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, oil or, rarely, wood) to heat water in the locomot ...

, a driving wheel is a powered

wheel A wheel is a circular component that is intended to rotate on an axle Bearing (mechanical), bearing. The wheel is one of the key components of the wheel and axle which is one of the Simple machine, six simple machines. Wheels, in conjunction wi ...

which is driven by the

locomotive A locomotive or engine is a rail transport vehicle that provides the Power (physics), motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, Motor coach (rail), motor ...

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 and is made gas-tig ...

s (or turbine, in the case of a steam turbine locomotive). 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 piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...

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

shunting locomotives A switcher, shunter, yard pilot, switch engine, yard goat, or shifter is a small railroad locomotive used for manoeuvring railroad cars inside a rail yard in a process known as ''switching'' (US) or ''shunting'' (UK). Switchers are not inte ...

. On an articulated locomotive or a duplex locomotive, driving wheels are grouped into sets which are linked together within the set.

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 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 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 flanges, which allowed them to negotiate tighter curves without binding. Some three-driving-axle locomotives also had flangeless wheels on the middle axle, such as the

NZR WH class The NZR WH class was a class of three steam locomotives built by Manning Wardle in 1884 for service on New Zealand's private Wellington and Manawatu Railway Company (WMR). They did not acquire their WH classification until 1908 when WMR was n ...

Balancing

On locomotives with side rods, including most

steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...

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 picture on the right.

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 A Triplex locomotive is a steam locomotive that Divided drive (locomotive), divides the driving force on its wheels by using three pairs of cylinders rigidly mounted to a single locomotive frame. Inevitably any such locomotive will be articulated ...

and

2-8-8-8-4 Under the Whyte notation for the classification of steam locomotives, a 2-8-8-8-4 has two leading wheels, three sets of eight driving wheels, and four trailing wheels. Other equivalent classifications are: UIC classification: 1DDD2 (also known as ...

locomotives. The largest number of coupled driving wheels was 14 (seven axles) on the ill-fated AA20

4-14-4 The AA20 was a one-off steam locomotive constructed by the Soviet Union under the dictatorship of Joseph Stalin. Wheel arrangement The AA20 was a "4-14-4" locomotive (using the Whyte notation classification of steam locomotives by wheel arrang ...

locomotive.

Other uses of the term ''driving wheel''

The term ''driving wheel'' is sometimes used to denote the ''

drive sprocket A sprocket, sprocket-wheel or chainwheel is a profiled wheel with teeth that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which radial projections engage a chain pa ...

'' which moves the

track Track or Tracks may refer to: Routes or imprints * Ancient trackway, any track or trail whose origin is lost in antiquity * Animal track, imprints left on surfaces that an animal walks across * Desire path, a line worn by people taking the shorte ...

on tracked vehicles such as tanks and

bulldozer A bulldozer or dozer (also called a crawler) is a large, motorized machine equipped with a metal blade to the front for pushing material: soil, sand, snow, rubble, or rock during construction work. It travels most commonly on continuous track ...

In popular culture

Many American roots artists, such as The Byrds, 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, 1931. It was later directly covered by Bob Dylan and Johnny Winter. Many versions of the American folk song " In the Pines" performed by artists such as Leadbelly,

Mark Lanegan Mark William Lanegan (November 25, 1964 – February 22, 2022) was an American singer, songwriter, and poet. First becoming prominent as the lead singer for the early grunge band Screaming Trees, he was also known as a member of Queens of the St ...

(on '' The Winding Sheet''), and Nirvana (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