Steam locomotive components explained

Main components found on a typical steam locomotive include:File:Diagram_of_steam_locomotive_components_(March_2021_version).tif|thumb|center|800px|The main components of a typical steam locomotive. Click or hover over numbers to see names.

rect 133 15 172 57 Tenderrect 538 17 575 57 Cabrect 667 15 708 57 Whistlerect 715 15 754 55 Reach rodrect 754 15 789 55 Safety valverect 797 13 836 57 Generator / Turbo-generatorrect 876 9 917 57 Sand domerect 923 9 963 55 Throttle lever / Regulatorrect 1009 11 1056 57 Steam domerect 1152 9 1194 57 Air pump / Air compressorrect 1220 9 1266 55 Smokeboxrect 1277 9 1324 57 Steam piperect 1338 11 1383 55 Smokebox doorrect 593 409 640 457 Trailing truck / Rear bogierect 645 409 688 457 Foot board / Run board / Running board / Tread platerect 695 409 741 457 Framerect 752 409 797 455 Brake shoe / Brake blockrect 849 412 891 455 Sand piperect 895 412 935 455 Coupling rods / Side rodsrect 993 410 1037 453 Valve gearrect 1041 410 1083 457 Connecting rod / Main rodrect 1102 410 1148 457 Piston rodrect 1150 410 1187 457 Pistonrect 1185 410 1231 457 Cylinderrect 1281 412 1322 455 Valverect 1320 409 1361 457 Valve chest / Steam chestrect 697 483 743 531 Fireboxrect 810 481 858 531 Boiler tubes and fluesrect 878 481 924 532 Boilerrect 930 481 974 532 Superheater tubesrect 1004 481 1052 536 Throttle / regulator valverect 1096 483 1143 536 Superheaterrect 1229 483 1277 531 Smokestack / Chimney / Funnelrect 1298 481 1350 531 Headlightrect 70 860 122 906 Brake hoserect 131 862 179 908 Water compartmentrect 368 860 414 908 Coal bunkerrect 571 860 617 908 Graterect 636 860 690 908 Ashpan hopperrect 784 860 834 906 Journal/axle boxrect 849 860 889 906 Equalising beams / Equalising levers / Equalising barsrect 897 860 935 906 Leaf springsrect 937 858 982 906 Driving wheelsrect 1035 862 1082 904 Pedestal / Saddlerect 1118 863 1165 908 Blast piperect 1215 860 1263 910 Pilot truck / Lead truckrect 1289 862 1338 906 Pilot / Cowcatcherrect 1348 862 1396 908 Coupler / Coupling

desc bottom-leftThe diagram, which is not to scale, is a composite of various designs in the late steam era. Some components shown are not the same as, or are not present, on some locomotives – for example, on smaller or articulated types. Conversely, some locomotives have components not listed here.

Details of the components

1. Tender
   Attached rail vehicle that holds both water for the boiler and fuel such as wood, coal, or oil.^{[1] [2] [3]}
2. Cab
   Footplate
   Compartment where the engineer (US+) / driver (UK+) and fireman control the locomotive and tend the steam supply and firebox.^[3] They achieve that using various devices, most of which are on the rear surface of the firebox, called the "backhead":^[4]
   • a throttle lever or regulator, which controls the amount of steam entering the cylinders^[3]
   • a reversing lever or (US term) Johnson bar, which controls the timing of the admission of steam into the locomotive's cylinders.^[3] This is required for two purposes. One is to reverse the locomotive's direction, e.g. when shunting. The other is to enable more fuel-efficient operation when the locomotive is running in a steady state: the throttle is set wide open and the power output is controlled by moving the reversing lever closer to its mid-point ("reducing the cut-off") to limit the amount of steam admitted to the cylinders.
   • a train brake lever, which controls the application of brakes throughout the length of the train and a locomotive brake lever, controlling brakes on the locomotive only^[3]
   • steam pressure gauges, which show the pressure of the steam in the boiler^[3]
   • injector valves, which allow steam to force water into the boiler when needed^[3]
   • water gauges, which allow the level of water in the boiler to be monitored^[3]
   • mechanical stoker controls (when fitted to larger coal-fired locomotives) or oil feed controls for oil-fired locomotives
   • lubricator glasses, which allow the flow of lubricating oil to be monitored^[3]
   • a blower valve, which regulates the steam supplied to the blower (11)^[3]
   • a whistle lever, which varies the steam supplied to the whistle (3)^[3]
   • blowdown (or blow-off) cocks, which allow water to be ejected from the boiler to avoid concentration of impurities remaining after evaporation of steam.^[3]

