Atrain wheel orrail wheel is a type ofwheel specially designed for use onrailway tracks. The wheel acts as a rolling component, typicallypress fitted on to anaxle and mounted directly on arailway carriage orlocomotive, or indirectly on abogie (CwthE) or truck (NAmE). The powered wheels under the locomotive are calleddriving wheels. Wheels are initiallycast orforged and then heat-treated to have a specific hardness.[1] New wheels are machined using alathe to a standardised shape, called a profile. All wheel profiles are regularly checked to ensure properinteraction between the wheel and the rail.[citation needed] Incorrectly profiled wheels and worn wheels can increaserolling resistance, reduceenergy efficiency and may even cause aderailment.[2] TheInternational Union of Railways has defined a standard wheel diameter of 920 mm (36 in), although smaller sizes are used in somerapid transit railway systems and onro-ro carriages.[3]
The running surface ("tread" or "rim") of most train wheels isconical, which serves as the primary means of keeping the train aligned centrally on thetrack while in motion. On curves, thewheelset becomes "self-steering" because of this coning: when one wheel is pushed closer to the adjacent (outer) rail, it presents a bigger circumference to the rail than when centred on the track. Simultaneously, the inner wheel presents a smaller circumference to its rail. 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.[4] If a strong sideways force is experienced – for example, when the radius of a curve is smaller than normal or there are defects in the track alignment – the wheelset will depart from its equilibrium. That is when a projection on the inner side of each wheel, called a flange, constrains the wheelset from moving further and derailing.[5]
The number of wheels per locomotive or car varies in both size and number to accommodate the needs of the vehicle. Regardless of these factors, pairs of identically sized wheels are always affixed to a straight axle as a singular unit, called awheelset.[4]
Wheels used forroad–rail vehicles are normally smaller than those found on other types ofrolling stock, such as locomotives or carriages, because the wheel has to be stowed 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.[citation needed]
Modern railway wheels are usually machined from a single casting, also known as monobloc wheels.[6] Some wheels, however, are made of two parts: the wheel core, and a tyre (CwthE) ortire (NAmE) around the perimeter. Separate tyres have been a component of some rolling stock, their purpose being to provide a replaceable wearing element – an important factor in the steam locomotive era with their costlyspoked construction. In modern times the tyre is invariably made fromsteel, which is stronger than thecast iron 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 asrubber, between the wheel and tyre. Failure of this type of wheel was one of the causes leading to theEschede high-speed train crash.[6]
The most common cause of wheel damage is severe braking, including sudden braking, braking on steep gradients and braking with heavy loads. Thebrake shoes (or brake blocks) are applied directly to the wheel surface, which generates immense amounts ofthermal energy: under normal operation, a wheel may reach a tread temperature of 550 °C (1,022 °F).[7] Under severe braking conditions, the generated thermal energy can contribute tothermal shock or alteration of the wheel's mechanical properties. Ultimately, acute thermal loading leads to a phenomenon calledspalling. Alternatively, severe braking orlow adhesion may stop the rotation of the wheels while the vehicle is still moving, which may cause aflat spot on thewheel-rail interface and localised heat damage.[citation needed]
Modern railway wheels are manufactured reasonably thickly to provide an allowance of wear material, since worn wheel profiles or wheels with a flat spot can be machined on a wheel lathe if there is sufficient thickness of material remaining.[8]
Rubber-tyred metros with a centralguide rail, such as theBusan Metro,Lille Metro and theSapporo Municipal Subway as well asrubber-tyred trams haveguide wheels.[citation needed]
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