1911 Encyclopædia Britannica/Parallel Motion
PARALLEL MOTION, a form of link-work invented by James Watt, and used in steam-engines (see Steam-Engine, § 88), to connect the head of the piston rod, moving up and down in a vertical path, with the end of the beam, moving in the arc of a circle. An ordinary form is shown diagrammatically in figure. MN is the path in which the piston-rod head, or crosshead, as it is often called, is to be guided. AB C is the middle line of half the beam, C being the fixed ,centre about which the beam oscillates. A link BD connects a point in the beam with a radius link ED, which oscillates about a fixed centre at E. A point P in BD, taken so that BP:: DP :: EN: CM, move in a path which coincides very closely with the straight line MPN. Any other point F in the line CP or CP produced is made to copy this motion by means of the links AF and FG, parallel to BD and AC. In the ordinary application of the parallel motion a point such as F is the point of attachment of the piston-rod, and P is used to drive a pump-rod. Other points in the line CP produced are occasionally made use of by adding other links parallel to AC and BD.
Watt's linkage gives no more than an approximation to straight-line motion, but in a well-designed example the amount of deviation need not exceed one four-thousandth of the length of stroke. It was for long believed that the production of an exact straight-line motion by pure linkage was impossible, until the problem was solved by the invention of the Peaucellier cell. (See also Mechanics: Applied Mechanics, §§ 77, 78.)