Part 32

Under this heading the author has grouped some information relating to a subject of wide interest and one sure to interest a student of mechanical drawing.

The diagrams are intended for exercises in drawing, i. e., to be redrawn as parts of practice; the text is to be studied not only for the good to be gained from the study of gearing, but as an example of the way in which written or printed descriptions are necessary to explain a subject illustrated by drawings.

A gear is primarily a toothed wheel; gearing is a train of toothed wheels for transmitting motions; there are two chief sorts of toothed gearing, viz., spur gearing and bevel gearing.

A spur wheel has teeth around the edge pointing to the center; commencing at the center, a spur wheel may be said to consist of a hole, square, octagonal or round, for its axle or shaft; a hub; the web, body or arms; a rim, and the teeth; see fig. 263.

A spur wheel has teeth on its circumference which run parallel to its shaft; wheels as shown in fig. 271 are termed helical wheels; these are similar to spur wheels except their teeth are arranged upon different angles to the shaft.

A bevel is a slant or inclination of a surface from a right line, hence a bevel wheel is one whose teeth stand beveling or at an oblique angle to the shaft, or towards the center; see fig. 267.

Miter wheels are bevel wheels of the same size, working at right angles with one another; see fig. 268.

The diameter of both spur and bevel wheels is measured and calculated neither from the outside nor from the bottom of the teeth, but on the pitch circle. When we speak of the diameter of a spur or bevel wheel, we mean the diameter of the pitch circle, without any reference to the form of tooth.

The addendum circle of a toothed wheel is as shown in illustration, fig. 264; addendum means “something added,” and, as shown in the figure, it is the part added beyond the pitch “line” or circle.

The pitch line is the most important one in gearing; the “pitch line” or “pitch circle” is supposed to be the working circle. This is shown in P--P in fig. 274.

The periphery of a wheel is the extreme circumference, as N in fig. 274.

All parts of gear-wheels consist of portions, to which have been given generally accepted names. Fig. 264 shows the “addendum circle” and the “pitch line” as marked. The teeth and rim are shown in white, and the other portions are indicated by the names.

The circular pitch line, as opposed to the diametral pitch, is the same as the pitch circle. It is a line which bisects all the teeth of a toothed wheel.

The rolling circle is the same as the circular pitch line.

Diametral means pertaining to a diameter or the length of a diameter; hence a diametral pitch is a system of measures or enumeration based upon the diameter instead of the circular pitch line; it is used very generally in spacing for fine tooth gear. Wheels of this description usually have their teeth cut in a gear-cutting machine, i. e., medium and fine tooth gears.

A cog wheel is the general name for any wheel which has a number of cogs placed around its circumference.

When the teeth of a wheel are made of the same material and formed of the same piece as the body of the wheel, they are called teeth; when they are made of wood or some other material and fixed to the circumference of the wheel, they are called cogs; see fig. 265.

A pinion is a small wheel. When two toothed wheels act upon one another, the smaller is generally called the pinion. The terms trundle and lantern are applied to small wheels having cylindrical bars instead of teeth. The teeth in pinions are sometimes termed leaves; in a trundle, staves. See fig. 273.

The wheel which acts is called a leader or driver; and the wheel which is acted upon by the former is called a follower or the driven. When a screw or worm revolves in the teeth of a wheel, the latter is termed a worm wheel or worm gear; see fig. 270. When a pinion acts with a rack having teeth, we speak of rack and pinion. When the teeth are on the inside of the rim, and not on the periphery, the wheel is termed an internal gear; see fig. 272.

Two wheels acting upon one another in the same plane are called spur gear; the teeth are parallel with the axis. When wheels act at an angle, they are called bevel gear.

Friction gear-wheels are those which communicate motion one to the other by the simple contact of their surfaces.

In frictional gearing the wheels are toothless and one wheel drives the other by means of the friction between the two surfaces which are pressed together.

Grooved friction wheels are used to give greater cohesion than can be obtained by the plain surface.

Fig. 263 shows a pair of spur-wheels in gear. The dotted circles which meet are the rolling circles, called the “pitch line” or “pitch circle.”

A spur mortise wheel is similarly shown in fig. 266; it is very like in appearance to a spur wheel; it differs essentially in that the teeth are separate cogs, fixed in singly to the rim; see also fig. 265, page 201.

NOTE.--The teeth of spur wheels cast from a pattern must of necessity be larger at one side than at the other, because the teeth must have taper to permit the extraction of the pattern from the mould; therefore, in fixing wheels to gear, the large side of one should meet the smaller side of the other; should the two large sides come together the teeth will meet only at the large side, and the teeth will probably break away from the excessive strain on that point.

Skew gearing are bevel wheels working out of center; the teeth do not form radial lines from the wheel center.

Fig. 267 shows a pair of bevel wheels in gear as described on page 199. A bevel mortise wheel, i. e., one having cogs inserted in its rim instead of teeth.