be to draw the plan of the plate, first draw engine centre line and centre lines of condenser and cylinder, then show the outline of the condenser and cylinder flanges which are to be fitted to the bed plate, and draw the plan of the facings for these, allowing about 4" beyond all round. Next show the outline of the feet of the guide standard, obtaining sizes and distance in front of cylinder centre from drawings of Figs. 179 and 190, and arrange facings in same way. It will then be better to draw the outline of the front cylinder cover, showing the lugs for the guide bars, and the guide block and crosshead in their nearest position to the cover. Then mark off along the centre line from the crosshead centre, a distance equal to length of connecting rod + length of crank (Ex. B 5 and 7), and this will give position of centre of crank shaft. Now mark off along the shaft centre line, the limit of the crank web in one direction, and the expansion eccentric in the other (see Figs. 181 and 196), then fix the centres for the shaft pedestals (Fig. 158), which may conveniently be as near to edge as ". Mark out the facing for the pedestal bases as before, and also for guide bracket for valverods. Next draw the plan of the recess for the crank, and for catching the oil, the crank recess should extend to cover all positions of the connectingrod end. The cross ribs may equally divide the length of the bed plate, the bosses for the bolts being about 31" within the edge; care must be taken that they do not foul with other bolts for the cylinder, condenser, or pedestals. Notice that the flange need not project for more than 1". Then draw a front elevation and an end section, and dimension.

(213) Length of Piston-Rod, Valve-Rod, and Crank Shaft. -From what has been already said we are evidently now in a position to exactly settle the length of these parts. For the piston-rod of Ex. A, its length from the crosshead centre to under side of piston will be equal to

(Distance from centre of crosshead to top of flange on standard when at back end of stroke + distance from under side of flanges on cylinder to лside of front end, + clearance + length of stroke of piston),

and for the rod of Ex. B, it will be equal to—

(Distance from centre of crosshead to inside of front cover, + clearance + stroke);

wh le to obtain the total length of the piston-rod since it passes through the back cover to join the air pump, we must add a length of―

(Distance from centre of cylinder to centre of condenser + length from centre of condenser to inside back end - clearance for plunger from end total length of plunger + length of piston-rod in plunger end for cottered joint half piston stroke.)

Allowance must be made for the length of rod required to fix to piston and crosshead.

To find the length of the valve-rod we must decide the position of the guide bracket and the length of the guide on the valve-rod (the enlarged part of valve-rod, see Figs. 184, 185, 186). Then we can find the distance from the centre of the valve at mid

stroke to the outer edge of the guide bracket, and add half the valve travel and the distance from centre of pin in forked end of valve-rod to edge of guide in nearest position to cylinder. The length of the eccentric-rods is best found from a drawing where the piston is shown at one end of the stroke and the eccentrics in their corresponding position upon the engine shaft (see Fig. 194).

Having determined the positions of the crank-shaft bearings, we are able to dimension the length of the crank shaft. It is purely a matter of convenience how far the shaft overlaps each end, except when a flywheel or driving wheel is fitted, and then, at least, a minimum length equal to the wheel boss is necessary. (214) General Arrangement Drawing.--A general arrangement drawing should show the engine complete, with the different parts in their correct relative positions. It should be something of the character of Figs. 178 and 179, only that more parts should be shown, and at least three views, two elevations and a plan, be drawn. In a finished drawing, parts may or may not be in section, usually not, and such details as bolts and nuts, lubricators, steam valves and pipes, and drain cocks may often usefully be included. The student should find no difficulty in making such a general arrangement drawing after having worked the previous questions, as the chief difficulties have been explained. If the cylinder is shown in section, it is usual to draw the piston at one stroke end, and the valve at mid-stroke, these being the easiest positions. There are other details, such as governors, feed pumps, which may be attached to the engine, but they may be so separate as not to affect any of the other work, and as space is limited they are omitted.

EXAMPLES.

EX. A 9. Make a general arrangement drawing, three views, of the vertical engine of Ex. A (Fig. 178). Scale 11" = 1'.

EX. B 10. Make a general arrangement drawing, three views, of the horizontal condensing engine of Ex. B (Fig. 179). Scale 11" 1'. Crank shaft pedestals as in Fig. 158.

SECTION XXXIII.

DESIGN OF MACHINE PARTS OF GIVEN

WEIGHT.

(215) IT is very desirable that the engineering draughtsman should be able to design parts which, when finished, shall not exceed a certain weight. The necessity for this frequently arises with such details as governors, counterbalance weights, and the flywheels for steam, gas, and oil engines, especially the last two, which depend so much upon the flywheels for their uniform running. Cases often occur, also, where probably a whole engine has to be produced under restrictions and penalties as to its total weight-such, for example, as the engines for war ships, or traction engines for service over light bridges in foreign countriesand the satisfactory fulfilment of the order may rest upon this question alone. That such examples present considerable difficulty is apparent, for the draughtsman has not only to fulfil all the usual conditions of strength and proportions, but he has to exercise a large amount of ingenuity to so distribute his necessary material in the most economical way as to secure the result of least weight.

