Description:Parallel Motion for Thomas Allingham, Rotherhithe.
Watt developed the parallel motion to keep the piston rod absolutely vertical as the beam moved up and down. If the piston was slightly at an angle, it would wear away the sides of the cylinder, causing steam to leak around it and seriously impairing the performance of the engine. For single-acting engines this was not a problem, as the piston rod could be connected to the beam via a chain – steam was not being used to push the piston up, so a rigid connection between piston rod and beam was not necessary. See some of the 1770s drawings for examples of chain connections. However, double-acting engines, which used steam to push the piston down and up needed a rigid connection. Watt tried various methods including the rack and sector and an arrangement of moving bars known as the 'three bar motion'. Watt then developed the parallel motion, a much more compact mechanism which quickly became standard on all Boulton & Watt engines. Watt said that he was more proud of this than any of his other inventions, and he included it in his patent of 28 April 1784. This drawing was prepared for an engine that was being planned for Thomas Allingham, a forge owner from Rotherhithe, who went into business with Folliott Scott & Co. at the King & Queen Foundry.
This drawing is part of the Archives of Soho collection, which is held by Birmingham City Archives [Ref. MS3147/Portfolio/5/1382]