Gaskets and Seals
An important challenge in engine design is that of providing leak-free joints at each of the component mating surfaces exposed to one or more of the working fluids. Minimizing the number and complexity of joints is one design goal. Another is that of prov
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Gaskets and Seals
14.1 Gasketed Joint Fundamentals An important challenge in engine design is that of providing leak-free joints at each of the component mating surfaces exposed to one or more of the working fluids. Minimizing the number and complexity of joints is one design goal. Another is that of providing durable, effective seals at each of the remaining joints. The working fluids include fuel, lubricant, coolant, intake air, and combustion products. Some joints must maintain separation between two or more of the engine’s working fluids, while others seal one of the working fluids from the atmosphere. Some joints are stationary, while others include the need for one component to move relative to the other. These moving joints include the interface between the pistons and cylinder walls, those between the valve stems and guides, and those associated with spinning shafts protruding from the engine (the crankshaft and the water pump are examples). A categorization of seal types is presented in Fig. 14.1. Piston rings and valve stem seals will be covered in Chaps. 15 and 17 respectively. Looking first at the static joints Fig. 14.2 provides a general summary of the characteristics of a gasketed joint. Sufficient clamping force must be applied to the gasket to ensure an effective seal. Any given combination of gasket material and surface finish will result in some minimum sealing load as shown in the figure. The sealing load is provided by screws or bolts located at various points along the gasketed surface. These fasteners provide a clamp force over the gasket area which results in an average sealing contact stress, or unit sealing stress, as shown in Eq. 14.1. Average Sealing Contact Stress =
# Fasteners ⋅ Clampload Each Total Gasket Area
© Springer Vienna 2016 K. Hoag, B. Dondlinger, Vehicular Engine Design, Powertrain, DOI 10.1007/978-3-7091-1859-7_14
(14.1)
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14 Gaskets and Seals
Fig. 14.1 Categorization of seal types
Fig. 14.2 General characteristics of the gasketed joint, showing upper and lower load limits
Assuming flat surfaces, the clamping force will be highest at each of the fasteners, and will drop to a local minimum between each of them. If the stiffness of both surfaces is constant along its length the local minimum clamping force will occur midway between the two surrounding cap screws. This can be represented by a beam fixed at both ends with a distributed load as shown in Fig. 14.3, and modeled in Eq. 14.2: W ⋅ L3 ymax = (14.2) 384 ⋅ E ⋅ I
14.1 Gasketed Joint Fundamentals
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Fig. 14.3 Fixed-Fixed Beam with uniform load
Where: ymax = W = L = E = I =
Maximum deflection at midspan of beam Total load Length of span between fastener centerlines Modulus of Elasticity of flange Moment of inertia of flange
A general guideline is to limit the maximum deflection, at midspan of the flange, to less than 0.034 mm of deflection per 1.0 MPa of sealing contact stress for flat gaskets and liquid sealants as shown in Eq. 14.3. This generally results in a fastener span of seven
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