LECTURE XXIV.-QUESTIONS. *

1. What is surface condensation? Distinguish between a surface condenser and a jet condenser. Describe the method of carrying out each system of condensation, making any sketches you think necessary.

2. Sketch a section through a surface condenser, showing the passage of the water and steam through it. By what means is the flow of water kept up? (S. and A. Exam., 1887.)

3. Mention some of the advantages of a surface condenser as applied to marine engines, and draw in section a surface condenser, showing the mode in which water is caused to circulate through it. How are the tubes fitted so as to avoid leakage? How is the vacuum ascertained?

4. A surface condenser consists of 1000 brass tubes, each 6 feet long and inch outside diameter. What amount of cooling surface does this give? Supposing that such a surface condenser is to be fitted to an engine, what pumps, valves, &c., would be required, and how would you arrange them in order to put the apparatus into working order? Ans. 1178 5 sq. ft.

5. A surface condenser has 1725 tubes, each 6 feet long and inch outside diameter, what amount of condensing surface do they give? Write down two numbers which express pretty nearly the relative conducting powers of copper and iron. How are the condenser tubes usually fitted and kept tight? Ans. 2202 sq. ft.

6. A surface condenser has 900 tubes, " internal diameter," thick, and 6' 6" long. Find the cooling surface in square feet. Ans. 1149 sq. ft. 7. A boiler has a heating surface of 1230 square feet. The cooling surface of the condenser is required to be 90 per cent. of the heating surface of the boiler. How many tubes " external diameter and 9′ 4′′ long will be required? Ans. 604.

8. If the thickness of the tubes in the last question be inch, find the weight of the tubes and the weight of water to fill them. (Here refer to Table at the beginning of Lecture III.) Ans. 1388 lbs. ; 751 lbs.

9. Sketch a section through a single-acting vertical air pump, showing the construction of the bucket and also the position and construction of the valves.

10. Sketch a section through the air pump and condenser of a condensing engine; name the several valves and essential parts, and point out their uses. Show the construction of the valves. (S. and A. Exam., 1888.) II. Sketch a section through a double-acting circulating pump for marine engines, and describe its action and give a complete index to parts. 12. Marine engines are fitted with a so-called air pump, circulating pump, feed pump, and bilge pump. What are the respective uses of these pumps? Show, with a sketch, the construction of any one of them, and explain how it acts. (S. and A. Exam., 1889.)

13. Sketch a section through a vertical air-pump, together with the foot and delivery valves. The bucket plunger has a valve in it, describe the construction of the valve and the packing of the plunger, and show by your drawing the method of attaching the pump rod to the bucket. Which valves are open during the descent of the bucket? (S. & A. Exam. 1890.) 14. Sketch a sectional elevation of a double-acting air-pump with foot and delivery valves, such as is suitable for a direct-acting engine. Describe clearly the construction of an india-rubber disc valve. (S. & A. Exam. 1891.)

See Appendix for more recent S. & A. Questions.

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LECTURE XXV.

CONTENTS.-Crank Shafts-Thrust Shaft, Block, and Bearing -Intermediate Screw Shafts-End Screw Propeller Shaft-Stern TubeOrdinary Form of Screw Propeller.

Crank Shafts.-As explained in Lecture XIV., the crank of an ordinary steam engine enables the reciprocating to-and-fro motion of the piston in the cylinder, to be converted into the circular motion of the crank shaft through the intervention of the piston rod and connecting rod. In the case of land engines employed to drive in factories and workshops, the power developed in the cylinder and transmitted to the crank shaft is usually conveyed to the shafting by means either of a spur wheel keyed to the crank shaft, or by a flat pulley with belting, or by a WW grooved fly wheel and rope drive. In marine paddle engines the crank shaft is invariably connected direct to the paddles, and in screw engines through intermediate shafting to the screw. In locomotives, the crank shaft is connected direct to the driving wheel. In the case of engines for driving dynamos, the power may be transmitted by pulley and belting, or the crank shaft may be coupled direct to the dynamo spindle, when high-speed engines are employed. Whichever plan be adopted, the main object aimed at is to transmit the power developed in the cylinder as direct and undiminished as possible to the machinery.

