Wheels and Bogies
A short description of the principles of the wheel/rail interface.
The Wheel on the Rail
Railway wheels sit on the rails without guidance except for the shape of the tyre in relation to the rail head. Contrary to popular belief, the flanges should not touch the rails. Flanges are only a last resort to prevent the wheels becoming derailed - they're a safety feature. The wheel tyre is coned and the rail head slightly curved as shown in the following diagram (Fig 1). The rails are also set at an inward angle.
Ideally, the wheel profile should be determined by agreement between the railway infrastructure owner and the rolling stock owner. Of course, it varies from place to place but it is rarely a simple angle. It's usually a carefully calculated compound form. With respect to the rail angle, in the UK for example, it is set at 1 in 20 (1/20 or 0.05). In the US and France it's usually at 1/40. Light rail systems operating over roadways will have special profiles.
Fig 1: The shape and location of wheels and rails on straight track.
This diagram is exaggerated to show the principal of the wheel/rail interface on straight track. Note that the flanges do not normally touch the rails.
On curved track, the outer wheel has a greater distance to travel than the inner wheel. To compensate for this, the wheelset moves sideways in relation to the track so that the larger tyre radius on the inner edge of the wheel is used on the outer rail of the curve, as shown in Fig 2.
Fig 2: The location of the wheels in relation to the rails on curved track.
The inner wheel uses the outer edge of its tyre to reduce the travelled distance during the passage round the curve. The flange of the outer wheel will only touch the movement of the train round the curved rail is not in exact symmetry with the geometry of the track. This can occur due to incorrect speed or poor mechanical condition of the track or train. It often causes a squealing noise. It naturally causes wear.
Many operators use flange or rail greasing to ease the passage of wheels on curves. Devices can be mounted on the track or the train. It is important to ensure that the amount of lubricant applied is exactly right. Too much will cause the tyre to become contaminated and will lead to skidding and flatted wheels.
There will always be some slippage between the wheel and rail on curves but this will be minimised if the track and wheel are both constructed and maintained to the correct standards.
A pair of train wheels is rigidly fixed to an axle to form a wheelset. Normally, two wheelsets are mounted in a bogie, or truck as it is called in US English. Most bogies have rigid frames as shown below (Fig 3).
Fig 3: A standard rigid bogie on curved track.
The bogie frame is turned into the curve by the leading wheelset as it is guided by the rails. However, there is a degree of slip and a lot of force required to allow the change of direction. The bogie is, after all, carrying about half the weight of the vehicle it supports. It is also guiding the vehicle, sometimes at high speed, into a curve against its natural tendency to travel in a straight line.
To overcome some of the mechanical problems of the rigid wheelset mounted in a rigid bogie frame, some modern designs incorporate a form of radial movement in the wheelset as shown below (Fig 4)
Fig 4: A bogie on curved track with radially steering wheelsets.
In this example, the wheelset "floats" within the rigid bogie frame. The forces wearing the tyres and flanges are reduced as are the stresses on the bogie frame itself. There are some designs where the bogie frame is not rigid and the steering is through mechanical links between the leading and trailing wheelset.
See also the Bogie Parts Page