SHEET TRANSPORT APPARATUS FOR AN INKJET SHEET PRINTER

Abstract

 The present invention provides a low costs sheet transport apparatus comprising an endless air permeable belt (101). An endless protrusion (102) is secured onto the belt and extending a predetermined height out of the surface of the belt. A plurality of rollers (110-113) comprising a driving roller (110) driveable by means of an actuator (122) and at least one support roller (111-113) positioned with respect to the driving roller to tension the belt over the plurality of rollers, such that a rotation of the driving roller moves the belt in a transport direction (D) is provided. At least one of the plurality of rollers comprises an endless recess (115) dimensioned to receive the protrusion, such that the protrusion is substantially prevented from transferring driving forces in the transport direction from the plurality of rollers to the belt. The protrusion is suspended within the recess such that it is prevented from transferring forces in the transport direction to belt.

Description

FIELD OF THE INVENTION

[0001] The present invention generally pertains to a sheet transport apparatus as well as to a sheet printer comprising such an apparatus.

BACKGROUND ART

[0002] A sheet transport apparatus is known from EP 3028969 A1 or EP 3023373 A1. The sheet transport apparatus comprises an endless belt supported on a plurality of rollers. Sheets of print medium are adhered to the belt by suction. The belt with the sheets on it passes below an inkjet print head assembly for printing an image on the sheet. The known sheet transport apparatus is provided with sensors for determining the position of the belt as well as actuators for adjusting the position of the belt if the determined position indicates that the belt deviates from its desired position. Though accurate and effective, the known sheet transport apparatuses are relatively expensive.

SUMMARY OF THE INVENTION

[0003] It is an object of the present invention to provide a low costs sheet transport apparatus.

[0004] In an aspect of the present invention, a sheet transport apparatus for transporting sheets of print media is provided. The sheet transport apparatus comprises:

• an endless air permeable belt with an endless protrusion secured onto the belt and extending a predetermined height away from the surface of the belt;
• a plurality of rollers comprising a driving roller driveable by means of an actuator and at least one support roller positioned with respect to the driving roller to tension the belt over the plurality of rollers, such that a rotation of the driving roller moves the belt in a transport direction, wherein at least one of the plurality of rollers comprises an endless recess dimensioned to receive the protrusion, such that the protrusion is substantially prevented from transferring driving forces in the transport direction from its respective roller to the belt while restricting movement of the protrusion and consequently the belt in a width direction perpendicular to the transport direction.

[0005] It is the insight of the inventor that an endless protrusion mounted on the belt in combination with a corresponding recess in one or more rollers provides a low cost system for controlling the lateral position of the belt. It is the further insight of the inventor that the protrusion should substantially only exert forces on the belt in the lateral direction and not drive the belt its transport direction. Thereto, the belt is tensioned along the driving roller, such that driving forces from the driving roller are transferred to the belt via direct contact between the belt and the circumferential surface of the driving roller. This manner of driving a tensioned belt by direct contact with a driving roller allows the belt to be formed of relatively cheap materials, such as plastics. Sheets are adhered to the air-permeable belt via suction and carried below an inkjet print head assembly. Any deformations such as wrinkles in the belt could deform the sheets and result in print artifacts in the print image or damage to the print head due to contact. To avoid wrinkling of the tensioned belt, the protrusion should be prevented from exerting any or too large forces on the belt in the transport direction. Thereto, the recess is dimensioned to receive the belt such that a spacing is present between the bottom of the recess and the protrusion. The protrusion can then substantially act on the belt in the lateral direction, thereby limiting its lateral movement range. While in the recess the lateral movement of the protrusion, and thus of the belt, is restricted, providing a low costs manner to control the lateral position of the belt which allows for the use of low costs belt materials. As such, a low costs sheet transport apparatus has been provided and the object of the present invention has been achieved.

