Train wheel

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Railroad car wheels are affixed to a straight axle, allowing both wheels to rotate at the same time. This is called a wheelset. Rollingstock axle.jpg
Railroad car wheels are affixed to a straight axle, allowing both wheels to rotate at the same time. This is called a wheelset.
Train wheels rolling on a train track in Japan, 2022

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 onto an axle and mounted directly on a railway carriage or locomotive, or indirectly on a bogie (in the UK), also called a truck (in North America). 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 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, 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

Railway wheel flange (left)
Tram wheel flange (right) TreinTramwielprofiel.svg
Railway wheel flange (left)
Tram wheel flange (right)

The running surface of most train wheels is conical, which serves as the primary means of keeping the train aligned with the track while 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. [4]

Flanged railway wheel Flanged wheel.jpg
Flanged railway wheel

Almost all train wheels have a projection, called 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 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

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 a wheelset. [4]

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. Land cruiser hyrail conversion.jpg
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 for road–rail vehicles are normally smaller than those found on other types of rolling 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 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. [5] Some wheels, however, are made of two parts: the wheel core, and a tire ("tyre" in British English, Australian English 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, 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 tire. Failure of such kind of wheel was one of the causes leading up to the Eschede high-speed train crash. [5]

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 (or blocks) are applied directly to the wheel surface which generates immense amounts of thermal energy. Under normal operation, a wheel may obtain a tread temperature of 550 °C (1,022 °F). [6] 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 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. [7]

Guide wheel

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.

Left: diagram of the Translohr guide rail (green) and the tram's guide wheels (red). Right: cross section of the guide rail and guide wheel of the Bombardier's GLT

See also

Related Research Articles

<span class="mw-page-title-main">Bogie</span> Chassis for wheels and suspension under vehicles

A bogie 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 transport. A bogie may remain normally attached or be quickly detachable. It may include a suspension component within it, or be solid and in turn be suspended ; it may be mounted on a swivel, as traditionally on a railway carriage or locomotive, additionally jointed and sprung, or held in place by other means.

<span class="mw-page-title-main">Wheel</span> Circular component rotating on an axle

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 moved easily facilitating movement or transportation while supporting a load, or performing labor in machines. Wheels are also used for other purposes, such as a ship's wheel, steering wheel, potter's wheel, and flywheel.

<span class="mw-page-title-main">Rubber-tyred metro</span> Form of rapid transit

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 traditional railway steel wheels with deep flanges on steel tracks for guidance through conventional switches as well as guidance in case a tyre fails. Most rubber-tyred trains are purpose-built and designed for the system on which they operate. Guided buses are sometimes referred to as 'trams on tyres', and compared to rubber-tyred metros.

As used in mechanical engineering, the term tractive force can either refer to the total traction a vehicle exerts on a surface, or the amount of the total traction that is parallel to the direction of motion. In railway engineering, the term tractive effort is often used synonymously with tractive force to describe the pulling or pushing capability of a locomotive. In automotive engineering, the terms are distinctive: tractive effort is generally higher than tractive force by the amount of rolling resistance present, and both terms are higher than the amount of drawbar pull by the total resistance present. The published tractive force value for any vehicle may be theoretical—that is, calculated from known or implied mechanical properties—or obtained via testing under controlled conditions. The discussion herein covers the term's usage in mechanical applications in which the final stage of the power transmission system is one or more wheels in frictional contact with a roadway or railroad track.

Rail transport terms are a form of technical terminology applied to railways. Although many terms are uniform across different nations and companies, they are by no means universal, with differences often originating from parallel development of rail transport systems in different parts of the world, and in the national origins of the engineers and managers who built the inaugural rail infrastructure. An example is the term railroad, used in North America, and railway, generally used in English-speaking countries outside North America and by the International Union of Railways. In English-speaking countries outside the United Kingdom, a mixture of US and UK terms may exist.

<span class="mw-page-title-main">Trackless train</span> Passenger transportation vehicle

A trackless train — or tram, road train, land train, or parking lot train is a road-going articulated vehicle used for the transport of passengers, comprising a driving vehicle pulling one or more carriages connected by drawbar couplings, in the manner of a road-going railway train. Similar vehicles may be used for transport of freight or baggage for short distances, such as at a factory or airport.