3. Whistle
   Steam powered whistle, located on top of the boiler and used for signalling (by the number and length of notes) and warning of approach.^{[5] [6] [3]}
4. Reach rod
   Rod linking the reversing lever (US: Johnson Bar) or reversing lever or wheel (UK+) in the cab to the valve gear.^{[5] [3]}
5. Safety valve
   Pressure relief valve to stop the boiler pressure exceeding the operating limit.^{[6] [7] [3]}
6. Generator / Turbo-generator
   Electrical generator driven by a small steam turbine, for the headlight and other locomotive lighting.^[3]
7. Sand dome
   Holds sand that is directed on to the rail in front of the driving wheels to improve traction, especially in wet or icy conditions or when vegetation is on the line, and on steep gradients.^{[5] [6] [3]}
8. Throttle lever
   Regulator
   Sets the opening of the throttle valve / regulator valve, (31) which controls the amount of steam entering the cylinders,^{[6] [7] [3]} hence the speed of the locomotive. It is used in conjunction with the reversing lever to start, to stop, and to control the locomotive's power output. When the regulator/throttle is closed, a vacuum valve (snifting valve)^[3] permits air to be drawn through the superheater and cylinders to allow the engine to coast freely. The throttle is not the only control that can limit the locomotive's power output: during steady-state running of most locomotives, the throttle is usually set wide open and the power output is controlled by moving the reversing lever (2) closer to its mid-point ("reducing the cut-off") to limit the amount of steam admitted to the cylinders.
9. Steam dome
   Collects steam at the top of the boiler (well above the water level) so that it can be fed to the engine via the main steam pipe, or dry pipe, and the regulator/throttle valve.^{[5] [6] [3]}
10. Air pump / Air compressor
    Westinghouse pump
    Powered by steam, it compresses air for operating the train air brake system.^[3] The Westinghouse air brake system is used world-wide;^[3] in Europe two systems that use the same principle are the Kunze-Knorr and Oerlikon systems. It can be a single-stage or, when larger capacity is needed, a two-stage cross-compound compressor.^[6] Vacuum brakes, used historically, do not employ compressors;^[3] as a result of their relative inefficiency they are no longer in large-scale use.
11. Smokebox
    Receives the hot gases that have passed from the firebox through the boiler tubes and, when the throttle/regulator is open, directs them and steam exhausting from the cylinders up the smokestack/chimney, sucking air through the firebed. The smokebox may contain a cinder guard to prevent hot cinders being expelled.^{[8] [6] [3]} Components in the smokebox are:

    Blower
    Vertical pipe below the chimney petticoat pipe, with holes to blow steam upwards. Provides a draught to maintain adequate combustion – and to prevent smoke and flames from entering the cab through the firebox door – when the blastpipe is insufficiently effective, for example when a locomotive is stationary or the throttle/regulator is closed such as when coasting into a station;^{[8] [3]} also helps to draw the fire when lighting up.

    Petticoat pipe / Apron
    Vertical pipe with a bellmouth-shaped lower end extending down from the smokestack into the smokebox; enhances and equalizes draft through the boiler tubes.^{[6] [3]}

