A train wheel or rail wheel is a type of

wheel A wheel is a circular component that is intended to rotate on an axle bearing. The wheel is one of the key components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axles, allow heavy objects to be ...

specially designed for use on

railway track A railway track (British English and UIC terminology) or railroad track (American English), also known as permanent way or simply track, is the structure on a railway or railroad consisting of the rails, fasteners, railroad ties (sleeper ...

s. The wheel acts as a rolling component, typically

press fit An interference fit, also known as a pressed fit or friction fit is a form of fastening between two ''tight'' fitting mating parts that produces a joint which is held together by friction after the parts are pushed together. Depending on the am ...

ted onto an

axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, beari ...

and mounted directly on a

railway carriage A railroad car, railcar (American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and UIC), also called a train car, train wagon, train carriage or train truck, is a ...

locomotive A locomotive or engine is a rail transport vehicle that provides the 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, railcar or power car; the ...

, or indirectly on a

bogie A bogie ( ) (in some senses called a truck in North American English) is a chassis or framework that carries a wheelset, attached to a vehicle—a modular subassembly of wheels and axles. Bogies take various forms in various modes of transp ...

(in the UK), also called a ''truck'' (in North America). The powered wheels under the locomotive are called

driving wheels On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons (or turbine, in the case of a steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled to ...

. Wheels are initially

cast Cast may refer to: Music * Cast (band), an English alternative rock band * Cast (Mexican band), a progressive Mexican rock band * The Cast, a Scottish musical duo: Mairi Campbell and Dave Francis * ''Cast'', a 2012 album by Trespassers William ...

or forged and then heat-treated to have a specific hardness. New wheels are machined using a

lathe A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece ...

to a standardized shape, called a profile, before being installed onto an axle. All wheel profiles are regularly checked to ensure proper interaction between the wheel and the rail. Incorrectly profiled wheels and worn wheels can increase

rolling resistance Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the motion when a body (such as a ball, tire, or wheel) rolls on a surface. It is mainly caused by non-elastic effects; that is, not all the energy ...

, reduce energy efficiency and may even cause a

derailment In rail transport, a derailment occurs when a rail vehicle such as a train comes off its rails. Although many derailments are minor, all result in temporary disruption of the proper operation of the railway system and they are a potentially ...

. The

International Union of Railways The International Union of Railways (UIC, french: Union internationale des chemins de fer) is an international rail transport industry body. History The railways of Europe originated as many separate concerns, and there were many border chang ...

has defined a standard wheel diameter of , although smaller sizes are used in some

rapid transit Rapid transit or mass rapid transit (MRT), also known as heavy rail or metro, is a type of high-capacity public transport generally found in urban areas. A rapid transit system that primarily or traditionally runs below the surface may be ...

railway systems and on ro-ro carriages.

Wheel geometry and flange

Almost all train wheels have a straight projection, a flange, on one side to keep the wheels, and hence the train, running on the rails when the limits or tests of alignment are reached: when a bend is taken at appropriate speed, when there are strong sidebreezes, and to withstand e.g. most common emergent defects in trackbed, rail and mild debris. See

Hunting oscillation Hunting oscillation is a self-oscillation, usually unwanted, about an equilibrium. The expression came into use in the 19th century and describes how a system "hunts" for equilibrium. The expression is used to describe phenomena in such diverse ...

. The running surface of most is conical, serving as the primary means of keeping the train's motion aligned with the track (the wheels are fixed on their axles, and as the mass of the train pushes it towards the outside of the curve, the outside wheel rides up to contact the rail at a larger diameter while the inside wheel drops down to contact its rail at a smaller diameter, travelling a smaller distance for each rotation of the axle, and steering the train round the curve). But sometimes wheels are cylindrical, such that the flanges are essential to keep the train on the

rail track A railway track (British English and UIC terminology) or railroad track (American English), also known as permanent way or simply track, is the structure on a railway or railroad consisting of the rails, fasteners, railroad ties (sleeper ...

Wheel arrangement

The number of wheels per locomotive or car vary in both size and number to accommodate the needs of the railcar or locomotive. Regardless of these factors, pairs of identically sized wheels are always affixed to a straight axle as a singular unit, called a wheelset.

Wheels for road-rail vehicles

Wheels used for

road–rail vehicle A road–rail vehicle or a rail–road vehicle is a dual-mode vehicle which can operate both on rail tracks and roads. They are also known as two-way vehicles (german: Zweiwegefahrzeug), hi-rail (from ''highway'' and ''railway'', or variation ...

s are normally smaller than those found on other types of

rolling stock The term rolling stock in the rail transport industry refers to railway vehicles, including both powered and unpowered vehicles: for example, locomotives, freight and passenger cars (or coaches), and non-revenue cars. Passenger vehicles ca ...

