View source for Locomotive frame

{{One source|date=May 2024}}
{{Short description|Chassis for a self-propelled railway vehicle}}
[[File:Frames in position, Pacific construction at Doncaster works (CJ Allen, Steel Highway, 1928).jpg|thumb|Locomotive frame of a [[LNER Gresley Classes A1 and A3|LNER Gresley Pacific locomotive]] during construction]]
A '''locomotive frame''' is the structure that forms the backbone of the [[railway]] [[locomotive]], giving it strength and supporting the superstructure elements such as a [[Cab (locomotive)|cab]], [[locomotive boiler|boiler]] or bodywork.  The vast majority of locomotives have had a frame structure of some kind. The frame may in turn be supported by axles directly attached to it, or it may be mounted on [[bogies]] (known as trucks in [[American English]]), or a combination of the two.  The bogies in turn will have frames of their own.

==Types of frame==
[[image:Dukedog.jpg|thumb|Preserved GWR 9017 showing outside frames]]
Three main types of frame on steam locomotives may be distinguished:<ref>{{Ransome-Wallis Encyclopedia}}, p 255.</ref>

===Plate frames===
These used steel plates about {{convert|1|-|2|in|1|abbr=on}} thick. They were mainly used in Britain and continental Europe. On most locomotives, the frames would be situated within the driving wheels ("inside frames"), but some classes of an early steam locomotive and diesel [[switcher|shunters]] were constructed with "outside frames".
Some early designs were '''double framed''' where the frame consisted of plates both inside and outside the driving wheels. Others were ''sandwich frames'' where the frame was constructed of wood sandwiched between two metal plates.

===Bar frames===
[[image:Bar frames U class locomotive WAGR.jpg|thumb|Bar frames of a [[WAGR U class]] locomotive]]
These are openwork girder structures built up from steel or iron bars which are usually {{convert|4|-|7|in|abbr=on}} thick, welded into a single load-bearing assembly. They were first used on the [[Bury Bar Frame locomotive]] during the 1830s, and were widely used in nineteenth century American locomotives (including those exported to Australia and New Zealand; see [[The Vogel Era#Vogel railways|Vogel railways]]).

===Cast steel beds===
[[Cast steel]] locomotive beds were developed in the latter years of steam locomotive design in the United States, from where they were also exported to Britain and Australia.

== Articulated locomotives ==
{{Main article|Articulated locomotive}}

An [[articulated locomotive]] with no fixed wheels (i.e. excluding the [[Mallet locomotive]] but including other articulated [[steam locomotive]]s, as well as most [[diesel locomotive|diesel]] and [[electric locomotive]]s) may have a separate frame beneath the superstructure, or the bodywork's internal structure may be load-bearing.   Rarely is a true [[monocoque]] structure used.

Diesel and electric locomotives with a traditional, full-width body, known as [[cab unit]]s in [[North America]], tend to have their strength in an internal structure.  This style of construction is still popular elsewhere, but North American locomotives nowadays are overwhelmingly [[hood unit]]s&mdash;with a strong frame beneath the superstructure that carries all the load, and bodywork made of removable panels for easy maintenance. Fully enclosed locomotives are used in some limited applications, mostly for [[passenger train]]s. These tend to be [[cowl unit]]s, in which the body is not load-bearing.

==See also==
* [[Chassis]]
* [[Steam locomotive components]]
* [[Underframe]]
* [[Rigid-framed electric locomotive]]

==References==
{{Commons}}

{{Reflist}}


[[Category:Locomotive parts|Frame, Locomotive]]


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