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The wheel arrangement of wheels on steam locomotives and small diesel locomotives is normally described using [[Whyte notation]] which specifies the number of powered and unpowered wheels, for example 0-6-0 or 4-6-2. This system does not work well for the more complicated arrangements found in large diesel locomotives or multiple units which may have bogies containing both powered and unpowered wheels and/or using single or multiple traction motors or hydraulic transmissions. Their wheel arrangement is therefore normally described using the more flexible '''UIC classification system, where letters are used to describe ''' which denotes the number of axles (as opposed to [[Whyte notation]] which describes the number of wheels)and specifies whether each is powered or unpowered. Those in use on the SVR are as follows:
==The international standard UIC notation==The international standard UIC notation includes the following conventions to describe axles:*BB: Four powered An '''upper case letter''' indicates the number of consecutive driving axles all mounted in the locomotive's frame, driven in pairs, istarting at A for a single axle.e. each pair of Thus a single C would indicate three powered axles is , connected by driving rods or gears.*Bo-Bo: Four axles in two individual bogies, all driven by their own traction motors.*Cmounted in the locomotive's frame (Whyte notation: Three 0-6-0), while BB would indicate four powered axles all mounted in the locomotive's frame, all connected by driving rods or gearsdriven in pairs (Whyte notation: 0-4-4-0).*C-C: Two bogies The '''lower case suffix o''' when applied to two or wheel assemblies under the unit. Each bogie has three powered more driving axles, mechanically connected denotes that they are individually driven by separate traction motors rather than by driving rods or gears.*Co-Co: Two six-wheeled bogies with all axles powered, with a separate shared traction motor per axleor other power source. *1CoA '''number''' indicates the number of consecutive non-Co1: Two eight-wheeled bogies, with three driving axles (six wheels) powered by separate motors for each axle, and an additional unpowered axle at the outer end of each bogie to reduce the axle load (the weight resting on each axle).
The wheel arrangement presence of bogies is denoted in one of two ways:*Where the bogie only contains one type of axle, a "'''prime sign'''" (a single quotation mark) is added to the relevant letter(s) to denote that they are mounted on steam locomotives a bogie. Thus Co'Co' denotes a pair of bogies, each with three powered axles individually driven by traction motors.*Where the bogie contains several different axle types, '''Brackets''' are used to group together letters and numbers describing the same bogie or truck. Thus (A1A)(A1A) indicates two three-axle bogies, each with the outer two axles driven by separate traction motors and small diesel locomotives is described using [[Whyte notation]]an unpowered central axle.
Although not present on the SVR, the UIC classification system also describes articulated vehicles (where the inner bogie is shared between two vehicles). So, 2'2'2 is used to describe an articulated pair of unpowered vehicles in the [https://en.wikipedia.org/wiki/British_Rail_Class_745 Stadler Class 745 units]. Where articulated sets are joined within a unit a '''plus sign''' sign is used, thus Bo'2'2+2'2'2. ===Commonwealth classification===The UK generally uses a modified version of this notation (sometimes referred to as "Commonwealth classification" or "British classification") where the prime sign (single axle) and brackets (multiple axles per bogie) are replaced by a '''hyphen''' denoting the separation between groups of axles in a manner more akin to Whyte notation. Thus B'B' internationally becomes B-B, Bo'Bo' becomes Bo-Bo, (A1A)(A1A) becomes A1A-A1A and so on. The SVR Wiki uses this classification system. ==Examples on the SVR==Examples found in diesels currently or formerly resident on the SVR are as follows (formerly resident classes in ''italics''): {| class="wikitable sortable"!Commonwealth classification !! UIC classification !! Description !! Examples|-|B || B || 2 powered axles connected by driving rods, no bogies||[[Ruston and Hornsby 165hp Diesel Shunter 319290|319290]], [[D2960]], [[D2961]], [[Former Residents|''Other early diesel shunters'']] |-|C ||C || 3 powered axles connected by driving rods, no bogies || BR Class 08, BR Class 09, [[BR Class 14 D9551|BR Class 14]]|-|A1A-A1A|| (A1A)(A1A)|| Two bogies or wheel assemblies under the unit. Each bogie has one powered axle, one idle axle, and another powered axle. The powered axles are individually driven by traction motors.|| [[BR Class 31 31466|BR Class 31]]|-|B-B|| B'B'|| Two bogies or wheel assemblies under the unit. Each bogie has two powered axles, connected by driving rods or gears.||[[ BR Class 35 D7029| BR Class 35]], [[BR Class 42 D821 Greyhound | BR Class 42]]|-|Bo-Bo||Bo'Bo'|| Four axles in two individual bogies, all driven by their own traction motors.|| [[Clayton Class 17 Bo-Bo D8568|BR Class 17]], [[Former Residents|''BR Class 20'']], [[Former Residents|''BR Class 25'']], [[Former Residents|''BR Class 27'']], [[BR Class 33 33108|BR Class 33]], [[Diesel Locomotives hired by the SVR|''BR Class 73'']]|-|C-C||C'C'|| Two bogies or wheel assemblies under the unit, each with three powered axles, mechanically connected by driving rods or gears.||BR Class 52|-|Co-Co||Co'Co'|| Two six-wheeled bogies with all axles powered, with a separate motor per axle. || [[BR Class 37 37308|BR Class 37]], BR Class 50|-|1Co-Co1||(1Co)(Co1)|| Two eight-wheeled bogies, with three axles (six wheels) powered by separate motors for each axle, and an additional unpowered axle at the outer end of each bogie to reduce the axle load (the weight resting on each axle).||[[ BR Class 40 40106 Atlantic Conveyor|BR Class 40]]|} ==Steam locomotives and small diesel locomotives==As noted above, the wheel arrangement on steam locomotives and small diesel locomotives is generally described using [[Whyte notation]], although it can of course also be described in the UIC classification system. Oliver Bulleid, the Chief Mechanical Engineer of the Southern Railway, used a modified version of the UIC method to number certain of his steam locomotives. A West Country/Battle of Britain class locomotive would be numbered in a pattern 21C XXX. The first two numbers referred to the number of 'unpowered' axles used for leading and trailing wheels, the third letter referred to the number of 'powered' axles used for driving wheels using the normal UIC classification substitution A=1, B=2, C=3 etc. XXX was the unique identification of the particular locomotive, hence [[SR 34027 Taw Valley | Taw Valley]] entered service as 21C127 and [[SR 34053 Sir Keith Park | Sir Keith Park]] as 21C153.
==See also==
[[Diesel Locomotives]]
==References==
==Links==
*[https://en.wikipedia.org/wiki/UIC_classification_of_locomotive_axle_arrangements UIC classification of locomotive axle arrangements] (Wikipedia)
*[https://en.wikipedia.org/wiki/Category:Commonwealth_classification Category:Commonwealth classification] (Wikipedia)