Kinematics of Mechanisms from the Time of Watt Part 6

There was an interesting cla.s.s of United States patents called "Mechanical Movements" that comprised scores of patents issued throughout the middle decades of the 19th century. A sampling of these patents shows that while some were for devices used in particular machines--such as a ratchet device for a numbering machine, a locking index for gunmaking machinery, and a few gear trains--the great majority were for converting reciprocating motion to rotary motion. Even a cursory examination of these patents reveals an appalling absence of sound mechanical sense, and many of them appear to be attempts at "perpetual motion," in spite of an occasional disclaimer of such intent.

Typical of many of these patented devices was a linkage for "multiplying" the motion of a flywheel, proposed in 1841 by Charles Johnson of Amity, Illinois (fig. 37). "It is not pretended that there is any actual gain of power," wrote Mr. Johnson; and probably he meant it.

The avowed purpose of his linkage was to increase the speed of a flywheel and thus decrease its size.[103]

[Footnote 103: U.S. Patent 2295, October 11, 1841.]

[Ill.u.s.tration: Figure 37.--Johnson's "converting motion," 1841. The linkage causes the flywheel to make two revolutions for each double-stroke of the engine piston rod B. From U.S. Patent 2295, October 11, 1841.]

An Englishman who a few years earlier had invented a "new Motion" had claimed that his device would supersede the "ordinary crank in steam engines," the beam, parallel motion, and "external flywheel," reduce friction, neutralize "all extra contending power," and leave nothing for the piston to do "but the work intended to be done."

A correspondent of the _Repertory of Patent Inventions_ made short work of this device: "There is hardly one a.s.sertion that can be supported by proof," he wrote, "and most of them are palpable misstatements." The writer attacked "the 'beetle impetus wheel,' which he [the inventor]

thinks us all so beetle-headed, as not to perceive to be a flywheel,"

and concluded with the statement: "In short the whole production evinces gross ignorance either of machinery, if the patentee really believed what he a.s.serted, or of mankind, if he did not."[104]

[Footnote 104: _Repertory of Patent Inventions_, ser. 3, October 1828, vol. 7, pp. 196-200, and December 1828, vol. 7, pp. 357-361.]

Although many of the mechanisms for which patents were taken out were designed by persons who would make no use of the principles involved even if such principles could at that time have been clearly stated, it is a regrettable fact that worthless mechanisms often got as much s.p.a.ce as sound ones in patent journals, and objections such as the one above were infrequent. The slanted information thus conveyed to the young mechanician, who was just acc.u.mulating his first kinematic repertory, was at times sadly misleading.

From even this sketchy outline of the literature on the subject, it should be fairly evident that there has been available to the mechanician an enormous quant.i.ty of information about mechanical linkages and other devices. Whatever one may think of the quality of the literature, it has undoubtedly had influence not only in supplying designers with information but in forming a tradition of how one ought to supply the background that will enable the mind to a.s.semble and synthesize the necessary mechanism for a given purpose.[105]

[Footnote 105: Some additional catalogs of "mechanical movements" are listed in the selected references at the end of this paper.]

Some of the mechanisms that have been given names--such as the Watt straight-line linkage and the Geneva stop--have appeared in textbook after textbook. Their only excuse for being seems to be that the authors must include them or risk censure by colleagues. Such mechanisms are more interesting to a reader, certainly, when he has some idea of what the name has to do with the mechanism, and who originated it. One such mechanism is the drag link.

After I had learned of the drag link (as most American engineering students do), I wondered for awhile, and eventually despaired of making any sense out of the term. What, I wanted to know, was being dragged?

Recently, in Nicholson's _Operative Mechanic and British Machinist_ (1826), I ran across the sketch reproduced here as figure 38. This figure, explained Mr. Nicholson (in vol. 1, p. 32) "represents the coupling link used by Messrs. Boulton and Watt in their portable steam engines. A, a strong iron pin, projecting from one of the arms of the fly-wheel B; D, a crank connected with the shaft C; and E, a link to couple the pin A and the crank D together, so the motion may be communicated to the shaft C." So the drag link was actually a link of a coupling. Nothing could be more logical. A drag link mechanism now makes sense to me.

[Ill.u.s.tration: 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]

[Footnote 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."]

[Ill.u.s.tration: 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 ill.u.s.trated 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.

[Ill.u.s.tration: 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).

