Train wheel

Not to be confused withTraining wheels.

Railroad car wheels are fixed rigidly to their axle, ensuring both wheels of thewheelsetrotate at the same rate

Atrain wheelorrail wheelis a type ofwheelspecially designed for use onrailway tracks.The wheel acts as a rolling component, typicallypress fittedonto anaxleand mounted directly on arailway carriageorlocomotive,or indirectly on abogie(in the UK), also called atruck(in North America). The powered wheels under the locomotive are calleddriving wheels.Wheels are initiallycastorforgedand then heat-treated to have a specific hardness.New wheels are machined using alatheto a standardized shape, called a profile, before being installed onto an axle. All wheel profiles are regularly checked to ensure properinteraction between the wheel and the rail.Incorrectly profiled wheels and worn wheels can increaserolling resistance,reduceenergy efficiencyand may even cause aderailment.TheInternational Union of Railwayshas defined a standard wheel diameter of 920 mm (36 in), although smaller sizes are used in somerapid transitrailway systems and onro-ro carriages.

Wheel geometry and flange

See also:Derailment,Adhesion railway,andHunting oscillation

A railway wheel'streadandflangeand its relationship to the load-bearing rail

The running surface of most train wheels is conical, which serves as the primary means of keeping the train aligned with thetrackwhile in motion. The wheels are fixed on an axle, and when rounding a curve the mass of the train pushes the wheelset towards the outside of the track. 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. The difference between the distances travelled by each wheel for each rotation of the axle causes the wheelset to follow the curve of the track.

Almost all train wheels have a projection, called a flange, on one side to keep the wheels, and hence the train, running on therailswhen the limits or tests of alignment are reached: when a bend is taken at appropriate speed, when there are strong side-winds, and to withstand most common defects in trackbed, rail and mild debris. Some wheels do not have a conical profile and instead are cylindrical, such that the flanges are essential to keep the train on the track.

Wheel arrangement

Main article:Wheel arrangement

The number of wheels per locomotive or car varies 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 awheelset.

Wheels for road-rail vehicles

The small rail wheels fitted to road-rail vehicles allow them to be stored away when the vehicle is in road-going mode.

Wheels used forroad–rail vehiclesare normally smaller than those found on other types ofrolling stock(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 245 mm (9.65 in) in diameter. In Australia, wheels forroad-rail vehiclesshould comply with the requirements of AS7514.4, which is the Australian standard for infrastructure maintenance vehicle wheels.

Railway wheel and tire

Main article:Railway 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 atire( "tyre" inBritish English,Australian Englishand 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 costlyspokedconstruction. In modern times the tire is invariably made fromsteel,which is stronger than thecast ironof earlier eras. It is typically heated and pressed on to the wheel before it cools and shrinks.Resilient rail wheelshave a resilient material, such asrubber,between the wheel and tire. Failure of such kind of wheel was one of the causes leading up to theEschede 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. Thebrake shoes(or blocks) are applied directly to the wheel surface which generates immense amounts ofthermal energy.Under normal operation, a wheel may obtain a tread temperature of 550 °C (1,022 °F).Under severe braking conditions, the generated thermal energy can contribute tothermal shockor alteration of the wheel's mechanical properties. Ultimately, acute thermal loading leads to a phenomenon calledspalling.Alternatively, severe braking orlow adhesionmay stop the rotation of the wheels while the vehicle is still moving, which may cause aflat spoton thewheel-rail interfaceand 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 metroswith a centralguide rail,such as theBusan Metro,Lille Metroand theSapporo Municipal Subwayas well asrubber-tyred tramshaveguide wheels.

Left: diagram of theTranslohrguide rail(green) and the tram'sguide wheels(red). Right: cross section of the guiderailandguide wheelof the Bombardier'sGLT

See also

• Adhesive weight
• Cycloidandtrochoid
• Wheelset (rail transport)#Conical wheel-tread

References

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

External links

• "APTA PR-CS-RP-003-98 Recommended Practice for Developing a Clearance Diagram for Passenger Equipment 5.3.2.1 Design tolerances"(PDF).APTA.com.American Public Transportation Association.1998-03-26.Retrieved2015-01-17.
• Train wheels