This section is included in order to give the student some examples of this kind, and it was for this purpose that the tables of pp. 162-165 were extended to include the weights of unit volumes of the ordinary materials of construction, and of the standard sizes of bolts and nuts. Of course, the results can only be approximately assured, owing to the difficulty of making a correct allowance for the fillets of corners, and similar parts, but after some practice, the designer acquires the knowledge of many ways in which these are very nearly accurately allowed for without undue complication. The examples chosen for working will not require anything beyond an elementary knowledge of the mensuration of surfaces and solids.

In working the examples, the student should make sketches of the part, filling in the sizes of the parts whose dimensions are fixed by other considerations than the weight. He should then try to complete the design roughly to fulfil the conditions of weight also, before beginning the actual working drawing. Then in making the drawing, he will see that there are certain sizes which must not be altered, and certain others—such as overlap of flanges, thickness and diameter of bosses-which can be altered without loss of strength.

EXAMPLE.

EX. 1.-Design a simple shaft pedestal, as in Fig. 154, Ex. 1, p. 227, so that its weight, when finished, shall be between 3 and 4 lbs.

(Here the only fixed size is the length and diameter of the biasses, and we can easily save weight in reducing the height from the base to the centre, since this is not stated, and in the base length. First find what weight the ordinary proportions will give, and then, if too heavy, cut away the least important parts.)

(216) Flywheels. In a steam engine having a crank and connecting-rod, the turning effort on the crank pin varies considerably during the stroke, owing to the different angle between the crank and rod, the changing pressure upon the piston, and the change in the velocity, and, therefore, in the momentum of the moving parts. At the two dead centres the turning effort is evidently nil. Hence, if the resistance offered to the engine is uniform, it follows that at some parts of the stroke the power of the engine is in excess of the resistance, while at other parts it is less, and this must mean that the velocity of the crank pin will slightly change. But if a heavy flywheel is fitted to the shaft, it decreases the amount of this change of velocity by absorbing energy when the work done on the crank is greater than required for the resistance, and by giving it out again when it is less. It does this by slightly changing its velocity, and in virtue of the fact that the energy of a rotating body depends upon its mass and its velocity; hence, since the mass of the wheel is constant, its velocity changes in order that the amount of its kinetic energy may change. But if the amount of the variation in the crank effort is found, as it can be by well-known methods, it is possible to design a flywheel of a certain size and weight, so that its actual variation from a uniform velocity may be of any desired value. This variation for well-balanced engines does not exceed 1 to 2 per cent.

We shall not deal with the methods by which these results are arrived at, but simply consider the question of designing the wheel when they have been decided. În doing this, we notice that the wheel is most economically designed when the greatest mass is at the greatest distance from the centre of the shaft, and this means that the mass should be concentrated as far as possible at the wheel rim. Purely practical conditions decide the diameter of the wheel in individual cases, and we shall assume the diameter is given as well as the sizes of the boss and spokes, and the greatest width of the rim.

EXAMPLES.

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EX. 2.-Make working drawings to a scale of 11" l', of a cast-iron flywheel for use with the engine of Ex. B, Fig. 179. Shaft, 33" diameter; boss, 7" diameter, 7" long; six spokes, elliptical section, 33" x 2" at boss, tapering to 3′′ × 111" at rim; outside diameter of wheel 5' 6", width 8", im to have inside flanges 13" wide from inside of rim and " thick. Total weight of wheel 8 cwts.

(In drawing the wheel it will be found that the spokes meet before reaching the boss. They must be made to curve into what may be called a second boss, having a width equal to the spokes at that part, and a diameter so that a curve of about 2" radius will join two spokes and the boss. After drawing as much as is given in the question, the weight of the parts drawn must be found, and the difference between the wheel weight and their weight will be the weight of the wheel rim, which must then be designed of a given thickness, knowing its width, so as to weigh that amount. It will be noticed that until the thickness of the rim is known, the real length of the spokes is not really known from which to find their weight, but it will be sufficiently near to assume that they reach to the outside of the wheel.

The drawing of a flywheel should show a view looking on all the spokes, and a second view looking on the wheel rim, half of which should be a section through the rim and boss.)

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EX. 3. Make working drawings to a scale of 11" 1' of a fly for a gas engine. Shaft 45" diameter. Boss 91" diameter, 83" long. Six curved spokes of elliptical section 48" x 23" at boss, 33" x 21" at rim; outside diameter of wheel 5' 10", width of rim 8", rim of T section central web about 3′′ thick and 54" deep. Total weight 22 cwts.

(217) Counterbalance Weights.-Quick speed engines run much more smoothly when weights are attached to the crank webs on the opposite side to the crank pin, to balance the weight of the parts acting at the pin. It is usual to take half the weight of the connecting-rod as acting at the crank pin, and the other half as acting at the crosshead. The weight of the crank pin, the crank webs beyond the shaft, and half the connecting-rod, are then taken as rotating about the shaft centre at a radius equal to the crank length, while the weight of the piston-rod, crosshead, and the other half of the connecting-rod are supposed to act together along the centre line of the engine. The former affect the balance both in the line of the stroke and also at right angles to it, but the latter only influence the balance in the direction in which they move—that is, in the line of the stroke.