On a

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, Fuel oil, oil or, rarely, Wood fuel, wood) to heat ...

, a driving wheel is a powered wheel which is driven by the locomotive's

piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder (engine), cylinder a ...

s (or

turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced can be used for generating electrical ...

, in the case of a

steam turbine locomotive A steam turbine locomotive was a steam locomotive which transmitted steam power to the wheels via a steam turbine. Numerous attempts at this type of locomotive were made, mostly without success. In the 1930s this type of locomotive was seen as a ...

). On a conventional, non-articulated locomotive, the driving wheels are all coupled together with side rods (also known as coupling rods); normally one pair is directly driven by the main rod (or

connecting rod A connecting rod, also called a 'con rod', is the part of a reciprocating engine, piston engine which connects the piston to the crankshaft. Together with the crank (mechanism), crank, the connecting rod converts the reciprocating motion of the p ...

) which is connected to the end of the piston rod; power is transmitted to the others through the side rods. On diesel and

electric locomotive An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a Battery (electricity), battery or a supercapacitor. Locomotives with on-board fuelled prime mover (locomotive), ...

s, the driving wheels may be directly driven by the traction motors. Coupling rods are not usually used, and it is quite common for each axle to have its own motor. Jackshaft drive and coupling rods were used in the past (e.g. in the Swiss Crocodile locomotive) but their use is now confined to shunter locomotives. On an articulated locomotive or a duplex locomotive, driving wheels are grouped into sets with wheels within each set linked together.

Diameter

Driving wheels are generally larger than

leading In typography, leading ( ) is the space between adjacent lines of type; the exact definition varies. In hand typesetting, leading is the thin strips of lead (or aluminium) that were inserted between lines of type in the composing stick to incre ...

or trailing wheels. Since a conventional steam locomotive is directly driven, one of the few ways to '

gear A gear or gearwheel is a rotating machine part typically used to transmit rotational motion and/or torque by means of a series of teeth that engage with compatible teeth of another gear or other part. The teeth can be integral saliences or ...

' a locomotive for a particular performance goal is to size the driving wheels appropriately. Freight locomotives generally had driving wheels between in diameter; dual-purpose locomotives generally between , and passenger locomotives between or so. The driving wheels on express passenger locomotives have come down in diameter over the years, e.g. from on the

GNR Stirling 4-2-2 The Great Northern Railway (Great Britain), Great Northern Railway (GNR) No. 1 class Stirling Single is a class of steam locomotive designed for Express train, express passenger work. Designed by Patrick Stirling (railway engineer), Patrick St ...

of 1870 to on the SR Merchant Navy Class of 1941. This is because improvements in valve design allowed for higher piston speeds.

Flangeless wheels

Some long-wheelbase locomotives (four or more coupled axles) were equipped with blind drivers. These were driving wheels without the usual

flange A flange is a protruded ridge, lip or rim (wheel), rim, either external or internal, that serves to increase shear strength, strength (as the flange of a steel beam (structure), beam such as an I-beam or a T-beam); for easy attachment/transfer o ...

s, which allowed them to negotiate tighter curves without binding. Some three-driving-axle locomotives also had flangeless wheels on the middle axle, such as Everett Railroad 11, and the NZR WH class.

Balancing

On locomotives with side rods, including most

steam Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...

and jackshaft locomotives, the driving wheels have weights to balance the weight of the coupling and connecting rods. The crescent-shaped balance weight is clearly visible in the illustrative image.

Whyte notation

In the Whyte notation, driving wheels are designated by the middle number or numbers in the set. The UIC classification system counts the number of axles rather than the number of wheels and driving wheels are designated by letters rather than numbers. The suffix 'o' is used to indicate independently-powered axles. The number of driving wheels on locomotives varied quite a bit. Some early locomotives had as few as two driving wheels (one axle). The largest number of total driving wheels was 24 (twelve axles) on the 2-8-8-8-2 and 2-8-8-8-4 locomotives. The largest number of coupled driving wheels was 14 (seven axles) on the ill-fated AA20 4-14-4 locomotive.

Other uses of the term ''driving wheel''

The term ''driving wheel'' is sometimes used to denote the '' drive sprocket'' which moves the track on tracked vehicles such as

tank A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and battlefield mobility provided by tracks and a powerful engine; ...

s and

bulldozer A bulldozer or dozer (also called a crawler) is a large tractor equipped with a metal #Blade, blade at the front for pushing material (soil, sand, snow, rubble, or rock) during construction work. It travels most commonly on continuous tracks, ...

In popular culture

Many American roots artists, such as

The Byrds The Byrds () were an American Rock music, rock band formed in Los Angeles, California, in 1964. The band underwent multiple lineup changes throughout its existence, with frontman Roger McGuinn (known as Jim McGuinn until mid-1967) being the so ...

, Tom Rush, The Black Crowes and the Canadian band Cowboy Junkies have performed a song written by David Wiffen called "Driving Wheel", with the lyrics "I feel like some old engine/ That's lost my driving wheel." These lyrics are a reference to the traditional blues song "Broke Down Engine Blues" by

Blind Willie McTell Blind Willie McTell (born William Samuel McTier; May 5, 1898 – August 19, 1959) was an American Piedmont blues and ragtime singer, songwriter and guitarist. He played in a fluid, syncopated finger picking guitar style common among many Eas ...

, 1931. It was later directly covered by

Bob Dylan Bob Dylan (legally Robert Dylan; born Robert Allen Zimmerman, May 24, 1941) is an American singer-songwriter. Described as one of the greatest songwriters of all time, Dylan has been a major figure in popular culture over his nearly 70-year ...

and Johnny Winter. Many versions of the American folk song " In the Pines" performed by artists such as Leadbelly, Mark Lanegan (on '' The Winding Sheet''), and

Nirvana Nirvana, in the Indian religions (Jainism, Hinduism, Buddhism, and Sikhism), is the concept of an individual's passions being extinguished as the ultimate state of salvation, release, or liberation from suffering ('' duḥkha'') and from the ...

(On '' MTV Unplugged In New York'') reference a decapitated man's head found in a driving wheel. In addition, it is likely that Chuck Berry references the locomotive driving wheel in "Johnny B. Goode" when he sings, "the engineers would see him sitting in the shade / Strumming with the rhythm that the drivers made."

References

* {{Locomotive running gear Steam locomotive technologies Train wheels