Pneumatic brake

Claim 1

In a pneumatic brake assembly for tensioning a data carrier in an electrostatic printing device where the data carrier passes over a brake surface of the pneumatic brake in contact therewith across substantially the full width of the data carrier, with a suction force being applied at the brake surface to draw the data carrier to the brake surface, the improvement of: the pneumatic brake including an enclosed vacuum chamber having at least one side thereof fromed by the brake surface, a plurality of suction orifices through the brake surface communicating to an interior of the vacuum chamber, the vacuum chamber being connected to a suction source, suction inhibiting means in the vacuum chamber, the orifices through the brake surface being distributed across the full width of the brake surface, the suction inhibiting means being such that the effective width of the brake element can be adopted to the width of the data carrier passing thereover without adverse reduction of the suction effect through the orifices covered by the data carrier when portions of the brake surface are not covered by the data carrier, the vacuum chamber being subdivided into subchambers by internal partition walls, each of the subchambers open to some of the brake surface orifices, the subchambers being connected to one another through openings in the partition walls between adjacent subchambers, the suction device being connected to a subchamber located at one side end of the vacuum chamber, the side end being adjacent a transverse edge of the data carrier passing over the pneumatic brake, the openings in the partition walls being of limited cross-section area effective to provide a pressure drop between adjacent chambers when one of the subchambers has a data carrier overlying the portion of the brake surface associated with a first of said adjacent subchambers and another of said adjacent subchambers does not have a data carrier overlying the portion of said brake surface associated with said another of said adjacent subchambers whereby an effective negative pressure is maintained in the first of said subchambers, the partition walls extending along the length of the vacuum chamber in a direction parallel to a path of movement of the data carrier along the brake surface, the vacuum chamber being closed except through the orifices and the connection to the suction device, there being a plurality of partition walls and subchambers transverse the width of the vacuum chamber.