Parallel motion
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.
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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.