An actuator for a fluid process control device, comprising: a housing comprising a first housing component and a second housing component; a diaphragm comprising an outer radial portion and an inner radial portion, the outer radial portion of the diaphragm carried between the first and second housing components (32, 34), the inner radial portion comprising an inner opening; a stem operatively coupled to the diaphragm for controlling the fluid process control device, the stem including a shoulder and a threaded end portion; and a plate assembly connecting the stem and the diaphragm, the plate assembly comprising a concave plate and a convex plate compressing the diaphragm therebetween, characterised in that a threaded means threaded onto the threaded end portion compresses the plate assembly against the shoulder and compresses and flattens inner radial portions of the concave plate and the convex plate into engagement with each other, the plate assembly applying a compressive load to the inner radial portion of the diaphragm and providing a line of contact with the stem to provide a fluid-tight seal between the plate assembly and the stem, wherein the inner radial portion of the diaphragm comprises a bead that extends around a circumference of the opening and that is disposed within a circumferential cavity formed between circumferential recesses in the concave and convex plates (152, 154); wherein the compressed and flattened configuration of the concave and convex plates (152, 154) applies a tensile load to a portion of the threaded end portion of the stem at a location between the concave and convex plates (152, 154) to provide a fluid tight sealing engagement between the concave plate and a shoulder surface of the shoulder of the stem. a housing comprising a first housing component and a second housing component; a diaphragm comprising an outer radial portion and an inner radial portion, the outer radial portion of the diaphragm carried between the first and second housing components (32, 34), the inner radial portion comprising an inner opening; a stem operatively coupled to the diaphragm for controlling the fluid process control device, the stem including a shoulder and a threaded end portion; and a plate assembly connecting the stem and the diaphragm, the plate assembly comprising a concave plate and a convex plate compressing the diaphragm therebetween, characterised in that a threaded means threaded onto the threaded end portion compresses the plate assembly against the shoulder and compresses and flattens inner radial portions of the concave plate and the convex plate into engagement with each other, the plate assembly applying a compressive load to the inner radial portion of the diaphragm and providing a line of contact with the stem to provide a fluid-tight seal between the plate assembly and the stem, wherein the inner radial portion of the diaphragm comprises a bead that extends around a circumference of the opening and that is disposed within a circumferential cavity formed between circumferential recesses in the concave and convex plates (152, 154); wherein the compressed and flattened configuration of the concave and convex plates (152, 154) applies a tensile load to a portion of the threaded end portion of the stem at a location between the concave and convex plates (152, 154) to provide a fluid tight sealing engagement between the concave plate and a shoulder surface of the shoulder of the stem.