Traction (Lat. trahere, to draw), the act of drawing or hauling. As used in this article the term refers to the methods of employing animal and mechanical power for transporting persons or things from place to place in wheeled vehicles.
The oldest form of motive power is that of animals, those most commonly employed for draught purposes on ordinary roads being horses, mules, donkeys and oxen. On the continent of Europe dogs are often harnessed to light carts or barrows, but in England their use in this way was prohibited by the Cruelty to Animals Act of 1854. Camels and elephants are only rarely used as draught animals in special circumstances.
L.
V.
- .
3 boo
LV.
LVT.
lb.
Feet
per
second.
Hours
per
day.
lb. con-
veyed
1 ft.
lb. conveyed
1 ft.
MAN
Walking unloaded, transport of own I
weight S
140
5o
10
700
25,200,000
Do. do
140
6o
to
840
30,240,000
Wheeling load L in two-wheeled barrow,
returning empty; V = i velocity. j
22 4
1 6
to
373
13,428,000
Do. one-wheeled barrow, do.. .
135
1 6
to
225
8,100,000
Travelling with burden
2.5
225
5,670,000
Conveying burden, returning unloaded
140
1.6
233
5,032,800
252
Carrying burden for 30 seconds only .
126
rr
_
14742
-
23.1
Walking with cart always loaded. .
1500
3.6
to
5400
594,400,000
Trotting do. do.
750
7'2
41
5400
87,480,000
Walking with cart, going loaded, re-
turning empty; V = } mean velocity
r 00
5
2.0
10
3000
108,000,000
Carrying burden, walking. .. .
270
3.6
to
972
.34,992,000
Do. trotting. .. .
180
1296
32,659,200
When men and animals carry burdens, or draw or propel loads in certain vehicles, it is difficult, and sometimes impossible, to determine the duty performed in foot-pounds of work, because of the uncertainty of the amount in pounds of the resistance overcome. In this case, for the purpose of comparing performances of the same kind with each other, a unit is employed called a foot-pound of horizontal transport, meaning the conveying of a load of lb i ft. horizontally. The following table, given by W. Macquorn Rankine, gives some examples of the daily duty of men and horses in units of horizontal transport, L denoting the load in lb, V the velocity in feet per second, and T the number of seconds per day of working: - For tramway service, horse, or occasionally mule, traction was formerly employed almost universally, but on account of limited speed and high cost it has been generally abandoned, except in a few localities, where the smallness of the line, low value of livestock, labour and feed, and long headway intervals, make it still profitable.
The tractive force required on a straight and level tramway is found to vary from v to of the load, according to the condition of the rails. On a tramway having grooved rails in average condition it is about - 5. The resistance is thus, at the best, nearly double that on a railway, and sometimes as much as on a good paved road. This is due to the friction of the flange of the wheel in the grooved rail, and to the fact that the latter is always more or less clogged with dirt. The clearance between the flange and the groove is necessarily small, as the former must have sufficient strength, and the latter must be narrow. The least inaccuracy of gauge, therefore, causes extra friction, which is greatly increased on curves. By removing the flanges from two of the four wheels of the tramway car H. E. Tresca (1814-1885) found that the resistance was reduced from T toto t s of the load. The resistance due to gravity is of course not lessened on a tramway; and if au of the load be the tractive force required on the level, twice as much, or D - of the load, will be required on a gradient of i in r oo and three times as much on a gradient of in 50. To start a tramcar, four or five times as great a pull is required as will keep it in motion afterwards, and the constant starting after stoppages, especially on inclines, is destructive to horses. Horses employed on tramways are worked only a few hours a day, a day's work being a journey of io or 12 m., and much less on steep gradients. In London a tramcar horse bought at the age of five years had to be sold at a low price after about four years' work. On the Edinburgh tramways, in consequence of steep gradients, the horses lasted a less time, and had to be constantly shifted from steep to easier gradients. The cost of traction by horses is generally 6d. or 7d. per mile for two horses, and more when the gradients are steep (see also Tramway).
The most universally used form of motive power is the steam engine, which has been constructed to work on ordinary roads, on tramways and on railways. The road or traction engine comprises a boiler mounted on wheels, and a steam engine usually placed on top of the boiler. The front axle is pivoted so that it may be moved by means of a steering wheel geared to it, and the rear wheels are geared to the engine. The wheel rims are made wide to prevent them from sinking in loose earth or muddy roads. The whole arrangement is similar to the ordinary wheeled portable boiler and engine with the addition of the steering wheel and a gear connexion from the engine to the rear wheels. The tractive power of these engines is high, but their speed low - usually 4 to 6 m. per hour.
A peculiar form of road motor is made by equipping the axles of a traction engine with the so-called " Pedrail " invented by B. J. Diplock. This is an arrangement whereby circular pads or " feet," fastened around the periphery of a wheel, come successively in contact with the ground, the motion approximating to a smooth, even stepping or walking along. Fourteen of these feet are placed around a wheel, and each is attached at the end of a spoke, free to slide radially toward and from the hub of the wheel. Each spoke has fastened to it a helical spring which tends to draw it inwards. On each spoke there is also a roller, which bears against a cam-shaped piece placed inside the periphery of the wheel. The engine is suspended by springs from the cam and is supported by it. The lower edge of the cam is practically straight and horizontal, the length of this straight portion being great enough to subtend an angle equal to the spacing of three spokes, or about 70°. By this means three of the feet are always resting on the roadway and support the engine, which really slides along on the rollers that are at any instant underneath the flat portion of the cam. The feet take successive positions on the ground as the movement of the engine proceeds, and the engine itself rolls along on the rail portion of the cam which rests on the rollers beneath it. Ball and socket joints are used to connect the feet to the spokes so that they may rest on any conformation they may encounter. This machine has shown a remarkable ability to pass over obstacles and rough roads, and even to climb roadless hills. It gives a maximum of adhesion of the drivers, and it is claimed that it will pass over rough roads with the expenditure of less energy than will an ordinary wheeled traction engine. Its speed is necessarily low - about 4 m. per hour.
FIG. 2. - Chain Track Tractor.
