A train wheel or rail wheel is a type of wheel specially designed for use on rail tracks. A rolling component is typically pressed onto an axle and mounted directly on a rail car or locomotive or indirectly on a bogie (UK), also called a truck (North America). Wheels are cast or forged and are heat-treated to have a specific hardness. New wheels are trued, using a lathe, to a specific profile before being pressed onto an axle. All wheel profiles need to be periodically monitored to ensure proper wheel-rail interface. Improperly trued wheels increase rolling resistance, reduce energy efficiency and may create unsafe operation. A railroad wheel typically consists of two main parts: the wheel itself, and the tire (or tyre) around the outside. A rail tire is usually made from steel, and is typically heated and pressed onto the wheel, where it remains firmly as it shrinks and cools. Monobloc wheels do not have encircling tires, while resilient rail wheels have a resilient material, such as rubber, between the wheel and tire.
Wheel geometry and flange
Most train wheels have a conical geometry, which is the primary means of keeping the train's motion aligned with the track. Train wheels have a flange on one side to keep the wheels, and hence the train, running on the rails when the limits of the geometry-based alignment are reached, e.g. due to some emergency or defect. See Hunting oscillation. Some wheels have a cylindrical geometry, where flanges are essential to keep the train on the rail track.
Wheels for road-rail vehicles
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 standard for infrastructure maintenance vehicle wheels.
The wheels of many rail vehicles, especially steam locomotives and older types of rolling stock, are fitted with steel tires (or tyres in British English, Australian English and others) to provide a replaceable wearing element on a costly wheel.
Replacing a whole wheel because of a worn contact surface is expensive, so older types of railway wheels were fitted with replaceable steel tires. The tire is a hoop of steel that is fitted around the steel wheel centre. The tire is machined with a shoulder on its outer face to locate it on the wheel centre, and a groove on the inside diameter of the flange face. The inside diameter of the tire is machined to be slightly less than the diameter of the wheel centre on which it is mounted, to give an interference fit.
The tire is fitted by heating to a controlled temperature, avoiding overheating. This causes the tire to expand. The wheel centre, usually already mounted on the axle, is lowered into the tire which is flange-side-up. The tire cools, and the retaining ring (a shaped steel bar rolled into a hoop) is fitted into the groove. Hydraulically operated rolls swage the groove down onto the retaining ring.
The tire is primarily held in place by its interference fit. The shoulder on the outside and the retaining ring also keep the tire in place if the interference fit is lost. This is most often due to severe drag braking down a gradient, or due to an error in the machining.
Removal of a worn tire is by machining out the retaining ring and heating the tire to relax the interference fit.
Some steam locomotive wheels had tires bolted through the rim, or with a second smaller shoulder machined on the inside face of the tire. This shoulder was severely limited in size as it had to pass over the wheel centre for assembly.
Tires of different designs were fitted to wheels with wooden centers (Mansell wheels in the UK) and to various other types.
The use of tires is becoming obsolete. The utilisation of traditional freight wagons was often so low that tires never needed renewal, so it was cheaper to fit a one-piece ("monoblock") wheel. Monoblock wheels are lighter and offer better integrity as there is no tire to come loose. Modern flow-line repair lines are disrupted by the inspection of the wheel centre once the tire is removed, possibly generating extra rectification work, and the need to make each tire fit its allocated wheel centre. Monoblock wheels are now more economical.
Causes of damage
The most usual cause of damage is drag braking on severe gradients. Because the brake blocks apply directly on the tire, it is heated up, relaxing the interference fit. It is not feasible to fit the tire with such a heavy interference as to eliminate this risk entirely, and the retaining ring will ensure that the tire can only rotate on the wheel center, maintaining its alignment. In rare instances the rotation could be so severe as to wear the retaining ring down till it breaks, which could result in derailment.
Severe braking or low adhesion may stop the rotation of the wheels while the vehicle is still moving, which can cause a flat spot on the tire and localized heat damage to the tire material.
Tires are reasonably thick, about 3 inches (76 mm), giving plenty of room for wear. Worn tires or tires with flats are re-profiled on a wheel lathe if there is sufficient thickness of material remaining.
- Liszewski, Andrew (15 March 2017). "The Reason Trains Have Angled Wheels Is Incredibly Clever". Gizmodo. Retrieved 14 March 2017.
ISO 1005 Parts 1-9 BS 5892 Parts 1-6 AS7414.4
- "APTA PR-CS-RP-003-98 Recommended Practice for Developing a Clearance Diagram for Passenger Equipment 184.108.40.206 Design tolerances" (PDF). APTA.com. American Public Transportation Association. 1998-03-26. Retrieved 2015-01-17.
- Train wheels