Читать книгу Railway Construction - William Hemingway Mills - Страница 7

	Goods Engine. Six wheels, all coupled, 4 ft. 6 ins. diameter. Cylinders, 17 ft. × 24 ft. Locked-down pressure on safety-valves, 140 lbs. per square inch. Assumed pressure at cylinders, 120 lbs. per square inch. Weight of engine 34 tons. ” tender 24 tons. 0000000000000 58 tons.
Assumed cut-off	¼	⅓	½	¾
” mean effective pressure, lbs.	45	56	76	100
” tractive force, lbs.	5780	7192	9760	12844
Speed in miles per hour	40	30	20	15
	Tons.	Tons.	Tons.	Tons.
Level	213	358	623	907
1 in 1000	187	310	532	768
” 800	181	299	512	739
” 600	172	285	482	695
” 400	157	257	432	621
” 300	143	233	390	560
” 250	133	216	361	519
” 200	120	195	324	467
” 150	101	165	276	397
” 100	74	123	208	302
” 90	—	113	191	279
” 80	—	101	172	253
” 75	—	95	163	240
” 70	—	88	153	226
” 60	—	74	131	196
” 50	—	—	107	163
” 40	—	—	—	127
” 25	—	—	—	67

Note.—The column loads in tons are exclusive of the weight of engine and tender.

From the above memoranda it will be seen how greatly the gradients affect the loads. For an important main trunk line, with a heavy and frequent train-service of passengers and goods, the introduction of steep gradients would not only reduce the speed of the train-working, but would probably involve the necessity of assistant engines over those parts of the line; and it may be prudent, where possible, to incur heavier earthworks, or considerable detours, or tunnels, to obtain more favourable gradients. For such a line the additional cost, and the extra distance caused by a detour of a mile or more, will be of far less importance than the interruption in the train service arising from a serious reduction in speed or taking on assistant engines. On many railways abroad there are very interesting examples of long detours of several miles, carefully studied out to obtain greater length and easier gradients, resulting in the construction of lines over which the traffic can be worked without necessitating auxiliary engine-power. On the other hand, there are situations where steep gradients cannot be avoided, where certain altitudes must be reached, and where there is no alternative but to face the inevitable.

On secondary lines, and short branch lines, where the traffic is not expected to be heavy, and where speed is not so important, it may be policy to economize outlay and introduce steeper gradients than on the main line.

Half a mile of a rather steep gradient is not felt so much when it is situate midway between two stations, because the attained speed of the train assists the engine over the short distance to the summit; but when it occurs as a rising gradient out of a station, it forms a great check to the working, particularly in bad or wet weather, when there is the risk of the engine slipping, and the entire train sliding back into the station.

Long steep gradients not only necessitate increased motive-power for the ascending trains, but also require increased brake-power, and precautionary measures for the descending trains. Where passenger trains are fitted with continuous brakes, the risk of losing control is minimized; but with goods trains composed of waggons, having only the ordinary independent side-lever brake, it will be found absolutely necessary in many cases to have additional heavy brake-vans for descending the inclines, and these special vans, unfortunately, will form so much extra non-paying weight to be hauled up on the ascending trains. Of course, it is quite possible—and, indeed, in many places it is customary—to pin down some of the side-lever brakes before commencing the descent, but once pinned down the brakes cannot be eased or taken off until the entire train is brought to a stand.

Every goods waggon should be fitted with a brake, and it would be of immense value if that brake could in all cases be applied and controlled when the train is in motion.

