Mechanical Drawing Self-Taught Part 17

[Ill.u.s.tration: Fig. 303.]

[Ill.u.s.tration: Fig. 305.]

The main valve, whose movement alternately opens the ports for the admission of steam to, and the escape of steam from, the main cylinder, is divided into two parts, one of which, C, slides upon a seat on the main cylinder, and at the same time affords a seat for the other part, D, which slides upon the upper face of C. As shown in the engravings, D is at the left-hand end of its stroke, and C at the opposite or right-hand end of its stroke. Steam from the steam-chest, J, is therefore entering the right-hand end of the main cylinder through the ports E and H, and the exhaust is escaping through the ports H 1, E 1, K and M, which causes the main piston A to move from right to left. When this _piston_ has nearly reached the left-hand end of its cylinder, the tappet arm, T, attached to the piston-rod, comes in contact with, and moves the valve rod collar O 1 and valve rod P, and thus causes C, together with the supplemental valves R and S S 1, which form, with C, _one casting_, to be moved from right to left. This movement causes steam to be admitted to the left-hand end of the supplemental cylinder, whereby its piston B will be forced towards the right, carrying D to the opposite or right-hand end of its stroke; for the movement of S closes N (the steam-port leading to the right-hand end), and the movement of S 1 opens N 1 (the steam-port leading to the opposite or left-hand end), at the same time the movement of V opens the right-hand end of this cylinder to the exhaust, through the exhaust ports X and Z. The parts C and D now have positions opposite to those shown in the engravings, and steam is therefore entering the main cylinder through the ports E 1 and H 1, and escaping through the ports H, E, K and M, which causes the main piston A to move in the opposite direction, or from left to right, and operations similar to those already described will follow, when the piston approaches the right-hand end of its cylinder. By this simple arrangement the pump is rendered positive in its action; that is, it will instantly start and continue working the moment steam is admitted to the steam-chest, while at the same time the piston is enabled to move as slowly as the nature of the duty may require. It will be noted that in Figure 305, the ports of C are shown through D, whose location is marked by dark shading. This obviously is not correct, because D being above C should be shaded lighter than C, and again the ports E 1 and K could not show dark through the port D. They might, of course, be shown by dotted outlines, but they would not appear to such advantage, and on this account it is permissible where artistic effect is sought, the object being to subserve the shading to making the mechanism and its operation clearly and readily understood.

[Ill.u.s.tration: Fig. 306.]

Figure 307 affords another excellent example for shading. It consists of an independent condenser, whose steam-cylinder and valve mechanism is the same as that described with reference to Figures 305 and 306.

[Ill.u.s.tration: Fig. 307. (Page 288.)]

[Ill.u.s.tration: Fig. 308. (Page 289.)]

[Ill.u.s.tration: Fig. 309. (Page 289.)]

[Ill.u.s.tration: Fig. 310--SECTION OF CYLINDER AND STEAM CHEST. (Page 289.)]

CHAPTER XV.

_EXAMPLES IN ENGINE WORK._

In the figures from 308 to 328 inclusive are given three examples in engine work, all these drawings being from _The American Machinist_.

Figures 308 to 314 represent drawings of an automatic high speed engine designed and made by Professor John E. and William A. Sweet, of Syracuse, New York. Figure 308 is a side and 309 an end view of the engine. Upon a bed-plate is bolted two straight frames, between which, at their upper ends, the cylinder is secured by bolts. The guides for the cross-head are bolted to the frame, which enables them to be readily removed to be replaned when necessary. The hand wheel and rod to the right are to operate the stop-c.o.c.k for turning on and off the steam to the steam-chest.

The objects of the design are as follows: Figure 310 is a vertical section of the cylinder through the valve face, also showing the valve in section, and it will be seen that the lower steam pa.s.sage enters the cylinder its full depth below the inside bottom, and that the whole inside bottom surface of the cylinder slopes or inclines towards the entrance of this pa.s.sage. The object of this is to overcome the difficulty experienced from the acc.u.mulation of water in the cylinder, which, in the vertical engine, is usually a source of considerable annoyance and frequently the cause of accident.

Any water that may be present in the bottom finds its way by gravity to the port steam entrance, and is forced out by and with the exhaust steam at or before the commencement of the return stroke.

To a.s.sist in the escape of water from the top of the cylinder, the piston is made quite crowning at that end, the effect of which is to collect the water in a narrow band, instead of spreading it over a large surface. This materially a.s.sists in its escape, and at the same time presents a large surface for the distribution of any water that may not find its way out in advance of the piston.

The piston is a single casting unusually long and light, and is packed with four spring rings of 3/8 inch square bra.s.s wire.

