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It has been suggested that Cant deficiency be merged into this article or section. (Discuss) |
The cant of a railway track (also referred to as superelevation) or a road (sometimes referred to as camber or cross slope) is the difference in elevation (height) between the two edges. A non-zero cant gives a banked turn, allowing vehicles to traverse the turn at higher speeds than would otherwise be possible.[clarification needed (only if outside edge higher, surely?)]
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Rail
On 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 amount of cant must be chosen for a given speed, and if trains traverse the curve at different speeds, the cant ceases to serve its purpose, and can lead to damage. As a result, a compromise value of cant must be chosen during design.
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. The cant 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 is determined by the following formula:[citation needed]
where Ea is the height in inches that the outside rail is "superelevated" above the inside rail on a curve and d is the degree of curvature in degrees per 100 feet (30 m). Vmax is in miles per hour.
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).[citation needed] For high speed railways in Europe, maximum cant is 180mm.[1]
Track unbalanced superelevation in the U.S. 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.[2] There are country-specific regulations that may apply, though.
Examples
In Australia, ARTC is increasing speed around curves sharper than 800 metres (2,600 ft) radius by replacing wooden sleepers with concrete ones so that the cant can be increased.
Roads
In civil engineering, cant is often referred to as cross slope or camber. It helps rainwater drain from the road surface. Along straight or gently curved sections, the middle of the road is normally higher than the edges. This is called "normal crown" and helps shed rainwater off the sides of the road. During road works that involve lengths of temporary carriageway, the slope may be the opposite to normal – i.e. with the outer edge higher – which causes vehicles to lean towards oncoming traffic: in the UK this is indicated on warning signs as 'adverse camber'.
On more severe bends, the outside edge of the curve is raised, or superelevated, to help vehicles around the curve. The amount of superelevation increases with its design speed and with curve sharpness.
See also
References
- ^ 2002/732/EC. *, Commission Decision of 30 May 2002 concerning the Technical Specification for Interoperability
- ^ Zierke, Hans-Joachim. "Comparison of upgrades needs to recognize the difference in curve speeds". http://Zierke.com/shasta_route/pages/05curve-curve.html. Retrieved 2008-04-10.
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