Idiosyncrasies of Valve Mechanisms and Their Causes 290023
AFTER mentioning the detrimental effects of valve bouncing and valve-spring surge upon the power and durability of an engine and on noise, the authors list four factors that contribute to perfect action of the valve mechanism. These are: the spring forces, as related to the speed and weight of the moving parts; the rigidity of the parts; the cam contour; and the design of the spring.
Four different methods of investigating valve behavior are then described in detail. The telescopic point-by-point indicator and the stroboscopic projector of the valve motion were the first of these to be developed. The former gives an accurate measure of the valve position at any point of the cycle, and the latter makes possible a visual inspection of the valve operation. These two instruments were used together, but they were found to be rather slow in operation.
The valve-lift-curve indicator, which was developed next, gives a photographic record of the valve-lift curve; and this was supplemented by a spring-vibration indicator which makes a record of the actual vibration of the spring on the same film with the valve-lift curve.
Spring surge is the direct result of resonance of the spring frequency with the harmonics of the valve-lift curve, according to the theory of the authors, which is checked experimentally. A formula, which includes both the harmonic analysis of the valve-lift curve and the characteristics of the spring, is given to aid in the selection of a good combination of spring and cam. Another formula is given for calculating the frequency of the spring in terms of its dimensions.
An outline for the procedure of selecting a good combination of spring and cam conclude the paper, the suggested order being: (a) harmonic analysis of the valve-lift curve; (b) determination of the spring frequency required to avoid resonance with bad harmonics; and (c) selection of the spring with reference to the limitations.
In an appendix are given a sample calculation of the harmonics of the cam and the mathematical derivation of the general equation for spring-vibration amplitude, as used in the paper.
Some of the discussers introduce other formulas than those given in the paper for spring vibration and discuss the application of the formula
in which C is the maximum amplitude of spring vibration; K is a constant of proportionality, dependent also on the spring material; d is the diameter of the wire; D is the mean diameter of the coil; N is the number of active coils; an is the largest valve-lift harmonic that can come into resonance with the spring; and Δ is a damping factor.
Experiences also are quoted by several discussers, much of it confirming the findings of the authors. However, one speaker is opposed to variable-pitch springs because of alleged noises and wear resulting from coil clash.
Materials, heat-treatment and hardness tests for springs are discussed by a member who favors the use of carbon steel because of more uniform results than have been secured with alloy steels of apparently higher properties.