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Tank engines and tender engines
Small locomotives, often used for industrial and shunting duties, generally carried their own water supply in an on-board tank. They would frequently carry a supply of coal as well. This was convenient, and made the locomotive self-contained. Industrial and shunting engines were seldom very far from a source of coal and water, so this arrangement generally worked quite well. However, most large locomotives -- whether used for passengers or freight -- had separate tenders. The tender carried water and coal, and was towed along behind the engine. The use of a tender considerably increased the range that can be covered between refuelling stops. However, a tender engine is significantly more complex to prepare than a tank engine, as the tender itself has to be prepared, load with consumables, and coupled to the engine. Moreover, tender engines are less self-contained than tank engines. Among tank engines, a number of different tank arrangements were in widespread use, as will be described later.Gauge
The gauge of the track, and therefore of the locomotive and train, is the distance between the inner edges of the rails. In the UK and the USA, `main line' railways -- railways that span large distances at high speed -- generally run on a gauge of 4' 8½" (`standard gauge'). The common story that this gauge derives ultimately from the wheel spacing of Roman chariots is almost certainly an urban myth. In fact, the Romans did not use chariots for transport, either in war or peace, although they did race them ceremonially. Chariots were a Greek idea, and were considered obsolete by the early days of the Roman empire. In reality, a number of different track gauges operated side-by-side in the UK when the early railways were privately owned.In contrast to the uniformity of main line gauge, at least after the early battles, there was historically far less agreement on a standard gauge for industrial locomotives. Frequently these were what would now be termed `narrow guage', that is, two feet or less. Gauges of 18 inches and, ocassionally, 12 inches were to be found in quarries and mines. Narrow gauge track is less expensive and easier to lay. However, the height of a locomotive is ultimately limited by its gauge. A tall loco on narrow rails will be unstable, particularly around curves.
Wheel plans
Many small industrial and shunting locomotives have only four or six wheels, all of which are driven. However, four or six wheels may not be sufficient for a high-speed passenger locomotive, whether they are driven or not. If the locomotive is long - and it will be if it is to house a powerful boiler - then more wheels will be needed to carry the weight. Where the wheels are merely load-bearing, they need not be driven. For wheels to be driven, they must be coupled onto the drive train in some way, which increases complexity and expense.For a train to achieve high speeds, the driven wheels must be quite large (see photo below). This is because the wheel rotates exactly once per stroke of the piston. To achieve high speeds with small wheels would require very rapid piston strokes, which is inefficient and prone to excessive wear. Occasionally, loco designers did experiment with really huge wheels, sometimes ten feet in diameter or more. The problem with these designs is that the axles of such wheels will need to be five feet above the track, which is where the boiler or firebox ought to be. In practice, only one such wheel could be driven, which could lead to poor traction on the rails. Such designs were rarely successful, and mostly did not catch on.
The wheel of a large express locomotive could be more than four
feet in diameter
So, in general, industrial locos had a small number of wheels, all
driven, while pasenger locos had a large number of wheels, some
of which were driven and some - smaller in size - which were not.
It is conventional to describe the wheel plan as three numbers,
writen `A-B-C', where A is the number of leading non-driven
wheels, B the number of driven wheels, and C the number of
trailing non-driven wheels. If the locomotive has only driven
wheels, then `A' and `C' are `0'. So, a locomotive with four
driven wheels only would be denoted `0-4-0'. The largest
passenger express locos had wheel arrangements as large
as 4-8-4.
Tank arrangement
Conventionally, the tank arrangement is indicated by an abbreviation after the wheel plan (see below). For example, in the designation `0-6-0T', the `T' simply stands for `tank', and denotes the standard tank arrangement. The following table describes some of the most common tank layouts and their abbreviations.
| Standard tank engine | T |
In the standard tank engine, the tanks are supported by the
engine's chassis, one on each side of the boiler. This construction
was very popular owing to its simplicity. However, it had the effect
of making access to the running gear
difficult. In addition, heat was lost needlessly from the top of the boiler,
which was somewhat wasteful.
|
| Well tank engine | WT |
In the well tank layout, the bulk of the water was carried low
down between the chassis members. This had the effect of
keeping the tank out of the way, but there was freqently not
enough room to store sufficient water. Consequently, as the
photo shows, well tank engines frequnently had auxilliary
side tanks in addition to the well.
|
| Saddle tank engine | ST |
In the saddle tank layout, the tank `hangs' from the boiler
like a saddle. This keeps the tank clear of the chassis, while
retaining the same capacity as a standard tank. However, the
construction of the tank is much more difficult than a standard
tank, and the weight of the water is carried higher on the
locomotive.
|
| Pannier tank engine | PT | In the pannier tank layout, the tank is raised above the chassis, but does not straddle the boiler like a saddle tank. The construction is simple and the centre of gravity kept lower, but the capacity is not as great as a standard or saddle tank. |
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