Full definition
Sealing principle where two lapped surfaces (flatness <1 μm, roughness Ra <0.4 μm) contact with a fluid film between them. Basis of mechanical seals. Hydrodynamic pressure in the film prevents solid contact and excessive wear. Stationary face: carbon graphite or SiC; rotating face: SiC or tungsten carbide. Face combinations selected for hardness differential and fluid compatibility.