The American type of long goods waggon, with a four-wheel bogie-truck at each end, is fitted with a brake very similar to those adopted on the ordinary horse tram-cars. On the top of the waggon a horizontal iron hand-wheel, about 18 inches in diameter, is fixed on to a strong vertical iron rod, which works in brackets, and extends down below the underside of waggon framing. One end of a short length of chain is secured to the foot of the vertical rod, and the other end is connected by light iron rods to the series of levers which pull on the brake-blocks. By rotating the horizontal hand-wheel the chain is coiled round the lower end of the vertical rod, the brake-levers are pulled over, and brake-pressure applied to the wheels of the waggon. The brakesman is supplied with a convenient seat and footboard, and on the floor-level of the latter there is a pawl and ratchet attached to the vertical rod, which permits the brakes to be applied to the extent required. The pawl retains the brakes in position until the brakesman with his foot pushes the pawl out of the notch of the rachet and releases the brake gearing, which is at once pulled off quite clear by strong bow-strings attached to the framework of the bogies.

This type of hand-brake is, perhaps, the simplest that can be made. The brakesman has merely to put it on, the pawl and ratchet keep it on, and the bow springs take it off when no longer required. Each one of these long, loaded goods waggons becomes a very serviceable brake-van, and for ascending and descending steep inclines all that is necessary is to take on a few additional brakesmen to manage the brakes of as many suitable waggons. These incline brakesmen, after going down, can return to the summit by the next ascending train, their small weight being a mere nothing as compared with that of special or extra brake-vans.

On some European lines it is the custom to sprag some of the goods waggon wheels when going down exceptionally steep inclines, as well as applying the brakes on the ordinary and extra brake-vans. The sprag is a piece of wood, circular in section, about 2 feet 6 inches long, and 5 to 6 inches thick in the middle, tapering off to about 2 inches thick at the ends. When the waggon-wheel is just beginning to move, the sprag is inserted between the spokes, and being caught against the waggon framework, the wheel is held fast, and being unable to revolve, remains fixed, and acts like a skid upon the rails. The skidding of the wheels upon the rails wears flat places on the wheel tyres, and it is needless to mention that the practice is only resorted to in very extreme cases. Although a very primitive means for checking the speed of a descending train, or for maintaining vehicles stationary on an incline, there have been many instances where lives have been saved and accidents prevented by the prompt use of a few sprags. Solid or close wheels cannot be spragged, only wheels which have spokes or openings, and for this reason alone it would be very desirable that in every passenger and goods train there should be some spoke or open wheels which could be spragged as a last resource, in the event of a sudden emergency of brakes failing or train becoming divided on an incline.

On ascending gradients there is always the risk of a coupling breaking, and the train becoming divided. If the detached portion left behind be provided with ample brake-power, hand-brakes, or otherwise, no harm may take place beyond a little delay; but if the brake-power be insufficient or defective, and if all the wheels are solid wheels incapable of admitting a few timely sprags, then the vehicles cannot be held, but must slide back, and running unchecked would soon attain such a velocity as would cause them either to leave the rails or dash into another train standing at the last station. Many lamentable accidents have taken place arising from portions of trains breaking away and running back, and the sad experience of those casualties should call forth every effort to avert a recurrence in the future. It may not always be possible to detect a hidden flaw in a coupling, or a defect in the brake-gearing until the actual failure occurs; but it is quite possible to guard against disastrous results from such failure, by providing means to hold the vehicles, and prevent them sliding back.

For some years the writer had the entire charge of an important railway abroad on which the gradients were very exceptional, and where it was absolutely necessary that he should organize the most complete precautions to prevent the possibility of trains, or portions of trains, running back down inclines. Starting from sea-level, the line, which was laid to the 4 feet 8½ inch gauge, rose to a summit of over 8000 feet, and on the mountain division there were many long gradients of 1 in 40, 1 in 33, and in one place a continuous gradient of 1 in 25 for 12 miles. The specially powerful engines reserved for these heavy inclines were each supplied with an ordinary hand-brake, a steam-brake, and a Westinghouse continuous brake. The passenger carriages, which were of considerable length, and carried on a four-wheeled bogie-truck at each end, were all fitted up with the Westinghouse brake, and in addition each carriage had its own hand-wheel brake with the pawl and ratchet gearing. All the goods waggons, which were of the American type, were fitted with hand-wheel brakes similar to those on the carriages. Special gangs of trained brakesmen took charge of the trains on these inclines, a brakesman to every carriage or waggon, and were always in readiness in case of the breakage of a coupling, or the failure in the Westinghouse brake or brakes on engine. The immunity from accidents justified the combined precautions adopted, and proved the possibility of working such severe gradients with perfect safety.