[0006] In an embodiment, the recess extends a predetermined depth radially inward from a circumference of the respective roller, and wherein the depth of the recess is greater than the height of the protrusion. The cross-section of the protrusion is smaller than the cross-section of the recess, specifically in the height direction of the protrusion when received in the recess. Thus a spacing is formed between the top surface of the protrusion and a bottom surface of the recess. The spacing prevents the protrusion from being pressed onto the bottom surface of the recess and experiencing sufficient friction to exert a force on the belt in the transport direction. The top surface of the protrusion when positioned in the recess is on an opposite side of the spacing in the radial direction of the roller with respect to the bottom surface of the recess.

[0007] In a preferred embodiment, the belt is formed of a plastic sheet. Generally sheet metal is preferred to form sheet transport belts due to its rigidity, which rigidity ensures a flat sheet support plane or area. A plastic belt is significantly cheaper to produce than a sheet metal belt, though the rigidity advantage is greatly reduced or even lost. It is another insight of the inventor that the sheet transport apparatus according to the present invention allows for the use of a belt made of a plastic sheet. By tensioning the plastic belt on the rollers a flat sheet support surface is obtained. During operation the flatness is not disturbed by the endless protrusion, though the protrusion still ensures an accurate positioning of the plastic belt in the lateral direction. An additional advantage of a plastic belt is that such a plastic may curved to a narrower curvature than a similar metal belt. The diameter of the rollers may thereby be reduced, which decreases amount of materials, volume, and weight of the sheet transport apparatus according to the present invention. In a preferred embodiment, the belt is fully or in its entirety formed of plastic while the endless protrusion is formed of a different material than the belt, for example rubber. Preferably, the material selected for the protrusion is flexible or deformable so as not to influence the tension in the belt at least in the transport direction.

[0008] In an embodiment, the sheet transport apparatus according to the present invention further comprises a suction box positioned between the plurality of rollers and adjacent the belt as well as a suction source, wherein the belt is provided with a plurality of through-holes for adhering sheets to the belt by means of an under-pressure applied by the suction source. The belt is provided with through-holes in fluid communication with the suction source via the suction box. Sheets can thus be adhered flatly to the belt.

[0009] In an embodiment, in a width or lateral direction perpendicular to the transport direction the driving roller engages the full surface of the belt with the exception of portions of the belt covered by the protrusion, such that driving forces in the transport direction are transferred from the driving roller to, specifically only to, portions of the surface of the belt engaged by or in direct contact with the driving roller. The belt is driven by a single driving roller. The driving roller drives the belt only through contact between the belt and the circumferential surface of the driving roller. The protrusion is prevented from driving the belt in the transport direction. The protrusion basically hangs above the bottom surface of the recess suspended from the tensioned belt. As such, the tension in the belt can be selected to be sufficiently large to maintain flatness during operation in a flexible plastic belt, without the protrusion affecting the driving of the belt.

[0010] In an embodiment, the sheet transport apparatus according to the present invention comprises only a single driving roller. One actuator or motor is provided and the actuator drives only a single one of the rollers. The remaining rollers are passive rollers driven via the belt.

[0011] In an embodiment, the protrusion and recess are tapered in their respective height and depth directions.

[0012] In another embodiment, a rotation axis of at least one of the rollers provided with the recess is skewed with respect to the one or more other rollers, thereby providing a force on the belt in the width direction. While the rotation axes of the other rollers are substantially parallel the roller with recess is skewed at a predetermined angle with respect to said rotation axes. The rotation axis of the skewed roller is offset with respect to said rotation axes by said angle. The angle is defined by rotation around an axis perpendicular to said rotation axes, e.g. an axis parallel to the transport direction at the skewed roller. By skewing the respective roller by a predetermined angle with respect to the other rollers a lateral force acts on the belt at the skewed roller. This force during operation continuously drives the belt against one of the side walls of the recess. Thereby, the positioning of the belt can be more accurately determined.

