GEARS, the moving parts of a mechanism for the transmission of motion. The commoner gears may be grouped into six classes : (1) spur gears; (2) bevel gears; (3) worm gear ing; (4) spiral gear; (5) helical gear; and (6) chain gearing.
Spur gears are toothed wheels in which the teeth are parallel with the axis. This is the commonest gear in use, and is employed to transmit motion between lines of shafting which are parallel. Bevel gears are toothed wheels in which the teeth are formed on the beveled sur face of cone frusta: they are used to transmit motion to shafting which intersects at right angles. Worm gearing is a combination of a screw and a toothed wheel: it is used in the transmission of motion where shafting crosses at right angles, but not in the same plane. Spiral gears are formed of wheels which are practically iections of screws of very steep pitch: they are used to transmit motion be tween shafting crossing at acute angles, and not in the same plane. Helical gears are composed of toothed wheels and pinions in which the teeth or cogs are cut on the lines of a spiral angle of 23 degrees. They are seldom used in the single form on account of the axial thrust de veloped, but they are cut in herring-bone fashion, right hand on one face of the wheel and left hand on the other, forming a tooth shaped liked a broad V. This cutting is known
as double helical gearing. It is regarded as the most perfect • of all gears, transmitting in some instalments 98 per cent of the power. Great speed is obtained, as a pinion may have as few as seven teeth to 140 in the wheel. Moreover, in operation the gear is nearly noiseless. Where the motion is necessary, triple helical gearing is used, the teeth being cut in three trends. Beveled helical gears are also in use, in both the single and double types. Chain gearing makes use of two spur wheels with peculiarly shaped teeth, on shafts at a con siderable distance apart. Motion is transmitted by means of an endless chain having flat open links which engage the teeth of the wheels.
Many forms of gearing have been devised for the transmission of interrupted motion, and for the conversion of continuous motion into special directions. These gears are of most in tricate and ingenious character. Illustrations and descriptions of some of the more important may be found in 'Mechanical Appliances; Me chanical Movements; and Novelties of Con struction' (New York 1914). by G. D. Hiscox. See WHEEL GEARING. Consult Flanders, R. E., 'Gear Cutting Machinery' (New York 1909); Ingham, A. E., 'Gearing) (London 1914).