Geneva mechanisms. Part 4: advanced examples.
Abstract:Example 1 demonstrates the analysis of an external Geneva mechanism in an existing machine, in which the crank angular velocity varies during each rotation. The external torque driven by the mechanism cannot be modelled adequately by one of the standard models provided in the ESDUpac A9611 software and so a user-defined external file is created and used. The results of the analysis indicate that the contact stress exceeds the stated limit. A method to identify the dominant factor causing the high contact force is described and various methods for eliminating operating problems caused by the excessive contact stress are considered. Example 2 describes how transient effects that can occur during start-up or shut-down of an external Geneva mechanism can be analysed. The mathematical approach for developing user-defined crank motion data files that represent the motion of the mechanism as it accelerates from rest to its operating speed, and from its operating speed to rest during shut-down, is described. Two alternative start-up times, i.e. the times taken to reach full operating speed, and two shut-down times, are analysed for an initial crank angle of zero. Using the information gained from these analyses, the initial crank angles that would result in the highest drive torque requirement for the two start-up/shut-down times are found. The mechanism performance for these 'worst cases' is analysed and recommendations are made for the control of the mechanism during start-up.
|Data Item ESDU 99001