Sheet handling apparatus with rotary drum
According to the present invention a sheet handling apparatus is provided which comprises a rotary drum with openings at its peripheral wall. A strip with perforations formed therein spirals circumferentially over an outer surface of the drum in a circumferential spiralling direction, such that a screen is formed over the drum. A suction system controls a flow of air through the perforations thereby to attract sheets towards the drum. The strip is biased by means of a tensioning assembly, which exerts a tensioning force on the strip substantially parallel to the circumferential spiralling direction of the strip.
1. Drying drum assembly for a sheet handling apparatus for holding sheets comprising:
a rotary drum having an outer peripheral wall provided with openings;
a strip with perforations formed therein, which strip spirals circumferentially over an outer surface of the peripheral wall of the drum in a circumferential spiralling direction, such that a screen is formed over the peripheral wall of the drum,
wherein the openings of the drum and the perforations of the strip are positioned with respect to one another for being in a fluid connection to one another and to a suction system, which suction system is arranged for controlling a flow of air through the openings of the drum and the perforations of the strip to removably fix the sheets on the screen,
wherein the strip is biased by means of a tensioning device, which exerts a tensioning force on the strip substantially parallel to the circumferential spiralling direction of the strip.
2. A drying drum assembly according to claim 1 , wherein the tensioning device is positioned at an end of the strip.
3. A drying drum assembly according to claim 1 , further comprising a tensioning assembly formed by a tensioning device at each end of the strip, wherein the tensioning devices are oriented in substantially opposite directions to one another in the circumferential spiralling direction of the strip.
4. A drying drum assembly according to claim 1 , wherein the tensioning device comprises a lever pivotably provided on the drum and connected to the drum via a spring element.
5. A drying drum assembly according to claim 4 , wherein during operation the spring element is arranged for exerting a continuous pulling force in the circumferential spiralling direction on the end of the strip via the lever.
6. A drying drum assembly according to claim 1 , wherein a tensioning device is positioned near either end of the drum.
7. A drying drum assembly according to claim 1 , wherein the tensioning assembly further comprises stop elements adjacent the edges of the screen for limiting the axial movement of the strip over the outer surface of the peripheral wall of the drum.
8. A drying drum assembly according to claim 7 , wherein the stop elements are positioned near the edges at the ends of the drum.
9. A drying drum assembly according to claim 1 , wherein the strip and drum are formed of materials having different thermal expansion coefficients.
10. A drying drum assembly according to claim 1 , wherein the strip and the peripheral wall of the drum are arranged for a free, preferably substantially frictionless, sliding motion of the strip over the outer surface of the peripheral wall of the drum.
11. A drying drum assembly according to claim 1 , further comprising air channels provided on the peripheral wall of the drum, which air channels are delimited by the screen.
12. A sheet handling apparatus comprising a drying drum assembly according to claim 1 , and further comprising a suction system for controlling a flow of air through the openings of the drum and the strip, thereby to attract sheets towards the peripheral wall of the drum, such that the sheets are removably fixed on the screen.
13. A sheet handling apparatus according to claim 12 , further comprising a heating system for heating the sheets on the drying drum assembly drum.
14. Printing system comprising a sheet handling apparatus according to claim 12 .
15. Method for producing a drying drum assembly for a sheet handling apparatus according to claim 1 , the method comprising the steps of:
attaching a first end of a longitudinal strip formed of a first material to a first tensioning device preferably at an outer surface of a drum formed of a second material,
wrapping the strip around the outer surface of the drum in a pattern spiralling over the outer surface of the drum, such that a screen is formed over at least part of the outer surface of the drum,
attaching a second end of the longitudinal strip to a second tensioning device preferably at the outer surface of the drum, such that the strip is biased in the circumferential spiralling direction of the strip by means of the tensioning devices.