Crank shafts are subjected to bending and twisting stresses, and must be made sufficiently strong to withstand these, as well as stiff enough not to spring and heat the bearings. Any remarks in an elementary treatise like this must be confined to merely describing the different forms of crank shafts, leaving an investigation of their dimensions to our more advanced courses of Steam and Applied Mechanics.

Single-cylinder land engines have but one crank, which is fixed either at the end of the shaft, as shown by the first figure, or with two crank webs and outer and inner bearings, as shown by the second figure.

Locomotive crank shafts for outside-cylinder engines simply consist of plain shafts, keyed firmly into the driving wheels at each end -the crank pins being fixed into solid bosses in the wheel at the

BESSEMER STEEL CRANK SHAFTS FOR LAND ENGINES.

proper radius from the crank-shaft centre for the piston's stroke. Inside-cylinder engines are fitted with cranks of the shape shown by the following figure, and the crank webs are generally fitted with weldless-steel or iron hoops, shrunk on to them, so as to strengthen them, and to prevent their coming to pieces and doing damage, should they crack or break through the neck.

Small launch engines or fast-speed electric-light engines are often fitted with very strong crank webs, as shown by the following figure.

CRANK SHAFT FOR LAUNCH ENGINE.

Marine paddle-wheel engines have their cranks usually arranged as shown by the following figure, taken from Seaton's Marine Engineering.

CRANK SHAFT FOR PADDLE-WHEEL MARINE ENGINE.

Marine screw engines are usually fitted with two interchangeable crank shafts. Up to 12 inches in diameter, the crank and the shaft are generally forged or compressed in one piece, but above that size they are frequently made as shown in the following figure. Where lightness combined with strength is desired, they are made hollow. (See p. 29 for figure of crank shaft for s.s. City of Rome.)

Specification for s.s. "St. Rognvald's" Crank Shaft.-Crank shaft to be built of best forged mild steel, made in two pieces, interchangeable and reversible, bolted together with bolts 2 inches diameter. Crank webs to be 10 inches thick, and carefully shrunk on shafts and crank pins, and keyed upon shafts with steel keys 1 inch deep, 24 inches broad. Crank

pin bearings to be 14 inches long, and 13 inches diameter; and crank-shaft bearings 18 inches long, and 13 inches diameter. The whole of the bearing parts to be turned perfectly true, after being fitted together, and the finished shaft to be carefully fitted into its bearings.

Thrust Bearings and Shaft.—In marine screw engines it is necessary to provide some means of taking up the longitudinal thrust of the screw propeller. In small engines this is often done by a simple collar on the shaft, which bears against the flange of the aft main bush in the sole plate; but when the thrust is great, this plan is not sufficient, and a "thrust block" is fitted. The form of thrust block which was chiefly in use until recently is shown in the following figure. An enlarged portion of the screw shaft next to the crank shaft has a number of rings cut in it, and these rings fit into corresponding recesses in a large brass bush. These recesses are formed either by having rectangular grooves bored out of the brass or by the insertion of brass rings into checks bored out to receive them, as shown in the figure. This

COMMON SMALL THRUST BLOCK.

arrangement has been found to work very well for small sizes, and when effectually prevented from heating. When it heats it gives great trouble, and adjustment cannot very well be effected at sea. A much better form of thrust block, T B, is shown in the next figure. In this block, the thrust is supported by independent horseshoe shaped pieces of cast iron, H S, which are faced with white metal. These thrust pieces fit between rings, C, to C,, turned on the screw shaft. They are secured on each side of the shaft to the thrust block, but are capable of independent adjustment by means of the nuts and set screws, A S, on each side, or, of adjustment as a whole, by the nuts at each end of the rods which support them at the sides. This block has some advantages over the old form, the principal of which are-(1) The horse-shoe pieces are separate and independent of each other, and they may be adjusted, or taken out separately for examination, without stopping the engines. (2) The lubrication is more easily effected, since the hollow casing