The distance between the inside edges of wheel flanges is generally kept less than the gauge of the track. So there is a gap between the wheel flanges and the running edges of the rails, i.e. 1 cm on either side. Normally, the tread of of wheels is dead center of the head of the rail; as the wheel is coned to keep it in this central position automatically. These wheels are coned at a slope of 1 in 20. This all is nothing but Coning of Wheels.
Advantages of Coning of Wheels
- Reduce the wear and tear of the wheel flanges and rails, which is due to the rubbing action of flanges with inside faces of rail head.
- Provide possibilities of lateral movement of the axle with its wheels on rail curves. (When inner rail distance is small, however outer rail distance is large).
- Prevent the wheels from slipping to some extent because of rigid axle.
Video animation – why conning of wheels is important ???
Theory of conning
On a level track, as soon as the axle moves towards one rail, the diameter of wheel tread over rail increases. While it decreases over the other rail. This prevents the further movement and axle retreats back to its original position. (It means with equal diameters on both rails and equal pressure on both rails).
On a curved path, due to rigidity of wheel base, it is seen that
If the tread diameter on both the rails is same, the amount of slip will be given by
Slip = Ѳ (R2 – R1)
Outer radius, R2 = R + (G/2)
Inner radius, R1 = R – (G/2)
G = Gauge
Ѳ = Angle at centre in radians
Therefore, Slip = Ѳ x G
For B.G. track, G = 1.676 m
Thus, Slip = (2πѲ° / 360) x 1.676
Ѳ° = angle at centre in degrees, say 1°
Slip = 0.029 (approx. for 1° of central angle)
Therefore, the slip is about 0.029 m per degree of the central angle.
Disadvantages of conning
Conning of wheels on curves is not of much use as the leading axle if due to centrifugal force moves towards the outer rail rear axle. This will move towards the inner rail and the full advantage of conning wheels can’t be availed. In other words, there is no free lateral movement of wheels and this leads to the following disadvantages –
- Pressure on the outer rail is more while on inner rail it is less. This results in wear of outer rail.
- Due to the centrifugal force, the horizontal components tend to turn the rail out and gauge has widening tendency.
- If no base plate is used under the voids sleepers under the edge of the rail are damaged.
In order to eliminate above demerits, tilting of rails is done. In tilting of rails, the base plate or sleeper is not laid horizontal but at a slope of 1 in 20 inwards. This is known as adzing of sleepers as shown.
To minimize the wear and tear of the rails and wheels on curves, the American practice is to reduce wheel conning to 1 in 40 and in some cases even to 1 in 100.