[0013] In a further aspect, the present invention provides a sheet printer comprising the sheet transport apparatus according to the present invention. Preferably the sheet printer according to the present invention further comprises an inkjet print head assembly mounted over the belt for printing images on a sheet held onto the belt. The inkjet print head assembly is positioned on a side of the belt opposite to the suction box.

[0014] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying schematical drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

Fig. 1 shows a schematic side view of a sheet printer according to the present invention;

Fig. 2 shows a schematic perspective view of an embodiment of a sheet transport apparatus according to the present invention;

Fig. 3 shows a further schematic perspective view of the sheet transport apparatus in Fig. 2;

Fig. 4 shows a schematic cross-sectional view of a steering roller of the sheet transport apparatus in Figs. 2 and 3;

Fig. 5 shows a schematic perspective side of another embodiment of a sheet transport apparatus according to the present invention; and

Fig. 6 shows a schematic cross-sectional view of a driving roller of the sheet transport apparatus in Fig. 5.

DETAILED DESCRIPTION OF THE DRAWINGS

[0016] The present invention will now be described with reference to the accompanying drawings, wherein the same reference numerals have been used to identify the same or similar elements throughout the several views.

[0017] FIG. 1 shows schematically an embodiment of a printing system 1 according to the present invention. The printing system 1, for purposes of explanation, is divided into an output section 5, a print engine and control section 3, a local user interface 7 and an input section 4. While a specific printing system is shown and described, the disclosed embodiments may be used with other types of printing system such as an ink jet print system, an electrographic print system, etc.

[0018] The output section 5 comprises a first output holder 52 for holding printed image receiving material, for example a plurality of sheets. The output section 5 may comprise a second output holder 55. While 2 output holders are illustrated in FIG. 1, the number of output holders may include one, two, three or more output holders. The printed image receiving material is transported from the print engine and control section 3 via an inlet

[0019] 53 to the output section 5. When a stack ejection command is invoked by the controller 37 for the first output holder 52, first guiding means 54 are activated in order to eject the plurality of sheets in the first output holder 52 outwards to a first external output holder 51. When a stack ejection command is invoked by the controller 37 for the second output holder 55, second guiding means 56 are activated in order to eject the plurality of sheets in the second output holder 55 outwards to a second external output holder 57.

[0020] The output section 5 is digitally connected by means of a cable 60 to the print engine and control section 3 for bi-directional data signal transfer.

[0021] The print engine and control section 3 comprises a print engine and a controller 37 for controlling the printing process and scheduling the plurality of sheets in a printing order before they are separated from input holder 44, 45, 46.

[0022] The controller 37 is a computer, a server or a workstation, connected to the print engine and connected to the digital environment of the printing system, for example a network N for transmitting a submitted print job to the printing system 1. In FIG. 1 the controller 37 is positioned inside the print engine and control section 3, but the controller 37 may also be at least partially positioned outside the print engine and control section 3 in connection with the network N in a workstation N1.

[0023] The controller 37 comprises a print job receiving section 371 permitting a user to submit a print job to the printing system 1, the print job comprising image data to be printed and a plurality of print job settings. The controller 37 comprises a print job queue section 372 comprising a print job queue for print jobs submitted to the printing system 1 and scheduled to be printed. The controller 37 comprises a sheet scheduling section 373 for determining for each of the plurality of sheets of the print jobs in the print job queue an entrance time in the paper path of the print engine and control section 3, especially an entrance time for the first pass and an entrance time for the second pass in the loop in the paper path according to the present invention. The sheet scheduling section 373 will also be called scheduler 373 hereinafter.

[0024] The sheet scheduling section 373 takes the length of the loop into account. The length of the loop corresponds to a loop time duration of a sheet going through the loop dependent on the velocity of the sheets in the loop. The loop time duration may vary per kind of sheet, i.e. a sheet with different media properties.