12. Steam pipe
    Carries steam to the cylinders.^{[6] [3]}
13. Smokebox door
    Hinged circular door to allow service access to the smoke box to fix air leaks and remove cinders.^{[6] [3]}
14. Trailing truck
    Rear bogie
    Wheels at the rear of the locomotive to help support the rear of the locomotive and improve riding qualities – see also Leading wheel (46).^[3]
15. Foot board / Run board / Running board / Tread plate
    Walkway around the locomotive, from the cab front, to facilitate inspection and maintenance.^{[5] [6] [3]}
16. Frame
    The strong, rigid structure that carries the boiler, cab and engine unit; supported on driving wheels (43) and leading and trailing trucks (14, 46).^[3] The axles run in slots in the frames.^{[5] [7]} Early American locomotives had bar frames, made from steel bar; in the 20th century they usually had cast steel frames or, in the final decades of steam locomotive design, a cast steel locomotive bed – a one-piece steel casting for the entire locomotive frame, cylinders, valve chests, steam pipes, and smokebox saddle, all as a single component. British locomotives usually had plate frames made from steel plate but some end-of-era designs included cast steel sub-frames.^[9]
17. Brake shoe / Brake block
    Cast iron or composite material that rubs on all the driving wheel treads for braking.^[3]
18. Sand pipe
    Deposits sand directly in front of the driving wheels to aid traction on steep gradients, when starting or when the rail surface is not dry and clean.^{[5] [3]}
19. Coupling rods / Side rods
    Connect the driving wheels (43) together.^{[5] [6] [3]}
20. Valve gear
    Motion
    System of rods and linkages synchronising the valves with the pistons and controls the running direction and power of the locomotive.^{[6] [3]}
21. Connecting rod / Main rod
    Steel arm that converts the reciprocating motion of the piston into a rotary motion of the driving wheels. The connection between piston and main rod is a crosshead, which slides on a horizontal bar behind the cylinder.^{[5] [3]}
22. Piston rod
    Connects the piston to the cross-head.^[3]
23. Piston
    Produces the motion for the locomotive from expansion of the steam. Driven backward and forward within the cylinder by steam delivered alternately, in front and behind, by the valve.^[3]
24. Cylinder
    Chamber that receives steam from the steam pipe.^[3]
25. Valve
    Controls the supply of steam to the cylinders. The valve gear, actuated by connection to the driving wheels, ensures that steam is delivered to the piston with precision. Types are slide valves, piston valves or poppet valves.^[3]
26. Valve chest / Steam chest
    Valve chamber next to the cylinder (24) containing passageways to distribute steam to the cylinders.^{[6] [3]}
27. Firebox
    Furnace chamber built into the boiler, which produces steam in surrounding water. Various combustible materials can be used as fuel; the most common are coal and oil but in earlier times coke and/or wood were used.^{[8] [6] [3]}
28. Boiler tubes and flues
    Carry hot gases from the front of the firebox to the front of the boiler, producing steam from the surrounding water. Flues are larger in diameter than tubes because they contain superheater units.^[3]
29. Boiler
    Horizontal tubular vessel, strong enough to contain high-pressure steam in a harsh working environment; closed at either end by the firebox and tube plate. Usually well filled with water but with space for steam – produced by heat from the firebox and boiler tubes – to be above the water surface.^{[5] [3]}
30. Superheater tubes
    Pass steam back through the boiler to dry and superheat it for greater efficiency. See Superheater (32).
31. Throttle valve
    Regulator valve
    Controlled by the Throttle Lever / Regulator (8), regulates the amount of steam delivered to the cylinders, which is one of two ways to vary power of the engine (throttle governing).^[7] For the other method, see Throttle lever (8).
32. Superheater
    Provides additional heat – as much as 300F-change hotter – to steam that has been generated in the boiler by sending it back through superheat tubes located in the boiler tubes (28), thus increasing engine efficiency and power.^[3] See also Superheater tubes (30).
33. Smokestack
    Chimney, Funnel
    Vertical pipe on top of and inside the smokebox that ejects the exhaust (smoke and steam) above the locomotive.^{[5] [3]}
34. Headlight
    Light on the front of the smoke box to illuminate track ahead and warn of the approach of the locomotive.^{[5] [6] [3]}
35. Brake hose
    Hose for conveying force to train brakes by a differential in air pressure. Contains either high-pressure compressed air or air at lower than atmospheric pressure (vacuum), depending on whether the locomotive has an air brake or vacuum brake system.^{[6] [3]}
36. Water compartment
    Tank for water to be used by the boiler to produce steam.^[3]
37. Coal bunker
    Compartment for storage of fuel before being directed to the firebox. When the fuel is coal (and in the distant past, coke or wood), the fireman shovels it manually through the firebox door or, in larger locomotives, by operating a mechanical stoker. When the fuel is oil, it is sprayed into the firebox from a sealed tank.^[3]
38. Grate
    Supports the burning fuel while allowing the products of combustion – ash and small clinker – to drop through.^[3]
39. Ashpan hopper
    Collects the ash from the fire.^{[5] [3]}
40. Journal box
    Axle box
    Housing for the bearing on the axle of a wheel (43).^{[6] [3]}
41. Equalising beams / Equalising levers / Equalising bars
    Part of the locomotive suspension system. Its function is to prevent inequalities in the track or roadbed putting an excessive load on an axle, which is especially necessary on uneven or poorly laid tracks. The beam is connected at each end to a spring on an axle or to the end of another equalising beam (usually via a vertical spring hanger) to distribute the locomotive's weight between two or more axles. The frontmost and rearmost components are secured to the locomotive frame. An equalising system links not only the driving-wheel axles but also the trailing and/or leading truck axle(s).^{[3] [5] [6]}
42. Leaf springs
    Main suspension springs for the locomotive. Each driving wheel supports its share of the locomotive's weight via leaf springs that connect the axle's journal box / axle box (40) to the frame.^[3]
43. Driving wheels
    Drivers
    Coupled wheels
    Wheels coupled to the main/side rods, through which the power developed in the cylinders (24) is transformed into tractive power at the rails.^{[5] [3]} The weight of bearings and coupling rods on the driving wheels is counterbalanced with cast-in weights to reduce "hammering" on the track when the locomotive is under way.^[3]
44. Pedestal / Saddle
    Connects a leaf spring to a journal box / axle box (40) on a wheel.^{[6] [3]}
45. Blast pipe
    Exhaust pipe
    Directs exhaust steam up the smokestack/chimney (33), creating a draught that draws hot gases through the firebox (27) and along the boiler tubes (28).^[3]
46. Pilot truck, Pony truck
    Leading bogie / Lead truck / Bissel truck
    Wheels at the front of the locomotive^[3] to guide the front driving wheels around curves, and minimise yawing at higher speeds with the attendant risk of derailment. The truck has some side motion and is equalised to the driving wheels (41). The names Pony truck (US+) and Bissel (or Bissell) truck (UK+) apply when there are two wheels; the others when there are four.
47. Pilot / Cowcatcher
    A shield made from bars, cast steel or sheet steel to prevent an object on the track from going under the locomotive and possibly derailing the train.^[3]
48. Coupler
    Coupling
    Device at the front and rear of the locomotive for connecting locomotives and rolling stock.^{[5] [3]}

