Read Ebook: How it Works Dealing in simple language with steam electricity light heat sound hydraulics optics etc. and with their applications to apparatus in common use by Williams Archibald

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rank is said to be at a "dead point;" for, were the crank turned to such a position, the admission of steam would not produce motion, since the thrust or pull would be entirely absorbed by the bearings.

DOUBLE-CYLINDER ENGINES.

THE FUNCTION OF THE FLY-WHEEL.

THE CYLINDER.

SLIDE-VALVE,

The use of a crank appears to be so obvious a method of producing this conversion that it is interesting to learn that, when James Watt produced his "rotative engine" in 1780 he was unable to use the crank because it had already been patented by one Matthew Wasborough. Watt was not easily daunted, however, and within a twelvemonth had himself patented five other devices for obtaining rotatory motion from a piston rod. Before passing on, it may be mentioned that Watt was the father of the modern--that is, the high-pressure--steam-engine; and that, owing to the imperfection of the existing machinery, the difficulties he had to overcome were enormous. On one occasion he congratulated himself because one of his steam-cylinders was only three-eighths of an inch out of truth in the bore. Nowadays a good firm would reject a cylinder 1/500 of an inch out of truth; and in small petrol-engines 1/5000 of an inch is sometimes the greatest "limit of error" allowed.

THE ECCENTRIC

The eccentric is in principle nothing more than a crank pin so exaggerated as to be larger than the shaft of the crank. Its convenience lies in the fact that it may be mounted at any point on a shaft, whereas a crank can be situated at an end only, if it is not actually a V-shaped bend in the shaft itself--in which case its position is of course permanent.

SETTING OF THE SLIDE-VALVE AND ECCENTRIC.

The subject of valve-setting is so extensive that a full exposition might weary the reader, even if space permitted its inclusion. But inasmuch as the effectiveness of a reciprocating engine depends largely on the nature and arrangement of the valves, we will glance at some of the more elementary principles.

"LAP" OF THE VALVE--EXPANSION OF STEAM.

HOW THE CUT-OFF IS MANAGED.

LIMIT OF EXPANSIVE WORKING.

COMPOUND ENGINES.

ARRANGEMENT OF EXPANSION ENGINES.

COMPOUND LOCOMOTIVES.

REVERSING GEARS.

The engines of a locomotive or steamship must be reversible--that is, when steam is admitted to the cylinders, the engineer must be able to so direct it through the steam-ways that the cranks may turn in the desired direction. The commonest form of reversing device is known as Stephenson's Link Gear. In Fig. 30 we have a diagrammatic presentment of this gear. E^1 and E^2 are two eccentrics set square with the crank at opposite ends of a diameter. Their rods are connected to the ends of a link, L, which can be raised and lowered by means of levers . B is a block which can partly revolve on a pin projecting from the valve rod, working through a guide, G. In Fig. 31 the link is half raised, or in "mid-gear," as drivers say. Eccentric E^1 has pushed the lower end of the link fully back; E^2 has pulled it fully forward; and since any movement of the one eccentric is counterbalanced by the opposite movement of the other, rotation of the eccentrics would not cause the valve to move at all, and no steam could be admitted to the cylinder.

"LINKING UP."

OTHER GEARS.

Before the introduction of Stephenson's gear a single eccentric was used for each cylinder, and to reverse the engine this eccentric had to be loose on the axle. "A lever and gear worked by a treadle on the footplate controlled the position of the eccentrics. When starting the engine, the driver put the eccentrics out of gear by the treadle; then, by means of a lever he raised the small-ends of the eccentric rods, and, noting the position of the cranks, or, if more convenient, the balance weight in the wheels, he, by means of another handle, moved the valves to open the necessary ports to steam and worked them by hand until the engine was moving; then, with the treadle, he threw the eccentrics over to engage the studs, at the same time dropping the small-ends of the rods to engage pins upon the valve spindles, so that they continued to keep up the movement of the valve." One would imagine that in modern shunting yards such a device would somewhat delay operations!

PISTON VALVES.

In marine engines, and on many locomotives and some stationary engines, the D-valve is replaced by a piston valve, or circular valve, working up and down in a tubular seating. It may best be described as a rod carrying two pistons which correspond to the faces of a D-valve. Instead of rectangular ports there are openings in the tube in which the piston valve moves, communicating with the steam-ways into the cylinder and with the exhaust pipe. In the case of the D-valve the pressure above it is much greater than that below, and considerable friction arises if the rubbing faces are not kept well lubricated. The piston valve gets over this difficulty, since such steam as may leak past it presses on its circumference at all points equally.