[0025] Resources may be recording material located in the input section 4, marking material located in a reservoir 39 near or in the print head or print assembly 31 of the print engine, or finishing material located near the print head or print assembly 31 of the print engine or located in the output section 5 (not shown).

[0026] The paper path comprises a plurality of paper path sections 32, 33, 34, 35 for transporting the image receiving material from an entry point 36 of the print engine and control section 3 along the print head or print assembly 31 to the inlet 53 of the output section 5. The paper path sections 32, 33, 34, 35 form a loop according to the present invention. The loop enables the printing of a duplex print job and/or a mix-plex job, i.e. a print job comprising a mix of sheets intended to be printed partially in a simplex mode and partially in a duplex mode.

[0027] The print head or print assembly 31 is suitable for ejecting and/or fixing marking material to image receiving material. The print head or print assembly 31 is positioned near the paper path section 34. The print head or print assembly 31 may be an inkjet print head, a direct imaging toner assembly or an indirect imaging toner assembly.

[0028] While an image receiving material is transported along the paper path section 34 in a first pass in the loop, the image receiving material receives the marking material through the print head or print assembly 31. A next paper path section 32 is a flip unit 32 for selecting a different subsequent paper path for simplex or duplex printing of the image receiving material. The flip unit 32 may be also used to flip a sheet of image receiving material after printing in simplex mode before the sheet leaves the print engine and control section 3 via a curved section 38 of the flip unit 32 and via the inlet 53 to the output section 5. The curved section 38 of the flip unit 32 may not be present and the turning of a simplex page has to be done via another paper path section 35.

[0029] In case of duplex printing on a sheet or when the curved section 38 is not present, the sheet is transported along the loop via paper path section 35A in order to turn the sheet for enabling printing on the other side of the sheet. The sheet is transported along the paper path section 35 until it reaches a merging point 34A at which sheets entering the paper path section 34 from the entry point 36 interweave with the sheets coming from the paper path section 35. The sheets entering the paper path section 34 from the entry point 36 are starting their first pass along the print head or print assembly 31 in the loop. The sheets coming from the paper path section 35 are starting their second pass along the print head or print assembly 31 in the loop. When a sheet has passed the print head or print assembly 31 for the second time in the second pass, the sheet is transported to the inlet 53 of the output section 5.

[0030] The input section 4 may comprise at least one input holder 44, 45, 46 for holding the image receiving material before transporting the sheets of image receiving material to the print engine and control section 3. Sheets of image receiving material are separated from the input holders 44, 45, 46 and guided from the input holders 44, 45, 46 by guiding means 42, 43, 47 to an outlet 36 for entrance in the print engine and control section 3. Each input holder 44, 45, 46 may be used for holding a different kind of image receiving material, i.e. sheets having different media properties. While 3 input holders are illustrated in FIG. 1, the number of input holders may include one, two, three or more input holders.

[0031] The local user interface 7 is suitable for displaying user interface windows for controlling the print job queue residing in the controller 37. In another embodiment a computer N1 in the network N has a user interface for displaying and controlling the print job queue of the printing system 1.

[0032] Fig. 2 shows in more detail the sheet transport apparatus 100 which forms the paper path section 33. The sheet transport apparatus 100 is positioned below the inkjet print head assembly 31, such that the print heads may jet ink onto a sheet S held by the sheet transport apparatus 100. Local height variations or deformations in the sheet S may result in print artifacts in the printed image. It is thus preferred that the sheet S while on the sheet transport apparatus 100 is substantially flat, i.e. in a planar form substantially free of local deformations. Thereto a holding force is provided over substantially the full surface of the sheet S.