The Hornsby " Chain Track Tractor " (fig. 2), patented by Mr David Roberts, is provided with two endless chains, one on each side, which constitute the track on which the machine travels. Each chain is carried on two sprocket wheels, placed at the extreme FIG. 3. - Links of Chain Track.
ends of the frame, and is formed of a number of links (fig. 3) so connected that it is free to bend in one direction, as required to pass round the sprocket wheels, but is locked into a rigid bar by pressure acting in the opposite direction. On their outer surfaces these links bear pads or feet, while their inner surfaces compose a track upon which roll the middle or weight-bearing wheels. Power applied to one of the sprocket wheels exerts a pull on the chain, but this being held fast by the weight of the engine pressing the feet to the ground, the effect is to roll the engine along the track, and as this happens the feet at the rear end are one by one lifted off the ground, carried round the sprocket wheels, and relaid at the front of the machine. This construction not only renders the whole weight available for adhesion, but also provides a long supporting base and thus enables the machine to pass over soft ground, loose sand, morasses, &c., in which an ordinary traction engine would certainly sink. Steering is effected by retarding or stopping the motion of the sprocket wheels on the side towards which it is desired to turn.
For tramway work steam is scarcely used at all now, though small locomotives - usually having their engines geared to the drivingwheels, instead of the connecting-rods being coupled direct to them - have been used in the past for this work. They were compactly designed and equipped with mufflers to deaden the sound of the exhaust, with other devices to decrease noise and smoke. In some instances, the engine and boiler were placed in the forward end of a car, a partition separating them from the main body of the car in which the passengers were carried.
For description of steam railway engines see Railways: Locomotive Power, and Steam Engine.
Fireless engines were first tried in New Orleans, and were in successful use on tramways in France and (From The Scientific American.) Position of the parts in overcoming Position of the parts on a level obstacles. road.
FIG. i. - Principle of the Pedrail's operation.
Batavia, Java, for some years. The' motive power was obtained from water heated under pressure to a very high temperature in stationary boilers and carried in a reservoir on the engine, where it gave off steam as the pressure and temperature were reduced. Two tons of water heated to give a steam-pressure of 250 lb to the square inch served for a run of 8 or 10 m., more than 1 9 0 of the water and a pressure of 20 to 25 lb above the atmosphere being left on returning to the boiler station. Large boiler-power was required to reheat the engine reservoirs quickly, and this could be afforded for only a few engines; but, when worked on a sufficient scale, the fireless engines were claimed to be economical, the economy resulting from the generation of the steam in large stationary boilers.
Compressed Air. - Compressed air as a motive power offers the advantage of having neither steam nor the products of combustion to be got rid of. In W. D. Scott Moncrieff's engine, which was tried on the Vale of Clyde tramways in 1876, air was compressed to 310 lb per sq. in., and expanded in the cylinders from a uniform working pressure to that of the atmosphere. There is a considerable loss of heat during the expansion of the air which is attended with a serious loss of pressure, and in L. Mekarski's system, which was in use for the propulsion of tramcars at Nantes for a number of years, the loss of pressure was considerably lessened by heating the air during expansion. The air, at a pressure of 426 lb per sq. in., was stored in cylindrical reservoirs beneath the car, and before use was passed through a vessel three-quarters full of water heated to 300° F., by which it was heated and mixed with steam. The heat of the latter was absorbed by the air during its expansion, first to a working pressure which could be regulated by the driver, and then to atmospheric pressure in the cylinders. At Nantes the average cost for three years of propelling a car holding thirtyfour persons was about 6d. per mile. Owing to the heat losses in compressing the air, and other considerable losses incident to its use, the compressed-air systems of traction have been found inefficient and have nearly all been replaced by the more flexible and efficient electric motor.
Moving steel cables, propelled by steam engines, have been used for traction. The street railway cars running from New York to Brooklyn, over the Brooklyn Bridge, were for many years propelled by a cable to or from which the cars could be attached or detached at will, and, though electric motors are now used on this line, the cables are still kept in place as a reserve in case of breakdown of the electrical system, and are used whenever an accident to the electrical plant occurs. Before the advent of electric traction, the tramways using cable propulsion were numerous and of great size, as at San Francisco, Chicago, Washington, Baltimore, Philadelphia and New York in America, at Highgate Hill (London) and Edinburgh in the United Kingdom, and at Melbourne in Australia. The Glasgow Subway is so equipped.
In the usual form, the motive power is transmitted from a stationary engine by a rope of steel wire running always in one direction, up one track and down the other, in a tube ?- =L midway between the rails, on pulleys (fig. 4) which are arranged so as to suit curves and changes of gradient as well as straight and level lines. Over the rope is a slot 4 in. wide, in which travels a flat arm of steel connecting the dummy car with the gripper (fig. 5) which grasps the cable. The flat arm is in three pieces, the two outer ones constituting a frame which carries the lower FIG. 4. - Cross-section Jaw of the gripperith grooved rollers at of Road. each e w end of it, over Which the cable runs when the gripper is not in action. The upper jaw is carried by the middle piece which slides within the outer frame, and can be depressed by a lever or screw, pressing the cable first on the rollers and then on the lower jaw until it is firmly held. The speed of the cable, which is generally 8 to io m. an hour, is thus imparted to the car gradually and without jerk. The arrangements for passing the pulleys, for changing the dummy and cars from one line to the other at the end of the road, for keeping the cable uniformly taut, and for crossings and junctions with other lines, are of considerable ingenuity. When the cars are cast off from the cable they must be stopped by hand brakes which, on steep gradients especially, must be of great power.
Explosive engines using gasolene (petrol) have been used for motive power, and this is the principal form employed in the road motor car. Certain railways in England and America have experimented with cars having a gasolene engine placed in one end to propel the car, the greater part of which is left clear for the accommodation of passengers. These cars are intended for short runs and may in effect be classed as belonging to extended tramway service. They have yielded encouraging results.
Electric traction, as treated here, will refer to the operation of vehicles for the transportation of passengers and goods upon tracks, as distinguished from what are known as telpherage systems on the one hand (see Conveyors), and automobiles intended to run on common roads on the other (see Motor Vehicles).
