What is a Crampton Locomotive?
When Thomas Crampton drew the detailed designs for the first Great Western Railway locomotives under the direction of Daniel Gooch, he understood clearly that a broad gauge of 7’0.25” had been chosen to enable hitherto impossible regular speeds of over 60mph to be reached without dangerous instability. In the 1830’s and 1840’s, sufficient power for sustained high speeds could only be obtained by using large diameter boiler with relatively low internal pressure: about 60 psi. This was because boilers were constructed from riveted plates of wrought iron which would not withstand higher pressures without the danger of explosion. These large boilers had to be mounted above the main frame to allow free movement of the valve gear beneath, both valve gear and cylinders being mounted between the frame plates. To further ensure safety at speed, the railway between Paddington and Bristol would be as straight and level as possible, with the trackbed well drained and the rails securely mounted on heavy baulks of timber.
While acknowledging he logic of the broad gauge, Crampton was sure that too much capital had already been invested in the so-called ‘Standard Gauge’ of 4’8.5” to enable the GWR to compete against it. It was already widespread in Britain, throughout Europe, and had been adopted in America. He accepted that higher operating speeds would be commercially necessary on standard gauge lines, and so, in his spare time, he began to work on an appropriate design of his own.
In 1843 he submitted his design to the Patent Office and received a certificate for ‘Crampton’s Patent Locomotive’. It included several radical departures from what had become known as the ‘Stephenson Locomotive’ arrangement:
1. The boiler was longer than previous designs, and was accompanied by a firebox with a large grate that spread outwards with a wide foundation ring. Earlier versions still made of wrought iron plates with a 60psi internal pressure were actually oval, with bracing bars running laterally along the centre line to prevent distortion. Fortunately, the development of high quality rolled steel sheet soon enabled higher pressure round boilers of smaller diameter to be used, especially in the highly successful French locomotives. However, some German Cramptons continued to use the oval option, and saw good service.
2. The frames had curved ‘cradle’ stretchers, enabling the boiler to be mounted far lower than before, thus greatly increasing the stability of the locomotive at speed. Because the boiler occupied this space, the cylinders and Gooch valve gear sets were mounted on he outide og the driving wheels. This, in turn, enabled the driving wheels to have a plain axle, rather than a crankshaft.
3. Crampton decided to power his locomotive from the rear, and the plain driving axle enabled the driving wheels to be mounted behind the firebox with the axle beneath the cab floor. The two pairs of smaller, non-powered wheels were placed in front, to support the elongated boiler.
4. In order to minimize loss of steam power, the cylinders were mounted midway along the boiler, directly under the team dome. Large exhaust pipes led forward to the smokebox flue.
On straight, level well-maintained track over long distances, there was nothing to touch a Crampton locomotive. For many years in the second half of the 19th Century, they dominated the express trains across northern Europe.
The French locomotives were widely exported, notably to Egypt, and to the St. Petersburg – Warsaw Railway. They enabled business trains to reach speeds touching 80mph on a regular basis in an era in which eminent scientists had gravely warned that the human body would sustain serious damage if it travelled at over 25mph! A large number were built in Germany, particularly for the Bavarian State Railway.
Thomas Crampton can justly claim to be one of the pioneers of high speed rail travel.