[0033] The sheet transport apparatus 100 comprises an air-permeable belt 101. The belt 101 runs as a loop around the rollers 110-113. The belt 101 is provided with a plurality of through-holes (not shown), preferably distributed in a pattern over the majority of or substantially the full surface of the belt 101 where sheets S are to be supported. On the opposite side of the belt 101 with respect to the inkjet print head assembly 31 a suction box (60 in Fig. 5) is provided. The suction box is positioned near or against the bottom side of the belt 101, such that a suction source (61 in Fig. 5) may draw in air through the through-holes via the suction box 60. As such an under-pressure is applied to sheets S on a portion of the belt 101 over the suction box 60. The through-holes ensure a holding down over substantially the full surface of the sheet S.

[0034] The belt 101 in Fig. 2 is formed of plastic, for example polyethylene, polypropylene, polyacetal, nylon, or other suitable plastic materials. While using plastic reduces the costs of producing a sheet transport belt 101 compared to e.g. a metal belt, the relatively flexibility of the plastic could result in wrinkles in the belt 101. These wrinkles could locally deform the sheets S on the belt 101 and result in print artifacts. To prevent wrinkling the plastic belt 101 is sufficiently tensioned over the rollers 110-113. When viewed in the width direction Y, the belt 101 comprises a sheet holding area HA or surface portion in which area through-holes are provided for adhering the sheet S to the belt 101. Each roller 110-113 defines or comprises a corresponding contact area which is positioned to be contacted or engaged by the sheet holding area HA. The contact area of the rollers 110-113 is formed by the outer circumferential surface (119 in Fig. 4) of the rollers 110-113. The contact area is preferably smooth and continuous in both the tangential and width direction Y of the rollers 1101-113. By sufficiently tensioning the belt 101 over the smooth contact surfaces of the 110-113 a flat sheet holding area HA is achieved.

[0035] One of the rollers 110 is provided with an actuator or motor (122 in Fig. 6) for driving the respective roller 110 around its rotation axis (Ra in Fig. 4). This driving roller 110 drives and controls the speed of the belt 101 in its transport direction D. The torque from the motor 122 is transferred to the belt 101 directly via the outer circumferential surface 119 of the driving roller 110. This contact surface of the driving roller 101 is smooth and/or continuous across the full width of the sheet holding area HA. The belt 101 is tensioned, such that driving forces from the driving roller 110 are only transferred via portions of the holding area HA in contact with circumferential surface of the driving roller 110. Thereby, the torque is distributed substantially evenly over the width of the sheet holding area HA.

[0036] During operation the belt 101 may move over the rollers 110-113 in the width direction Y, e.g. due misalignments between the rollers 110-113. To restrict movement of the belt 101 in the width direction Y, a protrusion 102 is mounted on the belt 101. The protrusion 102 is endless and runs as a loop over a surface of the belt 101 in the transport direction D. The protrusion 102 may be formed as an endless beam element 102 secured onto the belt 101. The protrusion 102 extends away from the surface of the belt 101 by a predetermined height H. The protrusion 102 has a predetermined width W significantly less than the width of the sheet holding area HA in the width direction Y. The height H and width W of the protrusion 102 are substantially constant or similar over its full length in the transport direction D.

[0037] In the embodiment in Fig. 2, the protrusion 102 is mounted on the "outer" surface of the belt 101. Only the steering roller 112 in Fig. 2 is then in contact with the side of the belt 101 on which side the protrusion 102 is mounted. The steering roller 102 is provided with a circumferential recess 103, as shown in Figs. 3 and 4. The recess 103 is formed as on endless angular groove 103 on the outer surface 119 of the steering roller 112. The recess 115 in the exaggerated view of Fig. 4 comprises a bottom surface 116 radially spaced apart from the circumferential surface 119 of the steering roller 112 by a predetermined distance D. Laterally to either of the bottom surface 116 side walls 117 extend radially towards the outer surface 119 of the roller 112. The sidewalls 117 are inclined with respect to the radial direction. The radially outer portion of the recess 115 is provided with a mounting strip receiving space 118, which extends laterally besides the side walls 117. The depth of the endless mounting strip receiving spaces 118 on either side of the bottom surface 116 is significantly less than the depth D of the recess 115.