[Ill.u.s.tration: Figure 41.--Scotch yoke, described as a "crank and slot-headed sliding rod." From W. J. M. Rankine, _A Manual of Machinery and Millwork_ (ed. 6, London, 1887, p. 169).]

[Ill.u.s.tration: Figure 42.--A "Scotch" supporting the top member of a string of well-drilling tools while a section is being added, 1876. From Edward H. Knight, _Knight's American Mechanical Dictionary_ (New York, 1876, p. 2057).]

It was surprising to me to find that the Ackermann steering linkage, used today on most automobiles, was patented in 1818 when Detroit was still a frontier town.[107] Furthermore, the man who took out the patent described himself as Rudolph Ackermann, publisher and printseller. I thought I had the necessary clue to the linkage's origin when I noticed that the first English translation of the Lanz and Betancourt treatise was published by Ackermann, but the connection finally proved to be more logical, if less direct. Ackermann (1764-1834), son of a Bavarian coach builder, had spent a number of years designing coaches for English gentlemen in London, where he made his home. One of his more notable commissions was for the design of Admiral Nelson's funeral car in 1805.

The Ackermann steering linkage was not actually Ackermann's invention, although he took out the British patent in his name and promoted the introduction of the running gear of which the linkage was a part (fig.

43). The actual inventor was Ackermann's friend George Lankensperger of Munich, coachmaker to the King of Bavaria. The advantage of being able to turn a carriage around in a limited area without danger of oversetting was immediately obvious, and while there was considerable opposition by English coachmakers to an innovation for which a premium had to be paid, the invention soon "made its way from its own intrinsic merit," as Ackermann predicted it would.[108]

[Footnote 107: British Patent 4212, January 27, 1818.]

[Footnote 108: Rudolph Ackermann, _Observations on Ackermann's Patent Moveable Axles_, London, 1819. It was interesting to me to note an abstract of W. A. Wolfe's paper "a.n.a.lytical Design of an Ackermann Steering Linkage" in _Mechanical Engineering_, September 1958, vol. 80, p. 92.]

[Ill.u.s.tration: Figure 43.--Ackermann steering linkage of 1818, currently used in automobiles. This linkage was invented by George Lankensperger, coachmaker to the King of Bavaria. From _Dinglers Polytechnisches Journal_ (1820, vol. 1, pl. 7).]

The Whitworth quick-return mechanism (fig. 44) was first applied to a slotter, or vertical shaper, in 1849, and was exhibited in 1851 at the Great Exhibition in London.[109] Willis' comments on the mechanism are reproduced in figure 44. I hope that Sir Joseph Whitworth (1803-1887) will be remembered for sounder mechanical contrivances than this.

[Footnote 109: The quick-return mechanism (British Patent 12907, December 19, 1849) was perhaps first publicly described in Charles Tomlinson, ed., _Cyclopaedia of Useful Arts and Manufactures_, London, 1854, vol. 1, p. cxliv.]

[Ill.u.s.tration: Figure 44.--Quick-return mechanism. _Top_, Early representation of the quick-return mechanism patented by Whitworth in 1849, from William Johnson, ed., _The Imperial Cyclopaedia of machinery_ (Glasgow, about 1855, pl. 88). _Middle_, Sketch by Robert Willis from his copy of _Principles of Mechanism_ (London, 1841, p. 264), which "shews Whitworth dissected into a simpler form"; it is as obscure as most subsequent attempts have been to explain this mechanism without a schematic diagram. _Bottom_, Linkage that is kinematically equivalent to Whitworth's, from Robert Willis, _Principles of Mechanism_ (London, 1841, p. 264).]

Mechanisms in America, 1875-1955

Engineering colleges in the United States were occupied until the late 1940's with extending, refining, and sharpening the tools of a.n.a.lysis that had been suggested by Willis, Rankine, Reuleaux, Kennedy, and Smith. The actual practice of kinematic synthesis went on apace, but designers often declined such help as the a.n.a.lytical methods might give them and there was little exchange of ideas between scholars and pract.i.tioners.

The capability and precision of machine tools were greatly enhanced during this period, although, with the exception of the centerless grinder, no significant new types of tools appeared. The machines that were made with machine tools increased in complexity and, with the introduction of ideas that made ma.s.s production of complex mechanical products economically feasible, there was an accelerating increase in quant.i.ty. The adoption of standards for all sorts of component parts also had an important bearing upon the ability of a designer economically to produce mechanisms that operated very nearly as he hoped they would.

