| LNWR Cornwall | |
|---|---|
| LNWR 3020 Cornwall c.1890 | |
| Power type | Steam |
| Designer | Francis Trevithick (son of Richard Trevithick), rebuilt by Ramsbottom |
| Builder | LNWR Crewe |
| Build date | 1847, rebuilt in 1858 |
| Total produced | 1 |
| Rebuild date | 1858 |
| Configuration | 4-2-2 rebuilt as 2-2-2 |
| Gauge | 4 ft 8 1⁄2 in (1,435 mm) |
| Leading wheel diameter |
3 ft 6 in (1.067 m) |
| Driver diameter | 8 ft 6 in (2.591 m) |
| Fuel type | Coal |
| Cylinders | Two, outside |
| Cylinder size | 17.5 × 24 in (445 × 610 mm), later 17.25 × 24 in (438 × 610 mm) |
London & North Western Railway 2-2-2 No. 3020 Cornwall is a preserved steam locomotive. She was built at Crewe in 1847. She was originally a 4-2-2 in 1847, but was extensively rebuilt, and converted to a 2-2-2 in 1858.
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In the 1840s, express passenger locomotive design was focussed on the need for single large-diameter driving wheels of around 8 foot (96"). The wheel diameter is effectively the "gearbox ratio" of a steam engine. Large driving wheels deliver the high linear tyre speed needed for fast locomotives, whilst keeping the axle bearing and piston speeds low enough to remain within the technology limits of the day.
Later on, increasing engine power would require better adhesion than could be achieved with single driving wheels, but that was not yet a problem at this time.
Along with needing large wheels for speed, stability required a low centre of gravity, and thus a low-slung boiler. The difficulty is that both of these needs are in conflict, requiring the driving axle position to overlap the boiler position.
One solution to this was the Crampton design, where the driving axle was moved behind the boiler's firebox. These engines were relatively long in comparison to their contemporaries and had long rigid frames, sometimes with as many as three carrying axles ahead of the driving axle, for a 6-2-0 wheel arrangement. Cramptons were most popular in France and Germany, but some were also used in England, by companies including the London & North Western Railway (LNWR).
One of these LNWR Cramptons, "Liverpool" was notably long, with an eight-wheeler (6-2-0) layout and rigid wheelbase of 18' 6". Although fast and capable of working heavy trains for long distances, it damaged the roadbed owing to the long rigid frame.[1]
Francis Trevithick had a notable pedigree as a locomotive engineer, being the son of Richard. He had moved North to become resident engineer, then Locomotive Superintendent of the Grand Junction Railway (GJR) (later formed into the LNWR). "Cornwall" was named after the county of his birth.
"Cornwall" was an attempt to avoid the damaging long wheelbase of the Cramptons, whilst still permitting large driving wheels. By moving the driving axle ahead of the firebox, one of the carrying axles could be moved backwards, giving a shorter overall wheelbase. The difficulty of how to fit the axles past the boiler recurred, to which Trevithick provided an "extremely complicated" solution.[1] The boiler was placed entirely underneath the driving axle.[2] Even then, it was necessary to recess a transverse channel across the top of the boiler, so as to provide clearance for the driving axle. The trailing carrying axle passed through a crosswise tube through the middle of the firebox. This made assembly difficult, but as it was only a straight carrying axle rather than a cranked driving axle, the tube diameter required was manageable. This use of a cross-firebox axle tube was part of Crampton's patent of 1842. As completed in 1847, and first numbered 173,[3] 'Cornwall' was a 4-2-2 with 8' 6" drivers, paired leading wheels of 3' 6", single trailing wheels of 4' and an overall wheelbase of 16' 6".[2] This is the condition in which she was exhibited at The Great Exhibition of 1851.
The Railway Gazette [4], cited in [1], suggests that there was an even earlier design for 'Cornwall', as a 2-2-2 with single 4' wheels both forward and back. It's uncertain if 'Cornwall' was ever built in this form. The drawing does show a considerable front overhang, with a high load placed on the front axle. If constructed like this, the likelihood is that it would suffer the same problems as its contemporary, Gooch's first 2-2-2 'Great Western' class of 1846, where a broken front axle led to re-design as a 4-2-2.
A typical Crampton feature, previously used on 'Liverpool', was the large diameter of the outside eccentrics used to drive the valve gear. These were so large as to be larger than the driving cranks, thus avoiding the need for an overhung (and potentially weak) crank. The 17½" × 24" cylinders were horizontal, fed by inclined steam chests above them.
In 1858, Ramsbottom redesigned Cornwall almost completely. Little survived unchanged, other than the outside frames and the centres of the drivers. The boiler was now moved entirely above the driving axle, without any notches, channels or tubes, to what would now be regarded as conventional practice.
New cylinders and valve gear were provided, fractionally smaller at 17¼" × 24". Wheel arrangement was now 2-2-2, shortening the wheelbase still further to 14' 10".[1] Ramsbottom also included his newly designed tamper-proof safety valve.
Another minor rebuild in the 1870s provided a typically LNWR style of cab, with a short roof and semi-open sides. She was given her current number, 3020, in June 1886.
'Cornwall' was a famously successful high-speed passenger express engine of its period. Charles Rous-Marten (cited in [5]) reported an 1884 run from Crewe to Chester behind 'Cornwall' (now far from new, and of antiquated design) at an average speed of 50.7 mph, reaching 70 mph down Whitmore bank. It remained in express service on the Liverpool-Manchester route until withdrawn in 1902. In 1925, Ahrons reports that it was still in service hauling the Mechanical Engineer's inspection coach.[1]
On final retirement, 'Cornwall' was deliberately preserved, one of the first locomotives to be so treated.
She is owned by the National Railway Museum and resides at Locomotion.[6]
E. L. Ahrons (1927). British Steam Railway Locomotive. Vol 1: from 1825 to 1925.