Steam locomotive components

Main components found on a typical steam locomotive include: <imagemap> 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. (Template:Plainlink)

rect 133 15 172 57 Tender rect 538 17 575 57 Cab rect 667 15 708 57 Safety valve rect 715 15 754 55 Reach rod rect 754 15 789 55 Whistle rect 797 13 836 57 Dynamo rect 876 9 917 57 Sand dome rect 923 9 963 55 Throttle lever / Regulator rect 1009 11 1056 57 Steam dome rect 1152 9 1194 57 Air pump / Air compressor rect 1220 9 1266 55 Smokebox rect 1277 9 1324 57 Steam pipe rect 1338 11 1383 55 Smokebox door rect 593 409 640 457 Trailing truck / Rear bogie rect 645 409 688 457 Foot board / Run board / Running board / Tread plate rect 695 409 741 457 Frame rect 752 409 797 455 Brake shoe / Brake block rect 849 412 891 455 Sand pipe rect 895 412 935 455 Coupling rods / Side rods rect 993 410 1037 453 Valve gear rect 1041 410 1083 457 Connecting rod / Main rod rect 1102 410 1148 457 Piston rod rect 1150 410 1187 457 Piston rect 1185 410 1231 457 Cylinder rect 1281 412 1322 455 Valve rect 1320 409 1361 457 Valve chest / Steam chest rect 697 483 743 531 Firebox rect 810 481 858 531 Boiler tubes and flues rect 878 481 924 532 Boiler rect 930 481 974 532 Superheater tubes rect 1004 481 1052 536 Throttle / regulator valve rect 1096 483 1143 536 Superheater rect 1229 483 1277 531 Smokestack / Chimney / Funnel rect 1298 481 1350 531 Headlight rect 70 860 122 906 Brake hose rect 131 862 179 908 Water compartment rect 368 860 414 908 Coal bunker rect 571 860 617 908 Grate rect 636 860 690 908 Ashpan hopper rect 784 860 834 906 Journal/axle box rect 849 860 889 906 Equalising beams / Equalising levers / Equalising bars rect 897 860 935 906 Leaf springs rect 937 858 982 906 Driving wheels rect 1035 862 1082 904 Pedestal / Saddle rect 1118 863 1165 908 Blast pipe rect 1215 860 1263 910 Pilot truck / Lead truck rect 1289 862 1338 906 Pilot / Cowcatcher rect 1348 862 1396 908 Coupler / Coupling

desc bottom-left </imagemap> The 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.<ref name=lindahall>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=railwaytech>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=Fowler>Template:Cite book</ref>Template:Rp
2. Cab Template:Font
   Footplate Template:Font
   Compartment where the engineer (US+) / driver (UK+) and fireman control the locomotive and tend the steam supply and firebox.<ref name=railwaytech /><ref name= Fowler/>Template:Rp They achieve that using various devices, most of which are on the rear surface of the firebox, called the "backhead":<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
   File:Backhead and cab of former South Australian Railways T class locomotive no 186, 19 May 2018 (DBinion).jpg

   Most controls are mounted on the boiler's backhead
   • a throttle lever or regulator, which controls the amount of steam entering the cylinders<ref name= Fowler/>Template:RpTemplate:Rp
   • a reversing lever or (US term) Johnson bar, which controls the timing of the admission of steam into the locomotive's cylinders.<ref name= Fowler/>Template:Rp 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<ref name= Fowler/>Template:Rp
   • steam pressure gauges, which show the pressure of the steam in the boiler<ref name= Fowler/>Template:Rp
   • injector valves, which allow steam to force water into the boiler when needed<ref name= Fowler/>Template:Rp
   • water gauges, which allow the level of water in the boiler to be monitored<ref name= Fowler/>Template:Rp
   • 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<ref name= Fowler/>Template:Rp
   • a blower valve, which regulates the steam supplied to the blower (11)<ref name= Fowler/>Template:Rp
   • a whistle lever, which varies the steam supplied to the whistle (3)<ref name= Fowler/>Template:Rp
   • blowdown (or blow-off) cocks, which allow water to be ejected from the boiler to avoid concentration of impurities remaining after evaporation of steam.<ref name= Fowler/>Template:Rp
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.<ref name=railwaytech /><ref name="Kirkman p.14">Template:Cite book</ref><ref name=Forney>Template:Cite book</ref>Template:Rp<ref name= Fowler/>Template:Rp
4. Reach rod
   Rod linking the reversing lever (US: Johnson Bar) or reversing lever or wheel (UK+) in the cab to the valve gear.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
5. Safety valve
   Pressure relief valve to stop the boiler pressure exceeding the operating limit.<ref name=railwaytech /><ref name=Forney/>Template:Rp<ref name=Kirkman>Template:Cite book</ref>Template:Rp<ref name= Fowler/>Template:Rp
6. Dynamo / Turbo-generator / Generator
   Electrical generator driven by a small steam turbine, for the headlight and other locomotive lighting.<ref name= Fowler/>Template:Rp
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.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
8. Throttle lever Template:Font
   Regulator Template:Font
   Sets the opening of the throttle valve / regulator valve, (31) which controls the amount of steam entering the cylinders,<ref name=Forney/>Template:Rp<ref name=Kirkman/>Template:Rp<ref name= Fowler/>Template:Rp 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)<ref name= Fowler/>Template:Rp 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.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
10. Air pump / Air compressor
    Westinghouse pump Template:Font
    Powered by steam, it compresses air for operating the train air brake system.<ref name= Fowler/>Template:Rp The Westinghouse air brake system is used world-wide;<ref name= Fowler/>Template:Rp 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.<ref name=Forney/>Template:Rp Vacuum brakes, used historically, do not employ compressors;<ref name= Fowler/>Template:Rp 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.<ref name=railwaytech /><ref name=Carlson> Free access subject to limited trial, subscription normally required registration: there are other constraints (such as a cap on daily views) to freely access this source. {{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp 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;<ref name=railwaytech /><ref name=Carlson/><ref name= Fowler/>Template:Rp also helps to draw the fire when lighting up.