[0038] The protrusion 102 is dimensioned to be received inside the recess 115. A cross-section of the protrusion 102 in a plane through the rotational axis RA of the steering roller 112 is smaller than a corresponding cross-section of the recess 115. Thereby, during operation the top surface 103 of the protrusion 102 is at all times spaced apart from the bottom surface 116 of the recess 115. Friction forces between the recess 115 and the protrusion 102 are thus small or absent, such that the movement and the speed of the belt 101 in the transport direction are not affected by the contact between the protrusion 102 and the recess 115. The side walls 117 of the recess 115 restrict movement of the protrusion 102 and thus of the belt 101 in the width direction Y. Contact between the side walls 117 of the recess 115 and the side walls 104 of the protrusion 102 results in a lateral force in the width direction Y preventing substantive lateral displacement of the belt 101. Preferably the side walls 117 of the recess 102 provide a close or tight fit for to the side walls 104 of the protrusion 102 to accurate define the position of the belt 101 in the width direction Y. The distance between adjacent or opposing sidewalls 104, 117 is substantially less than the spacing or distance between the bottom surface 116 and the top surface 103, preferably less than half, very preferably less than a tenth of said spacing. No active control systems are thus required for controlling the lateral position of the belt 101, reducing the costs of a sheet transport apparatus 100.

[0039] The steering roller 112 is further provided with mounting strip receiving spaces 118 on either side of the recess 102. The mounting strip receiving spaces 118 are positioned and dimensioned to receive a mounting strip 105 of the protrusion 102. The mounting strip 105 is wider than the elevated portion of the protrusion 102 which provides an improved holding of the protrusion 102 on the belt 101. The mounting strip receiving spaces 118 during operation preferably contactlessly receive the mounting strip 105 to avoid friction. Preferably, the protrusion 102 and the mounting strip 105 are formed from the same material or have been integrally formed.

[0040] Flatness of the sheet support surface HA is ensured by sufficiently tensioning the belt 101 over the rollers 110-113. The tension in the belt 101 ensures the belt 101 is and pressed against frictionally by the outer surface 119 of the rollers with the exception of the protrusion area of the belt 101 positioned over the recess 115. Due to the tension the belt 101 extends flatly across the recess 115 at least in the width direction Y. The protrusion 102 is spaced apart from the bottom surface 116 of the recess 102 and thus prevented from being pressed against the bottom surface 116. The bottom surface 116 does not frictionally engage the protrusion 102, such that the movement and speed of the belt 101 in the transport direction D is driven independent of the protrusion 102.

[0041] Fig. 5 shows an alternative embodiment of a sheet transport apparatus 100 according to the present invention. In Fig. 5 all rollers 110-113 are in contact with the protrusion side of the belt 101. The suction box 60 or vacuum table is positioned opposite the inkjet print head assembly 31. The suction box 60 is connected to a suction source 61, such as a pump or fan to apply an under-pressure to the sheets S via the through-holes in the belt 101. The passive rollers 111-113 (i.e. rollers not directly connected to a motor 122) are configured as shown in Fig. 4. The driving roller 110 is illustrated in Fig. 6. The driving roller in Fig. 6 comprises the components shown in Fig. 4 with the addition of a drive pulley 120 which is connected via a belt 121 to the motor 122. The driving forces from the motor 122 are transmitted to the driving roller 110 via the belt 121 and pulley 122. The driving roller 110 transfers the driving forces in the transport direction D through contact between the belt 101 and the outer surface 119 of the driving roller 110. The protrusion 102 is prevented from transferring forces in the transport direction D due to the spacing between the bottom surface 116 of the recess 115 and the top surface 103 of the protrusion 102.

[0042] Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. In particular, features presented and described in separate dependent claims may be applied in combination and any advantageous combination of such claims are herewith disclosed.

