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R.o.B Motive Power

Whyte Wheel arrangements

From the beginning of locomotive traction there has been the need for a specific amount of wheels to carry the weight of the locomotive, not all the time did the wheels actually produce the traction used to move the train. The Middleton railway in Leeds actually used a huge geared wheel running on a rack rail laid alongside the tramway. Further development of Locomotives saw chain driven and gear driven locomotives. One design even had a pair of legs pushing the engine and its train!! The most common form of transmission to develop however was the fore runner of what we today take for granted as the transmission for Steam locomotives and some diesel traction.

These early locomotives had their wheels directly connected to the cylinders via an intricate frame work situated over the boiler of the engine with connecting rods operating vertically on to the wheel slightly offset from the hub to give the necessary applied force to overcome the inertia of the locomotive and train. It was soon found though that locomotives were to light in their construction to produce the necessary friction on the rail to haul very heavy loads. This was partly due to the infant nature of locomotive design but also due to the quality and design of the track. Heavier locomotives had the tendency to crack and break the thin cast iron tramway rails. So any further locomotive development had to go hand in hand with tramway/railway development, this was the period when tramways with their light weight generally L shaped rails became the more commonly known T shaped rails that are in use today, but this subject will be covered in a different topic.

One way of overcoming the problem of weight friction ration was for the wheels of the locomotive to be coupled together using horizontal rods, or coupling rods with the connecting rods transmitting movement from the cylinders to the primary driving wheels and on to the coupled wheels. This may all sound easy but the calculations for weight/friction ratio are quite complex and have to take in to account many differentials such as type of rail, locomotive weight, speed, train weight, gradient, surface texture/conditions i.e wet, ice, dry, greasy etc. To overcome the coefficient of friction how much power is required by the engine so that too much power is not wasted in getting the train moving while too little can result in the engine not gripping the rails and its wheels slipping much to the delight of the enthusiast but embarrassing for the train crew/driver. 

The increase in applied frictional area, or the bit of the wheels that touch the rail certainly gave a much better performance to the early locomotives, but you may find that coupled wheels for the first 50 or so years of locomotive development were restricted to freight locomotives. The Stockton and Darlington railway which pre dominantly used its locomotives for freight were a good example of this with its 'Locomotion' 0-4-0 and its later 'Derwent' 0-6-0 one of the earliest post 1825 main line locomotives to have such a size and wheel arrangement. However George and Robert Stephenson's 'Rocket' of 1829 though blatantly after the period of transition from 2 wheel drive to coupled wheels does only have direct transmission via connecting rod to its front driving wheels. Why? To be honest I am not sure and I have trawled round a bit to find the answer, but I can only guess that the engine had sufficient weight and traction capability to handle the reasonably light loads it would be expected to haul as a passenger locomotive as it reaches 13mph with a load of 13 tons!!

However towards the end of the 19th Century locomotives became bigger and more powerful as speeds increased and train loads also increased. This was to result in locomotives having more coupled wheels as well as leading or trailing wheels to help distribute the weight of the locomotive rather than it been placed directly on to the driving wheels, which as mentioned above had to be finely balanced to give sufficient but not over frictional adhesion to the smooth steel rails. The practice of having non driving wheels had been around since the beginning of locomotives but from the 1830's classes of locomotives were designated not only by their size and power but also by their wheel arrangement. These extra wheels also helped guide the locomotive around corners at speed or just by acting as a guide in general.

By the end of steam a wide variety of wheel arrangements had been seen on the railways of Britain ranging from the early 2 driving wheel 2 extra wheels to the 10 coupled wheels with 2 extra wheels or the Garratt speciality of articulated locomotives with 2 loose wheels and 6 driving wheels on one part of the locomotive and then another 6 driving wheels and 2 trailing wheels on another part of the locomotive. Even today diesel shunters of the class 08 and 09 variety still use coupled wheels and are such powerful locomotives with this wheel arrangement, weight and engine they can move pretty much anything up to a speed of a grand geared 15mph!! However without much further ado, below is a chart of the different types of wheel arrangement according to the 'Whyte' principal which was established in America.

The way the Whyte principal works is that each set of wheels has its own notation in the form of leading wheels (uncoupled)- Coupled wheels (driving)- Trailing wheels (uncoupled).

Table of wheel arrangements seen on the Railways of Britain:

Wheel arrangement:  Notation: Name/Locomotive Type:

0o                               0-2-2        'Rocket'

o0o                             2-2-2        'Planet'

0-0                              0-4-0

0-0o                            0-4-2

0-0oo                          0-4-4

o0-0                             2-4-0

o0-0o                           2-4-2

oo0-0                           4-4-0

oo0-0o                        4-4-2       'Atlantic'

0-0-0                            0-6-0

0-0-0o                          0-6-2

o0-0-0                          2-6-0       'Mogul'

o0-0-0o                        2-6-2        'Prairie'

o0-0-0oo                     2-6-4

oo0-0-0                        4-6-0

oo0-0-0o                      4-6-2        'Pacific'

0-0-0-0                         0-8-0 

o0-0-0-0                       2-8-0        'Consolidation'

o0-0-0-0o                     2-8-2          'Mikado'

o0-0-0 0-0-0o              2-6-6-2       'Garratt'