This rack is connected with the piece b b in such a manner as to allow of a short transverse motion with respect to that piece, by which the pinion, when it arrives at either end of the course, is enabled by shifting the rack to follow the course of the guide-groove, and thus to reverse the motion by acting upon the opposite row of teeth.
The best mode of connecting the rack and its sliding piece is that represented in the figure, and is the same which is adopted in the well-known cylinder printing-engines of Mr. Cowper. Two guide-rods k c, k c are jointed at one end k k to the reciprocating piece b b, and at the other end c c to the shifting-rack; these rods are moreover connected by a rod m m which is jointed to each midway between their extremities, so that the angular motion of these guide-rods round their centres k k will be the same; and as the angular motion is small and the rods nearly parallel to the path of the slide, their extremities c c may be supposed to move at a right angle to that path, and consequently the rack which is jointed to those extremities will also move upon b b in a direction at a right angle to its path, which is the thing required, and admits of no other motion with respect to b b.
Fig. 215.
To multiply plane motion the construction shown in [Fig. 215] is frequently employed. a and b are two racks, and c is a wheel between them pivoted upon the rod r. A crank shaft or lever d is pivoted at e and also (at p) to r. If d be operated c traverses along a and also rotates upon its axis, thus giving to b a velocity equal to twice that of the lateral motion of c.
The diameter of the wheel is immaterial, for the motion of b will always be twice that of c.
Fig. 216.