of the rack the motors 6 are preferably arranged in two rows extending parallel to the longitudinal direction of the carriers with this arrangement it is possible to accommodate relatively large motors 6 for driving closely spaced carriers 12 it will be appreciated that in each group of motors 6 the motors 6 may be arranged in one row or in more than two rows a second embodiment 60 is illustrated in fig3 to 5 wherein parts similar to those in the first embodiment are referenced by the same reference numerals in embodiment 60 each motor 6 is arranged to directly drive an associated pinion gear 30 via a drive gear 61 accordingly in the second embodiment all motors 6 carried by the common support plate 51 are arranged in one row the plate 51 is inclined across adjacent carriers 12 to enable individual pinion gears 30 to mesh with an associated carrier in embodiment 60 sensor 40 for sensing the position of the associated carrier has been repositioned to co operate with the teeth 37 of the associated pinion gear 30 in this respect the sensor 40 is preferably an optical sensor which is arranged to detect the spaces between the pinion teeth 37 as the pinion gear rotates accordingly in embodiment 60 markers 41 on each carrier 12 have been.