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Parallel motion
The parallel motion is a mechanical linkage invented by the Scottish engineer James Watt in 1784 for the double-acting Watt steam engine. It allows a rod moving straight up and down to transmit motion to a beam moving in an arc, without putting sideways strain on the rod.
==Description==
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. He called it "parallel motion" because both the piston and the pump rod were required to move vertically, parallel to one another.
In a letter to his son in 1808 describing how he arrived at the design, James Watt wrote "I am more proud of the parallel motion than of any other invention I have ever made."〔Franz Reuleaux, ''The Kinematics of Machinery'' (1876), page 4.〕 The sketch he included actually shows what is now known as Watt's linkage which was a linkage described in Watt's 1784 patent but it was immediately superseded by the parallel motion.〔 Also available at https://www.gutenberg.org/files/27106/27106-h/27106-h.htm〕
The parallel motion differed from Watt's linkage by having an additional pantograph linkage incorporated in the design. This did not affect the fundamental principle but it allowed the engine room to be smaller because the linkage was more compact.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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