[0043] Further, it is contemplated that structural elements may be generated by application of three-dimensional (3D) printing techniques. Therefore, any reference to a structural element is intended to encompass any computer executable instructions that instruct a computer to generate such a structural element by three-dimensional printing techniques or similar computer controlled manufacturing techniques. Furthermore, such a reference to a structural element encompasses a computer readable medium carrying such computer executable instructions.

[0044] Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly.

[0045] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. A sheet transport apparatus (100) for transporting sheets (S) of print media, comprising:

- an endless air permeable belt (101) with an endless protrusion (102) secured onto the belt (101) and extending a predetermined height (H) out of a surface of the belt (101);

- a plurality of rollers (110-113) comprising a driving roller (110) driveable by means of an actuator (122) and at least one support roller (111-113) positioned with respect to the driving roller (110) to tension the belt (101) over the plurality of rollers (110-113), such that a rotation of the driving roller (110) moves the belt (101) in a transport direction (D), characterized in that at least one of the plurality of rollers (110-113) comprises an endless recess (115) dimensioned to receive the protrusion (102), such that the protrusion (102) is substantially prevented from transferring driving forces in the transport direction (D) from its respective roller (110-113) to the belt (101) while restricting movement of the protrusion (102) and consequently the belt (101) in a width direction (Y) perpendicular to the transport direction (D).

2. The sheet transport apparatus (100) according to claim 1, wherein the recess (102) extends a predetermined depth (D) radially inward from a circumference (119) of the respective roller (110-113), and wherein the depth (D) of the recess (102) is greater than the height (H) of the protrusion (102).

3. The sheet transport apparatus (100) according to claim 1 or 2, wherein the belt (101) is formed of a plastic sheet.

4. The sheet transport apparatus (100) according to any of the previous claims, further comprising a suction box (60) positioned between the plurality of rollers (110-113) and adjacent the belt (101) as well as a suction source (61), wherein the belt (101) is provided with a plurality of through-holes for adhering sheets (S) to the belt (101) by means of an under-pressure applied by the suction source (61).

5. The sheet transport apparatus (100) according to any of the previous claims, wherein in the width direction (Y) perpendicular to the transport direction (D) the driving roller (110) engages the full surface of the belt (101) with the exception of portions of the belt (101) covered by the protrusion (102), such that driving forces in the transport direction (D) are transferred from the driving roller (110) to portions of the surface of the belt (101) in contact with the driving roller (110).

6. The sheet transport apparatus (100) according to any of the previous claims, comprising only a single driving roller (110).

7. The sheet transport apparatus (100) according to any of the previous claims, wherein the protrusion (102) and recess (115) are tapered in their respective height (H) and depth (D) directions.

8. The sheet transport apparatus (100) according to any of the previous claims, wherein a rotation axis (RA) of at least one of the rollers (110-113) provided with the recess (115) is skewed with respect to the one or more other rollers (110-113), thereby providing a force on the belt (101) in the width direction (Y).

9. The sheet transport apparatus according to any of the previous claims, wherein the belt comprises a sheet holding area defined by through-holes on the belt, and wherein the protrusion is positioned on the belt outside of the sheet holding area.

10. The sheet transport apparatus (100) according to any of the previous claims, wherein the protrusion (102) is position at a lateral side of the belt (101).

11. The sheet transport apparatus (100) according to any of the previous claims, wherein the roller (110-113) comprising the recess (115) is positioned within a volume defined by the others of the plurality of rollers (110-113), wherein the protrusion (102) is provided on a surface of the belt (101)facing the recess (115) of said roller (110-113) and facing away from the circumference (119) of the others of the plurality of rollers (110-113).

12. A sheet printer (1) comprising the sheet transport apparatus (100) according to any of the previous claims.

13. The sheet printer (1) according to claim 12, further comprising an inkjet print head assembly (31) mounted over the belt (101) for printing images on a sheet (S) held onto the belt (101).

Drawing