A number of references show developing stations which are movable into position with respect to an image member as part of the operation of the machine. For example, U.S. Pat. No. 4,928,146 shows apparatus in which four linearly arranged development stations are sequentially moved to a single development position to apply different color toner to four consecutive electrostatic images. U.S. Pat. No. 4,622,916 shows four toner stations on a rotary carriage which rotates the stations through a single development position to also apply different color toners to four consecutive images. U.S. Pat. No. 4,801,966 is typical of a large number of references showing toning stations that are movable in and out of their own unique developer position to apply the correct color toner to the image being toned. U.S. Pat. No. 4,891,672 shows system in which one of a group of color stations is moved into a single toning position for a series of reproductions and then is replaced on demand from a storage position by another toning station of different color for another series of reproductions.
In most of these apparatus, a drum photoconductor is permanently fixed in the apparatus as is the supporting structure for each development unit. With such structure, critical positioning of each development unit with respect to the photoconductive drum can be managed by precise manufacturing and assembly of those parts and their supporting structure. It would be desirable to remove the need for such precision.
U.S. Pat. Nos. 4,922,302, issued to Hill et al on May 1, 1990; 4,884,109 issued to Hill et al on Nov. 28, 1989 and 4,797,704 issued to Hill et al on Jan. 10, 1989; show a development station having an applicator with a rotating magnetic core and a stationary nonmagnetic sleeve around which a developer mixture is moved by rotation of the core to pass the developer through a development position. The applicator is fed by a rotating paddle positioned below the applicator which both mixes developer and supplies it to the applicator. This particular structure requires that the applicator not be in contact with the image member carrying an electrostatic image to be developed, but that it be precisely spaced from it.
U.S. Pat. No. 4,801,966 (cited above) shows a developer applicator which is spaced from a photoconductive drum by a pair of rollers which engage the drum. This approach will provide accurate spacing only if other aspects of the relative position of the applicator and drum are precisely controlled.