The long-continued application of the brakes on heavy inclines naturally leads to the question as to the description of wheel to be adopted for the work. Not only are the wheels subjected to very severe torsional strains, but the temperature at the circumference is raised very high in consequence of the friction. Perhaps, theoretically, the safest wheel would be one made out of a solid piece of metal, similar to the chilled cast-iron wheels of the United States, or the steel disc wheels used on some lines in Europe, in either of which holes can be left for sprags. Wheels of this description can withstand very heavy wear and tear, they are not affected by increased temperature, and they certainly have the minimum of parts to work loose. Of the built-up wheels, the strong forged-iron-spoke wheel with steel tyres shows excellent results, and always gives due warning of loosening by indications at the tyre rivets. The suddenness with which the solid wooden centre wheels sometimes break up and fall to pieces does not commend them for a service where there must be a long-sustained application of the brakes. The increased temperature which expands the tyre, contracts the wood, and must loosen and weaken the entire wheel.

On all steep gradients the road-bed should be of the most substantial character, and the permanent way of a strong description, and maintained in perfect order, as the engines for working the traffic must necessarily be of a heavy type. The rails will be severely tested by the pounding and slipping of the engines on the ascending journey, and by the action of the brakes on the descending journey.

In the early days of the railway system, rope-haulage was adopted on some of the main lines for working the trains on steep inclines near the principal terminal stations. A powerful stationary engine, located at the highest point, was employed to work an endless rope which passed round large drums at the top and bottom of the incline, and was supported on sheaves or pulleys fixed between the rails. The connection between the carriages and endless rope was effected by means of a short piece of rope called the messenger, which was coiled round the main rope in such a manner as to be readily detached when the train reached the summit. There are many persons who will remember the time when the passenger trains were hauled by an endless rope up the 1 in 66 incline from Euston to Camden Town, a distance of about a mile and a half, and up the 1 in 48 incline from Lime Street, Liverpool, to Edge Hill, a distance of about a mile and a quarter, and several others. The rapid strides made in locomotive construction, and the increased pressure used in the boilers, enabled much more powerful engines to be built, until one by one the rope-haulage machinery has disappeared from nearly all the inclines where for years it had been considered indispensable. Rope-haulage on inclines is now very rarely met with, except at collieries and ironworks, where occasionally the rope may be seen so arranged that the loaded waggons descending pull up the empty waggons on the opposite or parallel line.

Curves.—The degree of curvature of a railway curve is generally expressed by giving the radius in feet, chains, metres, or other national standard measure.

When laying out a line of railway, the natural features of the country will necessitate the introduction of curves, and the question for consideration will be whether they are to be made of small or large radius. In some cases sharp curves are inevitable, except by incurring enormous works which would not appear to offer any corresponding prospective recompense. In others the curves may be made of easy radius, at a comparative moderate extra outlay, if the character of the line and description of traffic to be accommodated will warrant the expenditure. For main through lines, with heavy, high-speed traffic, it is advisable to have the curves of large radius, so as to avoid the necessity of reducing speed when passing round them. Although a high-class fast train may be allowed to run round an 80 chain (5280 feet) curve at almost unrestricted speed, safety demands that there should be a reduction of speed on curves of 40 or 30 chains radius, and a very much greater reduction for curves of 20 chains radius and under. A sharp curve will in some places form a greater check to fast trains than a length of moderately steep gradient on a straight line. In the former the trains running in either direction must slow down for some distance before reaching the curve, round which they should pass at greatly reduced speed, and then some distance must be run before they can attain their full speed again. On the other hand, with a rising gradient, on a fairly straight line, the acquired momentum of the train will materially assist in ascending the incline, and although the speed may be slackened as the train advances, there may not be any very great diminution in the running before the gradient is passed, and average level line reached again. A reduced rate of running must be maintained round curves of small radius, for, however substantial the works and permanent way, and however well devised and constructed the rolling-stock, there is an element of danger ever present when passing round sharp curves at anything more than moderate speed. In the great rush for fast through trains this point is very apt to be overlooked, and too little time allowed for the running. Even with the fastest trains on any line there are some portions of the route which must be traversed with greater caution and less speed than others, either on account of sharp curves or of gradients; and if those who are entrusted with the preparations of the time tables do not possess the technical information necessary to deal properly with the question of relative speeds, there is the strong probability that the programme prepared may be one both difficult and dangerous to fulfil. The spirit of rivalry is a strong incentive to fast running, but prudence and common sense should indicate that record speeds should only be attempted on the straight or favourable portions of the line. There is, unfortunately, the growing tendency to run faster and faster round the curved portion of our lines, heedless of the close approach to the limit of safety, and unless this excessive speed be controlled in time, the result must be disaster on a very large scale.

