Railway wheels are the wheels of railway vehicles and are part of the wheel set and have different designs. They were originally spoke wheels , later disc wheels as well as tires and solid wheels, rubber-sprung and in some cases pneumatic tires. Depending on the function, a distinction is also made between driven drive wheels and only load-bearing running wheels. As a rule, the wheel disks of a wheel set sit in a torsion-proof manner on the axle shaft, but there are also loose wheel sets with wheels that are independently mounted on the stationary axle and individually mounted half or stub axles. In the case of a gauge changing wheel set for standard vehicles, the wheel disks on the axle shaft can be laterally displaced and locked in a rotationally fixed manner.
The first railway wheels were influenced by the carriage-building tradition and often designed as wooden spoked wheels. The following original version of the drive wheels is known from the American locomotive John Bull , built in 1830 : the wheel hubs were made of cast iron , the spokes and rims were made of hard robinia wood , and the three-quarters of an inch thick wheel tires were made of wrought iron .
Replica of a running axle of the locomotive Adler with spoke wheels
(Original 1835 Replica 1935)
Spoked wheels with curved spokes on the replica of the Saxonia
(original 1838, replica 1988)
The large drive wheels of modern steam locomotives were later made entirely of cast steel, but were still designed as spoked wheels to save weight. The first full-line electric locomotives also ran on spoked wheels. In the case of some types of single-axle drives with spring-loaded, powerful motors, the spoke wheels were also a functional necessity. Here, outriggers from the large wheel on a hollow shaft led through the spokes to the outside of the wheels, where they were connected to the wheel center via spring elements (steel pot springs / " spring pot drive " or rubber segments / " rubber segment spring drive ").
The wheels can have a coat of paint, but this leaves out the treads and the side surfaces of the wheel tire (where the track brakes on freight wagons work). In addition to corrosion protection , the paint also fulfills the function of making thermal overload and cracks recognizable.
In the event of thermal overload caused by a hot runner or a permanently applied brake, the paint burns with significant smoke development. When it has cooled down, there are visible peeling of the paint.
Appropriate coloring of the wheel can help identify damage caused by cracks in good time. For spoked wheels of German steam locomotives, a red color was chosen in which hairline cracks in the spokes were clearly visible, as the dark grease residues that collected in the cracks were clearly separated from the bright red. An even lighter color would have been unsuitable for recognizing a shiny fresh break.
Four colored markings are attached to tires that show whether the tire has twisted on the wheel center. On the Rhaetian Railway, on the other hand, the black and white paintwork is used to detect blocked wheels. In this train is due to the relatively slow reacting vacuum brake without possibility of a slide protection provided, in connection with the elevation changes of the routes and in winter with snow and ice, a particularly high risk that block the wheel sets.
Paint coatings on wheel disks are generally prohibited on American railways.
An alternative to spoked by the US General Steel Castings Corporation (Granite, Illinois) patented "Boxpok" construction (= english "boxed spoke"), in which the hollow wheel of spokes with U-shaped cross-section with composed of approximately oval side recesses of different sizes. With a given load, they are lighter than “real” spoked wheels.
The Baldwin-disc wheel was similar to Boxpok wheel of the Baldwin Locomotive Works (Eddystone, Pennsylvania) and by the Art Deco -Industriedesigner Henry Dreyfuss designed Hudson - streamlined locomotives of the New York Central - Series J-3a known Scullin-Doppelscheibenrad the American Scullin Steel Co. (St. Louis, Missouri). The Bulleid Firth Brown wheel (BFB wheel) developed in Great Britain by Oliver Bulleid and Firth Brown is not hollow, but the wheel disc has a trapezoidal, corrugated cross-section.
Boxpok wheels were particularly popular in Europe after the Second World War , for example in the locomotives of the Soviet class П36 (P36) and the Mikado universal locomotives of the class 141 R, which were delivered in large numbers from the USA and Canada to France as a reconstruction aid . As the only German steam locomotives, eight express tractors of the DR series 01.5 were temporarily equipped with Boxpok wheels, which, however, did not prove themselves due to manufacturing defects and were replaced by newly cast, reinforced spoked wheel sets.