SPEED GOVERNORS.

Practically all engines except locomotives and those known as "donkey-engines"--used on cranes--are fitted with some device for keeping the rotatory speed of the crank constant within very narrow limits. Perhaps you have seen a pair of balls moving round on a seating over the boiler of a threshing-engine. They form part of the "governor," or speed-controller, shown in principle in Fig. 33. A belt driven by a pulley on the crank shaft turns a small pulley, P, at the foot of the governor. This transmits motion through two bevel-wheels, G, to a vertical shaft, from the top of which hang two heavy balls on links, K K. Two more links, L L, connect the balls with a weight, W, which has a deep groove cut round it at the bottom. When the shaft revolves, the balls fly outwards by centrifugal force, and as their velocity increases the quadrilateral figure contained by the four links expands laterally and shortens vertically. The angles between K K and L L become less and less obtuse, and the weight W is drawn upwards, bringing with it the fork C of the rod A, which has ends engaging with the groove. As C rises, the other end of the rod is depressed, and the rod B depresses rod O, which is attached to the spindle operating a sort of shutter in the steam-pipe. Consequently the supply of steam is throttled more and more as the speed increases, until it has been so reduced that the engine slows, and the balls fall, opening the valve again. Fig. 34 shows the valve fully closed. This form of governor was invented by James Watt. A spring is often used instead of a weight, and the governor is arranged horizontally so that it may be driven direct from the crank shaft without the intervention of bevel gearing.

MARINE GOVERNORS.

These must be more quick-acting than those used on engines provided with fly-wheels, which prevent very sudden variations of speed. The screw is light in proportion to the engine power, and when it is suddenly raised from the water by the pitching of the vessel, the engine would race till the screw took the water again, unless some regulating mechanism were provided. Many types of marine governors have been tried. The most successful seems to be one in which water is being constantly forced by a pump driven off the engine shaft into a cylinder controlling a throttle-valve in the main steam-pipe. The water escapes through a leak, which is adjustable. As long as the speed of the engine is normal, the water escapes from the cylinder as fast as it is pumped in, and no movement of the piston results; but when the screw begins to race, the pump overcomes the leak, and the piston is driven out, causing a throttling of the steam supply.

CONDENSERS.

Fig. 35 is a sectional illustration of a marine condenser. Steam enters the condenser through the large pipe E, and passes among a number of very thin copper tubes, through which sea-water is kept circulating by a pump. The path of the water is shown by the featherless arrows. It comes from the pump through pipe A into the lower part of a large cap covering one end of the condenser and divided transversely by a diaphragm, D. Passing through the pipes, it reaches the cap attached to the other end, and flows back through the upper tubes to the outlet C. This arrangement ensures that, as the steam condenses, it shall meet colder and colder tubes, and finally be turned to water, which passes to the well through the outlet F. In some condensers the positions of steam and water are reversed, steam going through the tubes outside which cold water circulates.

The bores of the cylinders are in the proportion of 4: 6: 9. The stroke of all three is the same.

The ends furthest from the eccentric.

"The Locomotive of To-day," p. 87.

THE STEAM TURBINE.

How a turbine works--The De Laval turbine--The Parsons turbine--Description of the Parsons turbine--The expansive action of steam in a Parsons turbine--Balancing the thrust--Advantages of the marine turbine.

More than two thousand years ago Hero of Alexandria produced the first apparatus to which the name of steam-engine could rightly be given. Its principle was practically the same as that of the revolving jet used to sprinkle lawns during dry weather, steam being used in the place of water. From the top of a closed cauldron rose two vertical pipes, which at their upper ends had short, right-angle bends. Between them was hung a hollow globe, pivoted on two short tubes projecting from its sides into the upright tubes. Two little L-shaped pipes projected from opposite sides of the globe, at the ends of a diameter, in a plane perpendicular to the axis. On fire being applied to the cauldron, steam was generated. It passed up through the upright, through the pivots, and into the globe, from which it escaped by the two L-shaped nozzles, causing rapid revolution of the ball. In short, the first steam-engine was a turbine. Curiously enough, we have reverted to this primitive type in the most modern engineering practice.

HOW A TURBINE WORKS.

THE DE LAVAL TURBINE.

