LSWR T7 class
Encyclopedia
The LSWR Class T7 4-2-2-0
was a prototype express steam locomotive
design by Dugald Drummond
for the London and South Western Railway
introduced in 1897.
3-cylinder compounds introduced in 1883 on the LNWR that employed two pairs of uncoupled driving wheels; the Drummond locomotives were always known as the "double singles".
). In the case of Drummond,the main motive in appears to have been to obtain maximum grate area in a period where low-pitched boilers were the norm and the firebox had to be set low between the frames. This limited the width of the grate whilst its length depended on the distance between the coupled axles minus the throw of eventual inside cranks; at the same time there was a reluctance to make the coupling rods too long due to concern about material resistance, for a broken coupling rod flailing away under a locomotive could wreak tremendous havoc. One way out of the impasse was to eliminate the coupling rods altogether and to have two independent pairs of driving wheels each pair driven by its own cylinders. The first engineer known to have adopted this solution was Francis Webb, followed by Alfred de Glehn in France who initially combined divided drive and independent driving axles, finally opting solely for the former whilst coupling the driving wheels. The aforementioned engineers locomotives were compounds and the layout was also was a way of separating high-pressure from low-pressure drive trains; Drummond's 720 worked with simple expansion so that the principle benefit sought would be the increased grate area. Compared with Drummond's standard boilers, barrel length was increased from 10 foot 6 inches to 12 feet.
4-2-2-0
Under the Whyte notation for the classification of steam locomotives, 4-2-2-0 represents the wheel arrangement of four leading wheels on two axles, four powered but uncoupled driving wheels on two axles, and no trailing wheels...
was a prototype express steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
design by Dugald Drummond
Dugald Drummond
Dugald Drummond was a Scottish steam locomotive engineer. He had a career with the North British Railway, LB&SCR, Caledonian Railway and London and South Western Railway...
for the London and South Western Railway
London and South Western Railway
The London and South Western Railway was a railway company in England from 1838 to 1922. Its network extended from London to Plymouth via Salisbury and Exeter, with branches to Ilfracombe and Padstow and via Southampton to Bournemouth and Weymouth. It also had many routes connecting towns in...
introduced in 1897.
Background
Number 720 was a prototype locomotive built in 1897. The layout was unusual and influenced by Webb'sFrancis Webb (engineer)
Francis William Webb was a British engineer responsible for the design and manufacture of locomotives for the London and North Western Railway .- Biography :...
3-cylinder compounds introduced in 1883 on the LNWR that employed two pairs of uncoupled driving wheels; the Drummond locomotives were always known as the "double singles".
Design features
Throughout locomotive history, this type of layout with independent uncoupled trains of driving wheels mounted on a common rigid chassis has been repeatedly tried with various aims in mind (the best-known more recent example is the Duplex locomotiveDuplex locomotive
A duplex locomotive is a steam locomotive that divides the driving force on its wheels by using two pairs of cylinders rigidly mounted to a single locomotive frame; it is not an articulated locomotive...
). In the case of Drummond,the main motive in appears to have been to obtain maximum grate area in a period where low-pitched boilers were the norm and the firebox had to be set low between the frames. This limited the width of the grate whilst its length depended on the distance between the coupled axles minus the throw of eventual inside cranks; at the same time there was a reluctance to make the coupling rods too long due to concern about material resistance, for a broken coupling rod flailing away under a locomotive could wreak tremendous havoc. One way out of the impasse was to eliminate the coupling rods altogether and to have two independent pairs of driving wheels each pair driven by its own cylinders. The first engineer known to have adopted this solution was Francis Webb, followed by Alfred de Glehn in France who initially combined divided drive and independent driving axles, finally opting solely for the former whilst coupling the driving wheels. The aforementioned engineers locomotives were compounds and the layout was also was a way of separating high-pressure from low-pressure drive trains; Drummond's 720 worked with simple expansion so that the principle benefit sought would be the increased grate area. Compared with Drummond's standard boilers, barrel length was increased from 10 foot 6 inches to 12 feet.