Discordant sounds from transmission gears can be avoided by using gear-tooth ratios that give pleasing combinations of tones; a 5:6 ratio produces a minor third note; a 4:5 ratio, a major third; a 2:3 ratio, a perfect fifth; and a 2:1 ratio, an octave. Careful attention to selection of relative tooth-numbers, therefore, will aid greatly in the production of quiet or un-objectionable transmissions.
Careful design and accuracy in the production of gears will not, alone, insure quiet operation; the shafts must be sufficiently rigid to hold the gears in proper operating position and large flat surfaces in transmission cases, which act as sound amplifiers, should be avoided. Bearings may also be noisy through faults of their own or because of improper mounting and alignment. Too much lubricant in the transmission case may be another cause of noise; the practice of filling the case full does more harm than good, although the large quantity of oil may serve to absorb the vibrations of the case. Sufficient space should be left for the excess oil to be squeezed out from between the gear teeth readily, as heat generated by forcing the oil out suddenly raises the temperature in the transmission case to a dangerous degree if the faces of the gears are wide, as in motor trucks.
Four characteristic sounds are produced by the gears. One is an intermittent clicking or irregular growl caused by poor spacing or irregular profiles of the teeth; another is a pulsating growl caused by eccentricity; a third is a high-pitched squeal due to rough tooth-surfaces; and a fourth is a tone that depends on the pitch of the teeth and speed of the gears. The remedy for the first three is accuracy in tooth generation and better workmanship; that for the fourth is to select a gear ratio that will produce a musical note.
Some sound is inevitable when power is transmitted through gears; the search for some modification of tooth profile that will obviate the need for accuracy has been fruitless and probably always will be, as the spacing between the teeth must be nearly perfect to insure quiet running. A solution of the noise problem that may be satisfactory today may not be satisfactory next year; other parts of the automobile have been made quieter and as a result the sounds from the transmission, which were not noticeable before, have become objectionable. Three courses are open to remedy the noisiness; (a) eliminate the gears, (b) reduce the amount of noise produced by them and (c) change the quality of the sounds so that they will not be annoying.