Rail
Further information: Rail tracks See also: Track class (United States) and Cant deficiencyOn railways, cant helps a train steer around a curve, keeping the wheel flanges from touching the rails, minimizing friction and wear.
The main functions of cant are to:
- better distribute load across both rails
- reduce rail- and wheel-wear
- neutralize the effect of lateral forces
- improve passenger comfort
The necessary cant in a curve depends on the expected speed of the trains and the radius. However, it may be necessary to select a compromise value at design time, for example if slow-moving trains may occasionally use tracks intended for high-speed trains.
Generally the aim is for trains to run without flange contact, which also depends on the tyre profile of the wheels. Allowance has to be made for the different speeds of trains. Slower trains will tend to make flange contact with the inner rail on curves, while faster trains will tend to ride outwards and make contact with the outer rail. Either contact causes wear and tear and may lead to derailment. Many high-speed lines do not permit slower freight trains, particularly with heavier axle loads. In some cases, the impact is reduced by the use of flange lubrication.
Ideally, the track should have sleepers (railroad ties) at a closer spacing and a greater depth of ballast to accommodate the increased forces exerted in the curve.
At the ends of a curve, the amount of cant cannot change from zero to its maximum immediately. It must change (ramp) gradually in a track transition curve. The length of the transition depends on the maximum allowable speed – the higher the speed, the greater length is required.
In the United States, maximum speed is subject to specific rules. The maximum speed of a train on curved track for a given cant deficiency or unbalanced superelevation is determined by the following formula:
where is the height in inches that the outside rail is "superelevated" above the inside rail on a curve, is the unbalanced superelevation or cant deficiency in inches and is the degree of curvature in degrees per 100 feet (30 m). is in miles per hour.
For the US standard maximum unbalanced superelevation of 3 inches, the formula is:
The maximum value of cant (the height of the outer rail above the inner rail) for a standard gauge railway is about 6 inches (150 mm). For high speed railways in Europe, maximum cant is 180mm (about 7 inches).
Track unbalanced superelevation in the US is restricted to 3 inches (76 mm), though 4 inches (102 mm) is permissible by waiver. There is no general maximum set for European railways, some of which have curves with over 11 inches (280 mm) of unbalanced superelevation to permit high-speed transportation. There are country-specific regulations that may apply, though.
Read more about this topic: Cant (road/rail)
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