Document B5KrdvvQKKy6JeB8D895op34E

TECHNICAL DATA Basic Theory of Sealing In order to provide a leak-free joint be tween any two members of an assembly, one condition must be met: The size of the po tential leakage paths must be smaller than the molecular size of the medium to be confinejJ. As long as this is true, total sealing will be achieved. If this condition is not met, the extent of the resulting leakage will be governed by the laws of fluid mechanics and thermodynamics. Simply stated, the extent of the leakage will be determined by the number of mole cules allowed to pass simultaneously through any given section of the joint and the driving force (differential pressure) behind them. Extension of Basic Theory to the Mechanism of Sealing the Cylinder Head The Dynamic Case In order to effectively seal the cylinder head of the typical reciprocating piston en gine, adherence to the basic theory of seal ing must be embraced. One further factor must be recognized, however. As the firing pressures in the combustion chamber pro duce assembly deflections of some magni tude, the cylinder head gasket must accom modate them while attempting to seal the combustion gases. The system deflections and the consequential gasket motion result in' a reciprocating stress (or load) condition on the gasket and its components. Unless the magnitudes of the operating stress limits on the gasket are considered, success of any gasket would be due strjctly to chance. The task relegated to the gasket, there fore, is to yield (or flow) sufficiently into the minute asperities of the two mating members of the joint in order to limit the size of the potential leakage paths to nothing larger than the molecular size of the confined medium. Having done this,' the gasket must retain that condition for the expected life of the assembly. To completely understand this mechan ism of operation, one should refer to an ideal free body force diagram of the cylinder head (FIG. 1). In this figure, L, & L2 are equal and represent the bolting forces. P|, P2, P3 & P4 are equal and represent the compressive forces common to the gasket. As the figure shows, the system is in equilibrium since the sum of the forces is equal to zero. As simple as the foregoing appears, the real problems encountered in any applica tion are caused by the environment of the joint. Such factors as chemical and thermal reaction between the confined medium and the gasket material, system vibrations, re lative motion between mating surfaces, and pressure of the confined medium all influ ence the performance of the gasketed joint. Identification of the magnitude of each of these factors is a necessity prior to success ful design of a gasket. As the engine is fired, however, the com bustion force must be added, Fv (FIG. 2). The combustion force can be calculated from the firing pressure (peak) and the area over which it acts, Fy = P A No. 10106 B 1 1-70