Axle Arrangement

The pattern of the wheels fixed to a locomotive decides its weight, its purpose & the characteristics of the line which it can run on. This pattern of which wheels are arranged is called Axle Arrangement. Wheel Arrangement or Axle arrangement is denoted by a code. There are several notations used by many countries all over the globe such as UIC classification & AAR classification. In Sri Lanka we use AAR classification.

AAR wheel arrangement was developed by the Association of American Railroads. It’s somewhat simple than the UIC classification. First of all we should know the symbols using to denote these arrangements.

There is a standard method to write the arrangement. E.g.: Bo-Bo, Co-Co, B-B, and A1A-A1A

English Capital Letters

English capital letters are used to denote the number of consecutive driving axles. A, B, C, D, E is used to denote 1, 2, 3, 4, 5 consecutive driving axles respectively. Driving axles are the axles which are connected to a traction motor or a shaft.

Numbers

Numbers represent the number of consecutive driven axles. 1, 2, 3 denotes 1, 2, 3 driven axles respectively. Driven axles are not connected to a traction motor or a shaft.

If all axles of the bogie has traction motors of their own it’s denoted by adding a simple “o” after the capital letter.

Hyphen (-)

This means that axles are fixed to a bogie. That means the axle system is connected to the chassis by only one place which can rotate around its own axis freely. Hyphen is used to separate the bogies when writing the axle arrangement. E.g.: Bo-Bo

Plus Mark (+)

This means the locomotive consist of articulated axle systems.

Axle Arrangements Used in Sri Lanka

Axel Arrangements of Diesel Locomotives & Steam Locomotives are different to each other. Let’s examine these two categories one by one.

Axle Arrangement of Diesel Locomotives

A1A-A1A

Locomotive has 2 bogies. A bogie has a powered axle, a driven axle & a powered axle respectively. All powered axles are connected to traction motors. This arrangement is used to minimize the axle load. The center wheel which is connected to the driven axle is slightly smaller than other 2 wheels to save space.

Sri Lankan Class M2, M2a, M2b, M2d & M6 locomotives have this arrangement.

Bo-Bo

This is the most popular axle arrangement in the world. There are 2 bogies in the locomotive & a bogie has 2 powered axles with traction motors. This is also known as the most suitable axle arrangement for the up country line due to the minimum length of the bogie & high axle load.

Sri Lankan Class M2c, M5, M7 & Diesel Electric DMUs (S9, S10, S11 & S12) have this arrangement.

Co-Co

Locomotive has 2 bogies. Each bogie have 3 powered axles which have separate traction motors. These bogies are longer than other bogies. This arrangement is often used for heavy load locomotives.

Sri Lanka Class M4, M8, M8a, M9, M10, M10a has this arrangement.

B-B

Locomotive has two bogies & a bogie has 2 powered axles. Those to axles are powered by one power supply & it’s divided among the axles by a mechanical method. Diesel hydraulic locomotives have this arrangement.

Sri Lanka Class W1, W2, W3 & Diesel Hydraulic DMUs (S2, S3, S4, S5, S6, S7 & S8) have this arrangement.

Axle Arrangement of Steam Locomotives

There were no method to classify the wheel arrangement of steam locomotives in the early days. After few years a system was invented to describe the arrangement of wheels by counting the number of wheels from front. It was noted in this order first the number of carrying wheels then the number of driving wheels & finally the number of trailing wheels. A hyphen (-) was used to separate these wheel sets & a plus mark (+) was used to denote articulated systems. Later French, German, US & UK notations came to act but all of them are somewhat similar to each other. In Sri Lanka we use UK & US notation. They counted the number of wheels instead of number of axles. Following are some arrangements used in Sri Lanka.

4-8-0

There are 4 carrying wheels (2 axles), 8 driving wheels (4 axles) & no trailing wheels. This arrangement was used in Sri Lankan Class A1, A1a, A2, A3, A3a, A3b & A3c locomotives.

4-6-0

Locomotive have 4 carrying wheels (2 axles), 6 driving wheels (3 axles) & no trailing wheels. This was used in Sri Lankan Class B1a, B1b, B1c, B1d, B2, B2a, B2b, B2c, B2d, B2e, B3, B3a, B4, B4a, B4b, B4c, B5, B5a, B6, B7 & B8.

2-6-4

There are 2 carrying wheels (1 axle), 6 driving wheels (3 axles) & 4 trailing wheels (2 axles). This arrangement was used in Sri Lankan Class D1, D1a, D2, D2a, D2b & D2c locomotives.

0-6-2

Locomotive have no carrying wheels, 6 driving wheels (3 axles) & 2 trailing wheels (1 axle). This was used in Sri Lankan Class E1, E1a, E1b, E1c, E1d, E1e locomotives.

4-4-0

There are 4 carrying wheels (2 axles), 4 driving wheels (2 axles) & no trailing wheels. This arrangement was used in Sri Lankan Class F1, F1a, F1b, F1c, F2, F2a & K1 locomotives.

0-4-0

There are no carrying wheels, 4 driving wheels (2 axles) & no trailing wheels. This arrangement was used in Sri Lankan Class G1 locomotive.

0-8-0

There are no carrying wheels, 8 driving wheels (4 axles) & no trailing wheels. This arrangement was used in Sri Lankan Class G2 locomotives.

4-6-4

Locomotive have 4 carrying wheels (2 axles), 6 driving wheels (3 axles) & 4 trailing wheels (2 axles). This was used in Sri Lankan Class J1, J1a, J1b, J2, J2a, J2b locomotives.

0-4-8

Locomotive have no carrying wheels, 4 driving wheels (2 axles) & 8 trailing wheels (4 axles). This was used in Sri Lankan Class L1 L1a & L1b.

2-6-2+2-6-2

There are two articulated axle systems. A set have 2 carrying wheels (1 axle), 6 driving wheels (3 axles) & 2 trailing wheels (1 axle). Used in Class C1 Garrets.

2-4-0+0-4-2

This arrangement was used in Class H1 locomotive which is also known as the Baby Garret. It also had two articulated axle systems. A system had 2 carrying wheels, 4 driving wheels & no trailing wheels. When we start from the front the whole arrangement is as follows. 2 carrying wheels, 4 driving wheels, no trailing wheels, no trailing wheels, 4 driving wheels & 2 carrying wheels.