1266
THEORY AND PRACTICE OF ENGINEERING.
Book II.
rod. These wheels and connecting rods being always in similar positions, on opposite sides
Fig. ‘.030. Cartwright’s parallel motion.
Fig. 2032.
CYCLOIDAL MOTION.
of the piston-rods, the obliquity of theiractions balances each other, and the rod ismade to describe a straight line ; to makethis arrangement work accurately the greatestnicety of execution is necessary.
Fig. 2031. PARALLEL MOTION.
The Cycloidal Parallel Motion , which is exceedingly beautiful, depends on the prin-ciple that an encycloidal curve, described by one circle rolling within another, approaches astraight line, as the inner circle becomes more nearly equal in diameter to the radius of theouter one. The large wheel, which has teeth on its inner circumference, is fixed on a frameconcentric with the axis and circle of the crank o : the small wheel, which has teethon its outer edge, is freely fixed on the crank pin, and p is the point at which the piston-rod is affixed. The small wheel is forced by the pressure of the piston-rod upwards, toroll round the great circle, ascending on the one side and descending on the other ; so thatthe distance of the end of the piston-rod from the point of contact of the circles is alwaysequal to the distance of the circle from the diameter.
PARALLEL MOTION.
The Sun and Planet-Wheel, (fig. 2033.) w.as contrived by Watt: a toothed wheel A is fixedcentrally with the large wheel to which the rotary motion is to be conveyed; B is anothertoothed wheel fixed firmly to the end of a vertical rod C ; a strong link connects the centresof the two wheels. If the shaft D be turned, the wheel B will revolve in the samedirection, and elevate the rod C till it arrives at the top of the wheel A, when, by continuingthe motion of the large wheel, B must descend to its first position, and so on continually.