by a cast-iron standard behind, which forms part of a framing of box section, containing the crankshaft bearings. The crosshead and guide is of the single slipper pattern, working between guides bolted to the back standard. The slide valve is of the piston valve type, worked by an eccentric placed outside the shaft bearing. The crank is made with solid counter-balance weights. Diameter of cylinder 71", stroke 53′′, revolutions per minute 400, about 35 I.H.P., with a boiler pressure of 115 lbs. per square inch.* The general arrangement of the engine is shown in Fig. 178. (See Folding-plate.)

(148) Examples B.-A horizontal engine, having a jet condenser of cast-iron box pattern, the cylinder and condenser being bolted to the same bed-plate, and suitable for the ordinary driving of machinery. The cylinder is steam jacketed, and is fitted with a main slide valve and an expansion valve, the eccentrics for which are fixed inside the crank shaft bearings. The crosshead is attached to double guide blocks, working between double guide bars. The piston-rod is extended beyond the back cylinder cover, and works the air pump, which is of the plunger pattern. Diameter of cylinder 81", stroke 12", revolutions per minute 150, about 25 I.H.P., with a boiler pressure of 105 lbs. per square inch. The general arrangement of the engine is shown in Fig. 179. (See Folding-plate.)

It must, therefore, be understood that all future references to Ex. A or Ex. B relate to these two engines. The questions forming the examples of each section are worded to show the relation of the different parts, as a result of which it will be necessary to refer to previous examples or to §§ 147, 148, in order to obtain all the necessary particulars. For instance, in working Ex. A 2, Section xxvii., the question of Ex. A 1, must be referred to in order to see the size of the cylinder to which the piston is to be fitted. It is only in this way that the interdependance of the several parts can be fully kept in mind.

(149) Order of Design.-The first proceeding in designing any complete machine is to decide, at least approximately, the chief sizes, form and arrangement of the leading parts. This is especially necessary in the case of a steam engine, as a desired power can be obtained from quite different proportions of the two leading dimensions, the piston diameter and the length of the stroke; and as the sizes of many parts depend so much upon the sizes of other parts; while equally good engines can be con* All steam pressures are stated as "pounds per square inch absolute."

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