LOW-PRESSURE SEALING INTEGRITY OF O-RINGS BASED ON INITIAL SQUEEZE AND COUNTERFACE FINISH

Sealing integrity of O-rings at low pressures is studied. For a given squeeze, this is found to be independent of hardness (modulus), i.e., squeeze alone determines the degree of infilling and the resulting lea kage path geometry for a particular surface finish. Relationships for footprint width and contact pressure as functions of squeeze size and modulus are presented and correlated with other published results. Inf illing of rubber into typical sawtooth machining finishes if calculate d, measured and presented as a function of O-ring squeeze, independent of rubber hardness. Sealing can be improved by increasing the squeeze (greater infilling) or the O-ring cord diameter (greater leakage path length), or by reducing the steepness and roughness height of the fin ish. Sealing is limited by an O-ring's upper limit of squeeze a test m ethodology to find this is described. Results are presented for a part icular fluorocarbon rubber, showing significant permanent damage begin ning at about 45 percent squeeze. Various approaches to calculation of leakage are discussed. For designers, the conclusions for better seal ing are to use fat, highly squeezed O-rings, and be aware that steepne ss (average slope) of the finish perpendicular to potential leakage pa ths is more important than a simple roughness number for the counterfa ce.