Possibly the first electric motor was that made by the Abbe Salvatore dal Negro in Italy in 1830. As early as 1835, Thomas Davenport, a blacksmith of Brandon, Vermont, U.S.A., constructed and exhibited an automobile electric car, operated by batteries carried upon it. Robert Davidson, of Aberdeen, Scotland, began experimenting about 1838 with the electric motor as a means of traction, and constructed a very powerful engine, weighing five tons and carrying a battery of forty cells. This locomotive made several successful trips on Scottish railways, but was finally wrecked by jealous employes of the railway while it was lying in the car sheds at Perth. In 1840 a provisional patent was granted in England to Henry Pinkus, which described a method of supplying electric energy to a moving train from fixed conductors. A little later, in 1845, French and Austrian patents granted to Major Alexander Bessolo described. practically what is to-day the third-rail system. In 1847 Professor Moses G. Farmer, of Maine, U.S.A., built a model. locomotive operated by electricity, which he exhibited at Dover, New Hampshire, and later at other places in New England.. Shortly afterwards Professor C. G. Page, of the Smithsonian Institution in Washington, constructed an electric railway motor, which made a trip on the 29th of April 1851, from Washington, D.C., to Bladensburg, Maryland, over the Baltimore & Ohio railway. This machine carried ioo Grove's cells, and attained speeds as high as 19 m. an hour. Perhaps the beginning of modern electric traction may be said to date from 1879, when the firm of Siemens & Halske put in operation the first electric railway at the Industrial Exposition in Berlin. In America it was not until a year later that real work began and T. A. Edison built an experimental line near his laboratory in Menlo Park, New Jersey. In 1880 a locomotive driven by -accumulators was constructed and operated at a linen-bleaching establishment at Breuil-en-Auge, in France; and in 1881 a similar car was worked upon the Vincennes tramway line. On the 12th of May 1881 the first commercial electric railway for regular service was opened for operations at Lichterfelde, in Germany. The FIG, 5. - Gripper.
first really noteworthy road was that constructed in 1883 at the Giant's Causeway at Portrush, in the north of Ireland. This line was 6 m. long, and the power was obtained from turbine wheels actuated by a cascade on the river Rush. The method of supply was, curiously enough, the third rail.
In 1883 invention in electric railways seems to have taken a decided advance in America. It was in this year that the conflicting interests of Edison and S. D. Field were consolidated; and at the same time C. J. van Depoele and Leo Daft began their experimental work, which later resulted in numerous commercial railways. Next year E. H. Bentley and Walter Knight opened to the public in Cleveland, Ohio, U.S.A., a railway operated by an open-slot conduit, and for the first time worked in competition with horse traction on regular street railway lines. For the next two years much experimental work was done, but it may be said with fairness that the first of the thoroughly modern .systems, in which a large railway was equipped and operated under service conditions by electricity, was the line built in Richmond, Virginia, U.S.A., by Frank J. Sprague in 1887. This railway had 13 m. of track, and started with an equipment of forty cars. It has been in continuous and successful commercial operation ever since. The original Richmond system was in all its essential particulars the overhead trolley system now in use. Many improvements have been made in the construction of the motors, the controllers, the trolleys and the various details of car equipment and overhead construction, but the broad principles have not been departed from. The success of the Richmond line called the attention of tramway managers to the advantages of electricity as a motive power, and its substitution for other systems progressed with astonishing rapidity.
The pioneer application of electricity to heavy electric traction was that of the Baltimore & Ohio railway tunnel at Baltimore, Md., U.S.A., and the system was put into operation in 1895. This tunnel is about 12 m. in length and passes under the 'city of Baltimore. Its route made the expense of ventilation prohibitive, and the smoke and gases from the locomotives made the use of the tunnel impossible without ventilation. The management therefore decided to attempt the use of electric locomotives to haul the trains through, despite the fact that there existed no prior applications of heavy electric motors for even far lighter service than that demanded by the conditions, namely, the propulsion of trains of over 2000 tons up a grade of 42 ft. to the mile. The engineering work and designing of the locomotives were undertaken by Dr Louis Duncan. The locomotives weigh 96 tons and have worked successfully since they were first put into commission. The electric service has been extended 6 m. from the mouth of the tunnel, making a total haul of nearly 8 m. for these locomotives. In 1907 many heavy electric locomotives using continuous current were constructed for the New York Central & Hudson River Railroad Company to operate a distance of about 5 m. from the New York terminus, and others for practically the same service, but using single-phase alternating currents, were put in for the New York, New Haven & Hartford Railroad Company.
It has been fully demonstrated that electricity is superior to its competitors - horses and moving cables - for tramway work. It is cheaper and more flexible. The relative cost of operation varies with the local conditions, but a fair average estimate would be that cable lines cost 25% more to operate than electric, and horse lines l00% more. The increased speed of the electric cars and the comfort rendered possible by larger vehicles always increase the receipts when horse traction is replaced by electric, while the latter, as compared with the cable, allows better and easier control of the car and a much greater possible speed variation. The installatipn of an overhead electric line costs less than a cable system, though the expense of a conduit electric line is about the same. By the extension of the urban tramway systems into the suburbs and the construction of inter-urban lines, electricity has come into competition with steam. Here the conditions are different. For ordinary suburban service, the electric cars, running through the city streets and on the highways, cannot, in speed, compete with steam trains operated on private rights of way. The fact that they run more frequently and can take up passengers anywhere along the line gives them an advantage, and within limited distances they have taken a large proportion of suburban traffic from steam railways. For long-distance service, in order to compete with steam a speed much greater than that used on ordinary tram-lines must be adopted, while owing to the time spent on the car more attention must be paid to the comfort of the passenger. Speed and comfort being equal, the great advantage of electricity is that, when it is used, the most economical way of transporting a given number of passengers between two points is in a larger number of small trains; with steam the converse is true. A frequent service is a great attraction to passengers.
For freight service, especially on railways having heavy grades, electricity also possesses many advantages, due principally to the peculiarity of the electric locomotive, which admits of its maintaining its tractive effort or so-called " draw-bar pull " when running at relatively high speeds. This steam locomotives cannot do. Thus a steam locomotive weighing loo tons may exert a draw-bar pull of say 45,000 lb at a speed of 6 m. per hour, while at 15 m. per hour the continuous draw-bar pull will not exceed about 25,000 lb. On the other hand, an electric locomotive weighing 75 tons and having a tractive effort of 34,000 lb at 6 m. per hour will exert a pull of about 27,000 lb at 25 m. per hour. From this it is clear that an electric locomotive may pull a heavier train at a fair speed than can a larger steam locomotive. This admits of more rapid movement of freight trains, and thus decreases the hauling cost. Another advantage the electric system has for freight service is the ability to couple several light locomotives in tandem, all under the control of one driver, and thus pull at a high speed larger trains than may now be drawn by steam locomotives of weights commercially admissible. Also, these lighter motors distribute the weight over the track instead of having it concentrated on a few wheels, and the heavy pounding due to the latter condition is obviated and the maintenance of the track and bridges reduced. Other savings arise from diminished fuel consumption, elimination of water and coal stations with their attendants, and greatly reduced repairs on motive power. The chief disadvantage is the stoppage of all trains on a section if the source of current supply should fail. With proper precautions in design and construction this should be a remote possibility, and since electric rail haulage, in any form attempted up to the present, has shown a reduced cost for a given service as compared with steam traction, it is not improbable that the future will witness great activity in the change from steam to electricity for long-distance railway work.
