Old Bess (beam engine)
Old Bess (beam engine)
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Old Bess (beam engine)

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Old Bess (beam engine)

Old Bess is an early beam engine built by the partnership of Boulton and Watt. The engine was constructed in 1777 and worked until 1848.

The engine is most obviously known simply for being an early example of an engine built by Boulton and Watt. However it also played a far more important role in the development of steam engines for being the first engine designed to work with an early cutoff, and so to use the expansion of the steam for greater efficiency.

It is now preserved in the Power Gallery of the Science Museum, London. It is the oldest surviving Watt engine, and the third-oldest surviving beam engine.

Watt's first engine at Kinneil in Scotland had been unsuccessful, and the parts were taken down and re-used at Boulton's Soho Manufactory in Birmingham. The reworked engine was more successful there, and encouraged Boulton to invest further in this developing steam technology and in Watt's inventions.

The Manufactory had been built to use a water wheel to drive its machinery, and the site had been chosen on that basis, but there were concerns over seasonal lack of water to power the wheel. Similar problems in the iron industry had inspired the development of the water-returning engine: a steam pump that could raise water to drive the wheel, in times of low water on the river. The Kinneil Engine had been built as a pump, for use in a coal mine, and so was suitable for this new task. Watt's rotative beam engine had not yet been considered and so the only way to produce rotary work to drive machinery in the Manufactory was by water power.

In 1777 Boulton and Watt decided to build a second engine for use at Soho, either to supplement the Kinneil Engine or primarily to experiment with Watt's new idea of expansive working of the steam. The new engine was also to be a water-returning engine Like the earlier Newcomen engines, it was only capable of pumping water rather than driving machinery directly.

As early as 1769, Watt was considering the possibility of working steam expansively, as recorded in a letter of 28 May to Dr. Small. Early engines were incapable of this, as they used a single valve for both inlet and exhaust. As Watt had already begun to use separate valves for each function, it would now be possible to control their timing independently, i.e. to apply lead to the timing of the inlet valve. Watt decided to construct a new engine to demonstrate this principle and was confident of the substantial savings in coal consumption to be offered.

Construction began in 1777 with the ordering of a 33-inch cylinder (84 cm). The engine was erected and working at Soho by August, although still incomplete.

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