Why are both the front wheels of a rail engine smaller than the remaining wheels of the engine?
SANATAN DAS, Cuttack, Odisha
In the modern electric and diesel locomotives, all wheels are driving wheels (as they carry an electric motor on each of the axles) and are of the same size. However in older steam locomotives, there used to be different sizes of wheels. Usually there were three sets of wheels, leading wheels, driving wheels and trailing wheels. The steam power was transferred directly from the piston to the driving wheels through connecting rod.
The size of these wheels used to be bigger for providing extended leverage to the connecting rod for giving required torque. Due to bigger size, these wheels were not able to provide adequate lateral stability to the locomotive to remain positioned on the track. For providing better stability, the front set of wheels, either two or four in number, were made of small size. Due to smaller size, the centre of rotation of these front wheels was very close to the ground which in turn provided better stability especially while negotiating a curve at higher speed. The front wheels were idle wheels as they derived no power from the pistons.
The third set of wheels (trailing wheels) was in the rear of the driving wheels and they were also smaller, usually of the same size as that of the front wheels. Main purpose of the trailing wheel was to share load of tender carrying coal and water for production of steam on board. For providing better adhesion, most of the locomotive’s weight was shared by the driving wheels. The usual nomenclature to represent wheels configuration was x-y-z. For example, 4-8-2 meant 4 front wheels, 8 driving wheels and 2 trailing wheels. For goods locomotives, the number of driving wheels used to be more than the passenger locomotives. However, the size of driving wheels was kept larger in case of passenger locomotives to achieve better speeds.
S.P.S. JAIN, Former Member, Engineering, Railway Board, Indian Railways, Greater Noida, Uttar Pradesh
