Description:Section and plan of 2 horse bell crank engine for G. Symonds, Reading, Berkshire, September 1799.
In the late 1780s, Boulton & Watt began to make small engines, of 3 horses power and under. They were encouraged in this by John Rennie, who perceived a growing market for such engines for small businesses that did not have much room and did not need large amounts of power. The first small engines were of the standard beam arrangement, but in 1799 Boulton & Watt began to make engines that were even more compact and cheap, as they did away with overhead beam. The crank shaft was placed close to the bottom of the cylinder, as can be seen in the plan. At the top of the piston rod was a cross-bar. Coming down from the cross-bar were vertical side rods, which were connected to triangular levers known as 'bell cranks'. The bell cranks were joined to the crank shaft by a linking rod. This arrangement can be seen in the side view. William Murdock had considerable input into the design of these engines.
The engine shown here, for G. Symonds, was the first one sold. Bell crank engines were made until 1814, and most of those sold were of 6 horses power. They were applied to a wide range of concerns that needed steam-driven rotative power, including collieries, blast furnaces, dockyards, breweries, textile mills and even a lapidary. Several were also exported to the West Indies and the Carribean to power sugar-grinding mills on the various plantations, and a few larger bell crank engines were made for powering boats.
This drawing is part of the Archives of Soho, held at Birmingham City Archives [Ref. MS3147/Portfolio/5/718]