Wheels with tires consist of a wheel body and a wheel tire surrounding it . These components each consist of a different steel alloy that is more suitable for their purpose and that can be processed separately before assembly (e.g. forging the wheel tires). This construction principle offered and still has some advantages over it today
- a homogeneous cast iron body (prone to breakage on the circumference),
- a homogeneous cast steel body (not yet economical to manufacture in the 19th century) or
- a homogeneous rotating body (not yet economical to manufacture in the 19th century) or
- a homogeneous forged body (not yet economical to manufacture in the 19th century)
In 1852/1853 Alfred Krupp invented the seamlessly rolled wheel tire in Essen : A forged, elongated piece of steel was split in the middle, driven apart in a ring, stretched and rolled. Krupp sold its wheel tires to most North American railways for decades , thus establishing the success of the later Krupp industrial empire . The three rings of the Krupp company symbol remind of this. At the same time, Jacob Mayer in Bochum succeeded in producing wheel tires directly as cast steel. Up to the beginning of the 20th century, both processes competed, but ultimately Krupp wheel tires formed from the block are more economical to manufacture and have better material properties due to the more intensive deformation of the steel. Before Krupp and Mayer invented the one-piece wheel tires, bars were bent round and welded - which led to frequent breaks at the welding point with the hard steel types required - or spirally wound from thinner bar material and then forged.
One advantage of wheel tires is that when the wheels are worn, the entire wheel disc does not have to be replaced. It is therefore not absolutely necessary to release the press connection between the wheel disc and the axle. Since the wheel tire rolls on the hard steel rails for an average of 600,000 kilometers , it has to be made of particularly resistant steel and attached extremely firmly to the wheel center. The disadvantage is the higher mass of tires with tires, firstly because of the amount of material required for the fit of the wheel center and wheel tire, for the stability of the press fit and because of the minimum thickness of the wheel tire
A rubber suspension ( rubber-sprung wheel set ) can be fitted between the wheel tire and the wheel disc . This increases driving comfort and has proven itself in trams and urban rapid transit vehicles. The ICE accident in Eschede , however, showed the limits of this system in high-speed traffic.
Assembly and disassembly
The connection of steel wheel tires to the wheel body is usually done by shrinking them on . For this purpose, the wheel tires are manufactured with a slightly smaller diameter than required for the operating state. The wheel tire is then heated to such an extent that its inner diameter is slightly larger than the outer diameter of the wheel body due to thermal expansion . The wheelset is used in this state. The wheel tire contracts again when it cools down and encloses the wheel body with a non-positive connection.
On the outside, wheel tires have a collar on the inner circumference, which serves as a stop when they are put on and prevents inward displacement. A groove is screwed into the inside into which a steel snap ring is inserted and rolled. It serves as a backup if the wheel tire loosens, in particular due to a fixed brake or a hot runner. There are also designs that are designed without an additional retaining ring groove. These wheel tires also have a stop collar on the inside, which prevents the wheel tire from moving sideways. The stop collar on the inside is made much smaller than on the outside, since it must be able to be pulled over the wheel body when the wheel tire is shrunk on after it has been heated.
For dismantling, a worn wheel tire is slit open, separated from the wheel body and disposed of. The wheel center will continue to be used after a test.
Load on wheel tires
Due to their shrink fit, wheel tires are constantly under tension. In addition to abrasion, small transverse cracks also appear on the treads of wheel tires on which the block brakes act. With high contact forces of ten tons per wheel, the rolling movement slowly shifts the material outwards and leads to rolling over on the outer edge. However, this rolling work on the running surface also relaxes the surfaces exposed to cracks from braking, so that there is no risk of breakage from the small transverse cracks. This is different when a brake block rubs on the outer tire edges and brings heat to the outer edge: This area is not relaxed by the rolling work, which leads to stress cracks on the outside. Wheel tires are examined for this cracking using ultrasound. Another hazard arises from the notch effect of stamps.
Railway wheels are loaded with wheel loads of up to over 11 t primarily in the central runway area. In addition to the weight forces, it is above all the drive and braking forces that have to be transmitted there. The flanges become thinner in medium-sized and narrow arcs on their flanks due to side wear. The wheel flange tops are usually not driven on.
In the case of tram vehicles, however, the wheel loads are less than 6 t. In contrast to railway wheels, it is primarily the wheel flange that is subjected to wear and tear and plastic deformation on its end face and also on its tip. Since radii of less than 20 m can occur in tram networks, side wear in particular is more pronounced than with railway wheels. In addition, especially in networks with a large proportion of road-flush stretches of road vehicles, road dirt or abrasion carried into the track channels of the grooved rails, which acts like an abrasive, especially in combination with moisture. This is expressed not least in shorter reprofiling intervals with mileages of 20,000 to 80,000 km between two wheelset machining.