The study of kinematics has been considered for nearly 80 years as a necessary part of the mechanical engineer's training, as the dozens of textbooks that have been published over the years make amply clear.

Until recently, however, one would look in vain for original work in America in the a.n.a.lysis or rational synthesis of mechanisms.

One of the very earliest American textbooks of kinematics was the 1883 work of Charles W. MacCord (1836-1915), who had been appointed professor of mechanical drawing at Stevens Inst.i.tute of Technology in Hoboken after serving John Ericsson, designer of the _Monitor_, as chief draftsman during the Civil War.[110] Based upon the findings of Willis and Rankine, MacCord's _Kinematics_ came too early to be influenced by Kennedy's improvements upon Reuleaux's work.

[Footnote 110: A biographical notice and a bibliography of MacCord appears in _Morton Memorial: A History of the Stevens Inst.i.tute of Technology_, Hoboken, 1905, pp. 219-222.]

When the faculty at Was.h.i.+ngton University in St. Louis introduced in 1885 a curriculum in "dynamic engineering," reflecting a dissatisfaction with the traditional branches of engineering, kinematics was a senior subject and was taught from Rankine's _Machinery and Millwork_.[111]

[Footnote 111: _Transactions of the American Society of Mechanical Engineers_, 1885-1886, vol. 7, p. 757.]

At Ma.s.sachusetts Inst.i.tute of Technology, Peter Schwamb, professor of machine design, put together in 1885 a set of printed notes on the kinematics of mechanisms, based on Reuleaux's and Rankine's works. Out of these notes grew one of the most durable of American textbooks, first published in 1904.[112] In the first edition of this work, acceleration was mentioned only once in pa.s.sing (on p. 4). Velocities in linkages were determined by orthogonal components transferred from link to link.

Instant centers were used only to determine velocities of various points on the same link. Angular velocity ratios were frequently noted. In the third edition, published in 1921, linear and angular accelerations were defined, but no acceleration a.n.a.lyses were made. Velocity a.n.a.lyses were altered without essential change. The fourth edition (1930) was essentially unchanged from the previous one. Treatment of velocity a.n.a.lysis was improved in the fifth edition (1938) and acceleration a.n.a.lysis was added. A sixth edition, further revised by Prof. V. L.

Doughtie of the University of Texas, appeared in 1947.

[Footnote 112: Peter Schwamb and Allyne L. Merrill, _Elements of Mechanism_, New York, 1904. In addition to the work of Reuleaux and Rankine, the authors acknowledged their use of the publications of Charles MacCord, Stillman W. Robinson, Thomas W. Goodeve, and William C.

Unwin. For complete t.i.tles see the list of selected references.]

Before 1900, several other books on mechanisms had been published, and all followed one or another of the patterns of their predecessors.

Professors Woods and Stahl, at the Universities of Illinois and Purdue, respectively, who published their _Elementary Mechanism_ in 1885, said in their preface what has been said by many other American authors and what should have been said by many more. "We make little claim to originality of the subject-matter," wrote Woods and Stahl, "free use having been made of all available matter on the subject.... Our claim to consideration is based almost entirely on the manner in which the subject has been presented." Not content with this disclaimer, they continued: "There is, in fact, very little room for such originality, the ground having been almost completely covered by previous writers."[113]

[Footnote 113: Arthur T. Woods and Albert W. Stahl, _Elementary Mechanism_, New York, 1885.]

The similarity and aridity of kinematics textbooks in this country from around 1910 are most striking. The generation of textbook writers following MacCord, Woods and Stahl, Barr of Cornell, Robinson of Ohio State, and Schwamb and Merrill managed to squeeze out any remaining juice in the subject, and the dessication and sterilization of textbooks was nearly complete when my generation used them in the 1930's.

Kinematics was then, in more than one school, very nearly as it was characterized by an observer in 1942--"on an intellectual par with mechanical drafting."[114] I can recall my own nave belief that a textbook contained all that was known of the subject; and I was not disabused of my belief by my own textbook or by my teacher. I think I detect in several recent books a fresh, less final, and less tidy treatment of the kinematics of mechanisms, but I would yet recommend that anyone who thinks of writing a textbook take time to review, carefully and at first hand, not only the desk copies of books that he has acc.u.mulated but a score or more of earlier works, covering the last century at least. Such a study should result in a better appreciation of what const.i.tutes a contribution to knowledge and what const.i.tutes merely the ringing of another change.

[Footnote 114: _Mechanical Engineering_, October 1942, vol. 64, p. 745.]