Apparatus is provided, in or for a screen separator for sieving materials, for clamping one or more sieve screen frames 78 within the confines of a chassis 40 of the separator to prevent egress of material from the separator. The apparatus comprises a sheath 50 extending around and embracing the chassis 40 and one or more expandable elements 66 for urging the sheath 50 against the chassis 40 and the screen frame(s) 78 to impart uniform clamping pressure to the clamped elements. The sheath 50 is designed to provide ease of mounting and release.