A sharp curve leading into or out of a terminal station or main-line stopping-station does not so much affect the train running as a sharp curve at an intermediate point between stations where the train may be expected to run at its maximum speed. Wherever it is possible it is very desirable to avoid sharp curves on inclines, because there are times when descending trains may acquire a considerable velocity, and wheels tightly gripped by the brakes have not the same facility for following the curves as when they are running free.

In rugged and mountainous districts sharp curves are almost unavoidable, except by introducing a series of tunnels; but in these districts both the gradients and curves are alike exceptional, the speed is necessarily slow, and special precautions are taken for the ascending and descending trains.

When setting out reverse curves on a main line a piece of straight line should always be laid in between the termination of the one curve and the beginning of the other, to allow of a proper adjustment of the rails to suit the super-elevation adopted on each of the adjoining curves. In station yards and sidings this is not so absolutely necessary, the sorting of the carriages and waggons and the marshalling of the trains being carried on at a low speed, which does not necessitate any super-elevation of the rails on the curves. The speed of the train regulates the amount of super-elevation to be given on any particular curve, and to ensure smooth and safe running this amount must be maintained uniform all round the curve. On curves of small radius, guard, or check, rails are frequently placed alongside the inner rail, as in Figs. 30 to 33, to check the tendency of the engine to leave the rails and run in a straight line. For the bull-head road a special chair is used, which holds both the running-rail and the check-rail, as shown on the sketch, the rails being kept the proper distance apart by the web portion in the centre, which forms part of the casting. For the flange railroad, check-rails are sometimes made of strong angle irons placed against the flange of the running-rail, and bolted to the transverse sleepers. This method is not nearly so strong or efficient as the arrangement shown on Fig. 33, with a cast-iron distance-block about six inches long, placed between the running-rail and check-rail, and all tied together with a strong through bolt. A bolt-hole is punched in the edge of the flange of check-rail, and a crab bolt and clip holds the two rails on the sleeper. The cast-iron distance-blocks are placed just outside the sleeper, so as not to interfere with the holding-down bolt. Doubtless these guard rails do good service, but if the leading wheels of the engine have sharp or worn flanges there is the possibility that the wheel, pressing against the high rail, may mount the rail, and throw the train off the line. A more secure method is to place the guard outside the high rail, as in Figs. 34 to 38. This can be done by securing a strong continuous longitudinal timber to the cross-sleepers—or to the cross-girders in the case of a girder bridge—with its outer or striking edge protected with a fairly heavy angle iron. The top of this outside guard above the rail level may be three inches or more, according to the height of any hanging spring, or portion of brake apparatus belonging to the rolling-stock. The distance between the striking-face of the guard and the inside of head of rail should be about 5 inches, or such width that before the flange of the wheel can mount on the top of the rail, the face of the wheel-tyre will be brought into contact with the striking-face of the outside guard, and thus effectually prevent the wheel leaving the rail. The sketches show some of the types applicable to the chair road, and to the flange railroad. In Figs. 34, 35, and 37, the outside brackets are of heavy angle iron cut off in lengths to correspond to the width of the sleeper. In Fig. 36 the cast-iron chair is lengthened, and has an end bracket to support the guard timber. In Fig. 37 a hard wood bolster is fastened on the top of each sleeper, and on this is placed the continuous guard timber. This method of increased security is frequently adopted on girder bridges and long iron viaducts which are on the straight, and in such cases it is usual to place the guards outside each of the rails forming the track.

