弁發條の衝撃に依る應力

抄録

Valve springs are known to fail when the limits of designed stress range based on static compression are far below the limits assigned as the fatigue limit range for the material in question. One reason for this is due to the phenomenon known as surging which occurs when the natural frequency of the valve spring is in resonance with the cam profile harmonic. Another is due to the additional instantaneous shock stress which occurs at the very begining of the opening of the valve.
However neither of these reasons is sufficient to account for the failures which occur with aero-engine valve springs. In the first place the resonance known as surging does not always occur on acconnt of the difference in the natural frequencies of the spring between the period the valve is opened and that when closed. This is of course due to the closing up of the end coils in these springs. Aero-engine valve springs are purposely designed thus weak to allow a quick opening to a very large lift. Again the instantanious shock stress at the very begining of the valve opening may amount to large values at very high speed but aero-engiug valve springs fail very often at much lower speed.
In this paper an attempt has been made to account for the failures by assuming that shock stresses originate at the moving end of the spring continuously from the time the valve is opened till the valve comes to a top at the position of maximum lift. The shock stresses are superposed in much the same way as is done in calculating the pressure waves in oil injection tubes where account is taken of the plunger motion and the reflection at both ends. The shock stress originating at each instant at the moving end is calculated from the velocity of the end as obtained from the actual lift curve. The reason why the actual experimental lift curve is used is due to the fact that the valve jumps off the cam profile at very low speed so that an analysis of the latter is not sufficient. In this way it was found that for the three springs tested the actual limits of stress range wear beyond those assigned as the fatigue limit range for the material of the springs although the range calculated from static compression between the limits of the valve lift was far below the assigned range.