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Gear couplings are widely used as connecting elements that can allow axis displacements caused between two axes in compact size. The recent demands to gear couplings are as follows, they are increase in load capacity, increase in allowable shaft angle and reduction in vibration and noise. So far, many studies have been conducted on relations of shapes of tooth surfaces, axis displacements, number of tooth pairs in contact, etc. However, there is no research that has analyzed tooth contact of gear couplings using the shape of tooth surface actually finished by various kinds of tools.In this paper, a method for calculating the tooth clearance of the gear coupling using the shape of tooth surface interpolated from many section profiles by means of parabolic approximation is proposed. The method was applied the gear couplings the gears of which are manufactured in the different ways and have different profiles. As the results, a new tooth profile that brings good tooth contact for gear couplings was developed.

New equations have been derived to calculate the power transmission efficiencies, at the meshing teeth, of two kinds of representative cylindrical gears, Novikov-Wildhaber and involute gears. The efficiencies could be calculated from the equations and the friction coefficients obtained by simple experiments with test rollers. In order to show clearly the difference in the power transmission efficiencies of the two kinds of gears, the authors designed and made two interesting gear pairs with a small number of pinion teeth (Z1 = 3) and with a high gear ratio (Z2/Z1 = 9) by applying new techniques developed by them. The effects of gear speed, tooth load, type of lubricant, etc. upon the power transmission efficiencies were clarified using two interesting techniques to measure the efficiency. The experimental results indicated that the efficiency of the Novikov-Wildhaber gears was appreciably higher at high speeds, while it was lower than the involute gears at the start of rotation and also at low speeds.

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