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The steel wheels of steam locomotives and other older types of rolling stock were usually 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 was 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 tyre 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 tyre is machined to be slightly less than the diameter of the wheel centre on which it is mounted, to give an interference fit.
The tyre is fitted by heating to a controlled temperature, avoiding overheating. This causes the tyre to expand. The wheel centre, usually already mounted on the axle, is lowered into the tyre which is flange side up. The tyre 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 on to the retaining ring.
The tyre is primarily held in place by its interference fit. The shoulder on the outside and the retaining ring also keep the tyre 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 tyre is by machining out the retaining ring and heating the tyre to relax the interference fit.
Some steam locomotive wheels had tyres bolted through the rim, or with a second smaller choulder machined on the inside face of the tyre. This shoulder was severely limited in size as it had to pass over the wheel centre for assembly.
Tyres of different designs were fitted to wheels with wooden centres (Maunsell wheels in the UK) and to various other types.
The use of tyres is becoming obsolete. The utilisation of traditional freight wagons was often so low that tyres never needed renewal, so it was cheaper to fit a one-piece ("monobloc") wheel. Monobloc wheels are lighter and offer better integrity as there is no tyre to come loose. Modern flow-line repair lines are disrupted by the inspection of the wheel centre once the tyre is removed, possibly generating extra rectification work, and the need to make each tyre fit its allocated wheel centre. Monobloc wheels are now more economical.
The most usual cause of damage is drag braking on severe gradients. Because the brake blocks apply directly on the tyre, it is heated up, relaxing the interference fit. It is not feasible to fit the tyre with such a heavy interference as to eliminate this risk entirely, and the retaining ring will ensure that the tyre can only rotate on the wheel centre, 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 can cause a flat spot on the tire and localised heat damage to the tire material.
Tires are reasonably thick, about 3 inches, giving plenty of room for wear. Worn tyres or tyres with flats are reprofiled on a wheel lathe if there is sufficient thickness of material remaining.
Some trains, mostly metros and people movers, have rubber tires, including some lines of the Paris Métro, the Mexico City Metro, the Caracas Metro, the Montreal Metro, Sapporo Subway, Seattle Center Monorail, Taipei Rapid Transit System and the Santiago Metro.
ISO 1005 Parts 1-9 BS 5892 Parts 1-6