The earliest form of railways used wooden spoked wheels surrounded by a flat plain metal rim or tyre that ran on an angled rail. These tyres would rapidly wear out until the design was altered so that the flange was fitted to the tyre instead of the rail. Various cone shaped tyre profiles were tried by the different railway companies and these studies showed that bogies with wheels that had a steep cone-shaped profile wobbled severely as the wheel tried to centre itself after leaving a sweeping curve or a set of points. If the movement was severe enough that the wheel flanges repeatedly grazed against the railhead, this movement was called hunting. In addition to causing excessive tyre and rail wear, it would also give a rough ride. Eventually the findings of the trials led to the use of cone-shaped tyres of a less steep profile and all English and Scottish railway companies agreed to standardise their wheel profiles.

     After a period of time, the tyre wears due to wheel slip on wet rails and the flange grinds against the railhead as the wheels go round curves. Thin flanges are potentially dangerous as they could derail a train on points. The allowed wear varies from one to three sixteenth of an inch (1.59 - 4.76 mm) according to the vehicle type, before the flange is said to be in need of reprofiling.

     To correct surface defects, the wheels are reprofiled on a huge lathe, while some wheel lathes allow complete sets of wheels to be turned at the same time without having to remove them from the locomotive. When the tyre has been reprofiled several times, new tyres are pressed onto the wheel to give it a new lease of life.

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Part 5: Valves and Pistons     Part 6: Valve Gear     Part 7: Compounding     Part 8: Cylinder Design
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