See also

• Glossary of boiler terms
• Glossary of rail transport terms
• Horsepower#Drawbar power
• Power classification
• Tractive effort

Further reading

• Web site: Steam locomotive introduction (the Great Western archive) . Daniel, John . 2008 . 2000 . 2008-05-14.
• Book: Locomotive cyclopedia of American practice . 1925–1947 . New York . Association of American Railroads and Simmons-Boardman Publishing Corp. .
• Book: How steam locomotives really work . Semmens . P.W.B. . Goldfinch . A.J. . Oxford University Press . Oxford, New York . 978-0-19-860782-3 . 2000.

External links

• List of US–UK terminology – Railway Technical Website

Notes and References

1. Web site: A glossary of 19th century railroad terms. The Transcontinental Railroad. Linda Hall Library. Kansas City. July 7, 2019.
2. Web site: Steam Glossary. The Railway Technical Website. July 7, 2019.
3. Book: Fowler, George Little . Locomotive dictionary; an illustrated vocabulary of terms which designate American Railroad locomotives, their parts, attachments and details of construction, with definitions and illustrations of typical British locomotive practice; five thousand one hundred and forty-eight illustrations . 1906 . New York . The Railroad Gazette. 9780912318202 .
4. Web site: Backhead definition . . 2022 . Your dictionary . 11 October 2022 .
5. Book: Kirkman, Marshall M.. The locomotive: its working described and illustrated. Google Books. Marshall Monroe Kirkman. Chicago. Cropley Phillips Company. 1917. 14.
6. Book: Forney, Matthias N.. Catechism of the locomotive. Internet Archive. 2nd. Matthias N. Forney. The Railroad Gazette. New York. 1891.
7. Book: Kirkman, Marshall M.. The locomotive: its working described and illustrated. Google Books. Marshall Monroe Kirkman. Chicago. Cropley Phillips Company. 1917.
8. [File:Lock-gray-alt-2.svg|9px|link=|alt=Free access subject to limited trial, subscription normally required|Free access subject to limited trial, subscription normally required]
9. BR drawings held by the National Railway Museum, including and