Systems of electric traction may be divided broadly into two classes, the one employing continuous, the other alternating currents to drive the motors. Both of these classes may be further divided with reference to the conducting system employed between the source of current and the motor. The system may also be divided according to operative units into three classes - the single car, the train pulled by one or more directly controlled locomotives or motor cars, and the train operated by two or more motor cars under a common secondary control. This last is called the " multiple unit system." Continuous-Current Systems. - The applications of continuous current to electric traction comprise six principal varieties, with numerous modifications and combinations. In all of them the motors are operated under a constant, or approximately constant, potential difference. The system in which cars were connected in series by automatic switches, in limited use in the United States in 1888 and 1889, has now disappeared, and the parallel system of connexion, in which the cars are bridged across between the two conductors of a parallel system, maintained at a substantially constant voltage, has become practically universal.
The overhead conductor and track-return construction is the standard for street railway work in most of the cities where electric traction is employed, though there are Construc- some notable exceptions. In its present development the system may be said to have grown out of the work of Sprague in Richmond in 1887. Over the track is suspended a bare wire, generally of hard-drawn copper, known as the trolley wire. The normal practice is to use a wire not less than 0.325 of an inch in diameter to assure permanence, since smaller wires wear out rapidly from the friction of the trolley and the burning of the surfaces of contact. The wire is usually of circular cross-section. Sometimes wires of other sections have been used, notably one having a crosssection similar to the figure 8, but the advantage of these forms is problematical, while the difficulty attending their proper installation is considerable. In some cases the working-conductor, or trolley wire, is suspended at one side of the track, connexion with it being made by a side-bearing trolley, but its usual place is directly over the track, as this arrangement leads to simpler and more efficient construction of the trolleys and their accessory parts. For certain special cases, where very large currents are employed, the overhead conductor is made of bar metal or structural shapes. In the Boston (Massachusetts) subway, where the traffic is very heavy, a bar of rectangular section is used, supported at frequent intervals from the roof. In the Baltimore & Ohio railway tunnel at Baltimore, Md., the steel working-conductor originally consisted of two Z bars forming a trough, the current being collected by an iron shoe, but this form has been replaced by a sectional third rail. But whatever the nature of the conductor, it is usually insufficient to carry the current necessary for the operation of the system without excessive loss. Recourse is therefore had to feeders or reinforcing conductors. These may be of any form, but are most frequently copper wires or cables of large section, connected at intervals of a few hundred feet to the working-conductor. They are sometimes carried on poles, but municipal ordinances frequently require their installation in underground conduits. In general, it is customary to divide the working-conductor into sections of from woo to 5000 ft. in length, insulated from one another and fed separately through manual or automatic cut-out switches, so that an accident causing a short-circuit or break in continuity on one section will not impair the operation of others.
In ordinary street railway construction two methods of suspending the trolley wire are in vogue. 'fhe most usual construction is to hang it from insulators attached to transverse wires running between pairs of poles set on opposite sides of the track. Bracket arms attached to poles are often used, especially on suburban lines; they are frequently double, or T-shaped, and placed between the two tracks of a double-track line. In the standard construction for either variety of suspension, the insulators are bell-shaped, and composed of some hard moulded or vitreous material. The trolley wire is supported by a clamp about 9 in. long, which embraces about three-quarters of its circumference. This clamp is usually made of bronze, and is now generally fastened to the trolley wire by a screw, causing the two parts of the clamp to close upon the wire as would the jaws of a vice, or is automatic, clamping the wire the more tightly as the strain upon it increases. It was formerly considered expedient to solder the wire into the clamp, but this practice is now generally abandoned. The insulating bell is so designed that its material is subjected only to compression stresses by the weight of the wire. It is provided at its upper part with a single catch for attachment to the transverse wire or to the bracket arm. If a span wire is used it is fastened to the poles, there being turnbuckles to tighten it, while a strain insulator on either side gives a double insulation between the trolley wire and the poles. With a bracket construction it was formerly the custom to attach the insulator directly to the bracket arm, but the blow of the trolley wheel broke great numbers of insulators, and it has therefore become the practice to adopt some more flexible method of attachment, a number of different forms being in use. The poles between which the span wires are stretched, or to which the bracket arms are attached, are of wood. or iron. They are firmly set in the ground, usually with concrete.
Another form of overhead construction for high speed service, brought out by the Westinghouse Company and known as the Catenary " Catenary " system (fig. 6), is designed to hold the Construc- contact or trolley wire in a horizontal position above Lion. the track without any dip or sag. Essentially it is FIG. 6. - Single Catenary Line.
made up of a supporting cable made of stranded galvanized steel wire i'in. in diameter which is firmly fastened to brackets attached to the supporting poles and from which the trolley wire is suspended by means of rigid iron hangers spaced about 10 ft. apart. A propersag is given the supporting cable, and the lengths of the hangers vary so that the trolley wire is held horizontal without sag. The construction resembles a single supporting cable and suspended chord of a suspension bridge. The trolley wire, the hangers and the suspension cable are all mechanically connected together and in metallic contact, so that the whole system acts as a conductor. The supporting cable is held by insulators at the points where it is supported on the brackets at the poles. For heavy work there the currents taken by the passing cars and locomotives are great, requiring a very large trolley wire, two supporting cables are strung from pole. to pole and the trolley wire suspended below and between the two..
FIG. 7. - Double Catenary Line.
In this case the hangers are triangular in form and hung with the. apex of the triangle downward. The two upper angles are fastened to the pair of supporting cables, while to the Iower angle is attached' the trolley wire. This arrangement is called the " double catenary construction (fig. 7).
