Abstract: A sieve screen deck comprising having a frame and a sieve screen resiliently mounted to the frame so as to allow a limited amount of movement between the sieve screen and the frame. The sieve screen has flexible side walls. The screen deck also has a rapping mechanism which itself has a ram fixed at one end relative to the frame and acting at its other end on a drive shaft via a crank arm. The drive shaft extends across the sieve screen and is rotatably journalled in the respective side walls of the sieve screen. The rapping mechanism also has a rapping bar connected to the drive shaft by resilient elongate arms. The rams is actuated so as to move the rapping bar at intervals from a rest position wherein the rapping bar is spaced a short distance from the sieve screen and the sieve screen is supported by the frame to a primed position.