摘要
In wrist watches gear drives with undercut pinions are used. Those pinions have got a tooth number as few as 5—7. The efficiency of such gear drives is important because of the limited energy supply. The mean efficiency of gear drives can be expressed as E<sub>m</sub>=1/（φ<sub>2</sub>-φ<sub>1</sub>）（integral from φ<sub>1</sub> to φ<sub>2</sub>（E（φ）dφ）） in which <sup>0</sup><sub>1</sub>, <sup><</sup>sub>2</sub> are the approach angle and the recess angle respectively. In the discussion we assume that the friction coefficient between the two tooth flanks is constant because the sliding speed is relatively low in wrist watches and the main purpose is to find out the relation between gear parameters and its efficiency rather than to calculate the exact value of the efficiency. In normal conditions <sup>o</sup><sub>1</sub> and <sup>o</sup><sub>2</sub> can be found from the gear geometry without much difficulty. However when the pinion is an undercut one we have to first find the forming diameter where the involute profile starts. In this paper equations for the computation of the efficiency and the contact ratio of those gear drives are discussed.
In wrist watches gear drives with undercut pinions are used. Those pinions have got a tooth number as few as 5-7. The efficiency of such gear drives is important because of the limited energy supply. The mean efficiency of gear drives can be expressed as in which ψ_1,ψ_2 are the approach angle and the recess angle respectively. In the discussion we assume that the friction coefficient between the two tooth flanks is constant because the sliding speed is relatively low in wrist watches and the main purpose is to find out the relation between gear parameters and its efficiency rather than to calculate the exact value of the efficiency. In normal conditions ψ_1 and ψ_2 can be found from the gear geometry without much difficulty. However when the pinion is an undercut one we have to first find the forming diameter where the involute profile starts. In this paper equations for the computation of the efficiency and the contact ratio of those gear drives are discussed.