<span class="mw-page-title-main">Wheelbase</span> Distance between the centers of the front and rear wheels

In both road and rail vehicles, the wheelbase is the horizontal distance between the centers of the front and rear wheels. For road vehicles with more than two axles, the wheelbase is the distance between the steering (front) axle and the centerpoint of the driving axle group. In the case of a tri-axle truck, the wheelbase would be the distance between the steering axle and a point midway between the two rear axles.

<span class="mw-page-title-main">Derailment</span> Form of train incident

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 serious hazard.

<span class="mw-page-title-main">Rolling resistance</span> Force resisting the motion when a body rolls on a surface

Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the motion when a body rolls on a surface. It is mainly caused by non-elastic effects; that is, not all the energy needed for deformation of the wheel, roadbed, etc., is recovered when the pressure is removed. Two forms of this are hysteresis losses, and permanent (plastic) deformation of the object or the surface. Note that the slippage between the wheel and the surface also results in energy dissipation. Although some researchers have included this term in rolling resistance, some suggest that this dissipation term should be treated separately from rolling resistance because it is due to the applied torque to the wheel and the resultant slip between the wheel and ground, which is called slip loss or slip resistance. In addition, only the so-called slip resistance involves friction, therefore the name "rolling friction" is to an extent a misnomer.

<span class="mw-page-title-main">Adhesion railway</span> Railway which relies on adhesion traction to move a train

An adhesion railway relies on adhesion traction to move the train, and is the most widespread and common type of railway in the world. Adhesion traction is the friction between the drive wheels and the steel rail. Since the vast majority of railways are adhesion railways, the term "adhesion railway" is used only when it is necessary to distinguish adhesion railways from railways moved by other means, such as by a stationary engine pulling on a cable attached to the cars or by railways that are moved by a pinion meshing with a rack.

Traction, traction force or tractive force is a force used to generate motion between a body and a tangential surface, through the use of either dry friction or shear force. It has important applications in vehicles, as in tractive effort.

<span class="mw-page-title-main">Variable gauge</span>

A variable gauge system allows railway vehicles in a train to travel across a break of gauge between two railway networks with different track gauges. For through operation, a train must be equipped with special bogies holding variable gauge wheelsets which contain a variable gauge axle (VGA).

<span class="mw-page-title-main">Hunting oscillation</span> Self-oscillation about an equilibrium that is usually unwanted

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 fields as electronics, aviation, biology, and railway engineering.

<span class="mw-page-title-main">Wheelset (rail transport)</span> Pair of railroad wheels fixed onto an axle

wheelset is a pair of railroad vehicle wheels mounted rigidly on an axle such that both wheels rotate in unison. Wheelsets are often mounted in a bogie – a pivoted frame assembly holding at least two wheelsets – at each end of the vehicle. Most modern freight cars and passenger cars have bogies each with two wheelsets, but three wheelsets are used in bogies of freight cars that carry heavy loads, and three-wheelset bogies are under some passenger cars. Four-wheeled goods wagons that were once near-universal in Europe and Great Britain and their colonies have only two wheelsets; in recent decades such vehicles have become less common as trainloads have become heavier.

<span class="mw-page-title-main">Railway tire</span>

The steel wheel of a steam locomotive and other older types of rolling stock were usually fitted with a steel tire or tyre to provide a replaceable wearing element on a costly wheel.

Wheel slide protection and wheel slip protection are railway terms used to describe automatic systems used to detect and prevent wheel-slide during braking or wheel-slip during acceleration. This is analogous to ABS and traction control systems used on motor vehicles. It is particularly important in slippery rail conditions.

<span class="mw-page-title-main">Flat spot</span>

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 caused by use of the emergency brake, or slip and slide conditions that cause wheels to lock up while the train is still moving. Flat spots are more common in the autumn and winter when the rails are slippery. Flat spots can also be caused by faulty brakes or wheelset bearings.

<span class="mw-page-title-main">Axle track</span> Distance across between the wheels of a vehicle

In automobiles, the axle track is the distance between the hub flanges on an axle. Wheel track, track width or simply track refers to the distance between the centerline of two wheels on the same axle. In the case of an axle with dual wheels, the centerline of the dual wheel assembly is used for the wheel track specification. Axle and wheel track are commonly measured in millimetres or inches.

A railway or railroad is a track where the vehicle travels over two parallel steel bars, called rails. The rails support and guide the wheels of the vehicles, which are traditionally either trains or trams. Modern light rail is a relatively new innovation which combines aspects of those two modes of transport. However fundamental differences in the track and wheel design are important, especially where trams or light railways and trains have to share a section of track, as sometimes happens in congested areas.


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