If the wheelbase is made too short, the locomotive becomes unsteady at high speeds. This was a problem with four-wheeled locomotives. (Clarke, 112-3).
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There are constraints on height and width, too. The so-called "loading gauge" (the clearances provided by bridges, tunnels, road cuts, stations and neighboring track) comes into play here. In America, the rolling stock can be as wide as 10'10" and as high as 16'2." (NOCK/RE, 208-9).
The width is constrained, not only by the loading gauge, but also by the track gauge (the distance between the inside edges of the rails), as a large vehicle on a narrow gauge track may tip over when running a curve. The standard American track gauge is 4'8.5."
Likewise, the height not only cannot be so great as to be "clipped" by the roof of a tunnel, it cannot be disproportionate to the width, or the locomotive will topple over.
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Increases in the dimensions of the locomotive will ordinarily mandate an increase in weight, too, unless a new, lighter structural material is employed. The materials presently available to the USE are wood, cast iron, wrought iron, steel, and a few other metals such as copper.
In nineteenth-century America, wood was used mostly in the cab and the tender frame, and as insulation. Copper was sometimes used for the heat exchange elements, because it conducts heat well, but it is structurally weak and thus copper tubing is thicker than the steel equivalent. Cast iron was used in cylinders, journal boxes, and valve boxes. For all other major components, the initial preference was for wrought iron, but this changed once the Bessemer process (1856) made steel affordable. By 1900, virtually the whole locomotive was made of steel. (White, 29-31).
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We cannot put into a locomotive the most powerful boiler and the most powerful engine available, only those whose power is greatest within weight and size constraints. And the engine and boiler compete for the mass and volume allotted.
Making Steam: Locomotive Boiler Design
The boiler is the stomach of the locomotive. It consumes fuel, air and water, and belches steam. The fuel is burnt to change chemical energy into heat energy; the air is necessary for combustion to occur, and the water is what is heated to generate steam. It is the expansion of steam which moves the pistons, and ultimately makes the wheels go round.
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Coal is shoveled onto a horizontal grate in the firebox, which receives air from the "ashpan" below, as well as, intermittently, through the firebox door.
The first fireboxes were mounted "inside" the wheel lines, and were long and narrow (grate area 17-18 square feet). Later, they were placed on top of the frame, and were wide but short (30 square feet). Long, wide fireboxes (up to 90 square feet) were made possible by relocating them behind the driving wheels. (Forney; Bruce, 36-43)
The smoke puffing from the steam locomotive is photogenic, but it is also evidence that fuel is being wasted. In 1859, engineers solved this problem with two new elements, a brick arch and a deflector plate. Together, they controlled the airflow so as to improve combustion.
"Monty" should be familiar with these two firebox features.
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There are two basic methods of using the released heat energy. Most railroad boilers were of the "fire tube" type, which means that the hot air rises from the coals and enters a multitude of pipes. These travel through the main section of the boiler, which holds the water. The heat brings the water to a boil, and the steam rises from the top of the water surface, ultimately collecting in the "steam dome." The fire tubes empty into the smoke box, and the smoke ultimately escapes through the smokestack. This creates a partial vacuum in the smoke box, which helps to draw in the air. EB11 "Boilers" shows two views of an express locomotive boiler (Fig. 10).
A few OTL locomotives were equipped with water tube boilers. Water is circulated in tubes through the firebox, rather than hot air through the water reservoir. Water tube boilers were much safer to operate, and potentially more economical, "but it was impossible to build efficient boilers of this type within the clearance limitations of the railway engine" (Sinclair, 691).
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The most efficient boiler operation is at a relatively low rate of combustion, e.g., 30-60 pounds of coal per square foot of grate per hour, resulting in evaporation of 11-13 pounds of water per pound of coal, and a boiler efficiency of about 80%. Burning 100-180 pounds per square foot of grate per hour, we obtain only about 6-8 pounds of water per pound of coal, and the boiler efficiency is about 40-50%. (EB11/R) Forney says that the most coal which can be burnt is about two hundred pounds per square foot of grate per hour, and then only at most six pounds of water would be evaporated by each pound of coal fired.
The size of the grate determines how much coal can be burning at one time. So a big grate seems like a good thing. However, there are problems of increasing its size. First of all, it means increasing the overall size of the locomotive. Secondly, once the grate exceeds a certain size, it becomes too difficult for a single "fireman" to keep it "fired" properly. (This was a problem with hand-fired "Pacific" locomotives, NOCK/RE 175.) You either need to provide two fire doors, for two firemen, or engineer a "mechanical stoker."
The firebox is positioned within the boiler so that there are water spaces to the sides and in back of the firebox, to maximize the direct firebox-to-boiler surface area (Alexander PL79). There is also water above the top of the firebox, the "crown sheet," and indeed the most common cause of a boiler explosion is that the crown sheet loses this protective blanket, and melts.
