Figure 38.—Drag link coupling used on Boulton and Watt portable engines. The link E drags one shaft when the other turns. From John Nicholson, The Operative Mechanic, and British Machinist (Philadelphia, 1826, vol. I, pl. 5).
Directly related to the drag link coupling were the patents of John Oldham (1779-1840), an Irish engineer who is remembered mainly for the coupling that bears his name (fig. 39). His three patents, which were for various forms of steamboat feathering paddle wheels, involved linkages kinematically similar to the drag link coupling, although it is quite unlikely that Oldham recognized the similarity. However, for his well-known coupling, which employs an inversion of the elliptical trammel mechanism, I have found no evidence of a patent. Probably it was part of the machinery that he designed for the Bank of Ireland's printing house, of which Oldham was manager for many years. "Mr. Oldham and his beautiful system" were brought to the Bank of England in 1836, where Oldham remained until his death in 1840.[106]
[ [106] Oldham's paddle-wheel patents were British Patents 4169 (October 10, 1817), 4429 (January 15, 1820), and 5445 (February 1, 1827). Robert Willis (op. cit. footnote 21, p. 167) noticed the existence of the coupling. Drawings or descriptions of the banknote machinery apparently have not been published though they probably still exist in the banks' archives. The quotation is from Frederick G. Hall, The Bank of Ireland 1783-1946, Dublin, 1949. John Francis in his History of the Bank of England (London, 1848, vol. 2, p. 232) wrote: "The new machinery for printing the notes, which was introduced by Mr. Oldham ... is well worthy of a visit, but would be uninteresting to delineate."
Figure 39.—Top, Original Oldham coupling built before 1840, using a cross (instead of a center disk), as sketched by Robert Willis in personal copy of his Principles of Mechanism (London, 1841, p. 167). Bottom, Oldham coupling as illustrated in Alexander B. W. Kennedy, Kinematics of Machinery, a translation of Franz Reuleaux' Theoretische Kinematik (London, 1876, pp. 315-316).
The Geneva stop mechanism (fig. 40) was properly described by Willis as a device to permit less than a full revolution of the star wheel and thus to prevent overwinding of a watch spring. It was called Geneva stop because it was used in Geneva watches. The Geneva wheel mechanism, which permits full rotation of the star wheel and which is frequently used for intermittent drives, was improperly called a Geneva stop in a recent textbook probably because the logical origin of the term had been lost.
Figure 40.—Geneva stop mechanism first used in Geneva watches to prevent overwinding. The starwheel B had one convex surface (g-f, dotted) so the wheel could be turned less than a full revolution. After Robert Willis, Principles of Mechanism (London, 1841, p. 266).
The name for the Scotch yoke seems to be of fairly recent origin, the linkage being called by a Scotsman in 1869 a "crank and slot-headed sliding rod" (fig. 41). I suppose that it is now known as a Scotch yoke because, in America at least, a "Scotch" was a slotted bar that was slipped under a collar on a string of well-drilling tools to support them while a section was being added (fig. 42).
