Counting rotor assemblies

A rotor assembly for counting sheets (11) in a stack (10) thereof and comprising a support (18), a rotor (12) rotatably mounted on the support to engage an edge region of the stack (10) and arranged upon rotation to transfer an edge portion of each sheet (11) in turn through a transfer groove (45) to the other side of the rotor (12), there being at least one suction port in the rotor (12) through which air is drawn in a timed relationship to the rotation of the rotor to assist in the separation of the sheet edge portions, the suction port being connected through the rotor to an inlet port (46) on an external surface (22) of the rotor, and a foot (21) mounted on the support (18) to bear on said external surface of the rotor and having a further port (47) which comes into and out of registration with the inlet port (46) upon rotation of the rotor, so that suction applied to the further port (47) is connected in said timed relationship to the suction port of the rotor (12), characterised in that the rotor (12) is movable axially with respect to the support (18) and detection means (36,60) are provided to detect the position of the foot (21) with respect to the support (18) thereby to give an output indicative of the position of the rotor (12) with respect to the support (18).