Heat transfer takes place not only at those walls of the firebox which are in contact with the water reservoir, but also at the walls of the tubes. So having lots of small diameter tubes is good—unless you are the fellow who has to make sure that those tubes are tight.
The longer the tubes, the greater the heat transfer area, but the weaker the combustion-promoting draft in the firebox. Having lots of tubes increases the heating area, but weakens the tube plate of the firebox.
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EB11/R discloses both the grate area and the total heating surface for 36 locomotive designs. Disregarding the Stephenson Rocket, the total heating surface ranged from ~1,400 to ~6,100 square feet, and the grate areas from 20 to 100 square feet. The average ratio was 71:1.
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The steam passes up into the steam dome, from which it is released to the cylinders by the throttle valve. Some locos had two steam domes, or other provisions for storing more steam.
The boiler pressure is a function of the rate at which steam is produced (evaporation rate), the rate at which steam is used, and the size of the steam reserve. Taking advantage of a large steam reserve to briefly make faster-than-normal speed or pull an extra-heavy load is called "mortgaging the boiler."
If you are producing a lot of steam quickly, the boiler pressure will increase. The pressure which the boiler can tolerate is dependent on the thickness of the walls, as well as the nature of its construction. Thicker walls can hold higher pressure steam, but the boiler will weigh more.
Alexander provides only limited boiler pressure data. An 1860 engine had 130 p.s.i. (PL47); locomotives built as late as 1882 had 125 p.s.i. pressure (PL76-7); three later locos were 180-190 (PL80, 85, 96). The highest pressure in the EB11/R table was 235.
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Bear in mind that since the cab is behind the boiler, a large boiler limits the crew's view of what is in front of them.
Putting Steam To Work: Locomotive Engine Design
Usually, the locomotive will have a pair of pistons, which operate one-quarter of a cycle out of phase, so when one is in "neutral" the other is ready to receive steam.
The cylinders can be mounted outside or inside the main frame. In general, during the nineteenth century, the British preferred to use inside cylinders, and the Americans, outside ones (Nock/RE 164).
There are some locomotives which have a second pair of cylinders, in which case it is very common to have one pair on the inside and
the other on the outside. However, both pairs can be on the outside. (EB11/R).
There was experimentation with other positions in the early days, but the cylinders of late nineteenth-century locomotives were mounted horizontally, and at axle level.
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Looking at the locomotive data in EB11/R, and ignoring both the primitive "Rocket," and engines with more than one pair of cylinders, we can see that the cylinders are 18 3/8-23 inches wide, and the piston stroke is 26-30 inches long. For the American locomotives in the Alexander book, if we ignore the pre-1840 models, cylinder diameter is 12-22 inches, and piston stroke 15-30 inches (save for one "13/54" locomotive).
For both the British and American locomotives, the stroke length was, on average, 50% greater than the cylinder diameter.
Cranking the Wheels: Locomotive Transmission Design
In the standard "rod" locomotive, the pistons are connected to cranks on the driving wheels, so two power strokes by the piston, make one turn of the crank, resulting in one revolution of the wheel.
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The driving wheels of a high speed locomotive may turn at a rate of more than five revolutions per second. During each half-revolution, each piston accelerates to full speed (say, 35 feet per second) and then decelerates to a full stop. The necessary force on it is the mass times the acceleration. The piston weighs, say, 500 pounds (Forney), and the maximum acceleration is proportional to the stroke length and the square of the wheel speed (EB11/SE 837). The piston transmits that force to the piston rods, cranks, and other elements. They all must be able to withstand the resulting stresses, and, unless they are balanced, they cause unpleasant, perhaps dangerous, vibrations in the locomotive structure.
The engine and running gear include both rotating and reciprocating masses (some parts do both). The perturbations caused by the rotating masses (e.g., crank pin) can be completely balanced by a wheel-mounted counterbalance.
However, the reciprocating masses (e.g., piston head, piston rod, crosshead, main rod, coupling rods) would still cause the locomotive to yaw right and left. This horizontal disturbance can be reduced by "overbalancing" the wheels, but at the price of causing a vertical imbalance (pitching up and down). This alternately hammers the rails, and lifts the locomotive.
Usually, the compromise is to balance all of the rotating mass and 25-50% of the reciprocating mass, so that there is both horizontal and vertical imbalance.
The vertical imbalance increases with the square of the wheel speed (Addendum). The rails have to be able to withstand this dynamic load, not just the static weight on the wheels. And, of course, when the disturbance is upward, the locomotive must be heavy enough, and the balanced reciprocating mass light enough, so that the locomotive remains on the track.
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Usually, with an outside cylinder, the piston rod fits into a crosshead, and the main rod connects the crosshead to the main crank pin, near the rim of one driving wheel. One driving axle is cranked directly, and the other driving axles are turned by the action of connecting rods, which run from one crank pin to another.