(such as locomotives or carriages). This is because the wheel has to be stored clear of the ground when the vehicle is in road-going mode - Such wheels can be as small as in diameter. In Australia, wheels for road-rail vehicles should comply with the requirements of AS7514.4, which is the Australian standard for infrastructure maintenance vehicle wheels.

Railway wheel and tire

Modern railway wheels are usually machined from a single casting, also known as monoblock wheels. Some wheels, however, are made of two parts: the wheel core, and a

tire A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineering), t ...

("tyre" in

British English British English (BrE, en-GB, or BE) is, according to Oxford Dictionaries, "English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in England, or, more broadl ...

Australian English Australian English (AusE, AusEng, AuE, AuEng, en-AU) is the set of varieties of the English language native to Australia. It is the country's common language and ''de facto'' national language; while Australia has no official language, Eng ...

and other variants) around the perimeter. Separate tires are a component of some modern passenger rolling stock. The purpose of the separate tire is to provide a replaceable wearing element – an important factor for steam locomotives with their costly spoked construction. In modern times the tire is invariably made from

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...

, which is stronger than the

cast iron Cast iron is a class of iron– carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuri ...

of earlier eras. It is typically heated and pressed on to the wheel before it cools and shrinks. ''Resilient rail wheels'' have a resilient material, such as

rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...

, between the wheel and tire. Failure of such kind of wheel was one of the causes leading up to the Eschede high-speed train crash.

Causes of damage

The most common cause of wheel damage is severe braking. This activity includes sudden braking, braking on steep gradients and braking with high weight loads. The

brake shoes A brake shoe is the part of a braking system which carries the brake lining in the drum brakes used on automobiles, or the brake block in train brakes and bicycle brakes. A device that is put on a track to slow down railroad cars is also called ...

(or blocks) are applied directly to the wheel surface which generates immense amounts of

thermal energy The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, de ...

. Under normal operation, a wheel may obtain a tread temperature of . Under severe braking conditions, the generated thermal energy can contribute to

thermal shock Thermal shock is a type of rapidly transient mechanical load. By definition, it is a mechanical load caused by a rapid change of temperature of a certain point. It can be also extended to the case of a thermal gradient, which makes different pa ...

or alteration of the wheel's mechanical properties. Ultimately, acute thermal loading leads to a phenomenon called

spall Spall are fragments of a material that are broken off a larger solid body. It can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ball ...

ing. Alternatively, severe braking or low adhesion may stop the rotation of the wheels while the vehicle is still moving, which may cause a

flat spot A flat spot, or wheel flat, also called spalling or shelling, is a fault in railroad wheel shape. A flat spot occurs when a rail vehicle's wheelset is dragged along the rail after the wheel/axle has stopped rotating. Flat spots are usually cau ...

on the wheel-rail interface and localized heat damage. Modern railway wheels are manufactured reasonably thick to provide an allowance of wear material. Worn wheels or wheels with a flat spot are machined on a wheel lathe if there is sufficient thickness of material remaining.

Guide wheel

Rubber-tyred metro A rubber-tyred metro or rubber-tired metro is a form of rapid transit system that uses a mix of road and rail technology. The vehicles have wheels with rubber tires that run on rolling pads inside guide bars for traction, as well as traditio ...

s with a central

guide rail A guide rail is a device or mechanism to direct products, vehicles or other objects through a channel, conveyor, roadway or rail system. Several types of guide rails exist and may be associated with: * Factory or production line conveyors * Pow ...

, such as the

Busan Metro The Busan Metro () is the urban rail system operated by the Busan Transportation Corporation of Busan, South Korea. The metro network first opened in 1985 with seventeen stations. The Metro itself consists of 4 numbered lines, covering of r ...

Lille Metro The Lille Metro (french: Métro de Lille) is a driverless light metro system located in Lille, France. It was opened on 25 April 1983 and was the first to use the VAL (french: véhicule automatique léger, en, light automated vehicle) sys ...

and the Sapporo Municipal Subway as well as

rubber-tyred tram A rubber-tyred tram (also known as tramway on tyres, french: tramway sur pneumatiques) is a development of the guided bus in which a vehicle is guided by a fixed rail in the road surface and draws current from overhead electric wires (either ...

s have guide wheels.

References

ISO 1005 Parts 1-9 BS 5892 Parts 1-6 AS7414.4

External links

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Train wheels
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