In the past, the wheels of stationary trains were struck by a wagon master with a long, light hammer - even when they stopped in train stations. Depending on the wheel set, he was able to identify loose wheel tires or the beginning of fatigue fractures on the wheel set by the sound.
Today, the wheel tires are monitored in the workshops for the consequences of overheating such as hairline cracks and loosening of the seat. Color markings can be used to check whether a wheel tire has twisted on the wheel center. Hairline cracks can be detected by ultrasound examinations and X-ray fine structure images. A wheel tire that has become loose can be distinguished from a stuck tire by means of a sound test: If the sound is as bright as a bell on 90% of the circumference and not muffled, the wheel tire can be regarded as tight. The wheel is provided with colored markings to check that it is firmly seated and may be used again, provided that no metal chips have escaped in the seat and the wheel tire does not twist again.
With a full wheel or monoblock wheel, the wheel disc and tread are made from one piece, which is the rule in modern vehicles. A heat treatment ensures that the soft wheel disc made of tough cast steel has a hard, wear-resistant running surface. Compared to a two-part wheel consisting of a wheel disc and wheel tire, there is a saving in mass with solid wheels; on the other hand, if the tread is worn, the entire solid wheel must be replaced. In contrast to two-part wheels, if a full wheel with block brakes overheats , there is no risk of loose wheel tires, but high internal stresses arise in the direction of the wheel circumference of the tread, which can lead to the wheel disc breaking. Solid wheels must therefore be checked regularly for traces of possible overheating during operation and checked for cracks using ultrasound during maintenance. In some cases, overheated wheels can be thermally regenerated so that they do not have to be scrapped. Full bikes can run between 1 and 2.5 million kilometers. The wheel is worn down to a diameter of 80 mm (see: DB series 101 ). A groove in the face of the wheel shows that the wear limit has been reached.
Assembly and disassembly
The solid wheel is pressed onto the wheelset shaft with a wheelset press. After the assembly of both wheels, the distances between the backs of the wheels are measured. Full wheel sets are balanced. To do this, the axle is clamped off-center and the disc on the inner rim rim is sickled. The wheel set is dismantled by pressing the wheel disks.
Loads and tests
A full wheel with block brakes is subject to similar loads as a tire with tires. The full pane is also examined for cracks on the outside. But there is an additional burden: If a full bike overheats, the running area expands. It pulls the middle, springy area of the wheel with it. After the bike has cooled down, the stretched middle area presses outwards onto the running area. The tread relaxes again through rolling work, but the flange does not. The forces are concentrated on the wheel flange and lead to deep cracks. For this reason, overheated solid gears should be replaced and the refurbishment relaxed. During refurbishment, the solid wheels are subjected to a full ultrasonic test including a residual stress measurement.
In the case of disc-braked solid wheels that achieve very high mileages without having to be turned off, in rare cases there can be a duplication of material in the running area. These cases are very rare and the causes are not yet fully understood. The material is peeling off a few millimeters below the running surface, which is why the full wheels are limited in running kilometers until the next refurbishment.
Wheel rim hardening / targeted hardening of running surfaces
The wheel runs on the rail and experiences greater wear in this pairing. This is due to the fact that the structure of the running surface of the finished wheel is pearlitic . For this purpose, only the tread and the flange of the forged wheel are hardened in a HEESS quenching bath by applying water in a targeted manner. The web and the hub of the wheel are not hardened. The heat treatment is basically carried out as follows:
- Heating the railway wheel in a high-temperature furnace to around 860 ° C (= austenitizing )
- Hold at 860 ° C (the hold time depends on the material and cross-section)
- Wheel rim hardening with water or a water-air mixture in the HEESS quenching bath
- Heating the train wheel in a low temperature furnace to around 550 ° C ( tempering = relaxing)
- Hold at 550 ° C (the hold time again depends on the material and cross-section)
- Cooling in air
As a result of the heat treatment, the following layers are created at the edge of the tread when viewed from the outside in
- A hard layer of pure bainite
- A mixed layer of bainite and perlite
- Basic structure: layer made of around 95 percent pearlite and five percent ferrite
The hard layer and the mixed layer are turned off in the subsequent hard machining so that the basic structure, namely essentially pearlite, remains. In Europe, low-alloy forged structural steels are used for the production of railway wheels. Common material names are R7, R8 and R9. The UIC-812-3 standard of the International Union of Railways specifies the required technical properties before and after heat treatment. Key specifications are made for:
- Brinell hardness at a depth of 30 millimeters
- Notched impact strength
- tensile strenght
- Structure of the turned part
- Level of residual stress
Low residual stress wheelsets
In order to counteract the additional warming that occurs when composite brake blocks are used and the resulting possible stress cracks, wheel sets with low residual stress have been used on a trial basis since the end of the 1980s and since the mid-1990s. The composite brake blocks are less able to dissipate the resulting brake heat than the gray cast iron brake blocks, so that the wheel disc has to dissipate more heat energy and is therefore exposed to greater temperature fluctuations.
