Steam spring

article - Steam spring

Steam springs or steam suspension are a form of suspension used for some early steam locomotives designed and built by George Stephenson. They were only briefly used and may have been used for fewer than ten locomotives.

Stephenson four-wheeled locomotive of around 1814[1]

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Cast-ironfishbelly rail

Early railways used cast-ironfishbelly rails. These were brittle and prone to cracking under shock loads. The new steam locomotives of the 1820s were much heavier than the horse-drawn wagons of earlier plateways. Locomotives of this period also used vertical cylinders set within the boiler. The vertical forces of the moving pistons further gave rise to hammer blow, which increased the load on the rails.

A further reason for suspension was to improve the frictional contact between the wheels and rail. This relied upon maintaining a good contact, thus requiring good suspension of the wheels over the uneven track.[2] The ability of an ‘adhesion-hauled’ locomotive to draw a train was much questioned at this time, as it was thought that the friction between a smooth iron wheel and the rail would be inadequate. Some designers, such as Blenkinsop with his Salamanca thought that a system of geared teeth would be necessary. Stephenson believed that, provided a good contact could be maintained between wheel and rail, frictional adhesion alone would be adequate.

1877 drawing of a claimed ‘1815 Stephenson locomotive’,[3] similar to The Duke but with flanged wheels, a plate chain and lacking the centre sprocket wheel.

At the time of these early locomotives there was not yet a way of forging an adequate steel spring to carry the weight of a locomotive. High quality steel had been available since Huntsman‘s crucible process, but it was still so expensive as to be regarded as ‘a semi-precious metal’.[4] It would be another forty years before Bessemer’s converter made cheap bulk steel available. A similar problem affected safety valves, causing them to rely on dead weights or Hackworth‘s bulky stack of leaf springs,[5] rather than the ubiquitous steel coil spring that would appear later.

Stephenson’s ‘steam suspension’ provided each wheel with its own ‘steam spring’. Vertical cylinders were set into the base of the boiler, above each axle and offset in pairs to the sides. The chassis or frames of Stephenson’s locomotives provided little structural strength, most of which came from the shell of the boiler. Inside each cylinder a piston carried the load of the axle and pressed upwards against steam pressure within the boiler.[6] A piston of only a few inches in diameter was sufficient to balance the locomotive’s weight. The axlebox bearings could slide vertically within hornblocks attached to the wooden frame beneath the boiler.

Piston seals were a perennial problem at this time. Those for large stationary engines, working at low pressures, were sealed by a variety of methods including leather cup washers, pools of standing water and even a poultice of cow dung. As working pressures increased, which had been an essential part of turning the stationary steam engine into the mobile steam locomotive, demands on the piston seal increased further. Pistons were now mostly sealed by having oakum rope wrapped around them in a groove, often smeared with tallow. Keeping the rope seal moist, thus swollen, was recognised as an important factor in achieving a good seal. As the steam spring cylinders were in the lower part of the boiler, below the water line, it was expected that they would seal well. Despite this, they continued to give trouble with leakage and were eventually removed and replaced with iron or steel leaf springs.[7]Wood in 1831 illustrates one of the Killingworth locomotives, now fitted with metal leaf springs and also coupling rods.[8][9]

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