From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Study of valve-mechanism idiosyncrasies, focusing on spring vibration and surge with experimental data graphs.
| Identifier | ExFiles\Box 56\2\ Scan083 | |
| Date | 15th January 1929 guessed | |
| 8 VALVE-MECHANISM IDIOSYNCRASIES mechanism, consisting of an eccentric and mushroom follower, was used to operate the spring up to 1850 r.p.m. of the shaft and no vibration whatever occurred. Fig. 19 shows three films taken with this eccentric, at speeds of 250, 1000 and 1850 r.p.m. Spring vibrations are visible in none of them, and the three records exactly superimpose. Results of experiments agree, therefore, with the theory that valve-spring surge is related directly to the harmonics of the valve-lift curve. SELECTING A COMBINATION OF SPRING AND CAM Up to the present, various writers have suggested different factors as influencing valve-spring vibration. Until all of these factors are known it is impossible to select a spring intelligently for minimum vibration. An attempt has been made, therefore, to collect these factors in a single equation, the derivation of which is given in the Appendix of this paper. This equation is C = (K/Δ) x (d⁵/DN) x aₙ (2) in which C is the maximum amplitude of spring vibra- VALVE-LIFT AND SPRING-VIBRATION CURVES The Valve Lift is Shown by the Solid Line and the Spring Vibrations by the Jagged Shadow Effect. Except at 850 R.P.M. the Speeds Are Those of Maximum Vibration Fig. 15—573, 603 and 640 R.P.M. Fig. 16—675, 717 and 765 R.P.M. Fig. 17—825, 850 and 885 R.P.M. Fig. 18—920, 965 and 1050 R.P.M. [Text from graphs on page 8] 573 R.P.M. 20 HARMONIC - 920 R.P.M. 640 R.P.M. 19 HARMONIC + 965 R.P.M. 12 HARMONIC + 603 R.P.M. 18 HARMONIC + 1050 R.P.M. 11 HARMONIC + 675 R.P.M. 17 HARMONIC + 250 R.P.M. 717 R.P.M. 16 HARMONIC + 1000 R.P.M. 765 R.P.M. 15 HARMONIC - 1850 R.P.M. VALVE-MECHANISM IDIOSYNCRASIES 9 [Text from graphs on page 9] 825 R.P.M. 14 HARMONIC - 1240 R.P.M. 850 R.P.M. 1280 R.P.M. 885 R.P.M. 13 HARMONIC - 1560 R.P.M. FIG. 19—VALVE AND SPRING DIAGRAMS MADE WITH HARMONIC CAM FIG. 20—RECORDS FROM A WELL-DESIGNED MECHANISM The Change in Spring Frequency and the Almost Complete Damping of the Vibration During the Valve-Closed Period Are Due to a Variable-Pitch Spring tion; 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. The equation reveals the fact that valve-spring vibration depends on three main factors: the nature of the valve-lift curve, the dimensions of the spring, and the characteristics of the spring as to material and damping. These factors will be discussed separately. Valve-Lift Curve.—The valve-lift curve influences the spring-vibration equation only by way of its harmonics. The harmonic to be considered is the largest one that comes into resonance with the spring within the speed limits of the camshaft. As the harmonics of lower order are in general of higher value, it follows that the design of the spring should be such that its frequency will be high enough to bring resonance with the dangerous harmonics beyond the maximum engine-speed. For example, an engine which has a maximum crankshaft speed of 3000 r.p.m., or camshaft speed of 1500 r.p.m., has its tenth harmonic in resonance at this speed with a spring whose frequency is 15,000 vibrations per minute. Of the camshafts we have studied, it has been found that harmonics up to the eleventh are dangerously high. In the foregoing example it would, therefore, be advisable to choose a spring with a frequency higher than 16,500 (11 x 1500). As the harmonics are a function of cam contour, a possibility of controlling them by contour design suggests itself as a method of reducing spring vibration. The primary factors of cam-contour design are valve area and valve timing; but it is possible within the limits of these requirements to change the values of the harmonics. An example of this is given in Table 1. Cams No. 4 and No. 5 give the same valve-lift and timing, but it can be seen that nearly all of the harmonics of No. 5 above the tenth are of appreciably less magnitude than those of No. 4. Tests on these shafts showed that cam No. 5 was correspondingly better as regards valve-spring vibration. Spring Dimensions.—The dimensions of the spring must be so selected that the value of C, in equation (2), is kept as small as possible without allowing the spring frequency to fall to such a value that it can come into resonance with the lower and more dangerous harmonics. The cube of the wire diameter in the numerator of the above equation indicates that the wire size is the most important factor. The smallest wire-size consistent with stress, load and frequency requirements should be used, to minimize vibration. The number of coils and their diameter appear in the denominator and should therefore be made as large as possible. Material and Damping.—Only steel wire is used for valve-springs; and, as neither the density nor the modulus of rigidity varies to any extent in the different kinds of steel used, factor K offers no possibility in the reduction of vibration. Spring damping is to a certain extent the unknown factor in the above equation. Some investigators have suggested various devices for mechanically damping the spring by means of friction, but none of these has as yet become commercially practical. It is, however, certainly within the realms of possibility that a simple and effective method of spring damping may be developed for reducing vibration. A spring having a pitch which varies in such a way that several coils close as the valve lifts has been found to bring about considerable damping by interference. Fig. 20 shows a valve-lift and spring-vibration record of a properly balanced valve-mechanism. The frequency of the spring, which is of variable pitch, is 15,600 vibra- | ||