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).
Article on the design, analysis, and failures of aircraft engine valve mechanisms, discussing issues like false motion and seating velocities.
| Identifier | ExFiles\Box 158\5\ scan0019 | |
| Date | 1st March 1939 | |
| Aircraft-Engine Valve Mechanisms By Vincent C. Young Chief Engineer, Wilcox-Rich, Division of Eaton Mfg. Co. NEMESIS of aircraft valve mechanism is false motion, Mr. Young believes. False motion in valve gear, he explains, is any motion of parts other than the theoretical, and it affects any part of the mechanism from the valve to the cam, since high loadings can be encountered which far exceed the calculated stresses. The result often is failure, either partial or complete, he contends, and illustrates this point by diagrams of actual valve motion under operating conditions. Emphasis is placed on other effects from the same cause, such as high seating velocities. In a discussion of the hydraulic lash adjuster, the author stresses its effect on seating velocities and maintenance of fixed timing, referring particularly to air-cooled radial engines. The importance of reduced operating temperatures of the valves is pointed out in regard to increased strength, and to improved fatigue and corrosion resistance. IN using the term “valve mechanism,” we think of it as covering all phases of the gear and the attendant parts that have to do with the induction and scavenging system of an internal-combustion engine. Obviously this is a vast subject, and any attempt to cover it in its entirety would require a book or perhaps several to do it proper justice. The topics dealt with, in consideration of the vastness of this subject, are confined to a few such parts as are affected by operation as well as original design, and from the viewpoint of the parts manufacturer rather than the engine designer. In designing for the future, one of the greatest aids in seeking improvement is careful study and analysis of results with current design, both from service as well as experimental operation. Failures – either partial or complete – play an important part in dictating design and material changes and, if they can be analyzed with a fair degree of accuracy, the correction is generally made in a direction that will lead to improved and more satisfactory results in future designs. It is quite conceivable, without stretching one’s imagination too far, that, if engine parts functioned always as designed, many of the problems that arise from service operation would not exist; but this is far from being a fact and makes necessary constant revisions to overcome unpredicted factors. Development work in connection with the application of hydraulic lash adjusters has emphasized the necessity for detail study in connection with the proper functioning of cam-operated parts. The mechanical type of lash adjuster apparently covers up a multitude of sins which, in normal operation, apparently are not considered the instigators of troubles that develop from time to time but, with careful study and investigation, are found to be generally the basic cause of failures at a later date. We define “false motion” in valve gear as being any motion of parts other than the theoretical. The cam is laid out to impart a certain motion to the valve. Calculation of stresses are based on this assumption and the parts designed accordingly, but the Nemesis of valve mechanism in the form of false motion creeps in and parts still misbehave. This departure from the true path affects any part of the mechanism from the valve to the cam, since high loadings can be encountered which far exceed the calculated stresses with consequent failure, partial or complete. It has considerable effect on seating velocities as affecting valve and cylinder-seat life. This particular item is an exceedingly difficult fact to run down as there are generally many other variables that affect it, and ofttimes these variables obscure the direct cause of the trouble. To illustrate this point, data have been developed to show the effect of cam design on the lift diagram. The diagrams shown are developed from point-to-point measurements of valve lift at the various cam angles, covering only that portion of the cam from the start of opening to seating. The lift actually is measured at each cam angle by a specially developed micrometer operating in conjunction with stroboscopes at the valve and at the camshaft – the first being necessary to measure the lift, and the second to locate the exact angle at which this lift is being measured. Fig. 1 is the record of a cam having a rather high acceleration; the full line represents the lift at comparatively low speed (500 rpm), and the dash curve that taken at 2050 rpm. The outstanding fault with this particular cam is that the loading is such that considerable deflection takes place which does not cause too much harm on the opening side, but can certainly cause a great many things to happen on the closing side. The seating velocity is increased on the order of 10:1 and, in this particular test, this velocity was apparently high enough to cause actual deflection of the seat, as it will be noted that the lift curve of the valve travels below the base or zero lift line. This section of the diagram is shown enlarged on Fig. 2, and here can be seen the extent to which false motion is imparted to the valve. Fig. 3 is the lift curve of a cam having approximately one-third the acceleration of that shown on Fig. 1; this condition is accomplished by maintaining the opening point at the same timing position, but by delaying the closing. Higher-speed operation was desired in the second case and, consequently, the whole lift curve was shifted to some extent. However, if less lag were desired, the centerline could be retarded which would bring the actual opening timing point somewhat earlier, but the effective lift would not be changed materially. [This paper was presented at the Annual Meeting of the Society, Detroit, Mich., Jan. 10, 1939.] March, 1939 109 | ||