The introduction of bogie engines and bogie carriages has conduced largely to the safe working of the train-service over the curved portions of many of our home railways, as well as to the economy in the wear and tear of permanent way and rolling-stock. The action of long rigid wheel-base vehicles passing round sharp curves is detrimental to all the parts brought into contact. Not only is there the constant tendency to mount the rails, and spread the gauge, but the tiny shreds of steel scattered all along close to the rail—particles ground off the rails, or off the wheel-tyres, or both—testify to useless wear, unnecessary friction, and great waste of motive-power.

The gradual increase of accommodation and conveniences in the carriage stock of European railways led to the gradual increase in the length of the vehicles. The six-wheeled carriage superseded the four-wheeled carriage, on account of its increased steadiness when running, but the introduction of long sleeping-cars, dining-cars, and corridor cars necessitated some better wheel arrangement than the ordinary six-wheel type could supply. The six wheels had been spread as far apart as was admissible for carrying weight and passing round curves, and something had to be done to meet the demand for still longer carriages. Many of the six-wheeled carriages at present running on our own home lines have a fixed wheel-base as long as 22 feet, and with this length the horn-plates must undergo a very considerable strain when adapting themselves for the passage round curves of small radius. On a curve of 15 chains radius (990 feet) a chord of 22 feet will have a versed sine or offset of 0·73 of an inch, and on a curve of 10 chains radius (660 feet) an offset of 1·10 of an inch. Fortunately, curves of the above small radius are not very numerous on our main lines; but wherever they do occur, the conflict between the long fixed wheel-base rolling-stock and the permanent way must be very severe to both. Several descriptions of eight-wheeled carriages have been tried on our home lines; but the system which is now most in favour is the ordinary bogie truck, which has been in use for so many years on all American railways. A bogie truck is really a short carriage frame complete in itself, with its wheels, springs, and brake appliances, and is attached to the under side of the carriage body by a central pivot, round which the truck can swivel or rotate sufficiently to adapt itself to the curved portions of the line. With a bogie truck at each end of a long carriage, the vehicle will pass as easily round curves as on the straight line, side pressure, or grinding against the rails, is obviated, and friction is reduced to a minimum. The bogie truck may consist of four wheels or six wheels, according to the length and weight of the carriage to be supported.

Figs. 39, 40, and 41 show sketch elevation, plan, and transverse section of one pattern of four-wheel bogie truck largely adopted in American carriage stock, and although there are other types varying in detail, the general principle remains the same in all. The diagram sketch (Fig. 42) represents the two bogie trucks slightly swivelled to adapt themselves to the curve round which the carriage is supposed to be passing.

For carriage or waggon stock with an independent bogie truck at each end, the central pivot and swivelling motion supply all the freedom that is requisite; but for locomotives it is necessary to provide for lateral as well as for swivelling movement. The driving and trailing wheels—and sometimes one or two other pairs of wheels—work rigidly in the frames, and as the normal position of the centre of the bogie truck must be in the centre line of the engine for the straight line, it is evident that some appliance must be introduced to allow the truck to move laterally when the engine has to traverse the curves.

Figs. 43, 44, and 45 give sketch elevation, plan, and transverse section of a swing-link bogie truck as applied to an ordinary American locomotive. Its recommendations are its simplicity, its efficiency, and its accessibility for inspection and lubrication. The swing-links, which provide for the lateral movement, are direct acting, and do not require any side springs of steel or indiarubber. All the principal parts of the bogie are visible and not mysteriously cased in with plate-iron boxwork.

