Steam locomotive distinct sound.
The exhaust steam is directed upwards through a nozzle called a blast-pipe which gives that familiar chuffing sound of the steam locomotive. The exhaust steam then goes out the chimney.
The blast-pipe is placed at a strategic point inside the smoke box that is at the same time traversed by the combustion gases drawn through the boiler and grate by the action of the steam blast.
The combining of the two streams, steam and exhaust gases, is crucial to the efficiency of any steam locomotive and the internal profiles of the chimney.
This has been the object of intensive studies by a number of engineers and almost totally ignored by others with sometimes catastrophic effect.
The fact that the draught depends on the exhaust pressure means that power delivery and power generation are automatically self-adjusting and among other things, a balance has to be struck between obtaining sufficient draught for combustion whilst giving the exhaust gases and particles sufficient time to be consumed.
The chassis or locomotive frame is the principal structure that the boiler is mounted and which incorporates the various elements of the running gear. The boiler is rigidly mounted on a saddle beneath the smoke-box and in front of the boiler barrel, but the firebox at the rear is allowed to slide forward and back, to allow for expansion when hot.
The steam locomotives wheels where coupled together which generally made it lack stability at speed. This made it desirable to include unpowered carrying wheels mounted on two-wheeled trucks or four-wheeled bogies centered by springs that help to guide the locomotive through curves.
These usually would reduce the weight of the cylinders in front or the weight of the firebox at the rear end. For multiple coupled wheels on a rigid chassis a variety of systems for controlled side-play exist.
Railroads typically wanted a locomotive with as few axles as possible. This would reduce the cost of maintenance. The number of axles required was dictated by the maximum axle loading of the railroad in question.
A builder would typically add axles until the maximum weight on any one axle was acceptable to the railroad's maximum axle loading. A locomotive with a wheel arrangement of two lead axles, two drive axles, & one trailing axle was in actuality a high speed machine.
Two lead axles were necessary to have good tracking at high speeds. Two drive axles had a lower reciprocating mass than three, four, five, or six coupled axles.
They were then able to turn very high speeds due to the lower reciprocating mass. A trailing axle was able to support a huge firebox.
Hence most locomotives with the wheel arrangement of 4-4-2 American Type Atlantic were free steamers and able to maintain steam pressure regardless of throttle setting.
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