1929-01-01

Electric Telemeter and Valve-Spring Surge 290022

THE electric telemeter presents an excellent means for investigating the phenomenon of valve-spring surge. Basically, the telemeter is composed of two differentially connected stacks of thin carbon discs so arranged that, when the apparatus is subjected to strain, the pressure is increased on one stack and decreased on the other. Each stack forms one arm of a Wheatstone's bridge, and the resistances of the stacks vary with the pressure on them, thus slightly upsetting the balance of the bridge. If an oscillograph galvanometer-element be substituted for the usual bridging instrument, the arrangement will be found suitable for making photographic records.
To study valve-spring surge, the telemeter is connected across the points of a stiff C-spring, one end of which is held against the valve-spring in such a way that vibrations of the spring are transferred to the C-spring and thence to the telemeter. This equipment has made it possible to identify the cause of valve-spring surge as a resonant condition at certain speeds.
Except at low engine-speeds, the stress conditions of a spring having resonant points are much worse than is indicated by the conventional stress-formula, both as to degree and as to the rapidity of the stress cycle. Surge and stress conditions are improved by designing the springs for high frequency, and by making the pitch variable so that the frequency is variable as the valve lifts, thus eliminating resonance.
Discussion of this paper includes warnings in regard to lack of uniformity of valve-spring material and to the possible low-frequency of the electric telemeter, and suggestions for further study of fatigue as a cause of failure and of internal friction as a remedy for surge.
Further discussion on the subject of valve-spring surge is included with that of the Jehle and Spiller paper,3 which was given and discussed at the same session of the Annual Meeting. The discussion of the latter paper immediately follows the discussion of this paper.

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