In the sketches several minor details are purposely omitted and only sufficient particulars shown to explain the method of working. The under side of the upper centre plate which carries the cylinder castings and smoke-box end of boiler is cup-shaped, and fits into an annular groove or channel in the lower centre plate, which is suspended from the framework of the truck by the four swinging links. Practically the entire carrying and swivelling work of the bogie truck is effected by the annular-groove casting moving round the cup-shaped casting, and the centre pin is merely passed down through each to guard against the risk of the one lifting out of the other from sudden shock or derailment.

The lateral movement of the truck is obtained by means of the four swing-links. When the engine is on the straight road the centre line of the bogie is on the centre line of the engine, and the links hang in the positions shown on the sketch, inclined towards the centre; but upon entering a curve they come into play, and allow the truck to move out sideways to the right or left, according to the direction of the curve, the one pair of links assuming a flatter angle, while the other pair approach nearer to the vertical, the extent of side movement depending on the amount of the curvature. When the engine enters the straight line again, the bogie truck resumes its central position.

The Bissell truck consists of one pair of wheels connected to a triangular framework, as shown in Fig. 46. The axle-boxes are attached to the side of the triangle which lies parallel to the axle, the other two sides terminate in a circular ring which works round a centre pin fixed to the engine. These two sides are practically the radii of a given circle, and permit a large amount of lateral movement, which can be controlled by placing suitable stop-pieces to limit the side play to the extent desired.

Radial axle-boxes have been tried on the engines of some railways. In the best types the opposite boxes are braced together by a diaphragm, or plate-iron framework, to ensure that both boxes work together. The curved faces of the horn-blocks, in which the radial axle-boxes slide, are struck from a centre taken at some point to the rear of the normal centre line of the axle, and stops are placed at proper distances to control the extent of lateral movement. Although the advocates of radial axle-boxes may urge some points in their favour, there are few engineers, if any, amongst those who have had practical experience of both systems, who would for a moment claim for the radial axle-box anything but a modicum of the many advantages obtained by the four-wheeled bogie truck.

As one of the principal functions of a four-wheeled bogie truck for an engine is to act as a path-finder, or guide, to the other wheels which constitute the fixed or rigid wheel-base portion of the machine, it follows, therefore, that the full benefit of the bogie truck can only be obtained when it is placed at the leading, or front, end of the engine. In this position the bogie, with its swivelling arrangement and smaller weights, is the first to pass over the rails, and in doing so shapes the course and prepares the way for the easy running of the heavier wheel weights which have to follow. When the bogie truck is placed at the rear end of the engine, its action is restricted to affording lateral movement only, and the driving and coupled wheels have to force or pound themselves round the curves in a jerky, irregular manner, as compared to their smooth running when following the leading or guiding influence of a bogie truck in front.

The wheel-base of a four-wheeled bogie truck for an engine should always be greater than the gauge of the line over which the bogie has to travel On the 4 feet 8½ inch gauge some of the best results have been obtained with bogies having wheel-bases varying from 6 feet to 7 feet. Where the wheel-centres have been less than 6 feet, the running has been found to be much less steady than with the wider spacing; and where the wheel-base is not more than the gauge, there is a tendency for the bogie to catch, or lock, when passing round sharp curves.

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Estimate of the Proposed (Railway).

Line No.

Length of Line:

Miles. F. Chs.

Whether single or double.

Cubic yards. Price per yard.

Earthworks:

Cuttings—Rock Soft soil Roads Total

Embankments, including roads, __ cubic yards

Bridges, public roads—number

Accommodation bridges and works

Viaducts

Culverts and drains

Metallings of roads and level crossings

Gatekeepers’ houses at level crossings

Permanent way, including fencing:

Miles. F. Chs. at

Cost per mile.

Permanent way for sidings, and cost of junctions

Stations

Contingencies __ per cent.

Land and buildings

Total £

Dated this day of 18__

Witness:

Engineer.