Elongation is the percentage increase in original length (strain) of a rubber specimen as a result of tensile force (stress) being applied to the specimen. Elongation is inversely proportional to hardness, tensile strength, and modulus. That is, the greater a material’s hardness, tensile strength, and modulus, the less it will elongate under stress. It takes more force to stretch a hard material with high tensile strength and high modulus than to stretch a soft material with low tensile strength and low modulus.

Ultimate elongation is the elongation at the moment the specimen breaks. Per ASTM D 412, ultimate elongation is generally noted along with tensile strength and modulus during tensile testing. Some elastomeric materials are much more forgiving in this area than others. Natural rubber can often stretch up to 700% before breaking. Fluorocarbons typically rupture at about 300%. Keep in mind that these figures highlight relative failure modes only and do not reflect shaft seal installation values. As noted in the discussion of modulus, the elastomeric lip of a shaft seal is typically stretched no more than 5% during installation. Greater stretch can be problematic, not because it pushes the bounds of the material’s properties, but because greater lip stretch upon installation means the lip will exert increased radial force on the shaft. This increased force will generate greater friction and heat, both of which can decrease the useful life span of the seal.

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