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The parallel motion was a mechanical linkage invented by James Watt in 1784 for his double-acting steam engine.
In previous engines built by Newcomen and Watt, the piston pulled one end of the walking beam downwards during the power stroke using a chain, and the weight of the pump pulled the other end of the beam downwards during the recovery stroke using a second chain, the alternating forces producing the rocking motion of the beam. In Watt's new double-acting engine, the piston produced power on both the upward and downward strokes, so a chain could not be used to transmit the force to the beam. Watt designed the parallel motion to transmit force in both directions whilst keeping the piston rod vertical.
 Schematic of Watt's parallel motion See the diagram on the right. A is the journal (bearing) of the walking beam ABC, which rocks up and down about A. H is the piston, which is required to move vertically but not horizontally. The heart of the design is the three-bar linkage consisting of AB, BE and EG. Point G is fixed to the framework of the engine. As the beam rocks, point F (which is drawn to aid this explanation, but which is not visible on the machine itself) describes an elongated figure-of-eight in mid-air. Since the motion of the walking beam is constrained to a small angle, F describes only a short section of the figure-of-eight, which is quite close to a vertical straight line.
It would have been possible to connect F directly to the piston rod, but this would have made the machine an awkward shape, with G a long way from the end of the walking beam. To avoid this, Watt added the parallelogram linkage BCDE to form a pantograph. This guarantees that F always lies on a straight line between A and D, and therefore that the motion of D is a magnified version of the motion of F. D is therefore the point to which the piston rod DH is attached.
As already noted, the path of F is not a perfect straight line, but merely an approximation. Watt's design produced a deviation of about one part in 4000 from a straight line. Later, in the 19th century, perfect straight-line linkages were invented, beginning with the Peaucellier-Lipkin cell of 1864.
References
- Linkages article in Encyclopedia Britannia, 1958.
- Parallel Motion article in Encyclopedia Britannia, 1911.
- Robert Stuart, A Descriptive History of the Steam Engine, London, J. Knight and H. Lacey, 1824.
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