In order to provide a proper return path for the current, the track: must be made electrically continuous. This is accomplished by bonding the individual lengths of rail together in some Rail way, or by actually welding them together to form Bonding.. a continuous length. There are many types of railbonding. In most of them holes are drilled in the ends of adjacent rails, and a copper conductor inserted between them, its ends being in some way forced against the walls of the holes. In one type the bond is in the form of a hollow cylinder, the ends of which are inserted in the holes in the rails, a tapered steel pin being driven in so as to, expand the cylinder out against the rail. In another form the end of the bond is a solid cylinder, which is upset by hydraulic pressure, forcing it against the rail. A semi-plastic amalgam of mercury has been used to give a contact between the adjacent rails and the fish-plate connecting them. The most usual practice is to use a short bond covered and protected by the fish-plates. Tracks used for a return circuit are cross-bonded at intervals. If the track return has too great an electrical resistance it is reinforced by conductors connected to it at intervals and extending back to the power-house. Neglect to provide a proper return circuit has often caused a great loss of energy, and, in many places, excessive electrolytic action on iron pipes, cable sheaths and other metallic bodies buried in the earth. The lightning arresters provided on overhead lines are placed on the poles at intervals determined by the location of the line.
In a few places the municipal authorities, in order to avoid the disturbances on telephone lines due to the fluctuation of a trolley current, and the electrolysis of gas and water pipes which Double may arise from a grounded return, have required the Trolley.. erection of a double overhead system. In this each track has two trolley wires forming a complete metallic circuit. The largest system of this kind is in Cincinnati, Ohio, U.S.A., where there are over 225 m. of tram-lines. The system has the advantages to which it owes its existence, but the multiplicity of wires at crossings, right-angle turnouts and switches is so complicated that automatic switching cannot be attempted. The man in charge of the car removes the double trolley from the wires at such points, and replaces it when they are passed. The construction adopted, except in respect to the points mentioned, is practically similar to that already described for the track-return system.
A number of patents have been granted in various countries for electric traction systems in which one or both of the fixed conductors are installed in a conduit underground, communication being had with them by means of an open slot, into which projects a current-taking device of some nature carried by the car as it moves along. A system of this character was installed at Blackpool, England, in 1885, and later one was very successfully operated in Budapest. The first large and important Open-slot Conduit. installation of this character to be made was in Washington, D.C., U.S.A., where a considerable system of street railways was changed from horse operation to this new method. The success of this system, and of experiments made on Lenox Avenue, in New York City, led to the construction of many miles of railways of the conduit type in the latter city. It is also used extensively in London. (For details of the construction of the conduits, see Tramway.) This system is much more expensive to install than the overhead trolley system, but experience has shown that it can be as economically operated. Most of the troubles that have occurred have been due to lack of experience, but on the whole they have not been more serious than those experienced with overhead systems.
The great expense of the open conduit has led numerous inventors to bring out systems of operating electric railways by means of closed conduits or sectional third rails, in which the Closed . working-conductor is laid on the surface of the ground between the rails, and is connected with the source of current only as the car passes over each section. In this way the immediate section or portion of the working-conductor under the, car is electrically active, but other sections are not, and all danger to the passage of street traffic is removed. Up to 1900, nearly one thousand patents for this type of street railway construction, known also as the " surface contact " system, had been granted by the United States patent office alone. So far the system has been introduced in but few places, but its performance has been more than promising, and it is thought that it will be more extensively adopted in the future. Among the more important railways at present equipped with it may be mentioned one in Paris, using the Diatto system, and one at Monte Carlo, where the Westinghouse system is installed. In both these the current is supplied by means of " buttons " or metallic disks laid flush with the surface of the street between the tracks, and connected through switches to a working-conductor. Under the car is installed a currenttaking device in the shape of a long runner or skate, which runs over the buttons and is appropriately connected with a storage battery on the car, so that when it touches one of the buttons current is sent from the battery through a system of electro-magnets operating the switches which connect that particular button to the feeding system, and thus the runners are enabled to receive current for the operation of the motors on the car. The various systems differ in the method of connecting the contact rail or button with the live conductors; in some a magnet on the car works a mechanism to make the desired contact, in others a current from batteries on the car actuates a switch located near the track. (See Tramway.) The third-rail system, which is a development of the overhead trolley and track-return system, has been applied to several large i rd-rail and important railway installations, especially in the Th . Un i ted States, and in the prolongation of the Orleans System railway in Paris from the Place Valhubert to the new station at the Quai d'Orsay. Its name almost sufficiently indicates its method of operation. A rail similar to the track-rails is laid upon insulators and forms the working-conductor. On the elevated railways in New York, Brooklyn, Boston and Chicago and the subway in New York, a pressure of about 600 volts is used between this rail and the track-rails which form the return circuit. Contact is made with the third rail by means of a bronze or cast-iron shoe, either resting upon the rail by its own weight, or pressed down upon it by springs. This is generally attached to some part of the truck of the car in preference to any part of the body of the car, so as to avoid any vibration or swaying due to the movement of the body upon its springs. The third-rail system has been adopted in many instances where large and powerful trains are to be operated on private rights of way, but it is nowhere in use for electric traction upon highways or in streets where there is any passing of foot passengers or vehicles. An excellent example of such construction may be found in the Albany & Hudson railroad, which connects the city of Albany with the city of Hudson, in New York state. Here the length of the road is about 32 m., the track being of standard gauge and laid with a 60-lb T-rail. A T-rail of the same size, raised about I ft. above the level of the running-rails, is used for the electrical conductor, and is installed on insulators situated 5 ft. apart on the ends of the cross-ties. All these rails are well bonded with copper bonds at the joints. At road crossings, which on this railroad are at grade, the third rail is omitted for a distance nearly equal to the length of a train. Appropriate cast-iron shoes, fixed to the trucks of the front and rear cars of a train, bridge the space, so that the forward shoes are running on the rail past the break before the rear shoes leave it. Upon this railroad motors of considerable size and power are used, and both passengers and freight in their original cars, as received from connecting steam railways, are transported. Other examples of third-rail construction occur in the extension of the Baltimore & Ohio railway tunnel at Baltimore, the New York Central Railway Company's New York terminal, the underground systems of the City & South London railway, the Waterloo && City railway, and the Central London railway in London, and the Versailles division of the Western railway of France. In some cases, as on the Metropolitan, the District, and several of the " tube " railways in London, the running-rails are not used for the return circuit, which is completed by a fourth rail similar to the conductor rail, laid outside the track.
One of the oldest forms of electric traction is by accumulators. In brief, its principle is that storage batteries, or accumulators, are carried on the car, which becomes a veritable automobile. - It has been the usual practice to instal about 80 cells, giving a pressure of 160 to 175 volts at the motors; these are recharged after the car has run about 25 m. In general, the accumulators are not charged in place, but the car is supplied with a new set, fully charged, at the end of a run of about the length mentioned. The system has been installed in a very large number of places in Europe and America, but has never shown the gratifying commercial success which the direct-conduction systems exhibit, on account of the high cost and depreciation of storage batteries. In some places, notably in Hanover, Germany, where legislative ordinances have forbidden the overhead conducting system in city streets, a combination has been used whereby accumulator cars run in the city districts from the energy stored in their batteries, and in the suburbs operate directly as overhead trolley cars, the batteries being charged at the same time from the overhead system.
