GEARS AND GEARING
For the purpose of showing how motion may be converted from a straight line or from a circular movement into any other form or direction, and how such change may be varied in speed, or made regular or irregular, the following examples are given, which may be an aid in determining other mechanical devices which can be specially arranged to do particular work.
While cams and eccentrics may be relied on to a certain extent, there are numerous places where the motion must be made positive and continued. This can be done only by using gearing in some form, or such devices as require teeth to transmit the motion from one element to the other.
The following illustrations do not by any means show all the forms which have been constructed and used in different machines, but they have been selected as types merely, in order to give the suggestions for other forms.
Racks and Pinions.—The rack and pinion is the most universal piece of mechanism for changing motion. [Fig. 101] illustrates it in its most simple form. When constructed in the manner shown in this figure it is necessary that the shaft which carries the pinion shall have a rocking motion, or the rack itself must reciprocate in order to impart a rocking motion to the shaft.
This is the case also in the device shown in Fig. 102, where two rack bars are employed. A study of the cams and eccentrics will show that the transference of motion is limited, the distances being generally very small; so that the rack and pinions add considerably to the scope of the movement.
The Mangle Rack.—The device called the mangle rack is resorted to where a back and forth, or a reciprocating movement is to be imparted to an element by a continuous rotary motion.