The valve is a simple rectangular plate, working between the valve face and a cover plate, the cover plate being held in its proper position, relative to the back of the valve, by steam pressure against its outer surface, and by resting against loose distance pieces between its inner surface and the valve seat. This construction admits of the valve leaving the seat, if necessary, to relieve the cylinder from water, as in the instance of priming, and also, by the reduction of these pieces, admits of ready adjustment to contact, should it become necessary.

[Ill.u.s.tration: Fig. 311--VALVE MOTION.]

The cover plate is provided with recesses on its inner surface which exactly correspond with the ports in the valve face, and the corresponding ports and recesses are kept in communication with each other by means of relief pa.s.sages in the valve. From this it will be seen that the valve is subjected to equal and balanced pressure on each of its sides, and hence, is in equilibrium.

The valve is operated through the valve motion, shown in Figure 311, the eccentric rod of which hooks on a slightly tapered block that turns on the pin of the rock arm, like an ordinary journal box.

The expansion, or cut-off, is automatically regulated by the operation of the governor in swinging the slotted eccentric in a manner substantially equivalent to moving it across the shaft, but is however favorably modified by the arrangement of the rock arm, which, in combination with the other motions, neutralizes the unfavorable operation of the usual shifting eccentric, and which, in connection with the large double port opening, provides for a good use of steam from 0 to 3/4 stroke.

The governor shown in Figure 312 is of the disc and single ball type, the centrifugal force of the ball being counteracted by a powerful spring. Friction is reduced to a minimum in the governor connection, by introducing steel rollers and hardened steel plates in such a manner as to provide rolling instead of sliding motion.

In order that a governor shall correctly perform its functions, it is unquestionably necessary that it have power largely in excess of the work required of it, and also that the friction shall represent a very low percentage of that power. In respect to this, especial means have been employed to reduce the friction; the valve being balanced, requires but little power to move it, while the governor ball being made heavy for the purpose of counterbalancing the weight of the eccentric and strap, its centrifugal force when the engine is at full speed is enormous, the spring to counteract it having to sustain from _two to three thousand pounds_. Under these circ.u.mstances, as might be expected, the regulation is remarkably good. This is a very important consideration in an engine working under the conditions of a roll-train engine.

[Ill.u.s.tration: Fig. 312--GOVERNOR.]

[Ill.u.s.tration: Fig. 313--SECTION OF PILLOW BLOCK.]

Figure 313 represents a section of the pillow block box, crank-pin and wheel, together with the main journal. It will be seen that the end of the box next the crank wheel has a circular groove around its outside, and that a corresponding groove in the crank wheel projects over this groove. From this latter groove an oil hole of liberal size extends, as shown, to the surface of the crank-pin. Any oil placed at the upper part of the groove on the box finds its way by gravity into the groove in the crank wheel, and is carried by centrifugal force to the outside surface of the crank-pin; so that whatever other means of lubrication may be employed, this one will always be positive in its action. This cut also shows the manner in which the box overlaps the main journal and forms the oil reservoir.

[Ill.u.s.tration: Fig. 314--CONNECTING ROD. (Page 295.)]

Another feature in the construction of this box is the means by which it is made to adjust itself in line with the shaft. It will be observed that it rests on the bottom of the jaws of the frame on two inclined surfaces, which form equal angles with the axis of the shaft when in its normal position, and that by moving longitudinally in either direction, as may be necessary, the box will accommodate itself to a change in the alignment of the shaft. In order that it may be free to move for this purpose it is not fitted with the usual fore and aft f.l.a.n.g.es. By this means any slight derangement, as in either the outboard or inboard bearing wearing down the fastest, is taken care of, the movement of the box on the inclined surfaces being for this purpose equivalent to the operation of a ball and socket bearing.

Figure 314 gives a side and an edge view of the connecting rod, the rod being in section in the edge view, and the bra.s.ses in section lined in both views.

The cross-head pin, it will be observed, is tapered, and is drawn home in the cross-head by a bolt; the sides of the pin are flattened somewhat where the journal is, so that the pin may not wear oval, as it is apt to do, because of the pull and thrust strain of the rod bra.s.ses falling mainly upon the top and bottom of the journal, where the most wear therefore takes place. The bra.s.ses at the crossed end are set up by a wedge adjustable by means of the screw bolts shown. The cross-head wrist pin being removable from the cross-head enables the upper end of the rod to have a solid end, since it can be pa.s.sed into place in the crossed and the wrist pin inserted through the two. The lower ends of the connecting-rod and the crank-pin possess a peculiar feature, inasmuch as by enlarging the diameter of the crank-pin, the ends of the bra.s.ses overlap, to a certain extent, the ends of the journal, thus holding the oil and affording increased lubrication. The segments that partly envelop the cross-head pin and crank-pin, and are section lined in two directions, producing crossing section lines, or small squares, show that the bra.s.ses are lined with babbitt metal, which is represented by this kind of cross-hatching. These drawings are sufficiently open and clear to form very good examples to copy and to trace on tracing paper.

