Gearing.
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 gearis 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 wheelhas 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; seefig. 263.
A spur wheelhas teeth on its circumference which run parallel to its shaft; wheels as shown infig. 271are termedhelical wheels; these are similar to spur wheels except their teeth are arranged upon different angles to the shaft.
A bevelis 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; seefig. 267.
Miter wheelsare bevel wheels of the same size, working at right angles with one another; seefig. 268.
The diameter of both spur and bevel wheelsis 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 bevelwheel, we mean the diameter of the pitch circle, without any reference to the form of tooth.
The addendum circleof a toothed wheel is as shown in illustration,fig. 264;addendummeans “something added,” and, as shown in the figure, it is the part added beyond the pitch “line” or circle.
DefinitionsFig. 264.
Fig. 264.
The pitch lineis 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 infig. 274.
The peripheryof a wheel is the extreme circumference, as N infig. 274.
All parts of gear-wheels consist of portions, to which have been given generally accepted names.Fig. 264shows 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 circleis the same as the circular pitch line.
Diametralmeans 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.
Cog wheelFig. 265.
Fig. 265.
A cog wheelis 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 calledteeth; when they are made of wood or some other material and fixed to the circumference of the wheel, they are calledcogs; seefig. 265.
A pinionis a small wheel. When two toothed wheels act upon one another, the smaller is generally called the pinion. The termstrundleandlanternare applied to small wheels having cylindrical bars instead of teeth. The teeth in pinions are sometimes termedleaves; in a trundle,staves. Seefig. 273.
The wheel which acts is called aleaderordriver; and the wheel which is acted upon by the former is called afolloweror thedriven. When a screw orwormrevolves in the teeth of a wheel, the latter is termed aworm wheelorworm gear; seefig. 270. When a pinion acts with a rack having teeth, we speak ofrackand pinion. When the teeth are on the inside of the rim, and not on the periphery, the wheel is termed aninternal gear; seefig. 272.
Two wheels acting upon one another in the same plane are calledspur gear; the teeth are parallel with the axis. When wheels act at an angle, they are calledbevel gear.
Friction gear-wheelsare 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. 263shows a pair of spur-wheels in gear. The dotted circles which meet are the rolling circles, called the “pitch line” or “pitch circle.”
Spur mortise wheelFig. 266.
Fig. 266.
A spur mortise wheel is similarly shown infig. 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 alsofig. 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.
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 gearingare bevel wheels working out of center; the teeth do not form radial lines from the wheel center.
Fig. 267shows a pair of bevel wheels in gear as described onpage 199. A bevel mortise wheel,i. e., one having cogs inserted in its rim instead of teeth.
Bevel wheelsFig. 267.
Fig. 267.
A bevel wheel and pinion must be made to suit one another by both having teeth forming together an angle of 90°, therefore they are pairs, or proportioned in the number of teeth one to the other. Any other proportion used would not exactly gear and would be termed a “bastard” gear.
Miter wheelsFig. 268.
Fig. 268.
Fig. 268represents a pair of miter wheels in gear; it will be noted that the shafts, when connected, will be at right angles to each other, the wheels being in all particulars of the same dimensions; the figure answers the purpose of a much longer description, if given in words.
A miter-wheel can easily be known by putting a square upon the face of the teeth, which are always at an angle of 45° with one another, irrespective of size.
A miter-wheelis a particular kind of bevel-wheel, the bevel being limited to an angle of 45° in each wheel.
The curve of the teeth in bevel-gears, when correctly formed, changes constantly from one end of the tooth to the other, therefore bevel-gears whose teeth are produced with a forced cutter are not theoretically correct.
Fig. 269represents a rack and pinion: the teeth in this form of gear are shaped similarly to those in the spur wheel, shown onpage 198, with the difference that the teeth of one are on a circle and on the rack are made on a straight line.
Rack and pinionFig. 269.
Fig. 269.
A flange or addition to the end of a tooth and the rim connecting them together is used to strengthen the teeth. This extends from the root to pitch line when the wheel and pinion are both flanged: if only one is flanged it extends from the root to the addendum.
Worm and worm wheelFig. 270.
Fig. 270.
Fig. 270illustrates a worm and a worm wheel, sometimes called screw gears. This is a slow but powerful method of transmitting power, one revolution of the worm only moving the wheel the distance of one tooth and space.
A worm gearis a spur wheel with teeth at an angle to the axis, so as to work with a worm which is ascrew, or has teeth shaped in the form of a spiral wound round its circumference; the screw or worm is called an endless screw, because it never comes to a stopping place in the circumference of the wheel.
Helical gearFig. 271.
Fig. 271.
Fig. 271represents a gear with helical teeth. It is similar to a spur wheel, and is used in place of same in heavy and slow moving machinery, the formation of teeth preventing—in large measure—the jar or concussion noticeable in common spur gears.
In recent years the speed at which gearing is run has been greatly increased. A striking instance is that of a pair ofcast-ironhelical wheels, 6 ft. 3 in. diameter, 12 in. wide, making 220 revolutions per minute, the speed of the pitch line being 4,319 feet per minute; these wheels are running continuously and with little noise. There is also acutgear in a mill in Massachusetts, 30 feet in diameter, and the speed of pitch line is 4,670 feet per minute.
Annular gearFig. 272.
Fig. 272.
An internal or annular gear wheelis one in which the faces of the teeth are within and the flank without the pitch circle, hence the pinion operates within the wheel. Seefig. 272.
In internal geared wheels there is almost an entire absence of friction and consequent wear of the teeth, as compared to ordinary spur gearing.
Fig. 273shows acrown-wheelwhich has pin teeth which are fixed by one end only, on its side face and gear into a trundle wheel.
Trundle wheelFig. 273.
Fig. 273.
A trundle wheelhas no teeth, properly speaking. Instead of teeth, it has pins as shown on illustration,fig. 273, arranged like the rungs of a ladder between two walls. Seepage 201.
Trains of Gears.—When two wheels mesh—that is, engage with each other—as infig. 263, one axle revolves in the opposite direction to the other; but when internal gears mesh as shown infig. 272, the shafts revolve in the same direction; three or more gears running together are often calleda train of gears.
Maximum speed of gears under favorable conditions for safety iscomparatively—
It is not, however, advisable to run gears at their maximum speeds, as great noise and vibration are caused.