In its earliest form this turbine was a modification of Hero's. The wheel was merely a pipe bent in S form, attached at its centre to a hollow vertical shaft supplied with steam through a stuffing-box at one extremity. The steam blew out tangentially from the ends of the S, causing the shaft to revolve rapidly and work the machinery mounted on it. This motor proved very suitable for dairy work, but was too wasteful of steam to be useful where high power was needed.

The normal speed of the turbine wheel is too high for direct driving of some machinery, so it is reduced by means of gearing. To dynamos, pumps, and air-fans it is often coupled direct.

THE PARSONS TURBINE.

DESCRIPTION OF THE PARSONS TURBINE.

The essential parts of a Parsons turbine are:-- The shaft, on which is mounted the drum; the cylindrical casing inside which the drum revolves; the vanes on the drum and casing; the balance pistons. Fig. 37 shows a diagrammatic turbine in section. The drum, it will be noticed, increases its diameter in three stages, D^1, D^2, D^3, towards the right. From end to end it is studded with little vanes, M M, set in parallel rings small distances apart. Each vane has a curved section , the hollow side facing towards the left. The vanes stick out from the drum like short spokes, and their outer ends almost touch the casing. To the latter are attached equally-spaced rings of fixed vanes, F F, pointing inwards towards the drum, and occupying the intervals between the rings of moving vanes. Their concave sides also face towards the left, but, as seen in Fig. 38, their line of curve lies the reverse way to that of M M. Steam enters the casing at A, and at once rushes through the vanes towards the outlet at B. It meets the first row of fixed vanes, and has its path so deflected that it strikes the ring of moving vanes at the most effective angle, and pushes them round. It then has its direction changed by the ring of F F, so that it may treat the next row of M M in a similar fashion.

THE EXPANSIVE ACTION OF STEAM IN A TURBINE.

On reaching the end of D^1 it enters the second, or intermediate, set of vanes. The drum here is of a greater diameter, and the blades are longer and set somewhat farther apart, to give a freer passage to the now partly expanded steam, which has lost pressure but gained velocity. The process of movement is repeated through this stage; and again in D^3, the low-pressure drum. The steam then escapes to the condenser through B, having by this time expanded very many times; and it is found advisable, for reasons explained in connection with compound steam-engines, to have a separate turbine in an independent casing for the extreme stages of expansion.

BALANCING OF THRUST.

The push exerted by the steam on the blades not only turns the drum, but presses it in the direction in which the steam flows. This end thrust is counterbalanced by means of the "dummy" pistons, P^1, P^2, P^3. Each dummy consists of a number of discs revolving between rings projecting from the casing, the distance between discs and rings being so small that but little steam can pass. In the high-pressure compartment the steam pushes P^1 to the left with the same pressure as it pushes the blades of D^1 to the right. After completing the first stage it fills the passage C, which communicates with the second piston, P^2, and the pressure on that piston negatives the thrust on D^2. Similarly, the passage E causes the steam to press equally on P^3 and the vanes of D^3. So that the bearings in which the shaft revolves have but little thrust to take. This form of compensation is necessary in marine as well as in stationary turbines. In the former the dummy pistons are so proportioned that the forward thrust given by them and the screw combined is almost equal to the thrust aft of the moving vanes.

ADVANTAGES OF THE MARINE TURBINE.

Absence of vibration. Reciprocating engines, however well balanced, cause a shaking of the whole ship which is very unpleasant to passengers. The turbine, on the other hand, being almost perfectly balanced, runs so smoothly at the highest speeds that, if the hand be laid on the covering, it is sometimes almost impossible to tell whether the machinery is in motion. As a consequence of this smooth running there is little noise in the engine-room--a pleasant contrast to the deafening roar of reciprocating engines. Turbines occupy less room. They are more easily tended. They require fewer repairs, since the rubbing surfaces are very small as compared to those of reciprocating engines. They are more economical at high speeds. It must be remembered that a turbine is essentially meant for high speeds. If run slowly, the steam will escape through the many passages without doing much work.

Owing to its construction, a turbine cannot be reversed like a cylinder engine. It therefore becomes necessary to fit special astern turbines to one or more of the screw shafts, for use when the ship has to be stopped or moved astern. Under ordinary conditions these turbines revolve idly in their cases.

Even at this speed the wheel has a circumferential velocity of two-thirds that of a bullet shot from a Lee-Metford rifle. A vane weighing only 250 grains exerts under these conditions a centrifugal pull of 15 cwt. on the wheel!

THE INTERNAL-COMBUSTION ENGINE.

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