OF
MY PARALLEL MOTION,
As applied to HEAVY Steam Engines.
While this Invention, as described in [page 30] of the first Part, is allowed to possess curious properties, and to be a pretty thing, opinions do not all concur in declaring it, essentially and generally, a good thing. Nor could I be unjust enough to insist that it is so, in every kind and magnitude of application. I have, however, convinced myself that it is susceptible of practical excellence, as a first motion to steam engines, whatever be their dimensions; and have, therefore, presumed to re-produce it, with those modifications which are required to make it so. In thus acting, I have again preferred the useful to the agreeable, and in some measure inverted the order of my subjects. But I trust this deviation will be excused, in favour of the motive and the result; on both which I feel a good degree of confidence.
To obviate the point of mechanical weakness in this Parallel Motion, (see [Plate 41], [fig. 3],) I have doubled it’s parts; and brought the piston rod a b, to act, at once, on two of the circulating wheels c d, placed exactly opposite each other, and rolling, as before, on the inside of the fixed wheels f e, so as to produce the rectilinear motion, by the action of the piston rod on them both. And to make their respective motions one, (as connected with the fly B A) this latter is fixed to a shaft common to the two wheels g h, and by which, therefore, the two other wheels i k, fixed to the crank shafts m n, are kept in due position. Thus, then, is all winding or twisting motion done away: and, therefore, can this System be employed in engines of every required power. Nor need I add, (what will be generally allowed) that much of the expence, and of the retardation, which a given engine suffers from the beam, the connecting rod, &c. will thus be completely obviated.
I must, however, stop every gainsaying mouth, on the circumstance of using geering between the engine and the fly—a system which I acknowledge to have been hitherto an evil; though, perhaps, a necessary evil—as giving (by a simple method) a double speed to the fly from a single motion of the piston. At all events, in this shape, I submit only to a very common difficulty—and might there rest my apology.
But I should have hesitated to go thus far, had I not foreseen that all the evil arising from this use of wheels, can easily be avoided by my geering:—by means of which I am bold to say, every vestige of shake or backlash may be destroyed; and this method of working a steam engine be made as silent as when a beam is used: in which case, considerable advantages must accrue from this method.
To come to the point:—the small [figure 4], in [Plate 41], relates to this subject. My geering is there seen in three forms or applications—each one intended to bring the above property into play. The part n o, represents the manner in which two wheels with singly-inclined teeth, work together when one of them is furnished with a cheek, as directed in [fig. 3] of [Plate 14]. But here, in addition to that, the teeth of both wheels are sloped more on one side than on the other, so as to assume a wedge-like form: insomuch, that in beginning to work, (if not perfectly formed) the wheels would not occupy the same plane. For, in fact, the cheek screws press home the cheek o against a number of thin washers all round the wheel, and thus only draw the wedge-formed teeth into each other as they become bedded, and successive washers are taken away. Hence, a good degree of precision is obtained—accompanied with little friction, and thus with great durability.