Cant (road/rail)
From Wikipedia, the free encyclopedia
The cant of a road (sometimes referred to as camber or cross slope) or railway (also referred to as superelevation) is the difference in elevation of the two edges; a cant which is not equal to zero results in a banked turn, allowing vehicles traveling through the turn to go at higher speeds than would otherwise be possible.
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[edit] Rail
In the case of railways, the cant helps a train steer around a curve and keeps the inside wheel edges from touching the rails, minimizing wear.
The amount of cant has to be chosen for a given speed, so that if trains travel through the turn at different speeds, the cant ceases to serve its initial purpose and will actually lead to damage to the wheels going both above and below the original design speed. As a result, a compromise value of cant must be chosen when turns are designed.
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]
Ideally, the track should have railroad ties (sleepers) at an increased rate per mile, and a greater depth of ballast to accommodate the increased forces exerted by the moving train.
When a straight becomes a curve, the amount of cant cannot change from zero to its maximum immediately. Rather, the cant must change (ramp) gradually in a track transition curve. The length of the transition curve depends on the maximum speed on the line – the higher the speed, the greater length is required.
The main functions of cant are
- to better distribute the load across both rails
- to reduce the wear of rails and rolling stock
- to neutralize the effect of lateral forces
- to provide comfort to passengers
[edit] Examples
In Australia, ARTC is increasing speed around curves sharper than 800 m radius by replacing wooden sleepers with heavier, concrete ones so that the cant can be increased.
[edit] Roads
In the case of roads, cant also helps rainwater drain from the road surface. In civil engineering, cant is often referred to as cross slope or superelevation. Cross slope is a strong factor in the design of a road based on its design speed.
