How a steam railway locomotive works, explained using photographs taken during maintenance or restoration of full-size steam locos at Ropley, Hampshire, England
using photos of the steam loco parts you normally cannot see
"If you can't explain it simply, you don't understand it well enough" - Albert Einstein
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Firehole
for the fireman to feed the fire (and access
for maintenance). The rivets around the hole join the inner and outer firebox. |
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The inner firebox is surrounded by water above and
on all sides. Stays
link inner and outer
firebox to stop the pressure making them bulge. This photo looks down inside the water-space between the inner and
outer firebox showing some of the stays. Crown stays also Cutaway corner of the firebox of a Bulleid West Country Class loco shows waterspace between inner and outer firebox sheets and stays between them. Stays may be made of (in ascending order by price and durability) steel, copper or monel (alloy). Because the material cost for a stay in monel more than doubles, these are used only in high-stress locations (e.g. throatplate). |
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| Rocking grate with just one 'finger' installed. The complete grate can be opened like a venetian blind to drop the ashes (or in emergency, the fire) into the ashpan below. | Rocking grate from a different
loco.
Grates normally slope down towards the front; coal is normally fed more to the back and sides, and works its way forwards as it burns. |
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It is only the cold air drawn through the grate* that prevents the intense heat of the firebox from melting it, so it is important that ashes in the ashpan are not so high as to block incoming airflow. *because of the partial vacuum created in the smokebox by the action of the blastpipe (Bernoulli's Principle, which is also used by injectors and ejectors) |
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| The smokebox end, with some tubes removed ... | ... 'beading' tube-ends to seal and to prevent burning by hot gasses | ||||
| Whole boiler undergoing steam test prior to refitting. |  |
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| Safety valves
prevent the boiler from exceeding safe pressure. Setting the safety valves: |
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| Correct water level in the boiler is maintained by
means of injectors,
normally controlled by the fireman. This photo by David Warwick of the exhaust injector of BR Standard 5 #73096 is useful for comparison with diagrams in BR textbooks See also |
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| Under the dome (between chimney and cab) is the regulator valve controlled by the driver. It allows steam from the boiler to enter a main steam pipe which leads to the .. | ... superheater header in the smokebox, a large casting below and behind the chimney. This header diverts steam through superheater elements (located inside large smoke-tubes) to heat it on its way from the regulator valve to the valves and cylinders. | ||||
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| Superheater header (upside down) Superheater header and elements on A4 Bittern More details about boilers and fittings
Fitting the header |
Superheater elements ready to
fit. Superheating converts into steam any water droplets carried over,
increasing steam volume by about 30%. On a run, this can
raise the engine's efficiency by more than 35%, so is well worth all
the added complication for locos other than shunters
Below:
Elements
installed behind the blastpipe. |
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| Valves and valve-spindle for one
cylinder
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... fit here in the valve chest. Each piston-valve covers the ring of ports leading to the cylinder-ends and controls admission or exhaust of steam depending on whether it is ahead of or behind the ports. | ||
| Slide-valve steam chest and ports (Hunslet J94) with front covers removed. In service, when the regulator is opened there is a 'clunk' as steam pressure throws the valves onto their vertical seatings |  |
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The vertical 'combination lever' produces 'lead' - valve events happen a little ahead of time to cushion potentially destructive hammer-blow at the connecting rod big end and gudgeon pin See it in action (web-movie, 1.3MB) See it in action again (web-movie, 6.4MB)
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The
banana-shaped expansion link (top centre) rocks about its centre
point*. The radius rod is connected to the expansion link by a brass
die-block which can be raised or lowered in the expansion link by the
driver to control forwards or backwards travel. In intermediate
positions, steam is admitted for only part of each stroke - 'cut-off' -
for greater economy and speed. (If steam were admitted for the whole
stroke, the ports would be unable to admit or exhaust steam quickly
enough and the engine would stifle, limiting its speed to about 40
mph).
*In Stephenson valvegear the expansion link is raised or lowered and the radius rod stays level |
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| Lemaitre blastpipe of a Bulleid West Country Class loco has five nozzles directing exhaust up a wide chimney to produce draught for the fire (applying Bernoulli's Principle) | Blastpipe
for a Urie S15
is like an inverted 'Y' Main steampipe is to its left. See also: Kylchap blastpipe fitted to Gresley A4 Bittern |
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| The
exhaust beats
of a steam loco depend on the 'timing' of the loco. On a two-cylinder
loco, the power strokes are arranged to be even, so a loco will have
equal intervals between chuffs. The cylinders are at equal height above
the rails, the cranks are arranged at
90° to each other (left-side, right-side), and each
cylinder is double acting (powers the piston in each direction by
turn.)
Things get a bit more complicated with extra cylinders. On a Bulleid Pacific, the middle cylinder is mounted higher than the outer cylinders, and the cranks are set at angles to allow for this, so there are six chuffs per rev at even intervals. On a King, the four cylinders are arranged so that the left outer cylinder and right inner cylinder power strokes occur together, and then the right outer and left inner occur together, so the (big) chuff is from two cylinders at the same time. This makes for four chuffs per revolution. On a Lord Nelson the cranks are set for even power strokes, and it produced eight evenly spaced chuffs per revolution. |
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| Vacuum brake cylinder
and piston as fitted on carriages, and some locos. Bubs is leaning on
the cover which fits on the bottom, and that is resting over the piston
and its spindle.
The O-ring rolls between piston and cylinder, and makes such a good seal that vacuum may remain after service for several months. Most locos also have a steam brake, operated by a simple cylinder and piston. |
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More details about steam locomotive valvegear
Simplified animated explanation
Signalbox (the website for bell codes, block system, etc.)
Re-tyring railway locomotive wheels (photos showing how it's done)
Driver's controls: BR Standard Class 5MT
Very Latest News from the Loco Shed
Very Latest News from Mid-Hants Railway
Page,
movies
and photos © Tony Wood or the contributor acknowledged
Note: Photos are of parts from various locos to illustrate
general principles
gif by Andy Attwell