With an inside cylinder, the main rod will act on a cranked axle, rather than a crank pin. One advantage of an inside cylinder was that it could be mounted close to the center line, reducing the disturbances caused by the piston action. Another advantage is that the cylinder is warmed by the smokebox, and insulated by the frame. However, Ellis (51) warns the up-timers that "persistent breakage of crank axles" bedeviled inside cylinder designs. Crank axles were also expensive, large, heavy, and difficult to inspect and repair (White 208-9; EB11/SE 841).
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The axle rotation also regulates the slide valve on the cylinders. EB11/R mentions five different mechanisms for this purpose, but for a description, you must turn to EB11/SE. The mechanisms are the Stephenson (Figs. 29, 32), Goochs (Fig. 30) and Allan (Fig. 31) type link motions, and the Joy (Fig. 36) and Waelschaert radial gears. EB11/SE also depicts the Hackworths (Fig. 33) and Marshalls (Fig. 34) valve gears. One 1887 valve control mechanism is depicted in Alexander (PL79); I believe this is a "link motion."
In the modern Waelschaert gear, the movement of (1) the crosshead, together with that of an "eccentric crank" connected to (2) the main crank pin, serves to move forward and back the valve rod (which directly controls which valve is open). However, the valve rod leads the piston rod.
Rolling Forward: Locomotive Wheel Design
In the EB11/R table, the driving wheel diameter ranged from 54 to 85 inches. Among Alexander's American locomotives, the range was 30 to 96 inches. In general, the bigger the wheel, the higher the intended operating speed of the locomotive. With typical locomotive designs, and adequate track, maximum speeds (mph) were usually 75-150% of the wheel diameter (inches).
Big wheels also have the advantage of a larger wearing surface (proportional to diameter). So the abrasion by the rail is spread more broadly.
However, if you increase the wheel size, you need to increase the size of the connecting rods, the cylinders, the frames, and so forth. Which means, given size and weight constraints, that much less room and weight allowance for the boiler. (Forney)
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The wheel is not a single piece construction. Rather, there is a wheel proper, over which is mounted a metal "tire." This is the "wearing surface" of the wheel, the part that is gradually worn away by the action of the rails.
The tire also includes a flange, a thin, flat, short metal projection. A flanged wheel looks a little bit like a stovepipe hat; the crown is the wheel, and the brim is the flange.
In nineteenth-century America, the tires were made of wrought iron, case-hardened cast iron, or, once the price came down, steel. Steel tires were preferred because they lasted at least five times as long. (White, 175-83).
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There are a number of little expedients used to make it easier for the driving wheels to hold onto curved track. One is to put non-driving pilot (leading) wheels in front of them.
Secondly, one or more of the axles may be allowed "sideplay," that is, the ability to shift left or right. The Bavarian Ep 3/6 had an inch or so of sideplay in several of its axles. Side play was even more marked in Baldwin's 1842 flexible-beam engine (Alexander PL14).
Thirdly, the wheels can be tapered. Wheels are slightly conical (standard "taper" is 1 in 20), with the narrowest diameter on the outside. As the train moves onto a curve, the wheels shift outward, so the outer wheel's diameter at the point of contact increases, and that of the inner wheel decreases. That corrects for the curve.
Finally, one or more pairs of driving wheels can be "blind" (flangeless)(Alexander PL20, 83, 84).
Locomotive Wheel Arrangements
Locomotive wheels are mounted on axles; the transmission system turns the axles, which in turn rotate the wheels.
Some of the axles are driven, directly or indirectly, by the engine. Others turn passively as a result of the action of the car on the wheel. If your car has front wheel drive, then the front axle is a driven axle, and the rear one isn't.
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Locomotives are described according to a standard wheel configuration nomenclature which, usually, but not always, uses three numbers, like so: X-Y-Z. The X value is the number of leading wheels. These wheels are not driven by the engine, but help to give stability to the ride. They are mounted on what is called a "truck" or "bogie," which can turn if the wheels encounter a curved track. X might be 4 for a passenger locomotive, 2 for a freight locomotive, and 0 for a switching yard locomotive. The American-style four wheeled leading bogie is mentioned in EB11/R.
The Y value is the number of driving wheels. Usually, the main rods directly drive one axle, to which the other driving axles are coupled. The driving wheels transmit the power of the engine to the rail and, by adhering to the rail (if there were no friction, the wheels would just spin in place), create the reactive force which impels the locomotive forward. A freight locomotive will usually have more driving wheels than a passenger locomo
tive of equal horsepower.
The Z value is the number of trailing wheels. Like the leading wheels, these are unpowered. However, by providing additional support, they permit a locomotive to enjoy a long, wide firebox. It can produce steam at a greater rate, and thus supply more power to the cylinders. Like the leading wheels, the trailing ones are mounted on a rotating truck.
If a train has both a leading and a trailing truck, that means that it can back easily into a curve. This can come in handy on a branch line serving a mining area.
Occasionally, a locomotive has a wheel configuration necessitating more than three numbers. This implies that there is more than one set of coupled driving wheels
For example, instead of a 4-8-4, you could have a 4-8-8-4, in which one pair of cylinders drives four driving axles, and a second pair drives the other four.