Chapter XII: 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.

The plain mangle racks are shown in Figs. 103 and 104, the former of which has teeth on the inside of the opposite parallel limbs, and the latter, Fig. 104, having teeth not only on the parallel sides, but also around the circular parts at the ends.

This form of rack may be modified so that an alternate circular motion will be produced during the movement of the rack in either direction. Fig. 105 is such an instance. A pinion within such a rack will turn first in one direction, and then in the next in the other direction, and so on.

If the rack is drawn back and forth the motion imparted to the pinion will be such as to give a continuous rocking motion to the pinion.

Controlling the Pinion.--Many devices have been resorted to for the purpose of keeping the pinion in engagement with the teeth of the mangle rack. One such method is shown in Fig. 106.

The rack A has at one side a plate B, within which is a groove C, to receive the end of the shaft D, which carries the pinion E. As the mangle rack moves to such a position that it reaches the end of the teeth F on one limb, the groove C diverts the pinion over to the other set of teeth G.

All these mangle forms are substitutes for cranks, with the advantage that the mangle gives a uniform motion to a bar, whereas the to and fro motion of the crank is not the same at all points of its travel.

Examine the diagram, Fig. 107, and note the movement of the pin A which moves along the path B. The crank C in its turning movement around the circle D, moves the pin A into the different positions 1, 2, 3, etc., which correspond with the positions on the circle D.

The Dead Centers.--There is also another advantage which the rack possesses. Where reciprocating motion is converted into circular motion, as in the case of the ordinary steam engine, there are two points in the travel of a crank where the thrust of the piston is not effective, and that is at what is called the _dead centers_.

In the diagram, Fig. 108, the ineffectiveness of the thrust is shown at those points.

Let A represent the piston pushing in the direction of the arrow B against the crank C. When in this position the thrust is the most effective, and through the arc running from D to E, and from H to G, the cylinder does fully four-fifths of the work of the engine.

While the crank is turning from G to D, or from I to J, and from K to L, no work is done which is of any value as power.

If, therefore, a mangle bar should be used instead of the crank it would add greatly to the effectiveness of the steam used in the cylinder.

Crank Motion Substitute.--In Fig. 109 the pinion A is mounted so that its shaft is in a vertical slot B in a frame C. The mangle rack D, in this case, has teeth on its outer edge, and is made in an elongated form. The pinion shaft moves up and down the slot and thus guides the pinion around the ends of the rack.

Mangle Wheels.--The form which is the most universal in its application is what is called the _mangle wheel_. In Fig. 110 is shown a type wherein the motion in both directions is uniform.

Mangle wheels take their names from the ironing machines called _mangles_. In apparatus of this kind the movement back and forth is a slow one, and the particular form of wheels was made in order to facilitate the operation of such machines. In some mangles the work between the rollers is uniform back and forth. In others the work is done in one direction only, requiring a quick return.

In still other machines arrangements are made to provide for short strokes, and for different speeds in the opposite directions, under certain conditions, so that this requirement has called forth the production of many forms of wheels, some of them very ingenious.

The figure referred to has a wheel A, on one side of which is a peculiarly-formed continuous slot B, somewhat heart-shaped in general outline, one portion of the slot being concentric with the shaft C.

Within the convolutions of the groove is a set of teeth D, concentric with the shaft C. The pinion E, which meshes with the teeth D, has the end of its shaft F resting in the groove B, and it is also guided within a vertical slotted bar G.

The pinion E, therefore, travels over the same teeth in both directions, and gives a regular to and fro motion.

Quick Return Motion.--In contradistinction to this is a wheel A, Fig. 111, which has a pair of curved parallel slots, with teeth surrounding the slots. When the wheel turns nearly the entire revolution, with the pinion in contact with the outer set of teeth, the movement transmitted to the mangle wheel is a slow one.

When the pinion arrives at the turn in the groove and is carried around so the inner teeth are in engagement with the pinion, a quick return is imparted to the wheel.

Accelerated Motion.--Aside from the rack and mangle type of movement, are those which are strictly gears, one of them being a volute form, shown in Fig. 112. This gear is a face plate A, which has teeth B on one face, which are spirally-formed around the plate. These mesh with a pinion C, carried on a horizontal shaft D. This shaft is feathered, as shown at E, so that it will carry the gear along from end to end.

The gear has cheek-pieces F to guide it along the track of teeth. As the teeth approach the center of the wheel A, the latter impart a motion to the gear which is more than twice the speed that it receives at the starting point, the speed being a gradually increasing one.

Quick Return Gearing.--Another much more simple type of gearing, which gives a slow forward speed and a quick return action, is illustrated in Fig. 113. A is a gear with internal teeth through one half of its circumference, and its hub B has teeth on its half which is opposite the teeth of the rim.

A pinion C on a shaft D is so journaled that during one half of the rotation of the wheel A, it engages with the rim teeth, and during the other half with the hub teeth. As the hub B and gear C are the same diameter, one half turn of the pinion C will give a half turn to the wheel A.

As the rim teeth of the wheel A are three times the diameter of the pinion C, the latter must turn once and a half around to make a half revolution of the wheel A.

Scroll Gearing.--This is a type of gearing whereby at the close of each revolution the speed may be greater or less than at the beginning. It comprises two similarly-constructed gears A, B, each with its perimeter scroll-shaped, as shown.

The diagram shows their positions at the beginning of the rotation, the short radial limb of one gear being in line with the long limb of the other gear, hence, when the gears rotate, their speeds relative to each other change, being constantly accelerated in one or decreased in the other.