    Petticoat pipe / Apron Template:Font
    Vertical pipe with a bellmouth-shaped lower end extending down from the smokestack into the smokebox; enhances and equalizes draft through the boiler tubes.<ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp

12. Steam pipe
    Carries steam to the cylinders.<ref name=railwaytech /><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
13. Smokebox door
    Hinged circular door to allow service access to the smoke box to fix air leaks and remove cinders.<ref name=railwaytech /><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
14. Trailing truck Template:Font
    Rear bogie Template:Font
    Wheels at the rear of the locomotive to help support the rear of the locomotive and improve riding qualities – see also Leading wheel (46).<ref name= Fowler/>Template:Rp
15. Foot board / Run board / Running board / Tread plate
    Walkway around the locomotive, from the cab front, to facilitate inspection and maintenance.<ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
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).<ref name= Fowler/>Template:Rp The axles run in slots in the frames.<ref name="Kirkman p.14"/><ref name=Kirkman/>Template:Rp 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<ref name=railwaytech /> but some end-of-era designs included cast steel sub-frames.<ref> BR drawings held by the National Railway Museum, including Template:Small and Template:Small</ref>
17. Brake shoe / Brake block
    Cast iron or composite material that rubs on all the driving wheel treads for braking.<ref name=lindahall /><ref name=railwaytech /><ref name= Fowler/>Template:Rp
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.<ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
19. Coupling rods / Side rods
    Connect the driving wheels (43) together.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
20. Valve gear
    Motion Template:Font
    System of rods and linkages synchronising the valves with the pistons and controls the running direction and power of the locomotive.<ref name=railwaytech /><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
21. Connecting rod / Main rodTemplate:Anchor
    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.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
22. Piston rod
    Connects the piston to the cross-head.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
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.<ref name=lindahall /><ref name= Fowler/>Template:Rp
24. Cylinder
    Chamber that receives steam from the steam pipe.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
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.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
26. Valve chest / Steam chest
    Valve chamber next to the cylinder (24) containing passageways to distribute steam to the cylinders.<ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
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.<ref name=lindahall /><ref name=railwaytech /><ref name=Carlson/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
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.<ref name=railwaytech /><ref name=Carlson /><ref name= Fowler/>Template:Rp
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.<ref name=lindahall /><ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
30. Superheater tubes
    Pass steam back through the boiler to dry and superheat it for greater efficiency.<ref name=railwaytech /> See Superheater (32).
31. Throttle valve Template:Font
    Regulator valve Template:Font
    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).<ref name=lindahall /><ref name=railwaytech /><ref name=Kirkman/>Template:Rp For the other method, see Throttle lever (8).
32. Superheater
    Provides additional heat – as much as Template:Convert 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.<ref name=railwaytech /><ref name=Carlson /><ref name= Fowler/>Template:Rp See also Superheater tubes (30).
33. Smokestack Template:Font
    Chimney, Funnel Template:Font
    Vertical pipe on top of and inside the smokebox that ejects the exhaust (smoke and steam) above the locomotive.<ref name=lindahall /><ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
34. Headlight
    Light on the front of the smoke box to illuminate track ahead and warn of the approach of the locomotive.<ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
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.<ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
36. Water compartment
    Tank for water to be used by the boiler to produce steam.<ref name= Fowler/>Template:Rp
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.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
38. Grate
    Supports the burning fuel while allowing the products of combustion – ash and small clinker – to drop through.<ref name=railwaytech /><ref name= Fowler/>Template:Rp
39. Ashpan hopper
    Collects the ash from the fire.<ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp
40. Journal box Template:Font
    Axle box Template:Font
    Housing for the bearing on the axle of a wheel (43).<ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
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).<ref name= Fowler/>Template:Rp<ref name="Kirkman p.14"/><ref name=Forney/>Template:Rp
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.<ref name= Fowler/>Template:Rp
43. Driving wheels
    Drivers Template:Font
    Coupled wheels Template:Font
    Wheels coupled to the main/side rods, through which the power developed in the cylinders (24) is transformed into tractive power at the rails.<ref name=lindahall /><ref name=railwaytech /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp 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.<ref name= Fowler/>Template:Rp
44. Pedestal / Saddle
    Connects a leaf spring to a journal box / axle box (40) on a wheel.<ref name=Forney/>Template:Rp<ref name= Fowler/>Template:Rp
45. Blast pipe
    Exhaust pipe Template:Font
    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).<ref name=railwaytech /><ref name= Fowler/>Template:Rp
46. Pilot truck, Pony truck Template:Font
    Leading bogie / Lead truck / Bissel truck Template:Font
    Wheels at the front of the locomotive<ref name= Fowler/>Template:Rp 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 Template:Font
    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.<ref name= Fowler/>Template:Rp
48. Coupler Template:Font
    Coupling Template:Font
    Device at the front and rear of the locomotive for connecting locomotives and rolling stock.<ref name=lindahall /><ref name="Kirkman p.14"/><ref name= Fowler/>Template:Rp

See also

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

Further reading

• {{#invoke:citation/CS1|citation

|CitationClass=web }}

• Template:Cite book
• Template:Cite book

External links

• List of US–UK terminology – Railway Technical Website