[Ill.u.s.tration: Fig. 315.]

[Ill.u.s.tration: Fig. 316. (Page 296.)]

[Ill.u.s.tration: Fig. 317.]

Figures 315, 316 and 317 represent, in place upon its setting, a 200 horse-power horizontal steam-boiler for a stationary engine, and are the design of William H. Hoffman. The cross-sectional view of the boiler sh.e.l.l in Figure 315 shows the arrangement of the tubes, which, having clear or un.o.bstructed pa.s.sages between the vertical rows of tubes, permits the steam to rise freely and a.s.sists the circulation of the water. The dry pipe (which is also shown in Figure 316) is a perforated pipe through which the steam pa.s.ses to the engine cylinder, its object being to carry off the steam as dry as possible; that is to say, without its carrying away with the steam any entrained water that may be held in suspension. Figure 316 is a side elevation with the setting shown in section, and Figure 317 is an end view of the boiler and setting at the furnace end. The boiler is supported on each side by channel iron columns, these being riveted to the boiler sh.e.l.l angle pieces which rest upon the columns. The heat and products of combustion pa.s.s from the furnace along the bottom of the boiler, and at the end pa.s.s into and through the tubes and thence over the top of the boiler to the chimney flue. There is shown in the bridge wall an opening, and its service is to admit air to the gases after they have pa.s.sed the bridge wall, and thus complete the combustion of such gases as may have remained unconsumed in the furnace. The cleansing door at one end and that lined with asbestos at the other, are to admit the pa.s.sage of the tube cleaners. The asbestos at the top of the boiler sh.e.l.l is to protect it from any undue rise in temperature, steam being a poorer conductor of heat than water, and it being obvious that if one side of the boiler is hotter than the other it expands more from the heat and becomes longer, causing the boiler to bend, which strains and weakens it. The sides of the setting are composed of a double row of brick walls with an air s.p.a.ce of three inches between them, the object being to prevent as far as possible the radiation of heat from the walls. The brick-staves are simply stays to hold the brick work together and prevent its cracking, as it is apt, in the absence of staying, to do.

[Ill.u.s.tration: Fig. 318. (Page 299.)]

Figures from 318 to 330 are working drawings of a 100-horse engine, designed also by William H. Hoffman.

Figure 318 represents a plan and a side view of the bed-plate with the main bearing and the guide bars in place. The cylinder is bolted at the stuffing box end to the bed-plate, and is supported at the outer end by an expansion link pivoted to the bed-plate. The main bearing is provided with a screw for adjusting the height of the bottom piece of the bearing, and thus taking up the wear. The guide bars are held to the bed in the middle as well as at each end.

Figures 319 and 320 represent cross sections of the bed-plate.

[Ill.u.s.tration: Fig. 319--CROSS SECTION OF BED PLATE NEAR JUNCTION WITH CYLINDER. (Page 299.)]

[Ill.u.s.tration: Fig. 320.]

[Ill.u.s.tration: Fig. 321--100 H.P. HORIZONTAL STEAM-ENGINE--ELEVATION OF CYLINDER--SCALE 1-1/2" = 1 FOOT. (Page 299.)]

[Ill.u.s.tration: Fig. 322--100 H.P. HORIZONTAL STEAM-ENGINE--END VIEW OF CYLINDER--SCALE 1-1/2" = 1 FOOT. (Page 299.)]

Figure 321 represents a side elevation of the cylinder, and Figure 322 an end view of the same, the expansion support being for the purpose of permitting the cylinder to expand and contract under variations of temperature without acting to bend the bed-plate, while at the same time the cylinder is supported at both ends. The cylinder and cylinder covers are jacketted with live steam in the steam-s.p.a.ces shown.

[Ill.u.s.tration: Fig. 323--100 H.P. ENGINE--OUTSIDE VIEW OF CYLINDER AND STEAM-CHEST. (Page 301.)]

[Ill.u.s.tration: Fig. 324--SECTIONAL VIEW OF CYLINDER AND VALVES--SCALE 1-1/2 INCHES = 1 FOOT. (Page 301.)]

[Ill.u.s.tration: Fig. 325--PLAN OF CUT-OFF DEVICE. (Page 301.)]

[Ill.u.s.tration: Fig. 326--WORKING DRAWING OF 100 H.P. ENGINE--DETAILS OF MAIN VALVE MOTION--SCALE 3" = 1 FOOT. (Page 301.)]