Hydraulic seal with pressure-actuated ridges

A high pressure hydraulic piston seal for sealing a piston to the wall of a cylinder comprising a substantially cylindrical elastomeric body provided with an integral circumferentially continuous, radially outwardly projecting, primary sealing ridge, and on one side of the primary ridge with a web of radially outwardly projecting secondary sealing ridges connecting with the primary ridge at a series of circumferentially spaced positions to define with the primary ridge a plurality of recesses on said one side of the primary ridge to act as fluid reservoirs, the radial height of the secondary ridges being chosen such that at low differential hydraulic pressures acting across the seal, in the direction from said primary ridge towards said one side, fluid in said reservoirs is in communication with the cylinder space on said one side of the seal by way of radial clearance between at least some of the secondary ridges and the cylinder wall, but that at high differential pressures acting in that direction the secondary ridges are pressed radially outwardly into firm sealing engagement with the cylinder wall through elastomeric deformation of the body to isolate the reservoir spaces from one another and from the pressure space on that other side.