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).
Analysis of valve lash variation resulting from the substitution of aluminum for malleable iron rocker arm brackets.
| Identifier | ExFiles\Box 124\3\ scan0427 | |
| Date | 23th December 1936 | |
| VALVE LASH VARIATION By the substitution of aluminum brackets for the malleable iron brackets used in previous designs, a marked reduction in valve lash variation has been obtained. In order to best understand why a more consistent lash is obtained with the use of aluminum brackets, consider the conditions at all temperatures from 0°F to the maximum operating temperature in an engine which has been properly lashed to .015" with approximately 150° oil and water temperatures and then thoroughly cooled to 0°F.{Mr Friese} While the engine was cooling after being lashed under conditions mentioned above the cast iron crankcase contracted a certain amount. The steel valves and steel tubing push rods, each of which have a greater coefficient of expansion than cast iron, contracted slightly more than the crankcase. With malleable iron rocker arm brackets, the lash at 0°F is more than .015", while with aluminum brackets the lash at 0°F is less than .015" caused by the higher coefficient of expansion of aluminum. When the engine is started at 0°F and is warming up, the lash increases in each case until after approximately 8 minutes when its peak is reached. At this point the lash starts to decrease and continues to decrease as long as the engine temperatures are rising. This type of lash curve is caused by the unequal heat distribution to the several engine parts which control the lash and to the different coefficients of expansions of these parts. The valves, cylinder head, and crankcase are first to start to expand, the rocker arm brackets following closely as they are heated from the cylinder head and water. The push rods are last to expand as they receive their heat from the oil which does not rise in temperature very rapidly. With malleable iron rocker arm brackets and no oil temperature regulator, the lash which is more than .015 at 0°F increases to a high peak before the push rods expand sufficiently to compensate for the crank-case and bracket expansion. As valve noise becomes objectionable at approximately .018" the high lash peak and the comparatively long time that the lash remains above .018" cause considerable objectionable noise. Then the lash decreases as the oil temperature and the push rod temperature rises until it becomes considerably less than .015" when the engine becomes very hot. With malleable iron rocker arm brackets and an oil temperature regulator in the rocker arm shaft, the lash is more than .015" at 0°F, and increases somewhat when the engine is warming up, but does not reach a very high peak because the water which rises in temperature rapidly heats the oil that goes to the push rods, causing them to start expanding early in the warm-up period. After the oil reaches a higher temperature than the water, some heat is transferred from the oil to the water and the total push rod expansion with very high oil temperature is thereby relatively reduced. These factors cause a fairly consistent valve lash. | ||