Alternating Current Systems. - Alternating current systems are now being used, both single-phase and three-phase. In the former case the newly-developed single-phase motors, later to l be described, are employed, while with three-phase systems induction motors are used. The polyphase current is much used as a means of distributing energy from a central powerstation over extended lines of railways, but is generally converted into direct current through the agency of rotary converters, and fed to the lines as such. There are, however, a few railways working directly with induction motors upon a three-phase system of supply. Prominent among these may be mentioned the Valtellina railway in Italy and the Jungfrau railway in Switzerland. Upon these lines the rails are used as one of the three conductors, and two trolley wires are suspended above the track. The locomotive is provided with two trolleys, one running upon each wire, and consists simply of an induction motor coupled through appropriate gearing to the mechanism of the truck. For starting a large resistance is introduced into the rotor or secondary circuit of the motors by means of collecting rings placed on its shaft, upon which bear brushes. This resistance is cut out as the speed increases, until it is all withdrawn and the rotor is short-circuited, when full speed is attained. It has been found that potential differences of about 500 volts in each phase can be safely handled, and it is claimed that the few railways which use polyphase currents have shown gratifying results in practice.
In the early years of the 20th century single-phase alternating current motors for electric traction were developed, and single phase systems were extensively installed both in Europe S i ngle- and in America. The simplest type of single-phase p hase. motor is a series motor provided with the usual commu tator and brushes, in which the current passes through both the field coils and the armature coils. The armature and field windings being traversed by the same current, the reversal of the field magnetization and that of the direction of current flow in the armature are coincident, so that the turning effort or torque, on the armature current produced by the interaction of armature and field magnetization is always in the same direction. Since the alternating current passes through both members of the motor, the armature and field cores are both laminated. In the later types of these motors the field coils are distributed and embedded in the field ring, so that the inner surface of the field ring presents a practically smooth surface to the armature. Troubles were at first experienced with commutation of the heavy alternating currents required for the operation of these motors, vicious sparking taking place at the brushes. This was overcome by the use of auxiliary or " compensating " coils, which are embedded in the field magnet ring, being placed between successive magnet coils. These compensating coils are usually connected in series with the main armature and field circuit. They may each, however, have their two ends joined together, (short-circuited), the currents in them being induced by the alternating magnetic flux of the fields.
Motors of the above types have the general characteristics of direct current series motors, and possess the same general relations between speed and torque that are such an important element in the success of direct current series motors. The efficiency of alternating current motors is not quite so good as that of direct current motors, on account of the rapid reversal of the iron magnetization in the field magnets, but their efficiency is high and their performance in practical work has been excellent (fig. 8).
There is another type of single-phase motor that has been used in Europe, but not in America, which is commonly called the repulsion motor. In these motors the armature is not directly included in the main circuit, but opposite points on the commutator are connected together through brushes. The working current is fed to the field magnets, and the rapid reversals of magnetization induce currents in the armature coils, which currents, working with the field magnetization, cause rotation. Several types of repulsion motors have been developed, and in general their characteristics are similar to those of the plain series type. They have not, however, come into extended commercial use. Single-phase motors for a given power are much larger, heavier and more expensive than the ordinary direct current motors, owing to the low magnetic densities at which the iron is worked. The power factor is between o75 and o85.
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The advantages of the single-phase alternating system lie in the fact that it combines the simplicity of the overhead direct current construction with the possibility of exceedingly high voltage. Where heavy traffic is to be handled, and especially where that traffic is scattered, a direct current system, which up to the present has been limited in its voltage, is not commercially possible, as the amount of copper used for distribution and the excessive amount of apparatus required to convert high tension alternating current into low tension direct current, would make the cost prohibitive. In direct current FIG. 9.
systems for lines of any length, it is the custom to use alternating current of high potential and to reduce it to direct current of low potential at different points along the line. This involves rotary converters, which by their nature require attendance in the substations, while if the traffic is scattered so that the load on the sub stations may at times be zero, and at other times may be very large, the capacity of the sub-stations must be equal to handling a maximum load, so that the total capacity of each sub-station would be based on the maximum instead of on an average condition. With the single-phase alternating current system, on the contrary, only static transformers in sub-stations along the line are required, and with the high voltages available (voltages as high as 11,000 volts are used at present) the distances between these sub-stations can be greatly increased as compared with the direct current sub-stations,. so that each sub-station feeding a much longer portion of the line would have a better average load than in the direct current case. The static transformers do not require attendance, and their efficiency is much higher than that of the rotary converters.
Electric motors for traction purposes have been highly elaborated and developed. At first they drove the car axles through belts or sprocket chains, the motor being sometimes attached to the car, sometimes to the truck. At Richmond, however, in 1887, the Sprague method of communicating the power from the motor axle to the car axle was put into practical operation, and this has with slight modifications been retained. It consists of sleeving one end of the motor on the axle, suspending the other FIG. I o. - Standard Railway Motor.
flexibly from the car body or truck, and driving from the armature through spur gearing. At first the motors were too small for the work demanded of them. Their high speed required a double reduction in gearing, their overheating caused continual burn-outs, and the sparking at the commutators necessitated constant repairs. These defects were gradually eliminated. The motors were made larger, the quality of the iron and insulation was greatly improved, and finally a four-pole motor requiring only a single-speed reduction by spur-gearing was produced. Since that time further improvements in material and design have been introduced, and the present motor has been evolved. Almost all the standard modern traction. motors are of the same general design. They are series wound, i.e. the same current passes through both the armature and the fields. This gives a strong starting torque or tractive effort, the torque diminishing as the speed increases. This characteristic is particularly suitable for traction. Fig. 9 shows the relation between speed and tractive effort of a standard railway motor of large size and power. The armature is built up of carefully tested iron disks, which are deeply slotted to make room for the coils. These are wound and insulated separately, and placed in the slots in the armature core; sometimes they are held in place by binding wire, sometimes by wedges. The commutator is put in place, the coil connexions soldered to it, and the proper end-coverings put on. The magnet frame is made in two parts, of cast steel, enclosing the entire armature. A lid in the top casting gives access to the brushes, which are of carbon. The field coils are wound on forms and properly insulated. When in operation it is practically water and dust proof, and with proper attention is a very durable piece of machinery (fig. io). Although the standard design of motors is at present based on a single-reduction gearing, there are in operation traction-motors which are not geared.
