Train wheel
A train wheel or rail wheel is a type of wheel specially designed for use on railway tracks. The wheel acts as a rolling component, typically press fitted on to an axle and mounted directly on a railway carriage or locomotive, or indirectly on a bogie (BrE) or truck (NAmE). The powered wheels under the locomotive are called driving wheels. Wheels are initially cast or forged and then heat-treated to have a specific hardness.[1] New wheels are machined using a lathe to a standardised shape, called a profile. All wheel profiles are regularly checked to ensure proper interaction between the wheel and the rail.[citation needed] Incorrectly profiled wheels and worn wheels can increase rolling resistance, reduce energy efficiency and may even cause a derailment.[2] The International Union of Railways has defined a standard wheel diameter of 920 mm (36 in), although smaller sizes are used in some rapid transit railway systems and on ro-ro carriages.[3]
Wheel geometry and flange
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The running surface ("tread" or "rim") of most train wheels is conical, which serves as the primary means of keeping the train aligned centrally on the track while in motion. On curves, the wheelset 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]
Wheel arrangement
[edit]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 a wheelset.[4]
Wheels for road–rail vehicles
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Wheels used for road–rail vehicles are normally smaller than those found on other types of rolling 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]
Railway wheel and tire
[edit]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) or tire (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 costly spoked construction. In modern times the tyre is invariably made from steel, which is stronger than the cast 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 as rubber, between the wheel and tyre. Failure of this type of wheel was one of the causes leading to the Eschede high-speed train crash.[6]
Causes of damage
[edit]The most common cause of wheel damage is severe braking, including sudden braking, braking on steep gradients and braking with heavy loads. The brake shoes (or blocks) are applied directly to the wheel surface, which generates immense amounts of thermal 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 to thermal shock or alteration of the wheel's mechanical properties. Ultimately, acute thermal loading leads to a phenomenon called spalling. 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 on the wheel-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]
Guide wheel
[edit]Rubber-tyred metros with a central guide rail, such as the Busan Metro, Lille Metro and the Sapporo Municipal Subway as well as rubber-tyred trams have guide wheels.[citation needed]
See also
[edit]References
[edit]- ^ Lewis, R.; Olofsson, U. (25 September 2009). Wheel–Rail Interface Handbook. Elsevier Science. ISBN 9781845694128. Retrieved 29 October 2020.
- ^ Lewis, Roger; Olofsson, Ulf (2009). Wheel-rail interface handbook. Boca Raton, Florida: CRC Press. ISBN 978-1-61583-153-1. OCLC 500906475.
- ^ Licitra, Gaetano (6 September 2012). Noise Mapping in the EU: Models and Procedures. CRC Press. ISBN 978-0-203-84812-8.
- ^ a b "Book: The Contact Patch". the-contact-patch.com. Retrieved 29 October 2020.
- ^ Richard Feynman (1983). Feynman: how the train stays on the track. Fun to Imagine. BBC TV – via YouTube.com.
- ^ a b Milne, Ian; Ritchie, R. O.; Karihaloo, B. L. (25 July 2003). Comprehensive Structural Integrity. Elsevier. ISBN 978-0-08-049073-1.
- ^ Peters, Carsten J.; Eifler, Dietmar (1 November 2009). "Influence of Service Temperatures on the Fatigue Behaviour of Railway Wheel and Tyre Steels*". Materials Testing. 51 (11–12): 748–754. Bibcode:2009MTest..51..748P. doi:10.3139/120.110094. ISSN 2195-8572. S2CID 135684020.
- ^ Nielsen, J. (1 January 2009). "Out-of-round railway wheels". In Lewis, R.; Olofsson, U. (eds.). Wheel–Rail Interface Handbook. Woodhead Publishing. pp. 245–279. doi:10.1533/9781845696788.1.245. ISBN 978-1-84569-412-8. Retrieved 29 October 2020.
External links
[edit]- "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. 26 March 1998. Retrieved 17 January 2015.
