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).
Page discussing aircraft valve mechanisms, material properties, fatigue failure, and corrosion resistance.
| Identifier | ExFiles\Box 158\5\ scan0024 | |
| Date | 1st March 1939 | |
| March, 1939 AIRCRAFT-VALVE MECHANISMS 115 itic is definitely superior to the hardenable type and, from 1000 F up, it is approximately double. This condition again points to the value of reduced temperatures. The fatigue value under alternating bending stress appears to be somewhat proportional to the ultimate strength, and it is this value that is of the utmost importance in valve life. One of the major causes of complete failure is bending of that section of the stem where it blends into the under-the-head radius. Fig. 13 illustrates a typical fatigue failure due to alternate bending of the valve head with relation to the stem and, while the basic cause may have been so severe that regardless of the type steel used, failure would occur, it is conceivable that, by increasing the fatigue strength by reduced temperatures, a definite improvement could be made. Specimen 1 on Fig. 13 indicates that the bending was mostly one way, with a small residuent zone. Cracks perpendicular to the “annual rings” indicate that, at some time during operation, they were borderlines, showing that the load must have been in the other direction and that the part had been stressed intermittently. During the stop periods the stress concentration in the impending cracks may have been relieved by plastic deformation. Specimen 2 shows bending under rotation, stressed on all fibers. This piece was over-stressed, and it will be noted that the residuent zone is of considerably greater area than that of Specimen 1, but both are products of the same basic cause, namely, excessive distortion between the head and stem of the valve. Corrosion resistance increases quite rapidly with temperature reduction, and it requires no argument to show that reduction in temperature would be of very great importance in reducing this type of attack. In consideration of these advantages, it is obvious that any steps that can be taken to improve cooling would be of material benefit. Ofttimes too great an emphasis is placed on the value of cooling the valve only, without due regard to the consideration that should be given to the guide boss and its cooling. The amount of heat that can be transmitted through the seat varies considerably and is subjected to so many different conditions that any data would be applicable only to a particular set-up. However, if the stem cooling is inadequate, the seat cooling is very ineffective in that the major portion of the valve head and stem will still attain high temperatures. Fig. 14 illustrates the benefit of carrying the guide-boss cooling as close to the valve seat as possible, consistent with the gas-flow through the port. It will be noted that, in both designs, the seat is reasonably cool, yet the portion of the valve requiring the highest resistance to corrosion, maximum strength, and so on – to say nothing of the effect of the mass of hot metal on combustion and volumetric efficiency – is up in the zone of reasonably high temperatures when the stem is cooled poorly. That the internally cooled valves have made possible the present high-output engines is a well-known fact, and the present trend is to develop cooling to the maximum amount permissible. This trend is in the direction of increasing the internal area exposed to the coolant, with larger so-called cavities in the head as compared to the straight-hole type. Where the port design is such that considerable distortion takes place, attempts have been made to incorporate some flexibility in the valve head by reduced sections which inherently prevent taking advantage of the maximum amount of cooling that is available with the hollow-head type of valve. If the design of the head cannot be modified to overcome the distortion, it would seem more logical to make use of the so-called “flexible” type of seat ring insert.⁵ Comparative tests of the straight-hole versus the hollow-head type of internally cooled valves indicate a marked reduction in operating temperatures as shown on Fig. 15. It is interesting to note how closely the present type of high-output exhaust valve follows the requirements previously mentioned. The physical properties at high temperature, resistance to warpage and brittleness, corrosion resistance, and so on, are obtained by the use of the high-chromium, high-nickel austenitic classes of steel. The seat proper, being faced with Stellite, resists both wear and corrosion. The stem life has been increased by nitriding the wearing portion, although this item should be given careful consideration in respect to the operating conditions as affected by lubrication, and so on, as nitriding the austenitic steels greatly reduces their resistance to cold corrosion resulting from the condensation of the products of combustion. The tip life has been increased to a marked ⁵See “Flexible Exhaust-Valve Seats,” by S. D.{John DeLooze - Company Secretary} Heron and A.{Mr Adams} L. Beall, presented at the National Aeronautic Meeting of the Society, Washington, D.{John DeLooze - Company Secretary} C., Mar. 12, 1937. Fig. 13 – These typical valve fatigue failures are due to alternate bending of the valve head with relation to the stem TYPICAL FATIGUE FAILURE OF VALVE STEMS Fig. 12 – The curves represent graphically the effect of temperature on the ultimate strength and hot hardness of two typical valve steels Graph Data: Title: Temperature, C / Temperature, F Legend: A-Hardenable Steel, B-Austenitic Steel Y-Axis Left: Ultimate Strength, lb per sq. in. Y-Axis Right: Brinell Hardness | ||