On the locomotives used on the New York Central, the New York, New Haven & Hartford, and the Baltimore & Ohio railways in America, the City & South London railway in England, the armatures surround the driving axles. In all the cases mentioned, except the Baltimore & Ohio railway, the armatures are set directly on and solid with the axles of the driving-wheels, while on the Baltimore & Ohio locomotives the motors are sleeved on the axles, there being a slight play between the sleeve and the axle, which allows a flexible support. The wheels are driven by arms projecting from the armature shaft.
There is no fixed method of rating the output of traction-motors. Most manufacturers, in giving a certain horse-power capacity, mean that at the given rating the motor will run an hour with a rise in temperature of a certain number of degrees, not that it can be run continuously at the power given. Another system of rating depends on the draw-bar pull which the motor can develop under normal conditions of voltage and speed. Uniformity is greatly needed.
One of the most important parts of the equipment of an electric car or locomotive is the controlling device. In the early days of Controllers . electric traction a number of different methods of regulat ing the speeds of the cars were used, but they have been reduced to practically one standard method. In the old Sprague system there were at first no resistances outside of the motors themselves, but the field coils of the motors were divided into sections, and by changing the relative connexions of these sections the total resistance of the circuit could be changed; at the same time the strength of the field for a given total current was either increased FIG. I I. - Controller (open).
or decreased. In other systems the fields and armatures of the motors were not changed in their relation to one another, but external resistances were cut out and in by the controller. Usually there are two motors on each car, and it is evident that if the speed of a car be changed within wide limits, all the other factors remaining constant, there will be a very considerable loss by either of these methods of regulating, unless the relative connexions of the motor armatures can be changed. This can be done by putting the two motors in series where low speed is' desired, and in parallel where the speed is to be increased. This method was tried in the early days of electric traction at Richmond, and discarded, but it has been again taken up, and is now the standard method of regulation in ordinary tramway work. Roughly speaking, when the car is started the controller connects the two motors in series with an external resistance, then cuts out the external resistance, then breaks the circuit, then connects the two motors in parallel. The external resistance is put again in series with them, and then is gradually cut out as the car speed increases. By this method a considerable range of speed is attained at a fair efficiency. The controller (fig. I I) consists of a cylinder having on it a number of copper segments so arranged that on rotating it different connexions are made between stationary fingers that bear on these segments. In the first types much difficulty was experienced from the burning of the segments and fingers, due to the sparking on breaking the circuit, but this has been to a large extent obviated by using magnetic blow-outs at the point of break. (A magnetic blow-out is simply a magnet so arranged that the arc caused by breaking the circuit takes place in the magnetic field.) There is a reversing lever on the controllers separate from the controller handle, and interlocking with the controller so that the reverse lever may not be moved except when the controller is in the " off " position.
When it is desired to run trains of cars and to accelerate them rapidly, it is sometimes necessary to have more than one car equipped with motors. In this case all the motors must be controlled from one point, and a number of ingenious devices have been evolved to accomplish such " multiple control." In general, each car has its own controller, and all the controllers are operated by electric power from switches on each platform of any of the motor cars.
A motor and controlling system designed to save and utilize the power produced by a car running down an incline has been developed and is termed the " regenerative system." A car run- R ning over a line having heavy grades must have sufficient tiv Control. energy given to it to overcome its frictional resistance to motion and also to lift the weight of car and load from the bottom to top of each up-grade. On the return trip, the car " coasts " or runs down the grade without the consumption of current, but is restrained from attaining too high a speed by the brakes, thus wasting the energy existing by reason of the position of the car.
With the regenerative system the motors are caused to act as dynamos which are driven by the motion of the car axles when descending a grade, and, as they are connected to the line by the trolley or contacting device, the current thus generated is fed to the line and may assist other cars climbing grades at some other point on the system. The delivery of electrical energy also puts a resistance on the car axles and produces a braking effect which almost automatically fixes the car speed. If the speed be too high, the excessive current generated will tend to retard the car and reduce its velocity, while if too low the small current produced will set up but little opposition to motion and the car will accelerate.
Obviously, series motors cannot be used for this service. The motors have shunt fields, and their speed is varied by varying the field strength. Motors of this type are larger, more costly and slightly less efficient than series machines, so that a regenerative system has no place on roads that have a fairly level contour. When, however, the grades are frequent and excessive, the power saved more than counterbalances these factors, and the system may prove a valuable one for such service.
For tramcars of ordinary sizes hand-brakes are used, these being generally spindle brakes, with leverage enough to handle the comparatively heavy cars. When the size and speed of the car increases, however, these hand-brakes do not give sufficient control, and power brakes have to be adopted. Of these there are several forms that have proved successful in practice. The one most extensively used in electric railways is the air-brake, which is similar in its mechanical operation to the air-brake used on steam railways. The compressed air required for the operation of the brake is obtained by means of an air-pump driven by an electric motor, the circuit of which is controlled by a switch actuated by the pressure of the air in the receiving tank. When this pressure rises to a predetermined value, the device acts and interrupts the supply of current to the motor, which is thus stopped. When the pressure falls below a determined minimum the device operates in the opposite direction, and the motor and pump start. Of electric brakes there are several varieties. One type consists of two iron disks, one keyed on the axle but capable of moving along it a short distance axially, and the other held firmly on the frame of the truck. By means of a coil, set in a recess of annular form turned in the face of the fixed disk, the disks are magnetized transversely, and are drawn together with greater or less pressure, dependent on the amount of current that is allowed to pass through the coil. It is customary to arrange the current connexions in this form of electric brake so that when the handle of the controller is turned beyond the stopping position the current is cut off from the source of supply, and the motor running as a dynamo furnishes the current to work the brake.
The magnetic track-brake, which is sometimes used on tramway cars, consists of a pair of steel shoes, suspended from the truck frame and hanging near and over the rail, a steel yoke connecting the two shoes together. On this yoke is wound a heavy magnetizing coil which, when energized, strongly magnetizes the two steel shoes and. causes them to draw against and adhere to the track. Bracing links connect these track shoes with brake shoes on the wheel rims, and the drag of the track shoes thus applies pressure also to the wheel shoes. The downward pull of the track shoes gives a greater pressure of the wheels against the track than that due to the weight of the car, and the sliding or " skidding " of wheels, with the consequent production of flats, is avoided. A further braking effect comes from the use of the motors as dynamos, driven by the motion of the car, to supply current to the brake magnetizing coils. This therefore is one of the most effective brakes that has been devised. It has, however, not been very extensively used owing to its high cost and difficulties that arise from the track shoes running so close to the rails that any uneven places - frogs, switches, crossings and the like - may rub against them and give a braking effect at times when the car is accelerating or running. A pair of shoes is applied on both sides of the car, one pair being hung over either rail.