In order to effectively combat stress cracks caused by temperature fluctuations, a wheel disc was developed that is less sensitive to such stresses. This wheel disc differs mainly in its pronounced S-shape between the wheel hub and the tread body, which results in better stress reduction than with flat wheel discs. As a side effect, the larger surface also improves heat dissipation. Such wheel sets are marked with an interrupted, vertical white line on the bearing housing on freight wagons.
Some subway networks and individual vehicles - for example the French Micheline - use wheels with pneumatic tires. The advantage of these wheels is that the tread material rubber on steel rails has a considerably higher coefficient of static friction than steel wheels. This allows for higher accelerations and braking decelerations, which allows for correspondingly shorter travel times and also more dense schedule cycles between stops that are close together. The higher static friction is also an advantage for routes with steep inclines, such as the Lausanne metro . In addition, the rubber wheels cause far fewer vibrations than conventional steel wheels, which is particularly noticeable on routes in simple low-lying areas in connection with a solid road surface , for example at the Métro Lyon . A disadvantage of wheel sets with pneumatic tires is the lack of self-centering in the track; in addition, the higher static friction between wheel and rail in drive wheel sets requires the installation of differential gears.
Depending on the railway system, the pneumatic tires run on standard railway tracks or on roadways specially designed for rubber tires. For travel on standard steel rails as well as over switches, the flanges of the additional classic railway wheels, parallel to the rubber tires, take the lead; they are also the emergency running elements in the event of a flat tire. In normal operation, the additional steel wheels do not touch the rail heads. In the case of roadways designed exclusively for rubber tires, additional other guide elements are required, such as lateral guide rails and horizontal guide wheels.
The comparatively complex design entails correspondingly higher acquisition costs. The construction costs for the track are around twice as high as for a conventional friction track. Conversely, the effort for maintaining the track is lower because due to the different hardness of the materials, only the tires on the train are subject to wear, but not the rails.
The "Howden-Meredith patent wheels" used in rail buses in Ireland are a special case . Richard Meredith and George Howden developed a railway wheel in which a gas-filled pneumatic tire was enclosed by a steel wheel tire. The Irish railway company Great Northern Railway (GNR) built in the 1930s for itself and other operators a number of rail vehicles on the basis of street buses, in which this system was used.
Rail vehicles derive a great economic advantage from the fact that they can convert the drive energy much more efficiently than many other vehicles. The low friction of steel wheels on the rail , which on the one hand causes the generally poorer braking and acceleration behavior of rail vehicles, on the other hand leads to an efficient use of the energy required for long journeys with heavy loads.
An optimal use of the running properties of rail vehicles, however, requires uniform standards for the geometric design of wheels and tracks. The different fits of wheels and rails are one reason why rail vehicles lose some of their technical efficiency as soon as they drive on track systems that have been built according to different standards (other countries, track designs and rail inclinations). Slower or different average speeds result in increased energy consumption, the stress on the flange (especially in bends) can lead to higher maintenance costs. In the European standard and broad-gauge networks, however, the dimensions for wheelsets and track are internationally standardized. However, there are deviations in the dimensions in the Chinese and North American standard gauge networks, for example in the important distance between the rear surfaces of the wheel discs and in the rail inclination and tread profiles. An exchange of vehicles between Europe and the Middle East and the Chinese or North American network usually requires, in addition to further adjustments, a wheelset exchange, and in some cases an outline editing. Vehicles that are supposed to run equally on road and rail networks are given wheels with compromise profiles. Their disadvantage is the lower tolerances and the more frequent reprofiling that is required.
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