(c) "Eccentric wheel" with external yoke hung from working beam. The wheel pivots at C.
(d) "Eccentric wheel" with internal driving wheel hung from working beam. Wheel B is pivoted at center of shaft A.
(e) Sun-and-planet gearing. This is the idea actually employed in Boulton and Watt engines. As the optional link JK held the gearwheel centers always equidistant, the annular guide G was not used.
The sun-and-planet arrangement, with gears of equal size, was adopted by Watt for nearly all the rotative engines that he built during the term of the "crank patents." This arrangement had the advantage of turning the flywheel through two revolutions during a single cycle of operation of the piston, thus requiring a flywheel only one-fourth the size of the flywheel needed if a simple crank were used. The optional link (JK of fig. 7e) was used in the engines as built.
From the first, the rotative engines were made double-acting—that is, work was done by steam alternately in each end of the cylinder. The double-acting engine, unlike the single-acting pumping engine, required a piston rod that would push as well as pull. It was in the solution of this problem that Watt's originality and sure judgment were most clearly demonstrated.
A rack and sector arrangement (fig. 8) was used on some engines. The first one, according to Watt, "has broke out several teeth of the rack, but works steady."[15] A little later he told a correspondent that his double-acting engine "acts so powerfully that it has broken all its tackling repeatedly. We have now tamed it, however."[16]
[ [15] James Watt, March 31, 1783, quoted in Dickinson and Jenkins, op. cit. (footnote 5), p. 140.