Another method of braking is by arranging the connexions of the two motors so that one acts as a dynamo driven by the motion of the car and supplies current to the other, which works as a motor, tending to turn the wheels in the direction opposite to that in which the car is moving. The production of current by the one motor and the reverse effort of the other give a powerful braking effect. The proper connexions are made by constructing the controllers with contacts additional to those required for motor control, which connect the machines in the desired manner when the controller handle is moved round past the " off " position.
Automatic brakes are always preferable to hand-brakes even though they cost much more, because the energy required to propel an ordinary tramcar is from 10 to 25% more with hand than with automatic brakes. The cause is the constant pressure of the brake shoes of a hand brake against the wheel rims, the shoes being so held by the operator to avoid having too long a hand movement in applying the brake. The maximum pressure possible for any brake should be about 90% of the weight of the car on the braked wheels. Less than this amount will give an inefficient brake; mote will produce sliding or " skidding" of the wheels, producing " flats" on them, and also causing loss of retarding effect.
Of the numerous accessories necessary in the operation of electric railways one of the most important is the trolley. For an overhead system this consists in general of a metallic rod or tube mounted upon the top of the car and pressed upward against the trolley wire by springs. At the upper end of this trolley pole is generally placed a bronze wheel which runs along the under surface of the wire. On the continent of Europe considerable use has been made of bow-trolleys, which consist of light metallic bow-shaped structures, sustained in place by springs and running along on the under side of the wire against which they rub. The designs patented for trolleys are almost innumerable. Besides the trolleys, cars are ordinarily equipped with switches which are used to break the trolley circuit, with fuses or automatic circuit-breakers, with electric lamps, with lightning arresters, and with the necessary car wiring. The fuses or automatic circuitbreakers guard against an excess of current being passed through the motors, and when they are fitted the ordinary platform switch can be dispensed with. These automatic breakers can be set for any desired current.
The question of the generation and the distribution of the current belongs to this article only in so far as electric traction has introduced peculiarities in the type of apparatus or the methods of its use. In a continuous current station the current is generated 'at an approximately constant potential, varying from 500 volts to 700 volts on different systems. As the load is apt to fluctuate, except in large stations, within wide limits, the machinery must be designed to stand the most severe usage. The engines are more massive than would be necessary for constant loads, and the dynamos must be built to stand sudden overloads without destructive sparking; usually, indeed, they are considerably over-compounded, not so much for the sake of raising the voltage as to strengthen the field and prevent sparking on overload. When a number of machines are to be run in parallel - as is usually the case - they are provided with " equalizing " switches, which serve to throw the series fields in parallel. As a result, if one of the machines tends to increase its armature current beyond the proper amount, the current in the series fields does not increase with it, but retains its normal proportion. The armature reaction and resistance fall of potential, in this machine, would both tend to increase, thereby decreasing its armature potential, and therefore its current would return to its proper value. From the dynamos the current from each machine goes through an ammeter and automatic circuit-breaker to the main " omnibus " bars, then through the station ammeter to the feeder " omnibus " bars, then through ammeters and circuit-breakers to the feed-cables. As a rule, watt-meters are provided to measure the output of the station, and, if an overhead system is being supplied, lightning arresters are installed. Where continuous currents are used to operate cars at considerable distances from the generating stations, " boosters " are used. These are series-wound dynamos driven at a constant speed, through which is passed the current that is to feed the distant section of the line. Usually the characteristic of the booster is so calculated that the amount by which it raises the voltage for a given current just equals the fall of potential in the feeding-line for the same current. The result is that the potential at the end of the line will be the same as that at the station. The question of economy, as between putting in additional copper and wasting energy in the booster, is easily calculated; the advantage is more and more on the side of the latter as the distance increases and the car service becomes more infrequent. It is necessary to the satisfactory operation of a system that the variations of voltage should not be too great, so boosters sometimes become a practical necessity, irrespective of the question of economy.
Accumulators are frequently installed in power stations to prevent the heavy load fluctuations which arise from starting and stopping of cars and ascending or descending grades. The generators give an approximately unvarying amount of current. When the load demand is less than that delivered by the generators, the excess current goes into the storage battery, and when the load is greater than the power from the generators the additional current required comes from the battery. The generators, engines and boilers may thus be proportioned for the average instead of the maximum load requirements, and the sizes of these units are thereby reduced.
As traction systems have been combined and extended, the area of operation of many of the companies has grown so that a number of direct-current stations are used for a single system. The limit of distance to which electric energy can be economically supplied at the comparatively low voltages employed is not great, and the advantage of having one or two large stations to supply a system, in place of a number of smaller ones, is evident. This fact has led to the use of high-potential alternating currents for the distribution of energy, the voltage being reduced at the points of consumption, and in most cases changed to a continuous current by rotary converters. If alternating currents are used for the car motors, the economical distribution of energy is greatly simplified, the rotary converters being eliminated and their first cost and losses and expense of operation saved. The expense of operating sub-stations containing rotary converters is necessarily large, and the capital outlay required for them is often greater than for the generating station.
As a rule, the cars used for electric traction have varied but slightly from the type of tramway car prevalent in different localities. The tendency, however, has been to increase their size. Cars. For electric railway work, as distinguished from tram way work, the cars generally follow the pattern that is standard on American steam lines. The trucks used for electric cars are made of steel, with heavy axles and suspension bars for carrying the electric motors. For smaller vehicles, a single four-wheel truck is used, the wheel base being limited by the curvature of the track, but not as a rule exceeding 72 ft. For the longer and heavier cars, two fourwheeled bogie trucks are employed. If two motors are used on a double-truck car, and if the grades on the road are very heavy, the trucks are made on the " maximum traction " pattern, in which one pair of wheels in each truck is of smaller diameter than the other and the greater part of the weight of the car is on the larger motordriven wheels. For very large high-speed cars, trucks are used of practically the same type and weight as are employed on steam railways. (See also TRAMWAY.) (L. Du.)
From Britannica 11th Edition (1911) 
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